The Irish Fossil Polyplacophora julia sigwart (Received 6 September 2006. Accepted 31 January 2007.) Abstract Three species of fossil polyplacophoran molluscs are known from Ireland. Two species were originally described in the nineteenth century: Helminthochiton griffithi Salter in M‘Coy, 1846 and Pterochiton thomondiensis (Baily, 1859), and an articulated specimen representing a third indeterminate species, described here for the first time. Previous work on the evolutionary context of these species has relied on published illustrations and descriptions without examination of the type material. As chitons are considered rare in the fossil record, these specimens represent an interesting and important aspect of Irish palaeobiology. Introduction The molluscan class Polyplacophora, commonly known as chitons or coat-of-mail shells (ciotón máille in the Irish language), is represented by about 1,000 living species worldwide. The polyplacophoran fossil record extends from Upper Cambrian to Recent (Smith 1960), with most fossils recorded from the Cenozoic. Chitons form a distinctive molluscan clade whose members normally have eight shell plates (valves) as adults (Fig. 1). These animals are found in oceans all over the world, primarily in the intertidal zone and shallow waters of exposed rocky shores, but ranging to depths of more than 7,000m (Gowlett-Holmes et al. 1998). Few articulated fossil chitons are known from the Palaeozoic (Hoare 2000). The majority of fossil chiton specimens are disarticulated (and often fragmented) individual valves. Many Recent species of chitons live in high-energy rocky intertidal environments where conditions are not conducive to fossilisation; however, identification remains a critical problem in finding fossil chitons, and additional material is frequently found in museum collections (Hoare 2002a). Three species of Palaeozolc fossil chitons have been discovered in Ireland, unrelated to each other except for
Fig. 1—Line drawing of typical valve elements of chitons, with arrows indicating measurements (l, longitudinal distance; w, lateral distance; h, height; dorsal elevation = h/w) and typical morphological features (N, apical notch, A, apophyses; D, diagonal on lateral plates; C, central area; L, lateral area; M, mucro on posterior plate). A, anterior (head) valve; B, intermediate valve; C, posterior (tail) valve; D, intermediate valve, view of anterior face.
their provenance from the same island. No other fossil chiton specimens are known from Ireland. Two of these species were described by Irish workers, while the third is figured here for the first time. Previous checklists and synoptic treatments of Palaeozoic fossil chitons
Irish Journal of Earth Sciences 25 (2007), 27–38.
© Royal Irish Academy
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28 Irish Journal of Earth Sciences (2007) have made reference to the two fossil chitons described from Ireland, but without examination of the specimens (Sirenko and Starobogatov 1977; Smith and Hoare 1987). In fact, the type specimens of both species have been kept safely in the Irish national collections, but have not been re-examined since their original descriptions in the nineteenth century. Smith and Hoare (1987) included both Helminthochiton griffithi Salter in M‘Coy, 1846 and Pterochiton thomondiensis (Baily, 1859) in their Checklist and Bibliography of fossil chitons. At that time, the whereabouts of the name-bearing type material for both species was not known to the authors. The holotype and only specimen of Helminthochiton griffithi is held in the Griffith collection in the National Museum of Ireland, Natural History Division (Geological collections, NMING). Material of Pterochiton thomondiensis has now been recognised in the collections of the Geological Survey of Ireland (GSI). This paper presents redescriptions of the material and discussion of conclusions made by other authors. This information contributes to knowledge of the palaeobiology of Ireland, and the study of polyplacophoran evolution. Systematic palaeontology Class POLYPLACOPHORA Gray, 1821 Subclass PALEOLORICATA Bergenhayn, 1955 Order CHELODIDA Bergenhayn, 1943 Suborder SEPTEMCHITONINA Bergenhayn, 1955 Family HELMINTHOCHITONIDAE Van Belle, 1975 Genus HELMINTHOCHITON Salter in M‘Coy, 1846 Type species Helminthochiton griffithi Salter in M‘Coy, 1846, by original designation. Recognised species Helminthochiton griffithi Salter in M‘Coy, 1846; H. aequivoca Robson, 1913; H. carpenteri Hoare, 2002a; H. grayiae Woodward, 1885; H. papilio Whidborne, 1892; H. secundus Horny in Spinar, 1965. The geologic al distribution is given in Table 1. Occurrence Lower Ordovician (Arenigian) and Devonian of the Czech Republic; Upper Ordovician of Scotland; Devon ian of England; Lower Silurian of Ireland. Remarks This genus and the type species were described by Salter in M‘Coy (1846) from the collection of Richard Griffith. Salter read his paper at the meeting of
