late ordovician- early silurian rh ynchonellid ...

LATE ORDOVICIAN- EARLY SILURIAN RH YNCHONELLID BRACHIOPODS FROM ANTICOSTI ISLAND

QUEBEC.

Jisuo JIN

Collection Universite

"BIOSTRATIGRAPHIE

DU PALEOZOIQUE"

Claude Bernard, Lyon I

0

Collection

�BIOSTRATIGRAPHIE DU PALEOZOIQUE.

Universite Claude Bernard, Lyon I

10

LATE ORDOVICIAN- EARLY S ILURIAN RH YNCHONELLID BRACHIOPODS FROM ANTICOSTI ISLAND, QUEBEC.

Jisuo JIN

Editeur: Universite Claude Bernard- Lyon I, Centre des Sciences de Ia Terre, 27-43, boulevard du 11 Novembre, F-69622 VILLEURBANNE CEDEX (FRANCE)

Impression:

Texte et reliure: Atelier offset, Centre des Sciences de Ia Terre, UCB-Lyon I. Couverture et planches photographiques: Atelier J.-M. PIERRE, Imprimeur Editeur, F-445 10 LE POULIGUEN

Dr. P.R. RACHEBCEUF

Commandes adressees a :

Universite Claude Bernard- Lyon I Centre des Sciences de la Terre 27-43, boulevard du 11 Novembre F-69622 VILLEURBANNE CEDEX (FRANCE)

Prix de vente: 180,00 Francs franr;ais (frais d'envoi non compris).

Redacteur: Patrick R. RACHEBCEUF

Comite de lecture:

Claude BABIN (Universire Claude Bernard, Lyon) Charles Howard C. BRUNTON (British Museum, London) Jean-Louis HENRY (Universite de Rennes) Francis LETHIERS (Universite Pierre et Marie Curie, Paris)

Adresse de I'auteur:

Dr. Jisuo JIN The University of Western Ontario Department of Geology LONDON, Ontario, CANADA N6A 5B7

Reference bibliographique:

Jin J. 1989 - Late Ordovician and Early Silurian Rhynchonellid Brachiopods from Anticosti Island, Quebec. Biostratigraphie du Paleozoique, 10 : 127 p., 71 fig., 30 pl.

Illustration de la couverture:

Hercotrema bulbicostatum n. gen., n. sp. Richardson Member of Jupiter Formation, x 4.

ISSN

:

0295-0146, ISBN

:

2-905431-09-1, Depot legal

:

15 novembre 1989

1

Born in 1957 in Hebei Province, People's Republic of China, Jisuo JIN

attended Beijing University between 1978 and 1982, graduating with the B.Sc. degree in geology. At the invitation of Professor Paul Copper of Laurentian University, Sudbury, Ontario, and with the support of the

Chinese Education Ministry, Jisuo JIN then went to Canada to study

towards the M.Sc. degree. His thesis dealt with Early Silurian pentamerid brachiopods of Anticosti Island in the Gulf of the St Lawrence. Having

met the requirements of the M.Sc. degree in 1984, Jisuo JIN then transferred to the University of Saskatchewan and began an investigation

of the Late Ordovician-Early Silurian rhynchonellid brachiopods of Anticosti Island for the Ph.D. degree, which he completed in 1988. The

results form the basis of this monograph. JIN's doctoral studies were supported by a University of Saskatchewan Graduate Fellowship and by

the Natural Sciences and Engineering Research Council of Canada.

Following a year as a Post-Doctoral Fellow at the University of Saskatchewan, Jisuo JIN is now a Professional Research Associate in the Department of Geology of the University of Western Ontario where, with the support of the Geological Survey of Canada, he is investigating the Late Ordovician-Early Silurian brachiopod faunas of the Hudson Bay Lowlands and other intracratonic basins of North Alnerica.

3

CONTENTS

Pages 5

RESUME/ABSTRACf

7

INTRODUCfiON

9

GEOLOGICAL SETIING

13

EXTERNAL MORPHOLOGY

19

INTERNAL STRUCfURES

29

PHYLOGENY OF THE EARLY RHYNCHONELLIDS

39

RHYNCHONELLID FAUNAL SUCCESSIONS

43

SYSTEMATIC PALEONTOLOGY Order RHYNCHONELLIDA Kuhn, 1949

43 43 43

44 45 49 51 53 54 56 58 60 61 62 65 65 67 68 70 72 72 73 74 75 77 79 79 80 82 82 84 84 85 87 88 90 91 91 92 95 97

Family RHYNCHOTREMATIDAE Schubert, 1913 Genus Rhynchotrema HALL, 1860 R. increbescens (HALL, 1847) RJringilla BILLINGS, 1862 R. nutrix (BILLINGS, 1866) R. parviseptatum n. sp. R. rhynchotrema sp. Genus Stegerhynchus FOERSTE, 1909 S. borealis (VON BUCH, 1834) S. vicina (BILLINGS, 1866) S. praecursor FOERSTE, 1909 S. concinna (SAVAGE, 1913) S. peneborealis (TWENHOFEL, 1928) Genus Lepidocyclus WANG, 1949 L. gigas WANG, 1949 Genus Hypsiptycha WANG, 1949 H. anticostiensis (BILLINGS, 1862) H.janea (BILLINGS, 1866) Family ORTHORHYNCHUUDAE Cooper, 1956 Genus Orthorhynchyllion n. gen. 0. prinstanum (TWENHOFEL, 1928) Genus Gnamptorhynchos n. gen. G. inversum n. sp. G. selliseptalicium n. sp. Family TRIGONIRHYNCHUDAE McLaren, 1965 Genus Rhynchotreta HALL, 1879 R. cuneata (DALMAN, 1828) Genus Rostricellula ULRICH & COOPER, 1942 R. transversa COOPER, 1956 R. sp. Genus Ancillotoechia HAVLICEK, 1959 A. eva (BILLINGS, 1866) Genus Hercotrema n. gen. H. bulbicostatum n. sp. H. sp Family CAMAROTOECHUDAE Schubert & LeVene, 1929 Genus Fenestrirostra COOPER, 1955 F. glacialis (BILLINGS, 1862) F. pyrrha (BILLINGS, 1866) F. primaeva n. sp. .

4

Genus Linguopugnoides HAVLICEK, 1960 L. cybelense n. sp. Genus Phoenicitoechia HAVLICEK, 1960 P. martinensis n. sp. Genus Astutorhyncha HAVLICEK, 1961 A. cf. astuta (BARRANDE, 1879) Family LEPTOCOELIIDAE Boucot & Gill, 1956 Genus Platyrochalos n� gen. P. crudicostatus n. sp. P. peninversum n. sp. P. pulvinatus n. sp. Family INCERTAE SEDIS Genus Leptolepyron n. gen:

99 101 103 103 104 104 106 106 107 107 111 113 113 113

L. argenteum (BILLINGS, 1866)

117

CONCLUSIONS

119

REFERENCES

127

PLATES

5

LATE ORDOVICIAN AND EARLY SILURIAN RHYNCHONELLID BRACHIOPODS FROM ANTICOSTI ISLAND, QUEBEC JisuoJIN

Abstract. The richly fossiliferous Upper Ordovician-Lower Silurian carbonate sequence of Anticosti Island contains unusually abundant and diverse rhynchonellid brachiopods which are classified in 5 families, 16 genera, and 32 species ; 5 genera and 10 species are new. The abundance and superb preservation of most of the species permit thorough analysis of their external and internal features and their range of variation. Interareas, deltidial plates, fold and sulcus, and growth lamellae emerge as the most reliable external features, and dental plates, cardinal process, and septalial plates as the most reliable internal features, for classification at the familial and generic levels. Shell ribbing, growth lines, muscle scars, teeth and sockets, median septum, and crura are additional external and internal features that commonly are useful in discriminating species. Analyses of morphological characteristics suggest a strong phylogenetic link between the orthorhynchulid rhynchonellids and orthids and between the leptocoeliid rhynchonellids and atrypids. Within the Rhynchonellida, these analyses also reveal the ancestral nature of the family Oligorhynchiidae to the families Trigonirhynchiidae and Camarotoechiidae. Compared to other rhynchonellid stocks, evolution of the Fenestrirostra lineage, endemic to the Anticosti basin, was unusually rapid. Evolution of the three distinct, successive, species took only about one million years of early Llandoverian time. The species could be used to define three interval (range) zones for the basin and for refined correlation of the formations in which they occur. Tne dominantly shallow-water, low-energy, substrate conditions of the Anticosti basin provided a refuge for survival of certain genera, hitherto known only from the Late Ordovician Epoch, into the Early Silurian Epoch, and nurtured ancestral species of many widespread later Silurian and Devonian genera. That the Anticosti basin was favourable to the rhynchonellids is reflected by the increased diversity of species from the Late Ordovician to the Early Silurian epochs. Among three successive rhynchonellid faunas recognized in the Anticosti sequence, the Ashgillian Hypsiptycha fauna is very similar in compositional species to its counterpart in the American mid-continental basins; the Early Llandoverian Fenestrirostra fauna consists mainly of endemic taxa; and the Late Llandoverian Stegerhy nchus-Lin guopugn oides fauna contains pioneer species of many subsequent cosmopolitan genera.

Resume. La succession carbonatee tres fossilifere de l'Ordovicien superieur-Silurien inferieur de l'ile d'Anticosti renferme une faune inhabituelle, tant par son abondance que par sa diversite, de Brachiopodes Rhynchonellides. Les taxons recenses appartiennent a 5 families, 16 genres et 32 especes ; 5 genres et 10 especes sont nouveaux. L'abondance et l'exceptionnelle preservation de la plupart des especes ant permis une analyse detaillee des structures externes et internes et de leur variations. Parmi les caracteres externes, les interaeas, les plaques deltidiales, le bourrelet et le sinus ainsi que les lamelles de croissance constituent les criteres taxonomiques les plus fiables au niveau de Ia famille et du genre ; tout comme les plaques dentales, le processus cardinal et les plaques septales parmi les caracteres internes. La costulation de Ia coquille, les !ignes de croissance, les empreintes musculaires, les dents et les fossettes dentales, le septum median, les crura sont des criteres externes et internes additionnels le plus souvent utiles pour differcncier les especes. Les analyses morphologiques suggerent de tres etroites relations phyletiques entre les Rhynchonellides orthorhynchulides et les Orthides d'une part, entre les Rhynchonellides leptocoelides et les Atrypides d'autre part. Au sein de l'Ordre Rhynchonellida, ces analyses revelent egalement le caractere ancestral de la Famille Oligorhynchiidae pour les Families Trigonyrhynchiidae ct Camarotoechiidae. Par rapport aux autres stocks de Rhynchonellides, !'evolution de la lignee de Fenestrirostra, endemique du Bassin d'Anticosti, est exceptionnellement rapide. L'evolution des trois especes distinctes et successives n'a necessite qu'un million d'annees au debut des temps llandoveriens. Ces especes ant permis de definir trois zones d'extension dans le bassin et de preciser les correlations au sein des formations qui les renferment. Les conditions generales dominantes d'eau peu profonde et de faible energie au niveau du substrat du Bassin d'Anticosti ont constitue un milieu refuge qui a permis a certains genres, qui n'etaient jusqu'ici connus que

6

de la fin des temps ordoviciens, de survivre au debut des temps siluriens. Cet environnement a egalement abrite les especes ancestrales de nombreux genres siluriens et devoniens a large distribution geographique. Le fait que le Bassin d'Anticosti ait ete favorable aux Rhynchonellides est atteste par la diversite accrue des especes depuis la fin des temps ordoviciens jusqu'au debut des temps siluriens. Parmi les trois faunes successives de Rhynchonellides reconnues dans la succession d'Anticosti, la faune ashgillienne a Hyp siptycha est tres semblable, par les especes

qui la composent, a son homologue des bassins intracratoniques americains ; la faune a Fenestrirostra du Llandovery inferieur est essentiellement constituee de taxons endemiques, et la faune a Stegerhynchu s­ Linguopugnoides de la fin du Llandovery contient les taxons precurseurs de nombreux genres cosmopolites plus recents.

7

INTRODUCTION

The Rhynchonellida is an order of the phylum Brachiopoda. Rhynchonellids first appeared in Middle Ordovician (Llandeilo) time, and they still live today in marine shelf environments. Like most other brachiopods, rhynchonellids are immobile filter feeders on the sea floor. A pair of feeding arms (lophophore), supported by two rod-shaped skeletal crura, create inhalant and exhalant micro-currents to supply the organism with nutrition. Rhynchonellid shells commonly are ribbed, with a rostrate posterior, and as fossils, the shells yield most profitable results when studied with emphasis on morphological changes in shell shape, crura and related cardinalia, and muscle scars. To many paleontologists, rhynchonellid brachiopods are notoriously small-sized shells with minute internal structures which show slow evolutionary rates. In short, rhynchonellids commonly are regarded as dismal raw material for any kind of exciting or sensational interpretation of functional morphology, paleoecology, or evolution. Understanding of rhynchonellid phylogeny was not helped by arbitrarily splitting the order into Paleozoic and Mesozoic-Cenozoic groups (Schmidt & McLar.en, 1965 ; Ager, 1965) ; this diminished the potential for discovering phylogenetic connections between them. The difficulty was exacerbated by paleontologists who worked on Paleozoic rhynchonellids often favouring a study technique (direct observation of disarticulated valves) different from that (serial sectioning) used by those who worked on Mesozoic rhynchonellids. The difference in technique has had an impact on taxonomy of the rhynchonellids, because different types of internal structures have been variably stressed in description and classification. For example, the morphology of muscle fields is greatly valued in the classification of Paleozoic rhynchonellids, because these features are readily observed in disarticulated valves (Cooper, 1956 ; Howe, 1965). But in Mesozoic rhynchonellids, the orientation and shape of the crura play a more important role in classification, because these structures usually are completely preserved in articulated shells and easily reconstructed from serial sections. Obviously, such discrepancies can be eliminated by employing both direct observation of internal structures and reconstruction from serial sections in all rhynchonellid studies. In reality, however, the technique used is determined by the type of preservation. Strongly silicified shells commonly have their detailed features, . especially microscopic lamellae, obscured by diagenetic changes. Thus they are less suitable for serial sectioning but ideal for direct observation. In comparison, complete calcareous shells commonly have their internal structures preserved at a microscopic scale (like the Anticosti specimens). Such fine-scaled structures can be reconstructed only by serial sectioning. Direct observation is virtually impossible, because it may be extremely . difficult to recover shells from the rock matrix without damaging the internal structures. Rhynchonellids commonly have small and delicate internal structures restricted to the cardinal area. The distal parts of the crura and dental plates only rarely are preserved intact in broken or disarticulated valves. One of the shortcomings of direct observation of single valves, therefore, has been that the internal structures commonly were incompietely documented. Moreover, some structural details, such as the relationship of median septum, septalial plates, crural bases, and hinge plates, are clearly exposed only by their microscopic structures, and these are conveniently revealed in serial sections. The general lack of a common language between Paleozoic and Mesozoic rhynchonellid specialists has been minimized in recent years by gradual adoption of serial sectioning as a common technique for studying all rhynchonellids (see, for example, St. Joseph, 1937 ; Schmidt, 1941 ; Havlicek, 1961 ; Nikiforova & Andreeva, 1961 ; Sartenaer, 1961a-b ; Rubel & Rozman, 1977). The classical pioneer works on the Paleozoic rhynchonellids of Bohemia (Barrande, 1879), Britain (Davidson, 1864-1871), and North America (Hall & Clarke, 1893-1895) have been reviewed by recent authors (notably Havlicek, 1961 ; Cocks, 1978 ; Cooper, 1956 ; and Schmidt & McLaren, 1965), with an increased emphasis on the taxonomic value of internal structures. But unfortunately many Paleozoic genera and species from the North American basins (Hudson, Michigan, Illinois, and Williston) still have not been serially sectioned to reveal details of their internal structures. In previous works too, little attention was paid to the phylogenetic relationship and paleoecological significance of the Early Paleozoic rhynchonellids. Studies of rhynchonellid evolution have been restricted to examination of changes in a

8

few morphological structures in certain taxa, such as the recent studies of reduction of the septalial cover-plate in Ancillotoechia (Rubel & Rozman, 1977) and gradual reduction of the septalial cavity in the Uncinulidae (Havlicek, 1983). Rhynchonellids from the Upper Ordovician and Lower Silurian sequence of Anticosti Island are among the world's most abundant and best-preserved fossil shells. In their associations, they show some degree of similarity to the faunas of the American mid-continental basins in generic and specific composition. The Anticosti specimens commonly are preserved as complete, unaltered, calcareous shells, and thus are ideal for investigation by serial sectioning. The present work is part of a continuing effort to bridge the gap between the study of Paleozoic and Mesozoic rhynchonellids by providing information on the detailed internal structures of Early Paleozoic genera and species. In addition to describing and illustrating the rhynchonellid taxa from Anticosti Island, attempts are made I) to clarify or re-define the technical terms commonly used in descriptions of rhynchonellids ; 2) to carry out a preliminary cladistic character analysis ofthe well-studied Ordovician and Silurian rhynchonellid genera to discover phylogenetic relationships amongst them ; 3) to revise in part the current classification of Paleozoic rhynchonellids at the familial and generic levels, so as to accommodate new information on internal structures ; and 4) to study the stratigraphical distribution and paleoenvironmental adaptation of the common Ordovician-Silurian rhynchonellids of the Anticosti basin, as well as possible paleogeographical links between these Anticosti brachiopods and the rhynchonellids of other basins, such as those of Michigan, Illinois, Gotland, and Bohemia. From the Upper Ordovician-Lower Silurian rocks of Anticosti Island, Billings (1862, 1866) described nine rhynchonellid species but did not illustrate most of them. Twenhofel (1928) provided photographic illustrations of most of Billings' species, but Twenhofel's redescriptions focussed mainly on the external structures and contained mistakes in the interpretation of internal structures. All Billings' type specimens were re­ examined and photographed in the course of the present work, and serial sections of his species were prepared from specimens in new collections, made mostly from the type localities or type horizons. Rhynchonellids from Anticosti Island were collected from 110 localities (mostly by Dr. Paul Copper of Laurentian University), and amount to about 8 000 specimens (Text-fig. 1). A total of 34 sets of serial sections were prepared for 26 species. Only those species represented by a small number of individuals were not sectioned, so as to preserve all available specimens for reference. In preparation of the serial sections, specimens were cut transversely with a Cutrock Parallel Grinder at 0,1 mm intervals. Each cross-section was impressed onto a peel of purple acetotutytatfolie (Triafol NB, Bayer GmbH), using acetone as a solvent. Text-figures of the serial sections were prepared by projecting the peel sections through a slide projector (normally x 20), and tracing the shell structures onto transparent Mylar. The diagrams then were reduced photographically to the size presented. All specimens used to prepare the photographic plates were coated with magnesium oxide. For the new rhynchonellid collections, all localities are defined precisely in terms of the National Topographic System (NTS) l/50 000 maps. Cross references between the maps and stereo-pair airphotographs make it possible to pin-point a given locality with an accuracy of 10 m. For example, Fenestrirostra g/acialis occurs in Baie Innommee, locality A317, NTS 12F/4W 87460-87620: 55480-55770. A change in the second smallest digit represents a lateral movement of 10 m. All type specimens established in the course of this study are deposited in the Type Collection of the Geological Survey of Canada and are designated by numbers following the abbreviation, GSC.

ACKOWLEDGMENTS. I would like to thank my supervisor, Dr. W. G. E. Caldwell of the Department of Geological Sciences, University of SaskatcHewan, for his encouragement and guidance.My thanks are extended also to Dr. P. Copper of the Geology Department, Laurentian University, who suggested the subject of the research, provided most of the specimens, and guided me in the field. Type specimens were made available through the courtesy of Dr. T. E. Bolton of the Geological Survey of Canada. Dr. P.M. Swan, professor of classics in the Department of History, University of Saskatchewan, kindly helped me with derivation of new generic and specific names from Greek and Latin. The Natural Sciences and Engineering Research Council of Canada provided funds to Dr. Copper for 12 field seasons on Anticosti Island, from 1966 to 1986. Further support was derived from NSERC Operating Grants to Drs. Caldwell and Copper.My sincere appreciation is extended also to the University of Saskatchewan for supporting me through College of Graduate Studies and Research Scholarships for three years.

9

GEOLOGICAL SETTING

Anticosti Island is located in the Gulf of St. Lawrence, on the eastern seaboard of Canada. Port Menier in Ellis Bay is the only permanent settlement on the island (Text-fig. 1). Characterized by low topography, the island exposes Ordovician and Silurian rocks principally in a series of coastal bluffs and cliffs. The inland area is densely forested, and only scattered outcrops occur in river valleys and roadcuts.

�. 0

o_

D D � :/""///:.-"'// :.-"'//

[2]

aux Anglais

Chicotte Fm.

Jupiter Fm.

Gun River Fm. �irrimack Fm. Btcscie Fm.

Ellis Bay Fm.

Vaureal Fm.

g:_

0 0.

10

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20 km

I

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49"rxf

Text-fig. 1

10 I

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Geological map of Anticosti Island. Black dots indicate rhynchonellid localities. Boundaries between formations along the coast are well controlled by outcrops, but inland, they are controlled by only a few localities in the eastern part of the island and are partly inferred. The thick solid lines are major roads.

10

The Anticosti basin had a depositional history from the Early Cambrian Epoch to the Early Devonian Epochs. Crescent-shaped, about 900 km long and 250 km wide, and extending from the present St. Lawrence River mouth to the western coast of Newfoundland (Poole et al., 1970), the basin was asymmetrical, with maximum subsidence on the eastern side. The present study deals only with the rocks exposed on the island, that is, those of Late Ordovician and Early Silurian age. The Ordovician rocks crop out mainly on the northern part of the island, and the Silurian rocks on the southern part. The complete stratigraphical sequence, measured from surface outcrops and well cores, has a maximum thickness of about 1 000 m. The rocks are little disturbed by regional tectonic events, such as the Taconic and Acadian orogenies. Beds have a regional southwesterly dip of 0.2-2.0 degrees, and minor faults rarely displace the rocks by a few meters. During Late Ordovician and Early Silurian times, the Anticosti basin was the site of a relatively stable carbonate phitform on the eastern margin of the North American Craton. It was separated, at least partly, from the Iapetus (Proto-Atlantic) Ocean by the Taconic Mountain belt to the southeast. To the northwest, the basin graded onto the Canadian Shield. It may have been connected from time to time with the Hudson, Michigan, and Illinois basins to the southwest during high stands of sea-level (Text-fig. 2). In its paleogeographical setting, the Anticosti basin generally had low-energy, shallow-water (deeper than intertidal) environments, favorable to the small-shelled rhynchonellid brachiopod faunas.

North

Text-fig. 2- Late Ordovician-Early Silurian paleogeography of the tropical continents (adapted from Petersen al., 1977 ; Seslavinskiy, 1984 ; Livermore et al., 1985 ; Scotese et al., 1985).

et

China

al., 1973

; Ziegler

et

The stratigraphy of Anticosti Island has been studied by many geologists during the last 130 years, notably by Richardson (1857), Twenhofel (1928), Bolton (1972), and Petryk (1979, 1981). The present work adopts the stratigraphy recently revised by Long & Copper (1987a, 1987b. Text-fig. 3). The Upper Ordovician strata are divided into the Vaureal and Ellis Bay formations. On the surface, only about 300 m of the 1 000 m thick Vaureal Formation is exposed. The upper part of the formation is divided into the Lavache, Easton, Hommard, Battery, Mill Bay, and Schmitt Creek members. These six members for a shallowing-upward sequence. The lower three consist mainly of bioclastic packstones, micritic mudstones, and calcareous shales. The upper three in eastern Anticosti are dominated by siliciclastic sediments, including sandstones and siltstones with hummocky, trough, and ripple cross-bedding, whereas their equivalents in western Anticosti consist mainly of limestones. The shallowing-upward sequence, associated with localized siliciclastic influx on the northeast side, probably was controlled by the growth of a Late Ordovician North African ice-cap which caused several large-scale drops in sea level. As a result, terrigenous clastic sediments from the Canadian Shield were carried into part of the Anticosti basin during low stands of sea-level. Only a few rhynchonellid species occur in the Vaureal Formation, including Rhynchotrema nutrix (Billings, 1866), Lepidocyclus gigas Wang, 1949, and Hypsiptycha anticostiensis (Billings, 1862).

11

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Merrimack

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Composite geological section of the Upper Ordovician-Lower Silurian sequence of Anticosti Island. Siliciclastic rocks (with dots and ripple marks) occur mainly in the upper Vaureal and Ellis Bay formations. Coral-algal-stromatoporoid bioherms or biostromes occur within limestones (brick symbols) and calcareous shales (black) of the Laframboise Member (Ellis Bay Formation) East Point Member (Jupiter Formation), and Babineau Member (Chicotte Formation). Thickness of the Chicotte Formation is not to scale. Total thickness of the composite section, as measured from various type sections, is less than the maximum thickness of the sequence.

The Ellis Bay Formation is currently divided into five members : the Grindstone, Velleda, Prinsta, Lousy Cove, and Laframboise members. Like the upper Vaureal Formation, the Grindstone and Velleda members in the lower Ellis Bay Formation are dominated by cross-bedded sandstones. Only one rhynchonellid species, Rostricellula transversa Cooper, 1956, has been found in the shaly units of the Velleda Member. In the Prinsta, Lousy Cove, and Laframboise members, there is a marked upward decrease in calcareous shales and an increase in limestones, culminating in the coral-algal-stromatoporoid biostromes of the Laframboise Member. In the Prinsta Member, two new rhynchonellid species are present: Orthorhynchyllion prinstanum (Twenhofel, 1928) and Gnamptorhynchos inversum n. gen. and n. sp. In the outcrops of western Anticosti, Hypsiptycha anticostiensis and Rhynchotrema nutrix occur as common rhynchonellid species in the Ellis Bay Formation. The Lower Silurian strata are divided into the Becscie, Merrimack, Gun River, Jupiter, and Chicotte formations. The lithology is mainly micritic mudstone to packstone (calcarenite), with interbedded calcareous shales. Siliciclastic rocks, intertidal carbonates (such as oolites, stromatolites, or limestones with fenestral porosity), and evaporites are virtually absent from the Lower Silurian sequence. Only in rocks occupying three intervals do high-energy and probably shallow-water sedimentary structures occur. First, in the Chabot Member of the Becscie Formation, ripple cross-bedding and current scouring increase towards the top. Second, the East Point Member of Jupiter Formation contains localized coral-algal bioherms. Third, the capping Chicotte Formation consists of coral-algal-stromatoporoid bioherms and encrinitic grainstones. The rapid accumulation of carbonates in the Anticosti basin is to be correlated with the Early Silurian (Llandoverian) marine transgression onto the paleocontinents of Laurentia and Baltica (McKerrow, 1979).

12

Rhynchonellid brachiopods occur in abundance only in the formations or members with dominant micritic mudstones and calcareous shales. For example, the shaly units of the Merrimack Formation contain pockets and layers with concentrated Fenestrirostra primaeva n. sp., F. glacialis (Billings, 1862), Rhynchotrema fringilla (Billings, 1862), R. parviseptatum n. sp., and Rhynchotreta cuneata (Dalman, 1828). Rhynchonellids also occur in great numbers in the Jupiter Formation, especially in the marly Goeland, Richardson, Cybele, and Pavilion members, including both cosmopolitan species and new species, such as Stegerhynchus borealis (von Buch, 1834), S. peneborealis (Twenhofel, 1928), S. concinna (Savage, 1913), Ancillotoechia eva (Billings, 1866), Linguopugnoides cybelense, Platytrochalos crudicostatus n. gen. and n. sp., and P. peninversus n. sp. In the Becscie, Gun River, and Chicotte formations, rhynchonellids are represented by only a few species and small number of specimens. In the Chicotte Formation, for example, only one species, Stegerhynchus vicina (Billings, 1866), has been found at scattered localities.

13

EXTERNAL MORPHOLOGY

Rhynchonellid fossils generally are small and preserved with valves articulated. Thus, in the nineteenth and early twentieth centuries, classification of the group was heavily dependent on observations of external morphology. Although the serial-sectioning technique for some time has allowed reconstructions of complex internal structures, many external features still reliably guide the discrimination and identification of species. In the present work, the following external characters are useful and described as a standard procedure in systematic descriptions: - Shell size and outline - Hingeline and commissure - Interarea -Umbo and beak(particularly in pedicle valve) - Fold and sulcus - Shell ornamentation The taxonomic value of each character varies from taxon to taxon, especially at the generic and familial levels.

Shell Size and Outline Paleozoic rhynchonellids generally are smalL Relatively large forms, such as Pugnax (Hall & Clarke, 1893), Pleiopleurina(Schmidt, 1964), and Paurorhyncha(Cooper, 1942) of Devonian age, barely exceed 50 mm in length; The largest species in the Upper Ordovician-Lower Silurian rocks of Anticosti Island, such as Lepidocyclus gigas and Rhynchotrema fringilla, reach a maximum length of only 35 mm. In the present work, shell size is described in terms of four catagories, based on shell length: small : 30 mm

Shell size rarely has been .considered to be of much value in the identification of taxa at the generic or higher levels. The size of adult forms, however, is useful in separating congeneric species. Studies of ontogenetic patterns and of population dynamics also may depend, at least partly, on analyses of shell size. When a large number of specimens are measured and studied statistically, shell size commonly is of v�1ue in defining species. For example, Fenestrirostra primaeva n. sp. is separated from F. glacialis (Billings, 1862) partly on the basis of shell size and outline. Study of several large fossil assemblages of the two species· (hundreds of specimens each), collected from single localities or even single beds, shows that the mean ,value of shell length is 16.3 mm for F. glacialis, but only 8.5 mm for F. primaeva, although it could be extremely difficult to separate large forms of F. primaeva from small forms of F. glacialis on external shape alone. Closely associated with variation in shell size is variation in shell outline. Two parameters commonly used to describe shell outline are the convexity (lateral view) and length/width ratio. The taxonomic significance of shell growth patterns recently has been discussed by Jones (1983). Statistical study of a number of rhynchonellid species from Anticosti Island shows that the length/width ratios generally decrease with age. The relationship between length(L) and width(W) can be described as follows: W

=

(L-b) tanS (9 > 45°, b > 0, L > b, W > 2 mm)

14

where b is a constant (termed the "initial growth index" by Imbrie, 1956). The equation shows that most rhynchonellid species tend to have elongate shells at their early growth stages but transverse shells at the adult stage (Text-fig. 4). This type of relationship is well demonstrated by the statistical data on Fenestrirostra glacialis, Stegerhynchus peneborealis, and many other species (refer to text-figures in systematic descriptions). It should be noted that the equation for curve A :

w

=

(L-b) tane

is only an approximation of the true bioallometric growth pattern B (see Imbrie, 1956; Jones, 1983) : W

A: w

w=

=

nLm (n > 0, m >1)

B:

( L-b)tane A

W=nLm

w

A

, , , I

Text-fig. 4

-

/ Ia I , ,

General models of ontogenetic growth in rhynchonellids. The linear relationship between shell length and width during ontogenetic development (curve A) is shown by the statistical data of shell dimensions for most rhynchonellid species from Anticosti Island (refer to the systematic section for detailed examples). The initial growth index b (Imbrie, 1956) has little biological meaning, for no species could have a shell beginning its growth at b mm length and 0 mm width. The linear relationship (curve A) is only an approximation of the real growth pattern (exponential curve B).

Therefore, the constants b and e do not have any biological value. But they can still be used as two parameters to define and describe the nature of ontogenetic change in outline for a given species. For example, high values for b and e mean that youngforms of a given species (e.g., Fenestrirostra glacialis) will have dominantly elongate shells, but adult forms will have transverse shells. When b approaches zero and e approaches 45°, both young and adult forms will have rather uniformly circular shells (that is, length= width). It is suggested that the two parameters, b and e, may be used as descriptive characters for species, especially when large number of specimens are available for measurement. In the present work, statistical data on shell dimensions are provided for species whenever possible, as a preliminary step in accumulating information for further study, especially for possible numerical analyses or numerical taxonomy. Interpretation of such data is often complicated by many factors, such as bias in collecting preservation. Theoretically, population dynamics can be studied or extrapolated from statistical data on shell size. Three types of population may be modeled hypothetically, without consideration of any complexities, such as preservation and collection bias.

Young Population (Pioneer Population). A young population is defined as one in the process of colonizing a substrate, dominated by young individuals emigrant from another area. If such a population was killed off by a "catastrophic" event and subsequently preserved, the statistical shell sizes of the population would show a left­ skewed distribution (Text-fig. 5B). In theory, then, a pioneer population should be distinguishable from a dwarfed population, which may result from unfavorable environmental conditions. A well-established dwarf population

15

should demonstrate its own normal distribution of shell-size frequency, even though the mean will be compressed to the left in comparison to a normal population. Such distinctions may be tested further by study of the relationships between the fossils and the host rocks. Left-skewed distribution of shell sizes has been observed rarely in fossil assemblages of Anticosti rhynchonellids. This may be explained in three ways : 1) pioneer populations may have been rare in the area at times when the shells were preserved ; 2) young individuals were destroyed in the process of preservation ; and 3) small shells,. difficult to extract from outcrops, were victims, _of preferential collection. The latter two explanations are more probable. The only clear example of a young population obtained from the Anticosti rhynchonellids is that of Ahcillotoechia eva (Billings, 1866 ; see Text-fig. 5B). The species generally is small­ sized, and large number of specimens were collected from Baie du Naufrage (locality A322-6). The preservation and collection bias is greatly reduced because the specimens used for Text-figure 5B were extracted from a single slab of rock in which large numbers of tiny specimens were well preserved.

mean

A :g

� •

a.

200

E

I.. 0

u Q,l a.

0 z

Ill 0

>u c: a. :::1 tT Cll L.

....

100

a. ..0

E :::1 c:

-

shell size ( length)

0

mm

5

length

mean

mean 100

Q,l

150

E u a. a. VI

-

0

10

D

c �

B

100 50

I.. a. .c

50

E :::1 c:

0

30

15

length Text-fig. S

•

mm

5

10

15

mm

shell length

Fossil population structures determined by distribution of shell sizes. A, hypothetical population with balanced young

and adult forms. B, a pioneer population of the fossil species Ancillotoechia eva ; 758 specimens from locality A322-6. C, an aged population of Fenestrirostra glacialis ; 332 specimens from locality A356-3.. D. an aged population of Fenestriroslra primaeva ; 927 specimens from locality Al50b.

Established Population. The term refers to a population which is well settled in a given area and is in the process of flourishing. Shell-size frequency will show ·a normal (non-skewed) distribution, because young and adult individuals will have reached a dynamic balance within the population (Text�fig. SA). In the rhynchonellid fossil assemblages from AnticostiJsland, however, development to this exact stage never has been observed.