the Geological Society (London) in 1846, and it was published in the proceedings in 1847. It was contemporarily included by M‘Coy in the ‘Addenda’ to A Synopsis of the Silurian Fossils of Ireland (Salter in M‘Coy 1846), which publication predates the published edition of Salter’s (1847) Geological Society presentation. M‘Coy’s Synopsis was later reprinted in London by an English publisher, but with the content unchanged (Salter in M‘Coy 1862). Helminthochiton griffithi Salter in M‘Coy, 1846 Fig. 2 1846 Helminthochiton griffithi Salter in M‘Coy. Addenda, p. 71, pl. 5, fig. 5a–e. 1847 Helminthochiton griffithi Salter in M‘Coy. Salter, p. 51, fig. 6. 1857 Chiton (Helminthochiton) griffithi (Salter). de Koninck, pp. 193, 196. 1859 Chiton (Helminthochiton) griffithi (Salter). Baily, p. 333. 1860a Chiton (Helminthochiton) griffithi (Salter). Baily, p. 96. 1860b Helminthochiton griffithi Salter in M‘Coy. Baily, pp. 42, 45. 1862 Helminthochiton griffithi Salter in M‘Coy. Addenda, p. 71, pl.5, fig.5a–e. 1882 Helminthochiton griffithi Salter in M‘Coy. Etheridge, p. 85. 1882 Helminthochiton griffithi Salter in M‘Coy. Dall, p. 280. 1883 Helminthochiton griffithi Salter in M‘Coy. de Rochebrune, pp. 18–19, pl. 3, fig. 7. 1960 Helminthochiton griffithi Salter in M‘Coy. Smith, pp. I52–3, figs 3a, 3b. 1977 Helminthochiton griffithi Salter in M‘Coy. Sirenko and Starobogatov, p. 33. 1987 Helminthochiton griffithi Salter in M‘Coy. Smith and Hoare, p. 32. 2002a Helminthochiton griffithi Salter in M‘Coy. Hoare, p. 95. Type Material Holotype NMING:F4522/A (part; mould of dorsal surface) and NMING:F4522/B (counterpart; cast of dorsal surface); holotype by monotypy (ICZN 1999: Art. 73.1.2). Diagnosis Salter in M‘Coy (1846) described the species as ‘Linear-oblong, smooth, carinate; plates bent at a low angle, thin, deeply emarginated behind; no distinct lateral area; cephalic plate half oval, longer than wide, marked with four or five faint radiations, and a few lines of growth’.
Sigwart—The Irish fossil Polyplacophora 29 Table 1—Currently recognised species in the fossil genus Helminthochiton; the type species of the genus is indicated with an asterisk. Helminthochiton species
Geological Age
Type locality
H. aequivoca Robson, 1913
Lower Ordovician (Arenigian)
Czech Republic (Sárka and Malé Prilepy)
H. carpenteri Hoare, 2002a
Devonian
Germany (Villmar)
H. grayiae Woodward, 1885
Upper Ordovician
Scotland (Ayrshire)
H. griffithi* Salter in M‘Coy, 1846
Lower Silurian
Ireland (Coolin, Co. Galway)
H. papilio Whidborne, 1892
Devonian
England (Devon)
H. secundus Horny in Spinar, 1965
Devonian
Former Czechoslovakia (exact locality unknown)
Description The holotype preserves a mould of the dorsal side of the valves in the counterpart, and a cast of this mould in the part; part and counterpart are split along the cavity of the mould. There is no shell material preserved. There are impressions of the head valve and five intermediate valves articulated together, presumably as in life, with the posteriormost of the intermediate valves broken diagonally. The valves are carinate and strongly arched, and there is a pronounced median ridge; the side slopes of the intermediate valves are straight. The whole fossil appears distorted somewhat so that the median line is slightly to the left of centre (Fig. 2). The body is elongate (more than four times long as wide) and the head plate is distinctively large. In the part (NMING:F4522/A), which is effectively a cast of the dorsal surface, radial ridges and concentric growth lines are visible, as illustrated by M‘Coy (1846, 1862). In addition, there is some evidence of dorsal sculpture, especially on the anterior edge of the head valve, but it is too poorly preserved to be distinguished with certainty from the texture of the rock. Growth lines are also visible on the right lateral edge of valve II. There is no distinction in elevation that separates lateral areas on the intermediate valves. The head valve has a V-shaped margin, but without an apical notch, and the valve is not highly elevated. The posterior margins of the intermediate valves appear to be straight, but are somewhat obscured. There is a breakage on the anterior edge of the impression (counterpart) of valve III and IV, which is manifest in the mould (part) as a false extension of the posterior edge of valve II and III.
Measurements Specimen length 25.0mm measured along median line, width 8.0mm. Head valve 5.1mm at apex, 7.9mm at posterior extent; intermediate valves 4.7–5.2mm long (on exposed surface). Preserved portion of specimen fully articulated. Occurrence The holotype represents the only known specimen of this species, and the only specimen of the genus Helminthochiton known from Ireland (Fig. 2). The single specimen is known from Coolin, Cong, Co. Galway, in the Kilbride Formation, Telychian, Llandovery, Lower Silurian. Salter in M‘Coy (1846) reported that the specimen had been found at Coolin ‘in flaggy mudstone’; Salter (1847) reported that ‘Mr. Griffiths found it a year or two back in the silty mudstone overlying the fossiliferous conglomerate of Cong, Co. Galway’. Discussion Salter (1847) and Salter in M‘Coy (1846) incorrectly interpreted the part of this fossil as representing a mould of the inner (ventral) surface of the chiton’s shell, when in fact it is simply a cast of the dorsal surface. This is clear from the corresponding evidence of dorsal shell sculpture on both part and counterpart. Because of this preservation of the unique specimen, it is impossible to see anything of the ventral shell features. Thus it is impossible to conclude what the apophyses (sutural laminae; Fig. 1) may have looked like; these are a critical diagnostic feature for paleoloricates. Sirenko and Starobogatov (1977), having only the original illustrations of the species to interpret and following Salter in M‘Coy (1846), assumed that the fossil
30 Irish Journal of Earth Sciences (2007)
Fig. 2—Helminthochiton griffithi, Silurian polyplacophoran from Coolin, Cong, Co. Galway. Holotype NMING: F4522/A (part), natural mould of dorsal surface; NMING: F4522/B (counterpart), impression of dorsal surface. Scale bar = 10mm.
represented an internal mould (i.e. preserving the ventral surface rather than the dorsal surface). They therefore concluded that Helminthochiton did not have any apophyses. This was a significant factor in their placement of the genus within the Palaeoloricata. Because of the nature of the preservation of this unique specimen, the presence or absence of apophyses cannot be resolved. Salter in M‘Coy (1846) and Salter (1847) included the feature of ‘apophyses widely separated’ in the description of the genus Helminthochiton, but, as has been observed by Hoare (2002a), the concept of the genus then included many other taxa now assigned to Gryphochiton Gray, 1847. However, Salter in M‘Coy (1846) and Salter (1847) also stated that Helminthochiton included three subdivisions, and that H. griffithi represented a new and ‘peculiar’ subgroup more closely allied to elongate forms of Recent chitons, particularly Stenoplax alata and Schizochiton incisus. H. griffithi does superficially resemble Stenoplax spp., or Schizochiton spp., genera in two families of the neoloricate order Chitonida (sensu Sirenko 1997), because of their elongate body form. However, these two families extend back only to the Palaeocene; it is unlikely that H. griffithi is related to these taxa as suggested by Salter in M‘Coy (1846) and Salter (1847). Smith (1960) and Reeve (1911) interpreted the fossil as preserving the posterior part of the animal, but
this was incorrect. Sirenko and Starobogatov (1977) noted that if H. griffithi did represent the posterior section of a polyplacophoran, its shells would have had to grow backwards. Salter in M‘Coy (1846) and Salter (1847) correctly referred to the terminal valve of the fossil as ‘cephalic’. Smith (1960) provided an inaccurate drawing of the counterpart (impression) slightly magnified, showing a crack on the anterior valve which is not present on the material, and a lateral drawing of the part (mould) at actual size, apparently taken directly from the illustration of Salter in M‘Coy (1846). Smith (1960) did not indicate that the two illustrations are of part and counterpart, only that they are in different views. As Smith’s (1960) contribution to the Treatise of Invertebrate Paleontology is the most widely cited work on fossil polyplacophorans, the fact that he was unable to consult the holotype of this species has no doubt added considerable confusion to its interpretation over the last 50 years. There are six species of Helminthochiton, of which H. griffithi is the type species (Table 1), known from the Lower Ordovician to Devonian in various localities across Europe. H. griffithi is from the lower Kilbride Formation, Co. Galway, which is middle Telychian (Llandovery) in age. The formation locally represents a nearshore environment with brachiopod faunas characterised by Eocoelia curtisi (Holland 2001). Of the other Helminthochiton species, the majority are from