16

Aged Population. An aged population is one which has lived in a given area for a considerable time, usually under favourable conditions. Prevervation and accumulation of large-sized individuals which died of old age

account for the right-skewed curve of size frequency (Text-fig. 5C-D). Preservation of this type of population generally requires low-energy substrate conditions, so that the dead shells will not be broken, transported, and redeposited as coquinas or calcarenites. Interpretation of a right-skewed curve of size distribution requires caution, however, because biased preservation (for example, preferential sorting and winnowing of small forms from the assemblage by weak currents) and biased field collecting may produce the same kind of size distribution as an aged population. The population analyses discussed above are based on the assumption that shell size is simply a function of age. In reality, of course, size variations commonly occur among individuals of the same age. Further study is needed to find ways of discriminating size differences related to ontogenetic growth from size variations among individuals of the same age.

Fold and Sulcus Development of fold and sulcus in the rhynchonellids always has been accorded important taxonomic value at the generic and specific levels. This is probably because the configuration of fold and sulcus is easily observed and takes on a variety of forms. Fold and sulcus can be divided into three main types according to their positions: (a) pedicle sulcus and brachial fold; (b) brachial sulcus and pedicle fold; (c) reversed fold and sulcus (posteriorly pedicle fold and brachial sulcus, but anteriorly brachial fold and pedicle sulcus). The first type is perhaps most common, especially in the families Rhynchotrematidae and Orthorhynchulidae. Commonly the fold and sulcus tend to be well developed in the anterior portion of the shell, but weak or even obscure in the umbonal areas. The second type occurs only in a small number of genera and species. Examples are the Devonian genera Plectorhynchella (Cooper & Muir-wood, 1951), andRozmanaria (Weyer, 1972; see Xu & Yao, 1984). Indeed, this type of fold and sulcus is so unusual in rhynchonellids that one family and one subfamily have been proposed chiefly on the basis of it, i.e., the Plectorhynchellidae (Rzhonsnitskaya, 1956) and Rozmanariinae (Havlicek, 1982a). It is observed from study of the Anticosti collections that the third type of fold and sulcus is closely related to the second. Some species (e.g., Fenestrirostra glacialis and Platytrochalos peninversus n. gen. and n. sp.) have a posteriorly developed pedicle fold and brachial sulcus, which are anteriorly reversed into pedicle sulcus and brachial fold. The small or young forms, however, maintain a pedicle fold (or carination) and a brachial sulcus (or median groove) throughout the length of the shell, thus resembling the second type. It would seem inappropriate, therefore, to use the position of fold and sulcus to define taxa at the familial level. In the present work, positions of the fold and sulcus commonly are regarded as significant characteristics at the generic level, and shape and ornamentation of the fold and sulcus as useful characteristics at the specific level. In some taxa, sculpture and configuration of the fold and sulcus show great infraspecific variation. For example, Stegerhynchus borealis from the Upper Silurian rocks of Arctic Canada may have one to seven costae in the sulcus (Jones, 1981). Fenestrirostra glacialis from Anticosti Island may have a variable number of three to ten costae or costellae on the fold. In such cases, the ornament of fold and sulcus cannot be considered taxonomically important unless the variation is statistically examined. In other taxa, however, the same morphological feature may show much less variation and be used conveniently as one of the characteristics for a given species. For example, Hypsiptycha anticostiensis (Billings, 1862), Rhynchotrema nutrix (Billings, 1866), and Stegerhynchus peneborealis (Twenhofel, 1928) have a constant number of three costae in the sulcus and four on the fold. Variants with more or less costae compose only about 5% of the fossil populations.

Shell Ornamentation Rhynchonellid shells generally have two types of surface ornament -- radiating ribs and concentric growth structures. Shells with a smooth surface or with nodules and spines have not been observed in the

17

Anticosti collections. In the present work, ribs are classified into three types for convenience in systematic description. (a) Plications are very broad, sparse ribs, commonly ranging from two to four on each shell flank. Typical examples are the ribs in such species as Oligorhynchia subplana (Cooper, 1935), Pleurocornu amissa (Barrande, 1879), and Platytrochalos crudicostatus n. gen. and n. sp. (see, for example, Pl. 22, fig. 6-20). (b) Costae are coarse, strong ribs, commonly ranging from four to eight on each shell flank. They are the dominant type in the rhynchonellids. In some families, such as the Rhynchotrematidae and Orthorhynchuliidae, the costae begin at the shell apex and cover the whole shell. In other families, especially in the Camarotoechiidae, the costae tend to be obliterated at the shell posterior, as in such camarotoechiid genera as Linguopugnoides (Havlicek, 1961), Phoenicitoechia (Havlicek, 1961), and Estonirhynchia (Schmidt, 1954). A particular type of ribbing is developed in the Anticosti genus F enestrirostra. Angular costae around the umbonal area increase in number anteriorly by bifurcation or intercalation and become rounded, flattened, and even obscure. So far, this type of rib has been found only in Fenestrirostra. (c) Costellae are fine ribs, numerous, commonly low and rounded, and ranging in number from 10 to 30 on each shell flank. In the P aleozoic rhynchonellids, costellae are common in the family Uncinulidae, as in Eoglossinotoechia (Havlicek, 1959) and Glossinulus (Schmidt, 1942). Leptolepyron n. gen. from Anticosti Island also exhibits typical costellae. The taxonomic value of ribbing in the rhynchonellids is difficult to assess : it seems to vary quite markedly even at the lowest ranks. For example, there is a high degree of infraspecific variation in the ribbing of Fenestrirostra glacialis. Among specimens collected from a single bedding plane at one locality (A356-3), some show only rare bifurcation and intercalation, and the shell is covered by angular to subangular costae from posterior to anterior. Others show such increase in number of ribs that, anteriorly, the shells become finely costellate, or may even develop a quasi-smooth appearance. In other species, the type of ribbing can be relatively constant and serves as a diagnostic feature. In the Anticosti collections, for example, Platytrochalos crudicostatus n. sp. has four strong costae (or plications) on each flank, one in the sulcus, and two in the fold. It is easily distinguished from Platytrochalos peninversus, which has six to seven subangular costae on each flank, two in the sulcus, and three on the fold. Strength of the ribbing determines the configuration of anterior commissure. A smooth commissure commonly is related to the presence of fine or anteriorly obliterated costellae. A denticulate commissure corresponds to the presence of strong, angular to subangular costae. An undulating commissure results from the presence of shell plications. Extremely strong or swollen ribs developed in the sulcus and on the fold may change the anterior margin from uniplicate to sulciplicate. The concentric growth structures in rhynchonellids also may be classified into three types: (a) Imbricate (growth layers), (b) Lamellose (growth lamellae), and (c) Fine (growth lines). Concentric sculpturing rarely has been regarded as having taxonomic value at the generic or higher levels, because the structures are normally delicate and easily modified or obliterated during preservation. Only in a few genera and species have concentric features been shown to have a diagnostic value. For example, Lepidocyclus gigas is typically imbricate (see Pl. 9, figs. 13-22), Hypsiptycha anticostiensis is lamellose (Pl. 9, figs. 1-12), and Rhynchotr ema nutrix has fine growth lines (Pl. 3, figs. 6-12). The Silurian species Stegerhynchus diodona (Dalman, 1828) exhibits peculiar rugae, which are as strong as growth layers but not imbricate (Bassett & Cocks, 1974; Rubel & Rozman, 1977).

Pedicle Umbo and Beak The convexity, height, and orientation of the pedicle umbo and beak commonly determine the basic shell outline. Like that outline, the configuration of umbo and beak changes greatly in most species, especially with ontogeny, and therefore intraspecific and ontogenetic variations have to be studied before the character of the umbo and beak can be used to help define species. This is illustrated by two Anticosti species for which large numbers of specimens were collected. In Fenestrirostra glacialis, specimens under 5 mm in length have relatively high and erect beaks. With ontogenetic development, however, the pedicle umbo becomes increasingly convex,

18

and the beak more strongly incurved and arched over the brachial umbo. In comparison, Stegerhynchus borealis maintains a suberect to erect beak from the young to the adult growth stages. Ontogenetic information of this type is important not only in the definition of species but also in the study of rhynchonellid phylogeny.

Interarea Well-defined interareas are rare in rhynchonellids because of the commonly rostrate posterior with small, narrow umbones. Only a few Early Paleozoic rhynchonellid genera developed rather prominent interareas in both valves, usually associated with apsacline orientation of the cardinal area. These genera are assigned mainly to the family Orthorhynchulidae, including Orthorhynchula (Hall & Clarke, 1893), Orthorhynchuloides (Williams, 1962), Tuvaerhynchus (Kulkov, in Kulkov et al., 1985),Gnamptorhynchos n. gen., and Orthorhynchyllion n. gen.

19

INTERNAL STRUCTURES

In the last few decades, many new genera and species of rhynchonellids have been erected on basis of new internal structures discovered by using serial-sectioning techniques. It is commonly believed that internal structures are more consistent, and thus more valuable, in taxonomy than external features. Hence, new types of internal structures have tended to be exploited to establish new taxa, even before the range of variation of the structures has been well assessed. The study of internal structures of the family Rhynchotrematidae (Howe, 1965), based on observation of interiors valves, has revealed that many internal structures also show a wide range of variation : hence, caution is called for in interpreting internal structures from serial sections. Convexity of the shell posterior may greatly affect the orientation of internal structures and thus control the angle of sectioning. For example, the height and length of dental plates and median septum, as well as the depth of a septalium, may appear quite different in serial sections dependent on the cutting angles.

In the present work, descriptions of internal structures are based mainly on serial sections and commonly include the following characters.

Pedicle valve :

1. Dental plates ; 2. Teeth and hinge fossettes ; 3. Delthyrium and deltidial plates; 4. Muscle field.

Brachial valve:

5. Septalium (septalial and hinge plates); 6. Cardinal process; 7. Median septum; 8. Crura;

9. Articulatory structures.

Dental Plates In rhynchonellids, dental plates are probably the most commonly, but at the same time the most variably, developed structures in the pedicle valve. Previous workers generally have relied on the development of dental plates for taxonomic discriminations at the generic and familial levels. For example, reduction of the dental plates was cited as one of the diagnostic characteristics of the Orthorhynchulinae (Cooper, 1956 ; Schmidt & McLaren, 1965). As most rhynchonellids develop a pair of dental plates to some degree, the absence of these plates is widely regarded as of particular importance. Many genera have been erected to include species lacking dental plates (see, for example, the discussion under Leptolepyron in the systematic section). Xu & Yao

(1984) proposed a new family, the Katuniidae, characterized by an absence of dental plates, median septum, and cardinal process. For those genera with well-developed dental plates, on the other hand, the various configurations of the structures have not been accorded any important taxonomic value at supraspecific levels, except that the union of the dental plates to form a spondylium has been considered a diagnostic feature of some families (for example, the Camerophorinidae, and Tetracameridae). The configuration and development of dental plates in Paleozoic rhynchonellids are hereby categorized into eight basic types (Text-fig.

6), although intermediate forms may be present.

20

configuration of dental plates

1.

parallel

5. sessile spondylium

Tcxt.-flg. 6

•

2.

convex

6. spondylium

3. concave

7. complex

fused

4.

8. absent

spondy!ium

Common configurations of dental plates in Paleozoic rhynchonellids.

l. Rhynchotrema nutrix (Billings, 1866), 1.6 mm from apex, x 5. 2. Fenestrirostra glacialis (Billings, 1862), 2.8 mm, x 3.5. 3. Stegerhynchus peneborealis (Twenhofel, 1928), 1.8 mm, x 5.5. 4. Lepidocyclus gigas (Wang, 1949, from Schmidt & McLaren, 1965), x 3.5. 5. Leiorhynchus quadracostatus (Vanuxem, 1842, from Schmidt & McLaren, 1965), 6. Camarophorina pachyderma (Quenstedt, 1871, from Schmidt, 1941), x 2. 7. Tetracamera subcuneata (Hall, 1858, from Schmidt & McLaren, 1965), x 1.5. 8. Leptolepyron argenteum (Billings, 1866), 1.3 mm to apex, x 5.5.

x

3.

Teeth and Hinge Fossettes The development of teeth is rarely considered to be of important taxonomic value. Size of teeth seems to be a function of shell thickness. For example, the thick-shelled Fenestrirostra glacialis, and Lepidocyclus gigas commonly have robust teeth, whereas the delicate-shelled Leptolepyron argenteum (Billings, 1866) has small teeth. Position of the teeth is perhaps controlled by the development of hingeline. In shells which have short hingelines and reduced dental plates, the teeth tend to be supported by the shell wall. Thus their position becomes lateral. In most rhynchonellids, a cavity or depression (fossette) is developed at various degrees on the inner side of tooth to receive the inner hinge plate, constituting a supplementary articulatory structure (see, for example, Pl. 28, figs.

7-8). The fossette may be homologous to the crural fossette in spiriferids and other brachiopods.

However, a crural fossette is defined as a "cavity on inner face of tooth receiving posterolateral edge of brachiophore or crural plate when valves are closed" (Williams & Rowell, 1965). Thus the fossette in most rhynchonellids cannot be called a crural fossette, because it has not been observed to accommodate the crus or septalial plate(= crural plate). In the present work, the fossette tentatively is called a hinge fossette.

21

Delthyrium and Deltidial Plates In the classification of Early Paleozoic rhynchonellids, the development of deltidial plates has been used to define some genera. For example, Lepidocycloides is distinguished from Lepidocyclus by its lack of deltidial

plates and a cardinal process. Hiscobeccus Amsden (1983) is separated from Lepidocyclus by its lack of medially conjunct deltidial plates. The presence or absence of deltidial plates, however, should be examined with care, using a large number of specimens, for the structures are normally very delicate and easily broken off. In the Anticosti material, real conjunct deltidial plates have been observed only in the genus Hypsiptycha. Various

forms of deltidial plates are shown in Text-figure 7A.

A.

delthyrium and deltidial plates

2

8.

pedicle pad and pedicle coll ar

2 Text-fig. 7

-

4

3

4

3

Morphology of the umbonal and delthyrial cavities. A-1. Conjunct deltidial plates. Hypsiptycha anticostiensis (Billings, 1862), 1.0 mm from apex, x 8.5. A-2. Disjunct duplex deltidial plates. Rhynchotremafringil/a (Billings, 1862), 2.2 mm from apex, x 4. A-3. Rudimentary deltidial plates. Fenestrirostra glacialis (Billings, 1862), 2.2 mm from apex, x 3.5. A-4. Open delthyrium. Stegerhynchus borealis (von Buch, 1834), 1.4 mm from apex, x 7. B-1. Pedicle pad. F. glacialis, 1.5 mm from apex, x 7. B-2. Incipient pedicle collar. F. glacialis, 1.0 mm from apex, x 7. B-3. Incomplete pedicle collar. Lepidocycloides baikiticus Nikiforova, 1961 (from Nikiforova & Andreeva, 1961), 1.0 mm from apex, x 5. B-4. Pedicle collar. Cirpa briseis (Gemmellaro, from Ager, 1965), 0.6 mm from apex, x 14.

The delthyrial cavity (or chamber) is defined as the space within the umbo, bounded by the dental plates or postero-lateral shell wall (Williams & Rowell, 1965). The function of the cavity is undoubtedly to accommodate and anchor the pedicle muscle. In a few species, the delthyrial cavity is partly or completely separated into two chambers, with the dorsal subchamber shaped like an oval tube (for example, Fenestrirostra glacialis, Lepidocycloides baikiticus, and Cirpa priminima in Text-fig. 7B). In Fenestrirostra glacialis, a pad-like structure (myotest) is observed in the delthyrial cavity, with a shallow median groove which extends anteriorly into the muscle field. The pad probably represents a primitive type of pedicle collar, a structure well described in some atrypid brachiopods (Coppe�.

1967).

Serial sections of Phoenicitoechia martinensis n. sp. have revealed a new type of attachment structure for the pedicle muscle. It consists of a deeply impressed scar with a pair of partly coalesced ridges (Pl. figs.

1-3). The structure is restricted to the extreme posterior of the umbonal cavity.

29,

22

Detailed structures associated with the delthyrial cavity until now have been documented poorly, especially in the Early Paleozoic rhynchonellids. Such structures are easily modified or obscured by secondary shell thickening and thus rarely have been given any significant value in taxonomy. Careful observation in future studies may prove that they are of some importance in classification at the specific level.

Muscle Field Rhynchonellids commonly have well-developed muscle fields in the pedicle valve. Muscle scars have been greatly valued in classification of rhynchonellids when observations have been based on disarticulated valves. This applies to many previous studies of the Paleozoic rhynchonellids (see, for example, Cooper, 1956, and Howe, 1965).

A pedicle muscle

cardinal

uctor

hinge axis

scar

adductor muscle

Text-fig. 8 .

•

General model of musculature for the early rhynchonellids. The reconstruction of adductor, diductor, and pedicle (protractor) muscles is based on the muscle scars obseJVed in fossil species from various families. A. Phoenicitoechia martinensis n. sp. 0.7 mm from apex, x B. Lepidocyclus laddi Wang, 1949 (from Howe, 1965), x l.

C. Stegerhynchus praecursor Foerste,

1909, 0.9 mm

10.

to apex, x

14.