Sigwart—The Irish fossil Polyplacophora 31 shallow marine habitats. H. carpenteri is known from shallow-water limestone representing a deltaic reef system (Beyrich 1868). H. papilio is known from another shallow-water limestone in the Tully Formation (House 2002). However, H. grayiae was found in deep marine shale deposits characterised by extensive trilobite faunas (e.g. Whittington 1950). This diversity, or inconsistency, in palaeoecology within the genus shows that there is considerable room for further study. Comparisons with other morphologic ally similar material (such as Septemchiton) and the type material of H. griffithi are necessary to settle this question. ‘Helminthochiton’ grayiae was originally referred to Helminthochiton when it was first described by Woodward (1885), but Rolfe (1981) subsequently considered it to be synonymous with Septemchiton vermiformis Bergenhayn, 1955, which revision made S. grayiae (Woodward, 1885) the type species of Septemchiton Bergenhayn, 1955. More recently, Hoare (2002a) included H. grayiae in a list of recognised Helminthochiton species without further discussion. All of these species belong to the extinct suborder Septemchitonina but it remains unclear whether this is a natural group or a polyphyletic assemblage of convergent elongate morphotypes. Subclass NEOLORICATA Bergenhayn, 1955 Order LEPIDOPLEURIDA Thiele, 1910 Suborder LEPIDOPLEURINA Thiele, 1910 Family LEPTOCHITONIDAE Dall, 1889 Genus PTEROCHITON Carpenter in Dall, 1882
Type species Subsequently designated by Carpenter in Dall (1882); Chiton eburonicus de Ryckholt, 1845. Recognised species Pterochiton eburonicus (de Ryckholt, 1845); P. absidatus Hoare, 2002a; P. concentricus (de Koninck, 1842); P. legiacus (de Ryckholt, 1845); P. mosensis (de Ryckholt, 1845); P. newelli Smith, 1976; P. sandbergianus (de Ryckholt, 1845); P. subgemmatus (d’Orbigny, 1850); P. thomondiensis (Baily, 1859). The geological distribution is given in Table 2. Occurrence Lower Carboniferous, Viséan, of Visé, Belgium, Permian of the United States of America, and the Lower Carboniferous limestone of Ireland. Remarks Carpenter in Dall (1882) erected this genus to include species predominantly from the Viséan of Belgium, characterised by large, thick plates with large apophyses and ornamented dorsal sculpture. The other diagnostic features are the ‘false beaks’ on intermediate plates, formed by converging anterolateral margins. Additional taxa in this genus have been recognised from the Permian of Texas and of Oregon, United States of America (e.g. Smith 1976; Hoare and Smith 1984; Hanger et al. 2000); however, these taxa differ morphologically from the European Pterochiton and may be better assigned to other genera (Hoare 2002a, 2002b).
Table 2—Currently recognised species in the fossil genus Pterochiton; the type species of the genus is indicated with an asterisk. Pterochiton sp.
Geological Age
Type locality
P. absidatus Hoare, 2002a
Lower Carboniferous
Visé, Belgium
P. concentricus (de Koninck, 1842)
Lower Carboniferous
Visé, Belgium
P. eburonicus* (de Ryckholt, 1845)
Lower Carboniferous
Visé, Belgium
P. legiacus (de Ryckholt, 1845)
Lower Carboniferous
Visé, Belgium
P. mosensis (de Ryckholt, 1845)
Lower Carboniferous
Visé, Belgium
P. newelli Smith, 1976
Middle Permian
Texas, U.S.A.
P. sandbergianus (de Ryckholt, 1845)
Middle Devonian
Vilmar, Germany
P. subgemmatus (d’Orbigny, 1850)
Lower Carboniferous
Visé, Belgium
P. thomondiensis (Baily, 1859)
Carboniferous
Co. Limerick, Ireland
unnamed Pterochiton sp. Hanger et al., 2000
Permian
Oregon, U.S.A.
32 Irish Journal of Earth Sciences (2007) Pterochiton thomondiensis (Baily, 1859) Fig. 3 1859 Chiton thomondiensis Baily. pp. 331–2, pl. 28, fig. 2, 2a–c. 1860c Chiton thomondiensis Baily. pp. 167–71, pl. 4, fig. 2, 2a–c. 1860a Chiton thomondiensis Baily, p. 95 (and footnote). 1860b Chiton thomondiensis Baily, pp. 43, 45. 1862 Chiton thomondiensis Baily. Kirkby, p. 237. 1882 Pterochiton thomondiensis (Baily). Dall, p. 281. 1883 Anthracochiton thomondiensis (Baily). de Rochebrune, p. 27. 1977 Pterochiton thomondiensis (Baily). Sirenko and Starobogatov, p. 36.
1987 Pterochiton thomondiensis (Baily). Smith and Hoare, p. 55. 2007 Pterochiton thomondiensis (Baily). Sigwart et al., Appendix. Type material Lectotype (designated herein) GSI:F02075 (Fig. 3); Paralectotypes GSI:F01752, GSI:F01753, GSI: F23464. Additional material Twelve additional isolated valves from seven localities in Co. Limerick, Ireland (Table 3). The material includes three head valves (dorsal aspect), one poster ior valve (ventral aspect; Fig. 3A), seven intermediate valves (dorsal aspect; Fig. 3B) and one fragment of the ventral surface of an intermediate valve.
Table 3—Details of Pterochiton thomondiensis material held in the Geological Survey of Ireland (GSI), from Co. Limerick. Measurements in brackets indicate the length of preserved material that may be fragmented or obscured by matrix. All measurements are in millimetres. Specimen number
Valve description
Length
Breadth
Height
GSI:F2075 Lectotype
Intermediate
22.9
32.5
20.3
GSI:F1753 Paralectotype
Intermediate (ventral fragment)
(15.0)
(15.4)
GSI:F23464 Paralectotype
Intermediate (ventral fragment)
(17.8)
(10.0)
GSI:F1752 Paralectotype
Intermediate (ventral fragment)
(25.9)
(12.6)
GSI:F1750
Posterior (ventral)
22.2
24.4
GSI:F23308
Intermediate
22.8
(26.2)
(18.3)
GSI:F2107
Head
17.0
22.8
17.8
GSI:F23304
Intermediate
24.8
27.9
20.7
GSI:F2078
Head
12.7
19.0
12.6
GSI:F11827
Intermediate
14.0
(13.5)
(6.0)
GSI:F23307
Intermediate
15.8
(16.6)
(12.5)
GSI:F23305
Intermediate
25.4
(29.0)
18.9
GSI:F1755
Intermediate
14.9
(21.0)
(11.0)
GSI:F1756
Head
(14.2)
20.8
12.5
GSI:F23306
Intermediate
17.8
(22.8)
(15.0)
GSI:F1754
Intermediate (ventral fragment)
(11.6)
(16.9)
Sigwart—The Irish fossil Polyplacophora 33
Fig. 3—Pterochiton thomondiensis, Carboniferous polyplacophoran from Lisbane and Rathkeale, Co. Limerick. A. Lectotype GSI:F2075 intermediate valve (anterior end to bottom); B. GSI:F23304 intermediate valve, showing anterior apophyses (bottom); C. GSI:F2107 head valve (anterior margin to bottom); D. GSI:F1750 tail valve (ventral aspect), anterior apophyses at top; E. Paralectotype GSI:F1752 intermediate valve, ventral fragment with impression of dorsal sculpture preserved (bottom); F. Paralectotype GSI:F1753 intermediate valve, ventral fragment; G. Paralectotype GSI:F23464 intermediate valve, ventral fragment. Scale bar = 10mm.
Diagnosis Baily’s (1859: 331–2) original description reads: Shell elongated. Plates subquadrate and very thick, broader than long, having a median elevation, or prominent ridge, with an accumulated apex; surface concentrically striated by lines of growth; which become broken into granulations on each side of the central ridge; about ten faint radiating lines proceeding from the apex to the posterior margin; apophyses widely separated. The figures of Baily (1859, 1860a) are stylised reconstructions rather than illustrating a particular specimen.
Description All material is embedded in hard limestone matrix, preserving shells with either ventral or dorsal surface visible. Valves are strongly arched, with very high dorsal elevation (height/breadth = 0.62 on lectotype and greater on other specimens; Table 3). There is a strong median ridge but without an excavated keel. The lateral areas of intermediate valves are slightly raised; outer pleural areas are slightly concave marking transition to lateral areas, but without differentiated sculpture. On all valves, the sculpture is uniformly radial—there are strong lateral growth lines on central and pleural areas and shallow radial ribs on lateral areas. The central (older) areas of the shell are relatively smooth, marked with few faint growth lines, but there are 7–10
34 Irish Journal of Earth Sciences (2007) closely concentric growth lines on the outer one-third of the dorsal surface, around the entire anterior edge. The pattern of sculpture is the same on the head valves. Apophyses, where visible, are rounded, either oval or trapezoidal in shape, visible as protrusions beyond the anterior margin. The ventral surface of intermediate valves (known only from fragments) is marked with strong, highly elevated ridges. Ventral aspect of the head valve is not known. However, the presumed tail valve is known only from the ventral aspect, which is round in shape, but with no strong ridges. The mucro of the tail valve is slightly anterior of centre. The posterior margin is strongly convex. Measurements Measurements for all material are given in Table 3. Occurrence Co. Limerick, Ireland, in Carboniferous limestone from eight localities. The type locality is at Lisbane, Co. Limerick. All localities are within the Waulsortian limestone (late Tournaisian to early Viséan). Discussion Baily’s (1859) original description reported six specimens found in Lisbane, Co. Limerick. Although he mentioned in a footnote that additional material had been found subsequently in Rathkeale, Co. Limerick, these first six specimens from Lisbane are considered
as the type series. The material from Rathkeale was mentioned as a footnote in the original description (Baily 1859) and revisited when he presented the finding again to the Geological Society of Dublin (Baily 1860c). A precise collecting locality for the type material is marked on GSI nineteenth century fieldsheet maps (6-inch 19/3; Table 4), corresponding to a large collection of fossils from an outcrop of Carboniferous limestone. The original register of the collection actually records seven specimens; four plates in dorsal aspect as reported by Baily (1859), and three fragments of valves in ventral aspect (not two as reported by Baily). Of the seven specimens, only four survive in the GSI fossil collections, and the three missing specimens are presumed lost (Dr M. Parkes, personal communication 2006). A single specimen was marked as a duplicate and sent temporarily to Dr Grainger in Belfast, but was apparently returned to C. Galvin (the GSI fossil collector) in March 1858, prior to its description as a new species by Baily (1859). The three missing specimens are, unfortunately, three of the four valves in dorsal aspect referred to by Baily (1859); the remaining material is a single dorsal valve (GSI:F02075) and the three ventral fragments (GSI:F01752, F01753, F23464). As Baily based his diagnosis primarily on the dorsal morphology, the single remaining valve from the original type locality is here designated as the lectotype (ICZN 1999: Art. 74). The three fragments of valves in ventral aspect cannot be assumed to be disarticulated parts of