The Anticosti rhynchonellids, however, are characterized by well-preserved articulated shells. For most species, the outline and configuration of a muscle field can only be inferred from serial sections. The diductor and adductor muscle scars commonly are not distinquishable unless very sharply defined. In the present work, the muscle field is not regarded as diagnostic at any taxonomic level, because it is difficult to observe its morphology and variation. The musculature so far documented for the Paleozoic rhynchonellids is modelled in Text-figure 8. The model is based mainly on the rhynchotrematids and a few species from other families. The adductor muscle scars are commonly much smaller than, and enclosed by, the diductor muscle scars, as is clearly shown in Rhynchotrema and Lepidocyclus (Howe, 1965, 1967, 1969). The pedicle muscle scar (previously called the protractor scar) is not easily recognized in most rhynchonellids. Serial sections of two Anticosti species have revealed the existence of pedicle muscle attachment structures in the form of a myophragm along the median line

23

of the extreme posterior of the muscle field; In Platytrochalos crudicostatus (n. gen. and n. sp.), the myophragm is represented by a unilobate ridge, partly obscured by secondary shell thickening (Pl. 30, figs. 6-8). The myophragm is best developed in Phoenicitoechia martinensis n. sp.;in which it is characterized by a pair of high median ridges at the apical portion of the muscle field (Text-fig. 8A ; Pl. 29, figs. 1-3). The adductor muscle scars in the brachial valve generally are not well impressed in rhynchonellids ; only in a few species have these scars been observed. The Anticosti species Gnamptorhynchos selliseptalicium (n. gen. and n. sp.), for example, has a pair of narrow, elongate, and shallow scars located around the mid-length of the valve, approximately corresponding to the commencement position of the brachial fold. Internal structures of the brachial valve in rhynchonellids vary much more than do those of the pedicle valve, and hence have greater significance in taxonomy. Median septa, septalia, cardinal processes, crural plates, and crura have been widely exploited to define families and genera. As pointed out by previous workers (for example, Schmidt & McLaren, 1965 ; Xu & Yao, 1984), however, internal structures of the Paleozoic rhynchonellids have been studied by different methods, some by serial sectioning, others by direct observation on internal molds, and still others by etching of silicified shells. Moreover, a great number of Paleozoic rhynchonellid species have not been well examined so far as their internal details are concerned. It is difficult, therefore, to organize the various interpretations into a single coherent systematic classification. In the following section, an attempt is made to summarize and discuss the variability of internal structures in the brachial valve, with special reference to Anticosti specimens.

Septalium The terms hinge plate and septalial plate need to be defined before discussing the septalium. A hinge plate, as used in the present work, is the "inner hinge plate" of Williams & Rowell (1965). The plate is fused to the crural base and extends laterally or ventro-laterally (Text-fig. 9). It can be regarded as a homologue of the inner plate iri pentamerids. A septalial plate, as defined by Williams & Rowell (1965), is the "crural plate forming floor of septalium and united ow:ith earlier-formed part of.median septum". The plate extends from the inner edge of crural base and unites medially with its counterpart to form a septalium. The septalial plate is equivalent, therefore, to the outer plate in some cruralium-bearing pentamerids. In reality, it is not always easy to separate the hinge plates from the septalial plates, because they represent different portions of a single pair of plates (see Text-fig. 9D ; Pl. 28, figs. 6-7). As shown by detailed microscopic structures, the hinge plate and septalial plate are conveniently separated at a point where the crural base is located. This is similar to the division of a brachial plate into an inner plate and an outer plate by a crural base in pentameracean brachiopods. In Williams & Rowell (1965), a septalium is defined as a "troughlike structure of brachial valve between hinge plates (or homologues), consisting of septalial plates (or homologues) enveloping and buttressed by median septum; does not carry adductor muscles".

The two terms, septalial plate and septalium, are misleading, because they give the deceptive impression that the septalium or septalial plate is closely related to the median septum, as was believed by some early workers. Leidhold (1928) first introduced the term septalium, based on the assumption that the trough-shaped chamber at the brachial posterior was formed by splitting of the median septum. More recent studies have shown that the assumption is invalid, and the septalium is formed by a pair of independent septalial.plates which are simply supported by a median septum (Cooper, 1959 ; Ager, 1965). Serial sections of Anticosti specimens also show that the microscopic lamellar structure of the septalial plate is more closely connected to that of the hinge plates than to that of the median septum. In the present work, however, the two terms, septalial plate and septalium, are retained to avoid using the term "cruralium" in rhynchonellids. A septalium is distinquished from a cruralium (commonly developed in pentamerids) by virtue of its much smaller size. A septalium normally does not extend anterior of the hingeline, and could not have served as a base of attachment for the adductor muscles. A cruralium in the pentameracean brachiopods, however, generally extends far beyond the hingeline, with its anterior portion housing the adductor muscles. In the present study, the septalium is defined as a trough-shaped chamber encompassed by a pair of hinge plates, crural bases, and septalial plates, commonly supported on the dorsal sitie by a median septum (Text-fig. 9).

24

hinge plate

"septalial" plate

B cardinal process

A

median septum

hinge plate

Text-fig. 9

-

c

Types of septalia. A-B. Rhyncholrema type. Large and thin septalial plates f orming a narrow septalium. C. Hypsiplycha type. A small septalium with vestigial septalial plates, elevated to the level of hingeline. D. Feneslriroslra type, with robust hinge and septalial plates. E. Plalylracho/os type. A sessile "septalium" with hinge plates convergent on the shell floor. All figures

x

14.

In some species, for example, Rhynchotremafringilla andR. nutrix, the septalial plates are subparallel to each other, and thus the bottom of the septalium is formed by the crest of the median septum (Text-fig. 9). This type of septalium was referred to as a pseudoseptalium by Rozman (1969; see also Havllcek, 1982). The term, however, seems to be restricted to a kind of tiny septalium. Two peculiar types of septalium have been observed in the Anticosti specimens. In Platytrochalos, the "septalium" sits directly on the shell floor, and the median septum is represented by a very low median ridge. In another new genus, Gnamptorhynchos, the septalial plates descend dorsally from the straight hingeline, forming a septalium subrectangular in cross-section (Text-fig. 10). There.has been confusion over the term notothyrial cavity in the description of some Paleozoic rhynchonellids. Williams & Rowell (1965) defined the notothyrial cavity (or chamber) as an umbonal cavity in the brachial valve corresponding to the delthyrial cavity in the pedicle valve, bounded laterally by brachiophore bases and the postero-lateral shell wall. The septalium in many Paleozoic (especially Ordovician and Silurian) rhynchonellids mistakingly has been called a notothyrial cavity. For example, the septalium in most species of Ferganella, Stegerhynchus, and Rhynchotrema is well formed by a pair of septalial plates and supported by a median septum, but traditionally it has been referred to as a notothyrial cavity (see Schmidt & McLaren, 1965 ;

25

Alberstadt, 1973 : Jones,1981). According to the definition of Williams & Rowell (1965), only shells lacking a

median septum and septalial plates could have a true notothyrial cavity, as for example in Ancistrorhyn.c�a (Ulrich & Cooper, 1942) and Katunia (Kul'kov, 1963). It should be pointed out that in most rhynchonellids the septalium extends to the posterior extreme of the valve as a narrow notch sitting directly on the shell floor. This

portion of the septalium could be called a notothyrial cavity for its small, notch-like appearance. To avoid confusion, however, the term notothyrial cavity is not used in descriptions of the Anticosti rhynchonellids.

The septalium in the Late Ordovician and Early Silurian rhynchonellids from Anticosti Island commonly is not covered by any plate. Only in one species, Ancillotoechia eva, is a septalial cover-plate present. This is the earliest rhynchonellid in which a cover-plate has been discovered. Variations in shape and microscopic structure of the septalium, as observed in serial sections of Anticosti rhynchonellids, are summarized in Text-figure 9.

Cardinal Process Mesozoic rhynchonellids usually lack a cardinal process, and hence the structure has not been regarded as taxonomically important within that group. In the Paleozoic rhynchonellids, however, the cardinal process is well developed in the majority of species and takes various forms. Not surprisingly, therefore, the structure has been given much greater taxonomic value in the Paleozoic rhynchonellid classification (see, for example,

Rzhonsnitskaya, 1958; Schmidt & McLaren, 1965; Bowen, 1966; compare to Ager et al., 1972).

1

4 Text. fig. 10

•

2

5

Types of cardinal process (figures not precisely to scale). 1. Septiforrn. Stegerhynchus peneborea/is (Twenhofel, 1928).

2. Septiforrne (anteriorly inflated). Hypsiptycha anticostiensis (Billings, 1862). 3. Branching septifonn. Orthorhynchyllion prinstanum (Twenhofel, 1928).

4.

Resilifonn. Glossinulus mimicus (Barrande, 1879). 5. Multilobatifonn. Eucharitina eucharis (Barrande, 1847). 6. Incipient multilobatifonn. Gnamptorhynchos inversus n. gen. and n. sp.

3

6

26

In Paleozoic rhynchonellids, the presence or absence of a cardinal process commonly has been used as a diagnostic feature of families and genera. Observation of the cardinal process in disarticulated valves may be unreliable, however, because the structure is generally delicate and easily broken. In contrast, reconstruction from serial sections can reveal a fairly complete picture of the process. The cardinal process in Paleozoic rhynchonellids can be classified into three major types

:

septiform,

resiliform, and multilobatiform (fext-fig. 10). The septiform cardinal process is perhaps the most common. It is developed along the median line of the septalium, as a vertical blade or septum. Its microscopic structure consists of secondary shell substance, with only poorly developed parallel growth lines. This type of process is seen in such common Ordovician and Silurian genera as Lepidocyclus (Pl. 26, figs. 4-5), Rhynchotrema, Stegerhynchus, and Ferganella. The resiliform refers to a type of tongue- or spoon-shaped cardinal process projecting into the pedicle valve. This type of process is best developed in the Silurian genus Boucotella (Bowen, 1966), and the Devonian genera Glossinulus (Schmidt, 1942), Uncinulus (Havlicek, 1959), and Markitoechia (Havlicek, 1959). It is notable that the resiliform cardinal process always develops in association with a shallow or poorly defined septalium. The spoon- or tongue-shaped structure, with its marginal accessory tooth-like projections, obviously served as a small platform to provide a firm attachment for the diductor muscles. In the Anticosti rhynchonellids, no resiliform cardinal processes have been observed. The multilobatiform cardinal process includes bilobate, trilobate, and quadrilobate types commonly found in the family Eatoniidae (Schmidt & McLaren, 1965). Within the multilobate type, the number of lobes seems to have little taxonomic significance; the number varies with the ontogeny. For example, young forms of Clarkeia (Kozlowsky, 1923) have a bilobate cardinal process which becomes trilobate, or even quadrilobate, in adult forms. ·

Among the Anticosti rhynchonellids, the multilobatiform cardinal process has been observed only in the species Gnamptorhynchos inversum. Intermediate forms and other variants of the three basic types of cardinal process commonly are observed in the Anticosti rhynchonellids. For example, serial sections of Hypsiptycha anticostiensis show that the posterior portion of its cardinal process resembles that of Rhynchotrema, but the anterior portion inflates into a thick ridge along the septalial bottom. The structure simply represents a variation of the septiform type. The branching form of cardinal process in Orthorhynchyllion prinstanum (Twenhofel, 1928) seems to represent an intermediate type between the septiform and multilobatiform types of cardinal process (Text-fig. 10 ; Pl. 27, figs. 6-8).

Median Septum Reconstruction of internal structures from thin sections of well-preserved shells from Anticosti Island shows that the rhynchonellids commonly have a very low median septum supporting the septalium. Height of the septum averages about 1/10 of the shell thickness. Transverse sections through the shell posterior commonly give the false impression that the septum is rather high, especially in strongly convex shells. This is because cross-sections through the umbonal area always cut the median septum as well as the dental plates along their longitudinal axes. In the same way, septalial depth in cross-section also varies with the cutting angle and the convexity of the brachial umbo. Observation of microscopic shell structure reveals that the median septa in rhynchonellids belong to two main types, the wedge-shaped and the parallel-sided. The wedge-shaped median septum is by far the more common in Early Paleozoic rhynchonellids. The dorsal (or basal) portion of the septum takes the form of a wedge with clear and closely spaced growth lines, whereas the ventral (or top) portion is composed of secondary shell material (callus) with only obscure growth lines. It is usually the callus that unites the septal wedge with the septalial plates to form the septalium. Dorsally, the fine growth lines of the wedge merge with the shell growth lines without any clear boundary between them. This type of median septum has been observed in most species of Rhynchotrema, Fenestrirostra, and Stegerhynchus (Text-fig. 9).

27

The parallel-sided median septum exhibits dominantly parallel growth lines. It may have a vestigial wedge. This type of median septum has been observed so far only in the Ordovician rhynchonellids Lepidocyclus and Hypsiptycha ( Text-fig. 9C ; see also Schmidt & McLaren, 1965). A median septum with mainly parallel growth lines may exist in other rhynchonellids, but lack of detailed line drawings or photographs of their septa makes it difficult to evaluate their septal structures. If, however, further work shows the parallel-sided median septum to be restricted to Lepidocyclus and Hypsiptycha, it will be supporting evidence for definition of the family Lepidocyclidae (Cooper, 1956; Howe, 1965; Amsden, 1983).

Crura In rhynchonellids, crura are commonly buttressed by a pair of plates, which are divided by the crura into septalial plates and hinge plates. The shape and orientation of the crura play an important role in interpreting the evolution and taxonomy of the Mesozoic-Cenozoic rhynchonellids (Ager, 1965; Ager et a/., 1972). In contrast, crural morphology in the Paleozoic rhynchonellids has been regarded as monotonous, and hence of little taxonomic value (Schmidt & McLaren, 1965 ; Ager et a/., 1972; Xu & Yao, 1984). In the present study of Anticosti ryhnchonellids, however, longitudinal reconstructions of the crura from serial sections reveal that the Ordovician-Silurian rhynchonellids developed various types of crura, some of which probably represent ancestral types to those observed in the Mesozoic and Cenozoic rhynchonellids. Within the Anticosti rhynchonellids, five types of crura are recognized, namely, radulifer, maniculifer, hamulifer, preseptifer, and prefalcifer.

Radulifer. The radulifer type of crura is the basic and most common in the rhynchonellids. The term refers to relatively long, slender crura which curve uniformly into the pedicle valve (Ager, 1965). Although Rothpletz (1886) mentioned distal processes in his original definition and description of radulifer crura, the exact configuration of the processes was not clearly demonstrated. From now on, the term radulifer should be restricted to slender and ventrally curved crura without any conspicuous distal modifications, as crura with distal processes commonly have been described by other terms (for example, the maniculifer, Cooper, 1959). In the Anticosti specimens, radulifer crura are commonly observed in Lepidocyc/us, Hypsiptycha, Stegerhynchus, Fenestrirostra, and Lepto/epyron n. gen. (refer to illustrations under each taxon for examples of radulifer crura). Maniculifer. The term was first used by Cooper (1959) to define a type of crura resembling the radulifer but with digital branches at the distal ends. He found this type of crura in the Tertiary rhynchonellid species Mannia nysti (Davidson) [see Ager (1965, p. 599)]. In study of the Early Silurian species Rhynchotremafringilla and Hercotrema bulbicostatum (n. gen. and n. sp.), a similar kind of crura has been observed. In comparison to the typical maniculifer crura figured by Cooper, the crura in the Anticosti species also are strongly curved, with four to five short, delicate branches at the distal end of each crus (Pl. 25, figs. 6-7; Pl. 30, figs. 3-4). If such delicate distal branches were broken off during fossilization, the maniculifer crura would be nearly identical to the radulifer crura. In the present study, two sets of serial sections were prepared for Rhynchotremafringilla. One specimen was found with maniculifer crura, the· other with radulifer crura. It is difficult, therefore, to determine the taxonomic value of this type of crura. As pointed out by Ager (1965), the maniculifer may be a simple variation of the radulifer. Hamulifer (hereby named from the Latin hamulus, masculine noun, meaning a small hook. Hamulifer crura are hook-bearing). The hamulifer type of crura is another variation of the basic radulifer type. Its basic morphology is similar to the radulifer, except that a small distal hook is developed through the incurvature of each crus. Hamulifer crura have been observed only in two species, Rhynchotrema nutrix and Fenestrirostra glacialis (Pl. 28, fig. 5).

·

Preseptifer (hereby named after the existing septifer). The term refers to a type of crura which arch towards the pedicle valve and flatten vertically in their anterior portions to form plate-like structures. Preseptifer crura differ from th� septifer type found in some Mesozoic rhynchonellids in that the septifer crura descend from the hingeline to the floor of the brachial valve and the septum-like crura are sessile. In the preseptifer type, the crura are buttressed by the hinge plates for the posterior half but are free for the anterior half and curved into the pedicle valve. So far, preseptifer crura have been found only in the Anticosti species Platytrochalos peninversus. It should be noted that the illustration of septifer crura by Ager (1959, 1965) lacks microscopic details. It is possible, therefore, that the "septifer crura" are a combination of crural bases and sessile crural plates which descend to the shell floor, as in the Paleozoic genus Pugnax. Prefalcifer (see Ager, 1965). The prefalcifer crura are nearly straight and connected to the crural plates which descend dorsally from the hingeline. This type was previously believed to occur only in a group of

28

Mesozoic rhynchonellids, the Cirpinae. The Late Ordovician species Gnamptorhynchos inversus from Anticosti Island, however, carries a pair of fairly typical prefalcifer crura.