Table 4—Details of Pterochiton thomondiensis material held in the Geological Survey of Ireland (GSI), from Co. Limerick. Modern grid-reference data for localities marked on archival GSI fieldsheets from the original collecting events. Specimen number
Townland locality in Co. Limerick
National Grid Reference
GSI:F2075, F1752, F1753, F23464
Lisbane—type locality (GSI 6 inch fieldsheet 19/3)
128088 145296
GSI:F1750
Tomdeely South (GSI 6 inch fieldsheet 10/4)
131207 150757
GSI:F23308
Altavilla (GSI 6 inch fieldsheet 19/2)
132809 146097
GSI:F2107
Ardlaman (GSI 6 inch fieldsheet 19/4)
132571 144522
GSI:F23304
Ballyvokoge (GSI 6 inch fieldsheet 20/2)
138384 148147
GSI:F2078, F11827
Doohylebeg (GSI 6 inch fieldsheet 20/3)
Approx. 137035 144178
GSI:F1754, F1755, F1756, F23305, F23306, F23307
Rathkeale Station (GSI 6 inch fieldsheet 20/3)
137177 144422
Sigwart—The Irish fossil Polyplacophora 35 the lectotype animal, and are designated as the paralectotypes (ICZN 1999: Art. 74.1.3). Additional material (from Rathkeale), although mentioned by Baily (1859), is not considered to have type status. Smith and Hoare (1987) reported that the type specimens were held in the Natural History Museum (London, BMNH) according to their communication with the then-Keeper of Palaeontology Dr H.W. Ball. Several additional intermediate valves are present in the London collection (J. Todd, personal communication 2006); however, the material has not been accessible to the present author. It is unclear if this material represents the ‘lost’ GSI material or was collected at another time. The additional material in the GSI collections includes another 12 separate valves. Apophyses are preserved on the lectotype, four specimens of intermediate valves (GSI: F23307, F23308, F23304, F1755), and the posterior valve (GSI:F1750). There is no specimen of an intermediate valve that preserves both apophyses associated with a single valve, although the lectotype preserves the right lamina and a small fragment of the left lamina. The overall shape of the intermediate valves of P. thomondiensis are very similar to other Pterochiton species, particularly P. eburonicus, and P. subgemmatus, but the apophyses of P. thomondiensis are distinctly smaller and less pronounced anteriorly. The sculpture on P. thomondiensis is also much smoother, without granular ornamentation. Terminal valves are not known from the type locality, but their general morphology and similarity to material of intermediate plates of P. thomondiensis in other collecting sites makes it probable that they are of the same species. One of the head valves (GSI:F2078) appears different from the other two, in its substantially thinner shell and smaller size. It may represent a juvenile specimen, and it is tentatively ascribed also to P. thomondiensis. The tail valve is known only from a fossil preserving the ventral aspect, which appears to have been flattened compared to the high dorsal elevation of the known intermediate and head valves. Collection of more material will elucidate whether some of these valves represent additional taxa or population-level variation. The collecting localities in Co. Limerick are all within the Waulsortian limestone (late Tournaisian to early Viséan), and represent complex marine slope environments of undetermined depth (Sevastopulo and Wyse Jackson 2001). Although the boundaries of the Waulsortian outcrop are not clear around all specific localities for the material, the matrix present with all specimens is from the same rock type. Baily (1859) believed that close comparisons could be made between
this material and the other species of Pterochiton from the Viséan of Belgium, and called it: an interesting fact confirmatory of Professor De Koninck’s observations on the Distribution of Carboniferous fossils, more especially as it is associated with Euomphalus Dionysii, and other characteristic fossils … from the Carboniferous Limestone of Visé. Smith and Hoare (1987: 55) designated Chiton thomondiensis as the type species of the genus Anthracochiton de Rochebrune, 1883, because C. thomondiensis is the first in the list of taxa included in the genus. Anthracochiton is considered a junior synonym of Pterochiton (Smith and Hoare 1987). In Baily’s (1859) original description, he suggested that P. thomondiensis might be closely related to Chiton gemmatus de Koninck, 1842 (= Pterochiton subgemmatus) and Chiton priscus Münster, 1839 (= Gryphochiton priscus), both from Carboniferous limestone deposits of Belgium. In modern terms, Pterochiton subgemmatus is congeneric with P. thomondiensis, along with seven other species and putatively additional undescribed material from the United States of America (Hanger et al. 2000; Table 2). Gryphochiton priscus is in the family Gryphochitonidae, which has been placed in a separate, wholly extinct suborder Cymatochitonina within the Lepidopleurida (Sirenko and Starobogatov 1977). Given the state of widespread flux in interpretation of fossil polyplacophoran material and the limited amount of material that was available in Baily’s time, his reference to these two taxa showed considerable insight. Subclass NEOLORICATA Bergenhayn 1955 Order indet. Suborder indet. Family indet. Genus indet. Fig. 4 Material GSI:F23302 Description The specimen consists of a tail valve and four intermediate valves, in ventral aspect. All of the valves are very badly broken. Apophyses are not present, or not preserved; from the breakage on the anterior ventral margin of each plate it appears that apophyses could have been present in life but have been broken off. Intact sections of the anterior margins (including the anterior margin of the head valve) are roundly convex. The lat-
36 Irish Journal of Earth Sciences (2007) interesting nonetheless because of the rarity of articulated Palaeozoic polyplacophoran fossils. Typically, the fleshy girdle that surrounds the eight plates disintegrates through decomposition, causing the plates to disarticulate and separate (e.g. Smith 1960, Gowlett-Holmes et al. 1998). The preservation presents a taphonomic curiosity, in that other known examples of articulated Palaeozoic chitons preserve the dorsal aspect, with the ventral surface embedded in the matrix (Dell’Angelo et al. 2003). Unfortunately, the ventral aspect is almost useless in determination of fossil polyplacophorans from our current knowledge. This specimen was tentatively ascribed to Pterochiton thomondiensis, but the paralectotype specimens of that species clearly preserve ventral morphology with distinctive ridges and muscle excavations referred to by Baily (1859) in his original description. What is preserved of the inner surface of these valves are smooth and the ridges of P. thomondiensis are entirely absent from this specimen. Discussion
Fig. 4—Loricata indet. Carboniferous (Dinantian) from Ashford, Cong, Co. Galway. Unique specimen, GSI:F23302. Ventral surface of plates, anterior end toward top. Scale bar = 10mm.
eral margins are also incomplete but in preserved marginal areas there were no insertion plates. Each valve has a ‘two-tone’ preservation with a lateral division marking a posteriorly thicker area; otherwise the shell venter is smooth, without apparent muscle excavations or badly worn. Measurements Total length of preserved specimen 40mm; each valve approximately 12mm in length, overlapping; maximum preserved width 20.5mm (approximately half the width of the total valves). Occurrence Ashford, Cong, Co. Galway, Ireland. Carboniferous (Dinantian). Discussion The preservation and poor state of this specimen mean that it is unfortunately impossible to diagnose, but is
Fossil polyplacophorans were first recognised in the first half of the nineteenth century. The first fossil chiton was discovered in 1802 from Middle Eocene deposits in the Paris Basin, France, and described by de Lamarck (1802). Not until 1834 were any other fossil chiton specimens found. The pace of discovery soon accelerated; what is now recognised as the first discovery of a palaeoloricate specimen was made in 1836 from the Carboniferous of Tournai (Münster 1839). The first Irish fossil chiton, Helminthochiton griffithi, followed soon after (Salter in M‘Coy 1846). When Helminthochiton was described by Salter (1847), he remarked that ‘it is, as far as we know, the earliest of the family’ and at the time it certainly was the oldest fossil chiton discovered. The new abundance of material was enough to prompt de Koninck (1857) to publish a history of the discovery of fossil polyplacophorans so far. Baily (1860a, 1860b), working at the Geological Survey of Ireland, translated de Koninck’s (1857) work into English. He published the translation in the Annals and Magazine of Natural History (Baily 1860a), but also presented the same paper at a meeting of the Zoological and Botanical Association of Dublin University (Baily 1860b). Baily’s (1860b) work updated de Koninck’s (1857) summary with additional species that had been named in the last few years of the 1850s, including his own report on the Irish fossil Pterochiton thomondiensis (Baily 1859). Salter (1847) had proposed three ‘sections’ of Helminthochiton (=Palaeoloricata), the oldest one
Sigwart—The Irish fossil Polyplacophora 37 of which included the monotypic H. griffithi. In an expansion on the diagnosis of these ‘sections’ Salter in M‘Coy (1846) stated that the other two were typified by Chiton eburonicus de Ryckholt, 1845, and Chiton priscus Münster, 1839 respectively. In modern terms, these taxa were sensible focal points for palaeoloricate taxonomy. Pterochiton eburonicus de Ryckholt, from the Lower Carboniferous of Belgium, is the type species of Pterochiton Carpenter in Dall, 1882, by original designation. Gryphochiton priscus (Münster, 1839) was the first-ever discovered palaeoloricate, and a species found abundantly in Europe (its type locality is also in Belgium). There is record in the literature of specimens of G. priscus from Ireland held in Britain (Smith and Hoare 1987:45) but these specimens have not been found in collections. Salter in M‘Coy’s (1846) reference to G. priscus