Articulatory Structures Rhynchonellids generally have strong articulating teeth and sockets. As observed by St. Joseph (1937) in his study of Rhynchotreta cuneata, the teeth fit so tightly into the sockets that very little movement between the valves is possible. Serial sectioning of the Anticosti specimens has shown that the articulation between teeth and sockets can be further enhanced by supplementary socket-floor recesses accomodating the jagged tips of the teeth (see, for example, the illustration of Hypsiptycha anticostiensis). In Gnamptorhynchos inversus, a pair of lateral accessory teeth and sockets are formed to strengthen the articulatory mechanism. Despite the teeth and sockets being well described in most species, the articulatory apparatus rarely has been regarded as important in taxonomy. In some species, hinge plates and hinge fossettes are so well formed as to constitute an important element of the articulatory apparatus. In Fenestrirostra glacialis (PI. 28, fig. 7) and Rhynchotremafringilla, for example, the hinge fossettes are not only large and deep but also rugged on their inner surfaces, which match the contact surface of the hinge plates. In the present study, morphology of the hinge fossettes is considered useful taxonomically at the specific level.

29

PHYLOGENY OF THE EARLY RHYNCHONELLIDS .

The earliest rhynchonellids made their appearance in Middle Ordovician (Llandeilo) time, which has been well established in the fossil record of both North America (Cooper, 1956) and Siberia (see, for example, Nikiforova & Adreeva, 1961). The rich and diverse rhynchonellid faunas of Anticosti Island spanned Late Ordovician and Early Silurian times, and their fossil record sheds much light on the evolution of the early rhynchonellids. Phylogenetic reconstruction, however, is always affected by subjectivity, because evaluation of characters or character states is involved. In the present work, no attempt is made to reconstruct rhynchonellid phylogenetic paths based on numerical analysis (sensu Sokal & Sneath, 1973, and Wiley, 1981), because many Ordovician and Silurian genera and species have not been well described, particularly with regard to their internal structures. Thus many potentially useful characteristics are not available for comparison. Rather, only a preliminary analysis is undertaken of some well-studied taxa, using the cladistic concepts discussed by Eldredge & Cracraft (1980). In the following discussion, two cladistic terms will be used, synapomorphy and synplesiomorphy. If a character possessed by two or more taxa is found in their immediate common ancestor and in no earlier ancestor, the character is said to be synapomorphous. In contrast, if a character possessed by two or more taxa is found not only in their immediate common ancestor, but also in their remote ancestor or ancestors, the character is said to be synplesiomorphous. In the rhynchonellids, for example, the anterior obliteration of shell costae is a synapomorphous character at the generic level for Fenestrirostra g/ai:ialis and F.pyrrha. Synapomorphous characters may offer strong evidence of close phylogenetic relationships. Characters (in alphabetical order) and character states (in numerical order) commonly referred to in comparing similarities among the Anticosti rhynchonellid genera are coded (Table 1). Among the earliest rhynchonellids of Middle Ordovician (Llandeilo-Caradoc) age, three main families are represented, the Oligorhynchiidae, Orthorhynchulidae, and Rhynchotrematidae. The rhynchotrematids are characterized by medium to large size, non-carinate pedicle umbo, rare inversion of fold and sulcus at young growth stages, and a well-developed septiform cardinal process. The rhynchotrematids became the most cosmopolitan rhynchonellids from Late Ordovician to Early Devonian times. Many orthorhynchulid genera developed clearly defined interareas on both valves and probably represent a link between the orthids and the rhynchonellids. Members of the Oligorhynchiidae were commonly of small shell size, with conspicuous inversion of fold and sulcus. They lack a cardinal process. Oligorhynchiids were closely related, and probably ancestral, to such post-Ordovician families as the Camarotoechiidae, Trigonirhynchiidae, and Leptocoeliidae. All these families are represented in the collections from Anticosti Island.

The Rhynchotrematid Group During Middle and Late Ordovician times, the family Rhynchotrematidae was represented by three common genera, Rhynchotrema, Lepidocyclus, and Hypsiptycha. Rhynchotrema increbescens (Hall, 1847) first appeared in Middle Ordovician time. It is the earliest recorded species in the family and probably was the ancestor of all other rhynchotrematids. In the American mid-continental basins, species of Rhynchotrema lived only during Ordovician time. In the Anticosti basin, however, the genus extended well into Early Silurian time, represented by the large-shelled Rhynchotremafringilla (Billings, 1862) and R. parviseptatum n. sp. Some unidentified species of Rhynchotrema also have been reported from Lower Silurian rocks of the Welsh Borderland and from Estonia (Temple, 1970; Rubel & Rozman, 1977). Serial sections of the Anticosti specimens suggest that all species of Rhynchotrema share one common character, a pair of well-developed septalial plates. The radulifer crura may be modified in some species with little hooks or digits at their distal ends.

30

Table 1. - Codes of characters and character states.

A. Adult or mean shell size 1. small(30 mm long)

D. Dorsal and ventral views 1.elongate(length > width) 2.transverse(length < width) 3. circular(length= width) 4, suboval 5.subtriangular 6.subrhomboidal 1. subrectangular 8.subpentagonal

H. Sulcus

I. absent 2. small 3.large 4. shallow 5.deep 6. narrow 7. broad 8.open 9.capped

1 Ventral beak and umbo

4.divergent 5.horizontal 6.divided 7. united 8. thin 9. thick

4.concavo-convex 5.convexo-concave 6.plano-convex 7. reversed convexities

1.erect 2. suberect 3.incurved 4. low 5. high 6.rounded 7. pointed 8.carinated 9.uniformly convex

D. Hingeline

J. Growth structures

1. short(< l/2 shell width)

1.fine(growth lines) 2.lamellose(growth lamellae) 3.imbricate(growth layers)

C. Lateral views 1.equibiconvex 2.unequibiconvex(b.v.deeper) 3.unequibiconvex(p.v.deeper)

2. long(> l/2 shell width) 3. straigh 4.curved E. lnterareas (p.v. and b.v.)

1. absent 2. present

F. Oelthyrium 1.open(no deltidial plates)

2.partially closed 3. completely close

K. Ribs 1. absent(smooth) 2.costellate(dense) 3.costate 4.plicate(sparse) 5. simple 6. intercalation 7. bifurcation 8.covering anterior 9.covering posterior

G. Fold

1. absent 2.dorsal , 3. ventral 4� anterior 5. posterior 6.low . 7,. high 8.narrow 9.broad 10.rounded 11.angular . 12. smooth 13.ribbed

N. Septalium

1. absent 2. dorsal 3. ventral 4.anterior 5. posterior 6. shallow 7. deep 8.narrow 9.broad 10.rounded 11.angular 12. smooth 13.ribbed

L. Dental plates-

1.absent 2.low 3.high 4.short 5. long 6. thin(weak) 7. thick(strong) 8.discrete 9.united(spondylium) M. Teeth

1. small 2.strong

0. Hinge plates 1. small 2.large 3.subparallel

P. Septalial plates

I. absent 2.low(small) 3. high(large)

Q.Cardinal process 1. absent 2.weak

3. strong 4. septiform 5.multilobatiform 6.branching 7. resiliform R. Median septum (p.v.) 1. absent

2.low 3. high 4. short 5. long 6. thin 7. thick S.Crura 1. short 2. long 3. thin 4. thick 5.radulifer 6.maniculifer 7. preseptifer 8. hamulifer 9.prefalcifer

31

In the Upper Ordovician and Lower Silurian sequence of Anticosti Island, only four rhynchotrematid genera have been found. Character analysis of the genera has not revealed any synapomorphous characters at the family level (that is, shared by all members of the family and not found in any other taxa), as the four character states shared by the genera can be observed also in genera from other families (Text-fig. 11).

Text-fig. 11

•

Character analysis of the Rhynchotrematidae. Only the genera from Anticosti Island are considered, for their internal structures have been studied by serial sectioning. See text for explanation of character codes.

Lepidocyclus and Hypsiptycha were separated from Rhynchotrema by Wang (1949). They are likely to have evolved from some species of Rhynchotrema. The close relationship between Lepidocyclus and Hypsiptycha is evident in two synapomorphous characters at the generic level, the development of growth layers and lamellae and conjunct deltidial plates. A simple process of strengthening the dense growth lines in Rhynchotrema could have led to the formation of imbricate growth layers in Lepidocyc/us and Hypsiptycha. The function of such prominent growth structures is not easily understood. In the Anticosti strata, Lepidocyclus gigas (Wang, 1949) commonly is found in micritic or marly limestone beds. This suggests that the species may have developed a kind of rough, imbricating shell surface for stabilization in a muddy substrate, as the shell commonly lacked a functional pedicle at the adult stage. Hypsiptycha anticostiensis (Billings, 1862), however, commonly is associated with shelly calcarenitic beds or intraformational conglomerates, indicating a relatively high-energy environment In both Lepidocyclus and Hypsiptycha, the septalial plates followed a trend to become obliterated. Thus the septalium became reduced to a small notch. The umbonal interior of the pedicle valve in Lepidocyclus commonly is strongly thickened by callus, probably an adaptative structure for stabilization in a relatively quiet environment.- that is, to keep the shell in a upright position and to prevent it from being swept away along the muddy substrate on which the shell sat. In the Anticosti succession, the genus Stegerhynchus shows a short stratigraphic overlap with Rhynchotrema, a rare phenomenon for the two genera. Previous descriptions have not clearly distinquished Stegerhynchus from Rhynchotrema, as pointed out by Amsden (1974). Character analysis based on Anticosti specimens shows that Stegerhynchus shares the same number of characters with Rhynchotrema as with Lepidocyc/us (Text-fig. 1 1). Stegerhynchus resembles Lepidocyclus and Hypsiptycha in its lack of well­ developed septalial plates, but it is also similar to Rhynchotrema in its possession of growth lines rather than growth layers or lamellae. Thus it is difficult to determine whether Stegerhynchus evolved from Rhynchotrema or Hypsiptycha. Study of the Anticosti collections has not revealed any phyletic trends among, or within, the rhynchotrematid genera..The large number of species so far documented for each genus is based mainly on shell outline, development of shell costae, and configuration of fold and sulcus. None of these characters shows any unidirectional change during the time-span of a given genus or species.

32

The Orthorhynchulid Group The orthid-like rhynchonellid stock also made its appearance in Middle Ordovician time, with species such as Orthorhynchula linneyi (James, 1881) as one of the earliest representatives. The development of prominent interareas on both valves, commonly associated with an apsacline orientation of the cardinal area, strongly suggests that the ancestor of this group was an orthid. The new species Gnamptorhynchos inversum from the uppermost Ordovician of Anticosti Island develops only a primitive type of short crura. The species shares a certain number of characters with some orthids (the Ordovician Platystrophia King, 1850, see Schmidt & McLaren, 1965)- for example, a straight hingeline and sharp interareas (Text-fig. 12). But the existence of a pair of short crura and dental plates clearly indicates that G. inversum is a rhynchonellid. It probably represents an intermediate form between the orthids and the rhynchonellids, although it could not have been the ancestor of the orthorhynchulids (Gnamptorhynchos inversum occurs only in the uppermost Ordovician rocks).

Text-fig. 12

-

A preliminary character analysis of some onhorhynchulids to show the phylogenetic relationship between the two new genera from Anticosti Island and the well-studied genus Orlhorhynchula. Gnamplorhynchos n. gen. shares three synapoinorphous character states with Orlhorhynchula, and these genera are more closely related to each other than is either of them to Orthorhynchyllion n. gen.

The discovery of the orthorhynchulids, Gnamptorhynchos and Orthorhynchyllion , in the uppermost Ordovician and Lower Silurian rocks of Anticosti Island has filled a gap in the stratigraphic range of the Orthorhynchulidae, that is, the gap between the isolated Middle Ordovician genera and the Devonian genera previously documented (Schmidt & McLaren, 1965). Compared to members of the Rhynchotrematidae, however, the genera assigned to the Orthorhynchulidae are only loosely connected to one another. Text-figure 12 shows that the orthorhynchulid genera share fewer common characters than the rhynchotrematids. The development of a trilobate cardinal process and accessory teeth in Gnamptorhynchos has not been observed in any other genus within the family. Orthorhynchyllion prinstanum is the only species in the family to have a branching form of cardinal process and prominent crural ridges. Within the Orthorhynchulidae, there seems to be a tendency toward reduction of the median septum in some genera and species. Orthorhynchula linneyi from the Middle Ordovician (Caradoc) has a thick and moderately high median septum in the brachial valve. The latest Ordovician (Ashgillian) Gnamptorhynchos inversum retains only a pair of low ridges supporting the septalium. In the Early Silurian species Gnamptorhynchos selliseptalicium, and in most Devonian species of Leptolepyron Cooper, 1942, Latonotoechia Havlfcek 1960, and Machaeraria Cooper, 1955, the septalium becomes completely sessile, without even a median ridge beneath the septalial cavity.

33

The Early Trigonirhynchiids In Ordovician-Silurian times, the family Trigonirhynchiidae was represented by only a few genera, commonly Rostricel/ula, Lepidocycloides, R hynchotreta, and Ancillotoechia. Most of these early trigonirhynchiids appear superficially similar to the rhynchotrematids, especially in the development of simple, strong costae, beginning at the umbonal apex, and of the anterior fold and sulcus. Internally, however, the trigonirhynchiids lack a cardinal process (Text-fig. 13). The diductor muscle attachment is reinforced either by a restricted septalial opening or by a cover-plate of the septalium. The absence of a cardinal process in the earliest trigonirhynchiid, Rostricellula, indicates that the ancestor of the family was closely related to some oligorhynchiids, such as Sphenotreta Cooper, 1956 and Dorytreta Cooper, 1956, which also lack a cardinal process. From Middle Ordovician to Devonian times, the only significant morphological change in the trigonirhynchiids seems to have been the development of a septalial cover-plate. The pioneer members of the family, Rostricellula and Lepidocycloides (Williams, 1962), had neither a cardinal process nor a cover-plate. Some species only developed a small, shallow, and non-restricted septalium to hold the diductor muscles (for example, Rostricellula transversa Cooper, 1956, and Lepidocycloides baikiticus (Nikiforova, in Nikiforova & Andreeva, 1961). In the Early Silurian species Rhynchotreta cuneata, a pair of small crural ridges formed to narrow the ventral opening of septalium. Ancillotoechia was probably the first genus to develop a septalial cover-plate. Most species of this genus previously were reported from Middle and Upper Silurian rocks (Havlicek, 1961 ; Rubel & Rozman, 1977). The Anticosti species, Ancillotoechia eva, indicates, however, that the genus first appeared in Early Silurian time. Rubel & Rozman (1977) suggested that there was a trend toward reduction of the cover-plate within the Ancillotoechia stock during Middle and Late Silurian times. Ancillotoechia bidentata (Risinger, 1826) and A. ancillans (Barrande, 1879) of Wenlockian age had a thick cover­ plate, which became delicate or covered only the anterior part of the septalium in the Ludlovian and Pridolian species. This observation seems to be in accordance with the internal structures of A. eva from the Lower Silurian rocks of Anticosti Island. In A. eva, the septalial cavity is very small but capped by a very thick and ventrally convex plate. If A. eva represents a primitive form of the genus, the Ancillotoechia stock may have originated in the Anticosti basin in Early Silurian time.

Tcxt·flg. 13

•

Character analysis of the four trigonirhynchiid genera from the Upper Ordovician and Lower Silurian of Anticosti Island. Not all the characters shared by the four genera are synapomorphous, for some of the character states can be observed in other families, such as

K(3-5-8-9).

See text for explanation of the character codes.

By Devonian time, some genera had evolved which retained a cover-plate capping only the anterior portion of the septalium, as in Trigonirhynchia Cooper, 1942, and Cupularostrum Sartenaer, 1961. In other genera, the septalimn again became completely open. Examples are the Early Devonian Bathyrhyncha (Fuchs, see Schmidt & McLaren, 1965) and the Late Devonian Ptychomaletoechia Sartenaer, 1961.

34

The morphological changes in the septalium and cover-plate of the trigonirhynchiids during Ordovician to Devonian times were far from simple, ubiquitous, and unidirectional. Many species of Hercotrema n. gen. reported from the Upper Silurian and Lower Devonian rocks of Bohemia (Havlicek, 1961), as well as two additional species from Anticosti Island, never developed a true septalial cover-plate but only a restricted septalial opening-typically Hercotrema nympha (Barrande, 1847) and Hercotrema pseudolivonicus (Barrande, 1847, see illustrations of Havlicek, 1961). On the other hand, the Late Devonian genus from the Altai Mountains, Nekhoroshevia (Bublichenko, 1956, see Schmidt & McLaren, 1965), retained a completely covered septalium. Character analysis for the four genera which occur in the Anticosti sequence is shown in Text-fig. 13.