as the type species of the ‘group’ seems ambiguous, since the type species of Gryphochiton Gray, 1847 is G. nervicanus (de Ryckholt, 1845) by original designation. Salter’s (1847) work marginally pre-dates Gray (1847) but is not taxonomically relevant. Gryphochiton was previously treated as a synonym of Helminthochiton (Smith 1960; Van Belle 1975), but was recognised as belonging to a separate subclass by Sirenko and Starobogatov (1977). The taxonomy, if not the evolution, of these two genera has been closely linked. Gryphochiton, the type genus of the family Gryphochitonidae has an additional posterior ‘half-valve’ VIII-b (sensu Sirenko and Starobogatov 1977); this shell is lacking in all members of the Lepidopleurida (including Pterochiton). The tail valve of Pterochiton thomondiensis is not known, but its morphology is otherwise very similar to other members of the genus Pterochiton which do not have the half-valve VIIIb. de Koninck’s (1857) synopsis of known fossil chitons, and Baily’s (1860a, 1860b) update of that summary, ignored Salter’s (1847) proposal for taxonomic partitions among fossil chitons. They included all taxa in the universal family Chitonidae, and genus Chiton, except to acknowledge Helminthochiton as a subgeneric taxon containing only the Lower Silurian ‘Chiton (Helminthochiton) griffithii’ (sic), the oldest fossil chiton then known. The history of these discoveries illustrates the small but important role Irish material plays in the international study of fossil chitons. Helminthochiton griffithi is a taxonomically important species that has caused confusion for other authors who did not access the type material. Pterochiton thomondiensis is an interesting record for a genus known from Belgium and dubiously from North America. The relative rarity of Palaeozoic fossil chitons from any locality means that these three
species from Ireland are important specimens to the polyplacophoran fauna of Northern Europe. Acknowledgements This research was funded by the Royal Irish Academy Praeger Fund for Natural History (2005). The author is grateful to Matthew Parkes (formerly of the GSI, now curator at NMINH) for access to collections and generous help with interpretation of GSI archives, and to George Sevastopulo for kind support with photographs for all figured material. George Sevastopulo and Hermann Strack provided helpful and improving reviews, and the author also thanks Matthew Parkes and Gareth Dyke for comments on the manuscript. References Baily, W.H. 1859 On the occurrence of detached plates of the shell of a new species of Chiton in the Carboniferous limestone at Lisbane, County of Limerick. Natural History Review and Quarterly Journal of Science 6, 330–4. Baily, W.H. 1860a Observations on two new species of Chiton from the Upper Silurian ‘Wenlock Limestone’ of Dudley. Annals and Magazine of Natural History ser 3 6, 91–8. Baily, W.H. 1860b On fossil Chitonidae, and their distribution in geological time. Proceedings of the Dublin University Zoological and Botanical Association 2, 40–7. Baily, W.H. 1860c On the occurrence of detached plates of the shell of a new species of Chiton in the Carboniferous limestone at Lisbane, County of Limerick. Journal of the Geological Society of Dublin 8, 167–71. Bergenhayn, J.R.M. 1943 Preliminary notes on the fossil polyplacophorans from Sweden. Geologiska Förenigen 1 Stockholm, Förhandlinger 65, 297–303. Bergenhayn, J.R. 1955 Die Fossilen Schwedischen Loricaten nebst einer vorlaufigen Revision des Systems der ganzen Klasse Loricata. Lund Universitet, Årsskrift, n.f. 2. Beyrich, H.E. 1868 Stringocephalenkalk bei Elburgerode. Zeitschrift fuer die Gesammten Naturwissenschaften 31, 396. Dall, W.H. 1882 On the genera of chitons. Proceedings of the U.S. National Museum 4, 279–91. Dall, W.H. 1889 Report on the results of dredging, under the supervision of Alexander Agassiz, in the Gulf of Mexico (1877–78) and in the Caribbean Sea (1879–80), by the U.S. coast survey steamer ‘Blake’, Lieutenant–Commander C.D. Sigsbee, U.S.N., and Commander J.R. Bartlett, U.S.N., commanding, 29, report on the Mollusca. 2, Gastropoda and Scaphopoda. Bulletin of the Museum of Comparative Zoology, Harvard University 18, 1–492. de Koninck, L-G. 1842 Description des animaux fossils, qui se trouvent dans le terrain carbonifère de Belgique. Le genre Chiton Linn. Dessain. Liège. de Koninck, L.-G. 1857 Sur deux nouvelles espèces siluriennes appartenant au genre Chiton. Bulletin de l’Académie Royale des Sciences, des Lettres et des Beaux-Arts de Belgique 11, 190–9. de Lamarck, J.P.D.M. 1802 Mémoires sur les fossils des environs de Paris, comprenant la détermination des espèces qui appartiennent aux animaux marins sans vertèbres et dont la plupart sont figurés dans la collection de vélins du Museum. Premier Mémoire, Genre I, Chiton. Oscabrion. Annales du Musée Nationale Naturelle 1, 308.
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JULIA D. SIGWART School of Biology and Environmental Science, University College Dublin, Belfield, Dublin 4, Ireland.
Merrion Street, Dublin 2, Ireland. E-mail:
[email protected]
and Natural History Division, National Museum of Ireland,
The subvention granted by University College Dublin towards the cost of publication of papers by members of their staff is gratefully acknowledged by the Royal Irish Academy.