The Silurian Camarotoechiids and the Anticosti Fenestrirostra Lineage The family Camarotoechiidae first appeared in Early Silurian (Llandoverian) time, represented by only two genera, Fenestrirostra and Plagiorhynchia (McLearn, 1918, see Schmidt & McLaren, 1965), and became more and more diversified during Late Silurian and Devonian times. Most genera in this family show two prominent morphological changes compared to other Ordovician-Silurian rhynchonellids. Firstly, costae tend to disappear, commonly in the anterior or posterior half of shell. Secondly, posterior of the shell generally is thickened by secondary shell material. Typical examples are Fenestrirostra, Calvinaria Stainbrook, 1945, and Hyborhynchel/a Cooper, 1955. These two morphological changes may have produced adaptive features to a low-energy muddy substrate. The thickened shell posterior, associated with a relatively long hingeline in some genera (as in Fenestrirostra and Calvinaria), obviously could have kept the shell in an upright feeding position and prevented it from being moved by weak currents or sinking into the muddy bottom. The Anticosti endemic genus Fenestrirostra shows relatively rapid morphological changes during early Llandoverian time compared to those of the other rhynchonellids in general. The pioneer species of the genus, Fenestrirostra primaeva, shows a number of morphological similarities to Dorytreta bella (Cooper, 1956) of the Oligorhynchiidae : 1) a conspicuously carinated pedicle umbo and grooved brachial umbo, which invert into a brachial fold and pedicle sulcus only near the anterior margin ; 2) lack of a cardinal process ; 3) a very low median septum ; and 4) a pair of strong and divergent hinge plates. These morphological similarities strongly suggest that the Fenestrirostra lineage stemmed from the Ordovician oligorhynchiids. Fenestrirostra primaeva occurs in the lower Merrimack Formation. It is characterized by angular to subangular costae which become rounded or weakly flattened towards the anterior. Intercalation and bifurcation of ribs are observed rarely. In the middle and upper beds of the Merrimack Formation, Fenestrirostra primaeva is replaced by Fenestrirostra glacialis. This species shows four morphological changes relative to the pioneer species (Text­ fig. 14). 1) Shell size becomes medium to large (with a mean shell length of 16.3 mm, compared to 8.5 mm for F. primaeva). 2) There is a gradation in strength of the costae. At one extreme, the shells show only a slight reduction in rib strength at the shell anterior, whereas at the other extreme, the angular to subangular costae in the umbonal portion become so intensely intercalated or bifurcated in the anterior half as to give the shell a quasi-smooth appearance. In the range of F. g/acialis, the proportion of quasi-smooth forms increases up section. 3) The strongly ribbed forms show a similar orientation of the hinge plates to those of F. primaeva, but in the quasi-smooth forms, the hinge plates are reduced in size and become nearly horizontal. 4) The median septum becomes rather high, probably associated with an increased convexity of the pedicle umbo. The end member of the lineage, F. pyrrha from the lower Gun River Formation, shows a tendency to become small again, with the median septum reduced to a low ridge. But the anterior obliteration of costae and the nearly horizontal hinge plates in F. pyrrha clearly indicate its close relationship to the quasi-smooth form of F. g/acialis. It is not clear whether the morphological changes observed in the Fenestrirostra stock represent an evolutionary trend or simply reflect ecological control. At any rate, the changes seem to record adaptation of the genus to a low-energy, muddy substrate. Reduction in the strength of costae, and in the size of hinge plates, teeth, and hinge fossettes must have greatly reduced the capability of the species to resist strong waves or currents. This postulate is further supported by the field observation that the species of Fenestrirostra predominantly occur in fine-grained micritic mudstones and shales.

35

shell and

size ribs

configuration of hinge plates around sockets

c 0 -

Cj

•

E

L.

0 u. L.

Ql >

FENESTRIROSTRA PYRRHA

a:: c :J (!)

1---+--?-

FENESTRIROSTRA GLACIALIS

c 0 -

Cj

E

'

....

0 u.

� u Cj

E

FENESTRIRO:JTRA PRIMAEVA

Text-fig. 14

•

Rapid morphological changes in the Feneslriroslra lineage during the early Llandoverian time.

The Fenestrirostra lineage disappears from the Anticosti sequence in the upper Gun River Formation. From information available, it seems that the genus was restricted to the Anticosti basin. Thus the stratigraphic and evolutionary implications of this fast-changing stock are very limited.

36

Text-fig. 15

•

Character analysis for the three early camarotoechiid genera from the Lower Silurian rocks of Anticosti Island. Fenestrirostra developed a number of unique characters, such as the anterior obliteration of costae, K(3-9), which point to its departure from the main evolution trend of the family. See text for explanation of the character codes.

In the middle Jupiter Formation, two other important genera, Linguopugnoides and Phoenicitoechia, representing the family Camarotoechiidae, made their appearance (Text-figure 15). The Anticosti species, Linguopugnoides cybelense n. sp. and Phoenicitoechia martinensis n. sp., resemble their congeneric species from Bohemia (Havlicek, 1961) in their posteriorly smooth and anteriorly low and rounded costae, and in their delicate internal structures. The Anticosti species, however, evolved much earlier (Llandoverian time) than the Bohemian

species (mainly Late Silurian-Early Devonian time). Thus it is possible that these semi-smooth camarotoechiids originated in the Early Silurian Anticosti basin in which a low-energy environment prevailed. The adaptation of Linguopugnoides cybelense to such an environment is indicated by species being preserved invariably in very fme-grained, homogeneous, micritic mudstones. Broken and disarticulated valves are extremely rare, even though the shells are typically thin and delicate. The ancestor of these posteriorly smooth camarotoechiids cannot be clearly established. They may have evolved from the same ancestral stock as Fenestrirostra but followed a different trend of transformation. Confusion arises, however, when the young forms of Fenestrirostra glacialis,Linguopugnoides cybelense, and

Dorytreta bella are compared. Linguopugnoides cybelense is smooth at the early growth stage, thus suggesting that the posteriorly smooth genera had an origin different from that ofFenestrirostra.

The Problematic Family Leptocoeliidae The Leptocoeliidae exhibit a number of morphological features reminiscent of some early atrypids, but it is uncertain whether these features are an expression of phylogenetic relationships or homeomorphy. The configuration of teeth and hinge fossettes, as well as the large, sessile hinge plates and sockets in Eocoelia

(Nikiforova, in Nikiforova & Andreeva, 1961) and Platytrochalos, resemble those of the Early Silurian atrypids Clintonella (Hall & Clarke, 1893), Alispira (Nikiforova, in Nikiforova & Andreeva, 1961), and of the Devonian (Eifelian) Carinatina (see Siehl, 1962, pl. 24, fig. 3). The family Leptocoeliidae previously has been assigned to the Spiriferida or Atrypida, but none of the genera in the family have been found to bear spiralia� In the present work (following Cocks, 1978), the family is transferred to the order Rhynchonellida (see further discussion under the description of the family; also Ma, 1984). In the Anticosti basin, the earliest species of the family, Platytrochalos peninversus, occurs in the basal beds of the Becscie Formation, that is, at the very base of Silurian sequence. P. peninversus shares a number of common characters with the Ordovician species Sphenotreta cuneata : 1) small shell size; 2) strongly reversed

37

fold and sulcus ; and 3) lack of a cardinal process. This suggests that the pioneer leptocoeliid species either could have originated from the oligorhynchiids or shared a common ancestor with them. Platytrochalos peninversus persists through nearly the entire Lower Silurian sequence (from the lower Becscie to the upper Jupiter formations) without showing any significant morphological change, but it may have given rise to the other Early Silurian leptocoeliid species, as it is the earliest species ever recorded in the family. The congeneric species, P. crudicostatus from the Richardson Member of Jupiter Formation (late Llandoverian), developed very strong and sparse shell costae. This species probably was adapted to a specific environment not well understood at present. The species lasted for only a short time. Platytrochalos pu/vinatus probably represents a link, or an intermediate form, between Platytrochalos and Eocoelia. Its internal structure is similar to that of P. peninversus, except for its well-developed myophore which posteriorly resembles a cardinal process. Externally, the semicircular and nearly plano-convex shell resembles that of Eocoelia. The strong muscle pad (myophore) and large and deep muscle field in the pedicle valve suggest that P. peninversus had strong diductor and adductor muscles, an indication that it was capable of living in waters of relatively high energy level and in various feeding positions. The closely related species of Eocoelia also have been common in shallow-water and high-energy environments (Ziegler, 1965, 1966 ; Ma, 1984). The phylogenetic relationships among the genera and species assigned to the family Leptocoeliidae require further study.

39

RHYNCHONELLID FAUNAL SUCCESSIONS

The Anticosti basin has particular importance for the development and migration of Late Ordovician and Early Silurian rhynchonellids. The Late Ordovician species of Anticosti Island show great similarity to those in the American mid-continental basins, but the Early Silurian rhynchonellids of the island are comparable to the coeval or younger faunas of the Baltic and Bohemian basins. From the information so far accumulated, there seem to be two striking characteristics in the development of rhynchonellid faunas in the Anticosti basin during the Late Ordovician and Early Silurian ages. 1) The well-established Ordovician rhynchonellid faunas became nearly extinct in the American mid-continental basins by the end of Ashgillian time, with the disappearance of such common genera as Lepidocyclus, Hypsiptycha, Rhynchotrema, and Orthorhynchula. In the Anticosti basin, however, some of the typical Ordovician rhynchonellids' survived and gave rise to the Early Silurian (Llandoverian) representatives of Rhynchotrema and Gnamptorhynchos. 2) Many genera found in the Middle and Upper Silurian strata of Bohemia and Estonia occur in the Lower Silurian rocks of Anticosti Island. These two characteristics imply that the Anticosti basin served as a critical locus for rhynchonellid evolution during the Ordovician-Silurian transition and served to link the rhynchonellid faunas of the American mid-continental and European basins. The Ordovician-Silurian rhynchonellid succession in the Anticosti basin may be divided, somewhat subjectively, into three stages, each represented by a characteristic assemblage of species : 1) the Ashgillian Hypsiptycha fauna; 2) the early Llandoverian Fenestrirostra fauna; and 3) the late Llandoverian Stegerhynchus­ Linguopugnoides fauna.

The Ashgillian Hypsiptycha Fauna The Hypsiptycha fauna is defined herein to be composed mainly of five common Middle to Late Ordovician genera, Rhynchotrema, Lepidocyc/us, Hypsiptycha, Orthorhynchu/a, and Rostricel/ula. These genera are widespread in the American mid-continental basins (Hall & Clarke, 1893-1895; Cooper, 1956; Howe, 1967, 1969). Rostricel/ula may well be a cosmopolitan genus, because it is also found as far away as Siberia (Nikiforova & Andreeva, 1961) and North China (Fu, 1982). In the Anticosti basin, nearly all the genera of the Hypsiptycha fauna were present in Late Ordovician time. An exception was Orthorhynchula which was replaced by two endemic genera, Gnamptorhynchos and Orthorhynchyllion. The close link between the Anticosti and American mid-continental rhynchonellids is shown clearly by the similarity of species (see Text-fig. 16). Rhynchotrema increbescens, Lepidocyclus gigas, Hypsiptycha anticostiensis (a probable senior synonym of H. hybrida), and Rostricellula transversa are species common to both Anticosti Island and American basins. Only three endemic species evolved during Ashgillian time, Rhynchotrema nutri.x, Orthorhynchyllion prinstanum, and Gnamptorhynchos inversum. From the evidence of the fossils and the host rocks, it is difficult to explain why Rostricellula and Rhynchotrema became more widely spread or cosmopolitan than the other genera of the Hypsiptycha fauna. Lepidocyclus, Hypsiptycha, Gnamptorhynchos, and Orthorhynchyllion were restricted to basins associated with the Laurentian (North American) paleocontinent or even to the Anticosti basin, except for rare and questionable species of Hypsiptycha reported from the Caradocian rocks of Ireland (Mitchell, 1977) and Estonia (Roomusoks, 1964). As far as shell morphology is concerned, none of these genera developed particular features which might have constrained them from spreading and living in various environments. Most of the species commonly occur

40

in micritic mudstones and shales, or biomicritic wackestones. Only Hypsiptycha has been observed to be associated with beds of conglomeratic limestones at one locality (A441, Anse aux Praises), which indicates a relatively high-energy environment. By the end of Ordovician time, four genera and seven species had become extinct in the Anticosti basin, marking the end of the Hypsiptycha fauna (Text-fig. 16). Disappearance of the majority of the taxa at that time may have been related to formation of the North African (Algerian) ice-cap (Berry & Boucot, 1973 ;McKerrow, 1979; Brenchley & Newall, 1984).

The Early Llandoverian Fenestrirostra Fauna The Anticosti Fenestrirostra fauna is composed mainly of endemic species. The major elements include Fenestrirostra primaeva, F. glacialis, F. pyrrha, R hynchotrema fringilla, R. parviseptatum, R hynchotreta cuneata, and Gnamptorhynchos selliseptalicium from the Becscie, Merrimack, and Gun River formations. Minor components are Stegerhynchus praecursor and Platytrochalos peninversus. It is important to note that the early Llandoverian Fenestrirostra fauna contains some hold-over species from the Ashgillian Hypsiptycha fauna, as well as some pioneer species of the late Llandovery Stegerhynchus­ Linguopugnoides fauna. Rhynchotrema became extinct in the American mid-continental basins by the end of Ordovician. In the Anticosti basin, however, the genus survived until early Llandovery time, represented by R. parviseptatum and the large-shelled R.fringilla. This indicates that the Anticosti basin provided environments favorable to the survival of these species. The explanation may be that the tectonically well-protected Anticosti basin of Early Silurian time provided the prevalent low-energy, relatively shallow-water (subtidal or deeper) environments preferred by the species (Text-fig. 2). Such an interpretation is supported by the field observation that R. fringilla occurs in abundance as articulated shells in the Merrimack Formation which is typically composed of micritic mudstones and shales. However, species of Rhynchotrema are rare in the upper Becscie Formation, which is commonly marked by ripples, current channels, scours, and other high-energy, shallow­ water, sedimentary structures. The adaptation of Rhynchotremafringilla to low-energy substrate conditions is also suggested by its co-existence with Fenestrirostra glacialis which shows a number of morphological features adaptive to a low-energy, muddy substrate. Some unidentified species of Rhynchotrema also have been reported from the Welsh Borderland (Temple, 1970) and from Estonia (Rubel & Rozman, 1977). This indicates that the survival of Rhynchotrema into earliest Silurian time was not a phenomenon unique to the Anticosti basin. Most of the Anticosti endemic species in the Fenestrirostra fauna are characterized by short time-spans (Text-fig. 16). The disappearance of R hynchotrema frin gilla, R. parviseptatum, and Gnamptorhynchos selliseptalicium from the Merrimack and Gun River formations probably marks the final extinction of the genera. The three short-lived species of Fenestrirostra are restricted to the Merrimack and Gun River formations. The intermediate nature of the Fenestrirostra fauna between the older Hypsiptycha fauna and the younger Stegerhynchus-Linguopugnoides fauna also is indicated by the occurrence within it of Stegerhynchus praecursor and Platytrothalos peninversus. Both Stegerhynchus and Platytrochalos are major components of the late

Llandovery fauna. The development of the Anticosti endemic species within the F enestrirostra fauna probably was controlled by local conditions of substrates and water regimes, rather than by paleogeographical barriers. This seems likely because the fauna, although dominated by endemic species, contains other widespread and cosmopolitan species. For example, Stegerhynchus praecursor is a common species in the American mid­ continental basins (Foerste, 1909). Rhynchotreta cuneata has been well documented in the Silurian rocks of North America, the Welsh Borderland, Gotland (St. Joseph, 1937b; Bassett & Cocks, 1974), Estonia (Rubel & Rozman, 1977), Bohemia (Havlicek, 1961), the Altai Mountains (Kul'kov, 1967), and North China (Fu, 1982). The presence of R. cuneata in the Fenestrirostra fauna suggests that there existed a fair paleogeographical ' potential for faunal migration during early Llandoverian time. It is to be noted also that the presence of R. cuneata in the lower Llandovery rocks of Anticosti basin marks the earliest occurrence of the species. In other basins, the same species has been reported only from the Wenlockian and Ludlovian strata. Thus the Anticosti basin may have been close to the locus of origin of the species

41

The Late Llandoverian Stegerhynchus-Linguopugnoides Fauna The late Llandoverian rhynchonellid fauna of the Anticosti basin consists of many widespread or cosmopolitan genera, such as Stegerhynchus, Ancillotoechia, Linguopugnoides, and Hercotrema. At the species level, however, the Anticosti species pre-date most of their congeneric species or counterparts in other basins. The Stegerhynchus-Linguopugnoides fauna is defined herein to include, as major elements, such common species as Stegerhynchus borealis, S. peneborealis, S. vicina, Ancillotoechia eva, Platytrochalos peninversus, P. crudicostatus, Linguopugnoides cybelense, Leptolepyron argenteum, as well as a few other species which occur in small numbers. This fauna is nearly restricted to the Jupiter Formation (Text-fig. 16).

ASHGILL

SERIES

LLAND OVERY

RAWTHEYAN - HIRNANTIAN

STAGE

VAUREAL

FORMATION

ELLIS

' '

MEMBER

' '

AERONiAN

RHU DDANIAN BECSCIE

BAY

GUN

MM.

TELYCHIAN

- increbescens I

cc.

JUPITER

RIVER

' ' ' '

' '

:

' '

:

' '

nufrix

Rhynchofrema

parv i sepfafum

- fringilla - gigas

Lepidocyclus

I

H ypsiplycha

anficosfiensis - praecusor borealis peneboreolis

Sfegerhynchus

concinno vic ina Orfhorhynchyi/Jon Gnampforhynchos

prinsfonuminversumsel/isepfoliclum

Rhyncholrela

transversa

Rosfriceflula

-

cuneafa

-

- eva

Ancifloloechia bulbicosfafum

Hercofrema

•

primoeva

�

-

pyrrha marfinensis

Phoenicifoechia

I

I

Asluforhyncha peninversus

�----- -- pu/vinafus

•

asfufa

•

-

crudicosfafus

Text-fig. 16

-

cybelense

Linguopugnoides

Plafyfrochalos

-

- glacio/is

Fenesfrirosfra

Lepfo/epyron

-

I

-

orgenfeum-

Range chart of the Anticosti rhynchonellid species. The rhynchoneilid faunas are divided into the Ashgillian Hypsipt ycha fauna, the early Llandoverian Fenestrirostr a fauna, and the late Llandoverian Stegerhynchus­ Ling uopugnoides fauna.

42

Like the Fenestrirostra fauna, many species in the S.-L. fauna show morphological features adaptive to low-energy and probably muddy substrates. Most species developed an incurved pedicle beak which closed off the delthyrium and foramen, indicating that these species lacked a functional pedicle at their adult stage. The loss of a pedicle muscle was common in a great number of brachiopod groups adapted to low-energy, muddy substrate conditions. Examples are some free-lying strophomenaceans, most Silurian atrypids, and posteriorly weighted pentameraceans. Obliteration of the foramen is well shown by two closely related species in the Anticosti basin, Stegerhynchus borealis and S. peneborealis. S. borealis is a cosmopolitan species. It generally developed a suberect or erect pedicle beak, with the delthyrium well-exposed and the foramen open (St. Joseph, 1937a; Rubel & Rozman, 1977; Jones, 1981). This suggests that the species possessed a functional pedicle during life, and thus was able to attach itself to various substrates, even in relatively high-energy environments. In comparison, S. peneborealis lacked a pedicle opening at the adult stage due to its strongly incurved pedicle beak. To compensate for the loss of a pedicle, the shell became much extended transversely, and the hingeline much longer than inS. borealis. In addition, The shell posterior became so thickened as to obliterate the dental cavities. These were common adaptational modifications for low-energy and soft substrates. The common occurrence of the species in the micritic mudstones of the Goeland Member supports such an interpretation. S. borealis from the biohermal East Point Member and S. vicina from the biohermal-encrinitic Chicotte Formation commonly retained a suberect beak and an exposed pedicle opening. The unusually early appearance of certain taxa in the middle and upper Jupiter Formation (Richardson through Pavilion members) is the most interesting feature of the S.-L. fauna. Species such as Ancillotoechia eva,Linguopugnoides cybelense, and Phoenicitoechia martinensis are the earliest recorded for these genera. By way of contrast, Ancillotoechia bidentata and A. ancil1ans occurred at much higher stratigraphic levels (Wenlockian to Ludlovian) in Gotland, Estonia, and Bohemia (Havllcek, 1961 ; Bassett & Cocks, 1974). Within the Laurentian paleocontinent, Ancillotoechia conspicuua (Amsden, 1978) from Oklahoma occurs in Wenlockian rocks. In the same way, other genera, such as Linguopugnoides and Phoenicitoechia, appear much later in Bohemia and the USSR, represented by the Wenlockian-Ludlovian species L. carens and L. dulcissima and by the Devonian species L. praecox and P. phoenix (Havlicek, 1961). Relating the Anticosti species to their congeneric counterparts in other basins and other continents leads to two important conclusions. 1) The Anticosti late Llandoverian S.-L. fauna contains the earliest known, and possibly the ancestral, species of a number of genera which became widespread in other basins and other continents during Wenlockian and later times. 2) There was a good faunal link between the Laurentian and Baltic paleocontinents during the Middle and Late Silurian epochs. Most species in the S.-L. fauna disappear within the Jupiter Formation, except for S. vicina which is the only rhynchonellid species to occur in the Chicotte Formation. It is not clear at present whether the fauna became extinct at the end of Llandoverian time or survived for longer. The biohermal-encrinitic environment represented by the Chicotte Formation may have been unfavorable to the late Llandoverian rhynchonellid fauna. Only future work in the surrounding region will show whether the fauna migrated to other parts of the Anticosti basin or even to other basins, as the Chicotte Formation marks the top of the stratigraphic sequence on Anticosti Island. The three rhynchonellid faunas are concentrated in three separate formations, that is, the Ellis Bay, Merrimack, and Jupiter. A general trend, however, seems to have been for an increase in species diversity with time. The Ashgillian Hypsiptycha fauna consists of only seven species. The early Llandoverian Fenestrirostra fauna is composed of nine species, and the late Llandoverian Stegerhynchus-Linguopugnoides fauna of 13 species. This general increase in species diversity suggests that the Anticosti basin became more and more favorable for rhynchonellid evolution through the Late Ordovician and Early Silurian epochs. It may also explain why the Anticosti basin provided a kind of "shelter" for survival of some of the Ordovician taxa (Rhynchotrema and Gnamptorhynchos) and also nurtured the earliest, and perhaps the ancestral, species of such genera as Ancil/otoechia, Linguopugnoides, and Eocoelia.

43

SYSTEMATIC PALEONTOLOGY

RHYNCHONELLIDA Kuhn, 1949 Order Superfamily RHYNCHONELLACEA Grey, 1848 Family RHYNCHOTREMATIDAE Schuchert, 1913 Diagnosis : Costae generally simple, angular to subrounded, beginning at apex of umbones, crossed by fine to imbricate concentric growth structures. Pedicle sulcus and brachial fold present. Delthyriuril open to completely covered by deltidial plates. Dental plates present, rarely fused to shell wall. Septalium developed to varying degrees, with or without well-formed septalial plates. Hinge plates usually large and thick. Cardinal process septiform. Crura commonly radulifer.

Remarks

: Schmidt

& McLaren (1965) divided the family Rhynchotrematidae into tWo subfamilies, the

Rhynchotrematinae and Orthorhynchulinae. The latter was said to have reduced dental plates and median septum. Study of the Anticosti rhynchotrematids has revealed, however, that dental plates in a single genus vary from

normal to reduced, as in Stegerhynchus and Lepidocyclus. The Orthorhynchulidae, erected by Cooper (1956) and subsequently degraded to a subfamily by Schmidt & McLaren(1965), is hereby resurrected as a separate family. It differs from the Rhynchotrematidae in its costae of irregular strength, fold and sulcus of more variable shape and position, better-defined interareas, commonly reduced dental plates and median septum, and more variable cardinal process (septiform, trilobate, or branching). Many genera in the Orthorhynchulidae have not been well studied by serial sectioning. Future work may show that some genera previously regarded as orthorhynchulids could belong to the Rhynchotrematidae.

Genera asssigned: Rhynchotrema HALL, 1860

Stegerhynchus FOERSTE, 1909 ( Stegorhynchella RZHONSNITSKAYA, 1959) Ferganella NIKIFOROVA, 1937 Lepidocyclus WANG, 1949 Hypsiptycha WANG, 1949 Pleurocornu HAVLICEK, 1960 ? Hiscobeccus AMSDEN, 1983 =

Age : Middle Ordovician----Late Devonian.

Genus Rhynchotrema HALL, 1860

Type species: Atrypa i.ncrebescens HALL, 1847, p. 146, pl. 33, figs. 13a-d (non 13e-y). Trenton Limestone of New York. Middle Ordovician (upper Caradoc).

Diagnosis :(Herein emended from Wang; 1949, Cooper, 1956, Howe, 1965, and Schmidt & McLaren, 1965). Shell small to large. Pedicle umbo uniformly convex, non-carinate, with suberect or incurved beak. Delthyrium

44

partly closed by marginal deltidial plates. Pedicle sulcus developed anterior of umbo. Brachial fold inverted into minute median groove on umbo. Shell costae simple, subangular to subrounded. Dental plates varying from low and stout to high and thin. Muscle field flabellate, commonly striated. Septalium small to large, with well­ developed septalial plates. Cardinal process septiform. Crura of radulifer type, or modified into hamulifer type.

Remarks: Howe (1965) compared Wang's (1949) and Cooper's (1956) definitions of Rhynchotrema, and noticed six contradictory character descriptions, such as the presence or absence of deltidial plates, cardinal process, and development of dental plates. The delthyrium in Rhynchotrema commonly carries marginal deltidial plates. In some specimens, however, the plates may be so rudimentary that the delthyrium appears completely open. Wang (1949) mentioned that the presence of a cardinal process in the genus was questionable. This was probably due to his observations being based mainly on disarticulated valves in which the delicate blade-like processes were broken. Serial sectioning of well-preserved shells of Rhynchotrema has always revealed a well-formed cardinal process. Rhynchotrema differs from Lepidocyclus and Hypsiptycha in its lack of growth layers or lamellae and in its more rudimentary deltidial plates. Internally Rhynchotrema has well-delimited septalial plates, whereas Lepidocyclus has short septalial plates, and Hypsiptycha lacks such plates. Species assigned :Atrypa increbescens HALL, 1847 ( Rhynchotrema wisconsinense FENTON

& FENTON, 1924 ; Rhynchotrema inaequivalvis WINCHELL & SCHUCHERT, 1895, Synonymy discussed in Weiss, 1955 ; Cooper, 1956, and Alberstadt, 1973). Atrypa dentata HALL, 1847 Rhynchonellafringilla BILLINGS, 1862 Rhynchonella nutrix BILLINGS, 1866 Rhynchotrema kentuckiense FENTON & FENTON, 192 ( R. inaequivalvis WINCHELL & SCHUCHERT, 1895, in part ; see synonymy in Cooper, 1956) Rhynchotrema plicata COOPER & KINDLE, 1936 Rhynchotrema iowense WANG, 1949 Rhynchotrema parviseptatum n. sp. =

=

=

·

Species questionably assigned : Rhynchotrema borealis NASEDKINA, 1973

Rhynchotrema uralica NASEDKINA, 1973 Rhynchotrema yaoxianensis FU, 1983 The Middle Ordovician R. borealis and Upper Ordovician R. uralica from the Ural Mountains develop a medium-sized shell and septiform cardinal process. Nasedkina (1973) did not provide illustrations of internal structures for the two species from the Urals, and distinquished R. borealis from R. increbescens by its larger shell, more pronounced fold and sulcus, and more convex pedicle umbo. She also made comparison between R. uralica and R.windermeris WILSON 1926 (formulation of the specific name by Wilson will be discussed in a separate paper). Examination of R. windermeris from the Beaverfoot Formation of the southeastern Canadian Rocky Mountains, however, shows that the species belongs to Lepidocyclus, as it has medially conjunct deltidial plates and strong growth lamellae.

Age : Middle Ordovician-Early Silurian.

Rhynchotrema increbescens (HALL, 1847) Pl. 3, figs. 23-27

1847 1860 1894

Atrypa increbescens Hall, p. 146, pl. 33, figs. 13a-d. Rhynchonella (Rhynchotrema) increbescens HALL, p. 66. Rhynchotrema i n c rebescens (HALL) ; Hall & Clarke, p. 182, (no illustration, only a revision of

1895

Rhynchotrema inaequivalvis (CASTELNAU) ; Winchell & Schuchert, .p. 459, pl. 34, figs. 12-14, 20-23,

1924 1949 1955

Rhynchotrema wisconsinense FENTON & FENTON, p. 71, pl. 71, figs. 6-8 (synonymy by Weiss, 1955). Rhynchtorema increbescens (HALL) ; Wang, p. 11 (lectotype selected). Rhynchotrema increbescens increbescens (HALL) ; Weiss, p. 772, pl. 70, figs. 3-7.

description). 24-25 ?).

45

1956 1965

Rhynchotrema increbescens (HALL) ; Cooper, p. 628 (the four original type specimens re-examined). Rhynchotrema increbescens (HALL); Schmidt & McLaren, p. 555, figs. 422-3a-c.

Type specimens : Hall's original types were a mixture of R. increbescens and Lepidocyclus capax (CONRAD, 1842), for he believed that "R. increbescens has no higher value than a designation for an earlier and somewhat modified type of R. capax". that is, "Rhynchotrema increbescens HALL = R. capax CONRAD"(Hall & Clarke, 1894, p. 183). Wang(1949) selected the specimen figured in Hall(1847, pl. 33, figs. 13c-d) as the lectotype for R. increbescens. Cooper(1956) examined the four original types deposited in the American Museum of Natural History and emended the definition of the species. The problem also was discussed by Weiss (1955) and Howe (1965). Unfortunately, no one seems to have photographed the four original types, nor have serial sections been made of any of the original types or topotypes. Type locality and type horizon: Broadly defined by Hall as the Trenton limestone(= the Trenton Formation), New York. Caradoc, Middle Ordovician(Schmidt & McLaren, 1965; Cocks, 1978). Remarks : The only complete specimen found is 7.6 mm long, 7.1 mm wide, and 4.8 mm deep. It is small, subtriangular, unequally biconvex, with a deeper brachial valve. The hingeline is very short and curved. The pedicle umbo is narrow, with a suberect beak. The pedicle sulcus and brachial fold are gentle, beginning 2-3 mm from hingeline. The costae are simple and subangular, with four on the fold, three in the sulcus, eight to nine on each flank. Growth lines are very fine and well developed anteriorly. Internal structures have not been studied due to a lack of specimens for serial sectioning. The Anticosti specimen resembles Rhynchotrema increbescens (HALL, 1847) in external morphology, particularly in its slightly elongate, subtriangular outline and development of costae, fold, and sulcus. More specimens need to be collected for study of internal structures, especially the development of septalial plates and cardinal process. Collections: Vaureal Formation. A405,

(I

specimen), NTS 12E/8W 48580: 80500, Cap Robert.

Rhynchotremafringilla (BILLINGS, 1862) Pl. 1, figs. 1-15; Pl. 2, figs. 1-15; Pl. 25, figs. 1-8; Text-figs. 17-20.

Rhynchonellafringilla n. sp. ; Billings, p. 141, figs. 118a-b. Camarotoechiafringilla (BILLINGS) ; Hall, p. 190, pl. 56, figs. 28-30. 1928 Camarotoechiafringilla (BILLINGS); Twenhofel, p. 209, pl. 21, figs. 7-9. 1972 Camarotoechia (?)fringilla (BILLINGS); Bolton, p. 12, pl. 7, figs. 9, 13 ; pl. 8, fig. 6 . 1981 Anci/lotoechiafringi/la (BILLINGS) ; Bolton, pl. 13, fig. 25. 1862

1894

Type specimens : The lectotype, GSC2370a, selected as the "holotype" by Twenhofel (1928, note that his illustration of the specimen in Pl. 21, figs. 7-9 was actual shell size, not x 2 as he explained), is illustrated herein in Pl. 1, figs. 1-5. Twenhofel(1928) mentioned that the types consisted of the holotype, 2370a, and the paratypes, 2370b-n. In the Geological Survey of Canada collections, however, only six original types were found. One of the syntypes, GSC2370(not the lectotype, GSC2370a), is most similar to Billings' illustration. But Billings' line drawings were probably based on more than one specimen, as the figures do not match precisely any of his types. Thus Twenhofel's selection of GSC2370a as the lectotype is regarded herein as valid. In addition to the five paratypes (GSC2370 and GSC2370b-e) assigned by Billings, two more specimens (GSC2371b,k), originally regarded as belonging to Rhynchonella glacialis, should be assigned to Rhynchotrema

fringilla. Type locality and type stratum : "Gull Cape, Anticosti. In Division 4, Anticosti Group, Middle Silurian" (Billings, 1862, p. 142). Billings' Gull Cape is now called Merrimack Point. Detailed stratigraphic measurements and fossil collections made during the last decade indicate that the precise type locality is .the coastal cliffs forming the east side of Baie Innommee, adjacent to Pointe Merrimack, NTS 12F/4W-5W 8740088100: 54850-55700. The species occurs in abundance in the upper Merrimack Formation(bed A356-4, about 2 m from the top). Description : Shell large, transverse, subpentagonal to elliptical, nearly equibiconvex, or with slightly deeper brachial valve in strongly convex, very large forms. Mean shell length 23.1 mm, width 26.0 mm, thickness

46

4

width (mm)

LOCALITY' Al56 141

TOTAL' 76 SPECIMENS MEASURED

35

3

25

x=0.89

20 L/W 0.1

15

.a

.9

1.0

1.1

2.2

2·6

X=t.40

! 10 LIT (,Q

(.4

1.8

length 5

10

15

20

25

30

mm

Text-fig. 17- Shell dimensions of Rhynchotremafringi/la (Billings, 1862). The sample measured is from a single bed in locality

A356-4, Baie Innommee, Merrimack Formation.

'

16.5 mm. Smallest shell 2.8 mm long, 2.7 mm wide, 1.0 mm thick ; largest 32.5 mm long, 38.4 mm wide, 26.2 mm thick. Length/width ratios decreasing slightly, thickness/width ratios increasing greatly, with increasing shell size (Text-fig. 17). Hingeline weakly curved, attaining one-third to two-thirds shell width. Pedicle umbo moderately and uniformly convex, with suberect to incurved beak. Delthyrium partly covered by disconjunct duplex deltidial plates (Pl. 25, figs. 2-3 ; Text-fig. 18, 2.2 mm from apex). Sulcus deep, wide, with gentle slopes, beginning anterior to umbo, marked by three simple, subrounded costae increasing to five near anterior margin of some large specimens by bifurcation or intercalation. Brachial umbo moderately to strongly convex, with beak curved into delthyrial cavity. Minute, shallow median furrow in brachial umbo changing into median fold 3-5 mm from apex of valve. Fold low to moderately high, attaining about one-third shell width at anterior margin, generally marked by four simple, subrounded costae increasing by bifurcation to eight at anterior margin of large forms. Costae strong, simple, ten to twelve on each flank, with lateral four fine to obscure in relatively small shells. Bifurcations and intercalations restricted to anterior margin of large shells. Dental plates short, low, stout, fused posteriorly to shell wall, becoming free distally. Teeth strong, with fairly large hinge fossettes (Text-figs. 18-20). Muscle field large, deep, flabellate in outline, striated in anterior part, without clearly differentiated adductor and diductor scars. Septalium narrow, moderately deep, posteriorly sessile, anteriorly supported by relatively low, thick median septum. Septalial plates well developed,

'

47

1.2

-����

+:��

_;-·�

/

J\ �

\

tF

A j

mus cle field

'

crura

Text-fig. 18

·

Serial sections of Rhynchotremafringilla (Billings, 1862). Hypotype GSC 102401, from locality A356-4, Baie Innommee, Upper Merrimack Formation. Note the disjunct duplex type of deltidial plates (labeled as "deltidium"), the high septalial plates, and the reduction of dental plates. x 4.

Q.9 �� .' •

deltidial plate

·

.

: .

'

.

..

1·5

.

:.

..

dental plate

median �septum

monieutlfe-r type

of

r&curved crura

crura

�/@ A

Text-fig. 19

·

5·3

,/ t

�

4·3

3·1

Serial sections of Rhynchotremafringilla (Billings, 1862). Hypotype GSC 102402, from locality A356-3, Baie Innommee, Upper Merrimack Formation. Note the well-delimited septalial plates dorsally fused to each other to form a septalium, and the maniculifer type of crura. The disjunct duplex deltidial plates are not well formed (1.9 mm from apex). x 4.

48

concave or subparallel, descending dorsally from hingeline for about half valve depth to meet median septum. Hinge plates relatively small, fitting tightly into hinge fossettes. Cardinal process thin, septiform (Pl. 25, figs. 1, 4-5). Crura long, radulifer, strongly curved, with distal ends modified into three. to four subparallel branches to become maniculifer in some specimens(Pl. 25, figs. 6-8).

median septum

crus

median septum

Text-fig. 20

•

Rhyncholremafringi/la (Billings, 1862) from the serial 18 and 19). Note that the low median septum appears deceptively high in cross sections around

Longitudinal reconstruction of the internal structures for sections (text-figs.

the umbo, mainly due to the cutting angle. x 4.

Remarks :In his description of the species, Twenhofel(1928) noted that R .fringilla had "no cardinal process."

Twenhofel was wrong, possibly because he based his observations on some strongly ribbed specimens of Fenestrirostra glacialis, which may superficially resemble young shells of R.fringilla, or possibly by inference, because he assignedfringilla to Camarotoechia, which lacks a cardinal process. Serial sections of well-preserved shells of R.fringilla show that the species invariably possesses a septiform cardinal process. R.fringilla differs from other congeneric species in its large size at the adult growth stage and its disconjunct duplex deltidial plates. Among the Ordovician-Silurian rhynchonellids, the maximum size of R. fringilla is approached only by Lepidocyclus gigas. Although R.fringilla differs from Fenestrirostra glacialis in its possession of a cardinal process, it may take careful observation to separate the two species, especially their young forms. The superficial resemblance of R.fringilla to the strongly ribbed forms of F. glacialis may explain why Billings and Twenhofel misplaced a few of the type specimens for the two species. Externally, F. glacialis tends to have costae of uneven strength, even in the strongly ribbed forms, a carinated pedicle umbo, and strongly

inverted fold and sulcus. There is some ontogenetic variation in the orientation of pedicle beak. In small shells (< 15 mm in length), the beak is suberect to weakly incurved, but in larger forms(> 15 mm in length), the beak becomes strongly curved and appressed onto the brachial umbo. Collections: (total 363 specimens). Merrimack Formation. A356

=

A317 NTS 12F/4W-5W 87400-88100:

54850-55700, Baie Innommee. A356-3(10 specimens); A356-4(103); A356-8(2); A317-1(5); A317-5(14); A317-ll(1); A317-13(21); C683 A22(207), NTS 12E/12E 47120: 99100, La Loutre Road. =

49

Rhynchotrema nutrix (BILLINGS, 1866) Pl. 3, figs. 1-22 ; Pl. 25, fig. 9; Pl. 26, figs. 1-2; Text-figs. 21-23.

1866 1928

?

1973

Rhynchonella nutrix n. sp. ; Billings, p. 43 (no illustration). Rhynchonella (?) nutrix (BILLINGS) ; Twenhofel, p. 123, pl. 23, figs. 18-20. Rhynchotrema increbescens (HALL) ; Alberstadt, p. 51, pl. 7, figs.6�7.

Type specimens : The holotype by monotypy, GSC2278, was first illustrated by Twenhofel (1928) and also is shown in the present study (Pl. 3, figs. 1-5). Type locality and type stratum : "Gamache Bay, Division 1, Anticosti Group" (Billings, 1866, p. 43). The old name Gamache Bay was used for the present Baie Ellis. In the course of the present study, no specimens were found at the type locality. Among the new localities for R. nutrix, the Junction Cliff (Anse aux Praises to Cap de la Vache-qui-Pisse) is the nearest to Ellis Bay and is selected herein as the locus typicus restrictus. At this locality, R. nutrix is relatively common and well preserved. It constitutes a minor component of the Parastrophinella beds, which are assigned to the Prinsta Member of the Ellis Bay Formation in the uppermost part of the Ordovician sequence.

E E

Rhynchotrema nutrix

14

•

c 697

s:

-o 13 c :

iii il; l5

15

100 80

60 � 40 � 20 z

LENGTH

TOTAC '

10

332 �PECJMENS

30

� 2S � 20 � 1 6 5 UJ tt 10

5

0-6

5 Text-fig. 51

-

10

15

20

.7

.s

.9

tO

1.1

1-2

l/W RATIO

25

1.3

mm

length

Shell dimensions ·of Fenestrirostra glacial is (Billings, 1862). Measured in sample collected from a single bed at locality A356-3, Baie Innommee, Merrimack Formation.

93

crural) "/

sto ctkhet

� dettidium

crus (with a distal hook)

hincje

Text-fig. 52

•

Serial sections and longitudinal reconstruction of internal structures of Fenestrirostra glacialis (Billings,

102 847,

Hypotype GSC X

strongly ribbed from (variant

A),

locality

A356-3,

1862).

Baie Innommee, Merrimack Formation.

4.

UL O���, QOOOO I fltJ 1) : � \ �eptaltu(!J.�

"detlldoum·

Cl1\ �

���:

...,.--....

I

!

Q.J

OA

��

� /

;'

\

-

'

I

\

0.6

--

09

10

�

'

crura

, -� .

,

mufietsdcle

,

·

12

' '

1.4

Text-fig. 53

•

Serial sections of Fenestrirostra glacialis (Billings, locality

A 356-3,

1862).

Hypotype GSC

102 488,

cutting angle through the strongly convex brachial umbo. The hamulifer specimen. x 4.

quasi-smooth form (variant B),

Baie Innommce, upper Merrimack Formation. The septalium appears deceptively deep due to the crura are not well developed in the

94

Description : Shell medium-sized, with mean length 16.3 mm, width 18.5 mm, thickness 9.9 mm. Smallest shell 2.4 mm long, 2.1 mm wide, 0.6 mm thick; largest 26.0 mm long, 28.3 mm wide, 17.5 mm thick. Shells less than 8 mm long generally elongate, with mean length/width ratio 1.15 ; shells more than 8 mm long commonly transverse, with mean length/width ratio 0.84 (Text-fig. 51). Adult shell nearly equibiconvex. Hingeline nearly straight or weakly curved, attaining one-third to one-half shell width. Anterior commissure weakly or moderately denticulate, uniplicate. Pedicle umbo low, rounded, with sharp median carination. Beak strongly incurved, appressed onto opposite umbo in relatively large specimens, erect or suberect in small forms. Delthyrium open or with rudimentary deltidial plates (seen in serial sections). Umbonal median carination of pedicle valve changing into sulcus 5 mm anterior of apex. Sulcus angular to rounded, deep to shallow, attaining two-fifths shell width anteriorly, commonly marked by three major costae increasing by intercalation (rarely bifurcation) to six to ten at anterior margin. Brachial umbo strongly convex, with beak buried in delthyrial cavity. Median groove of brachial umbo changing into fold 5-6 mm anterior of hingeline. Fold occupied by four major costae increasing to eight to ten by bifurcation (rarely intercalation) near anterior margin.

Shell generally costate from beaks to anterior

commissure. Costae high, angular in posterior one-third of shell, low, rounded towards anterior. Posterior shell flanks each with five to seven costae increasing by bifurcation (common in b. v.) or intercalation (common in p. v.) anteriorly.

'

hinge plate

f \I

tooth & socket

Text-fig. 54

·

Serial sections of Fenestrirostra glacial is (Billings, 1862). Hypotype A356-3, Baie Innommee, Merrimack Formation.

x

8.

GSC 102489, weakly ribbed form, locality

95

Dental plates low, thick, dividing umbonal interior into deep delthyrial cavity and small dental cavities. Pedicle muscle pad or incipient pedicle collar well formed in some specimens (Pl. 28, figs� 1-4 ; Text­ figs. 52-54). Dental plates convex to each other, descending from hingeline to one-half valve depth, then curving laterally to rest on shell wall or merge with shell thickening. Teeth strong, with well-developed hinge fossettes. Muscle field deep, adductor and diductor scars not readily identifiable. Septalium shallow to moderately deep, about one-quarter to one-third valve depth, with short, thick septalial plates. Hinge plates relatively large, divergent from each other at 80-160°, fitting tightly into hinge fossettes (Pl. 28, figs. 6-7). M�ian septum rather low. Cardinalprocess absent. Crura long, radulifer or hamulifer.

Remarks : Fenestrirostra glacialis shows great intraspecific variation in the strength of its costae. In the collection of F. glacialis, two extreme variants can be recognized, with a series of gradational forms between them. One is the strongly ribbed form (named variant A for convenience ; see Text-fig. 52). The angular costae begin at the beaks, becoming slightly flattened around the anterior margin but still forming a strongly to moderately denticulate commissure. The number of costae is relatively constant: six to eight on each flank, three to four in the sulcus. Only one or two of the lateral costae may increase by bifurcation or intercalation near the anterior margin. The other is the weakly ribbed or quasi-smooth form (named variant B ; see Text-fig. 54). In this variant, angular or subangular costae are retained only in the umbonal area, and they become intensely flattened and bifurcated or intercalated at mid-length. Around the anterior margin, the costae become numerous and costellae-like, giving some shells a smooth appearance. It is difficult to determine the precise number of ribs in this form, because of their disappearance, with concomitant increase in number, in the anterior half of the shell. The anterior commissure becomes linear, resembling that of a smooth shell. Examination of large numbers of specimens (over 100 in each sample) shows that gradational forms are common between the two extreme variants. Internally, the only marked difference between the extreme variants lies in the development and orientation of the hinge plates and hinge fossettes. In variant A, the hinge plates are large and strong, divergent from each other at 60-90°, and fit tightly into a pair of deep crural fossettes. In variant B, weaker hinge plates are divergent from each other at obtuse angles (150°).

Collections : (total 2 862 well-preserved shells from 11 horizons in the Baie Innommee section). Merrimack Formation. A356 A317 A411 NTS 12F/4W-5W 87400-88100 : 54850-55700. Baie Innommee. A356-3 (810 specimens), A356-4 (69), A356-6 (1), A356-8 (71), A356- ll (10). A317-1 (213), A317-5 (636 + 1 slab with numerous young forms). A317- ll (63), A317-13 (507). Gun River Formation. A356�2 (354), A411 (137). =

=

Fenestrirostra pyrrha (BILLINGS, 1866) Pl. 20, figs. 1-24 ; Pl. 28, fig. 9 ; Text-figs. 55-56

1866 1928

Rhynchonella Pyrrha [sic] BILLINGS, n. sp., p. 44 (no illustration). Camarotoechia (?) pyrrha (BILLINGS); Twenhofel, p. 211, pl. 21, figs. 11-12.

Type specimens : Billings' original types consist of six slightly silicified complete shells, GSC2368, a-e. The specimen GSC2368c was selected by Twenhofel (1928) as the "holotype" and thus is to be regarded as the lectotype (herein illustrated in Pl. 20, figs. 1-5).

Type locality and type stratum : "One mile east of Otter River; Division 2, Anticosti Group" (Billings, 1866, p. 44). Otter River was the old name for Riviere ala Loutre. There are outcrops in low bluffs along the coast east of Rivi�re ala Loutre (e.g. A23 NTS 12E/12W 44630 : 95950), but no new specimens were collected from these outcrops. The strata belong to the lower part (about 8-15 m above base) of the Gun River Formation. On the north coast, numerous specimens were collected from two Baie Innommee localities in the Sandtop and Chute members of the Gun River Formation. Description : Shell small to medium-sized, subcircular to subpentagonal, nearly equibiconvex, with moderate convexity. Mean length 9.5 mm, width 10.5 mm, thickness 4.9 mm. Specimens smaller than 7 mm length slightly elongate ; those 7-9 mm circular ; those larger than 9 mm dominantly transverse (Text-fig. 55). Hingeline generally shorter than one-half shell width. Anterior commissure vaguely denticulate, gently to strongly uniplicate. Pedicle umbo low, carinate, 1-1.5 mm above hingeline. Beak small, incurved, appressed onto opposite umbo, suberect only in specimens less than 8.0 mm length. Delthyrium open. Median ridge low, angular to subangular, fold-like, with two to four fine, subangular costae, changing anteriorly into low, rounded sulcus

96

E E 15 14 13 12 11

LOCALITY :, A 235 164 SPECIMENS

'X=9.1mm

WIDTH

1

so �40 .. E ·o 30 .. "" ..

- 20 0

l E

10

� c

a.o 10 11 12 shell length (mm)

4.06-0

10 9 8 7 6

0/o

5

t 30 z � 20

4

0 UJ

..

3

e: 10

2

0

2

3

4

x=o.9e

5

6

7

8

9

1·1 L/W RATIO

10

11

12

13

14 mm

SHELL LENGTH Text-fig. 55

•

Shell dimensions of Fenestrirostra pyrrha (Billings, 1866). The measurements are based on a sample from Baie lnnommee (A235), lower Gun River Formation.

\

\

\

crura

hinge plate Text-fig. 56· Serial sections of Fenestrirostrapyrrha (Billings, 1866). Hypotype GSC 102501, locality A151b, Baie Innommee, lower Gun River Formation. Note the small septalium, low median septum, and nearly horizontal hinge plate.

x

11.

97

4-5 mm anterior of apex. Sulcus with two low major costae and one fine, intercalating costa becoming very obscure anteriorly. Brachial umbo weakly convex, marked by minute median furrow changing into low, rounded fold 3-4 mm anterior of hingeline. Two to three fine costae of umbonal furrow extending onto fold, becoming obscure near anterior margin. Umbonal part of shell moderately costate. Lateral costae tending to disappear anteriorly. Radiating lines, three to four in 1 mm, visible anteriorly, crossed by equally fine concentric growth lines (Pl. 20, figs. 25-26). Dental plates well developed, posteriorly low and thick, anteriorly high and thin, with small dental cavities. Teeth moderately strong, with small hinge fossettes. Muscle field very deep, without clearly divided adductor or diductor scars (Text-fig. 56). Septalium small, shallow, with poorly defined septalial plates, supported by low median septum. Hinge plates stout, divergent from each other at about 150° (Pl. 28, fig. 9). Cardinal process absent. Crura slender, radulifer.

Remarks : F. pyrrha is closely related to the quasi-smooth form of F. glacialis in its sharply delimited median carination in the pedicle umbo, median furrow in the brachial umbo, and intense diminution of costae at the shell anterior. F. pyrrha differs from F. glacialis in its much smaller size (see Text-fig. 55), and in having its median septum reduced to a low ridge in the brachial valve.

Collections : (total 323 slightly silicified specimens). Gun River Formation. Chute Member: A235 (216 specimens extracted from a single slab), NTS 12F/4W 87970: 54840, Baie Innommee. Sandtop Member: Al51 (107 specimens extracted from one slab), NTS 12F/4W 87530: 55120, near the mouth of Chute Creek.

Fenestrirostra primaeva

n.

sp.

Pl. 18, figs. 6-20; Pl. 19, figs. 1-18; Pl. 28, fig. 8; Text-figs. 57-60.

Derivation of name :Latin primaeva, a feminine, singular adjective, meaning "of early period". The term implies that the new species is the ancestral species of Fenestrirostra. The Latin noun rostra is the neuter, plural, and nominative form of rostrum (beak). According to the International Code of Zoological Nomenclature, however, "a genus-group name must be or be treated as a noun in the nominative singular" (Article 11g, ICZN, third edition, 1985, p. 29), and "a genus-group name that is or ends in a Greek or Latin word takes the gender given for that word in the standard Greek or Latin dictionaries, unless the Commission rules otherwise" (Article 30a, ICZN, third edition, p. 57). In his formulation of the name Fenestrirostra for Billings' species Rhynchonella glacialis (both terms feminine), Cooper (1955, p. 56) did not explain why he chose rostra instead of rostrum as the ending. In the present study, Fenestrirostra is provisionally treated as a feminine, singular, and nominative Latin noun, so as to conform with Billings' species, glacialis and pyrrha.

Type specimens

: Eight specimens are selected herein as the types, including the serially sectioned paratype, GSC 102496(Text-fig. 58), and one slab (paratype, GSC 102497) with shells of F. primaeva preserved in life

position (Pl. 19, figs. 15-18). The holotype, GSC102494, is illustrated in Pl. 19, figs. 5-9.

Type locality and type stratum : Locality AlSO, NTS 12F/5W 86950-87010 : 56150-56250, Baie Innommee, Anticosti Island. At the type locality, the new species occurs in abundance in the lowermost 3.7 m of the Merrimack Formation, characterized by gray calcareous shales with minor thin, slabby beds of limestone.

Diagnosis : Small shells of Fenestrirostra of low convexity; angular costae only slightly flattened anteriorly, increasing rarely by bifurcation and intercalation ; hinge plates relatively large and thick ; median septum very low.

Description

: Shell dominantly small, subcircular, nearly equibiconvex, generally of low convexity, with mean length 8.9 mm, width 9.7 mm, thickness 5.1 mm. Smallest shell in collection 3.0 mm long, 2.5 mm wide, 1.1 mm thick ; largest 15.0 mm long, 16.7 mm wide, 10.0 mm thick. Small shells (