(conodonta: ibexian, lower ordovician), and the ...

J. Paleont., 74(1), 2000, pp. 92–100 Copyright q 2000, The Paleontological Society 0022-3360/00/0074-0092$03.00

STIPTOGNATHUS NEW GENUS (CONODONTA: IBEXIAN, LOWER ORDOVICIAN), AND THE APPARATUS OF STIPTOGNATHUS BOREALIS (REPETSKI, 1982) R. L. ETHINGTON, OLIVER LEHNERT,

AND

J. E. REPETSKI

Department of Geological Sciences, University of Missouri–Columbia 65201, Institut fu¨r Geologie und Mineralogie, Universita¨t Erlangen-Nu¨rnberg, Germany, and United States Geological Survey, Reston, Virginia 20192

ABSTRACT—Collections from upper Ibexian (Tulean Stage) rocks of western United States, from the Canning Basin in western Australia, and from the Argentine Precordillera contain a seximembrate apparatus of multidenticulate conodonts whose elements have been included by authors in species of Prioniodus Pander, 1986, and Reutterodus Serpagli, 1974. The individual elements as well as the complete apparatus are not consistent with assignment of the species to either of these genera or to any other extant genus. A new generic name, Stiptognathus Ethington, Lehnert, and Repetski, is proposed with Reutterodus borealis Repetski, 1982, as type species. The apparatus consists of Pa, Pb, Sa-c, and M elements; the genus represents either the Prioniodontidae or the Periodontidae.

western Utah; from the Pogonip Group in eastern Nevada by Sweet and Tolbert (1997); from the El Paso Group in the Franklin Mountains of west Texas by Repetski (1982); from the Black Rock Limestone member of the Smithville Formation in northeastern Arkansas by Ethington and Repetski (1986); and from the sponge/algal limestone unit of the upper part of the Knox Group in the subsurface of northeastern Mississippi by Alberstadt and Repetski (1989). Because these elements are not associated with denticulate elements comparable to the unibranched and bibranched forms described by Serpagli, they were identified by some of those authors with query as ?Reutterodus andinus. Ethington and Clark (1981) described a transition series of four kinds of denticulate ramiform elements from the Fillmore Formation of western Utah in which they occur a considerable stratigraphic distance beneath the lowest occurrence of the seemingly conelike elements mentioned above. They observed that the denticulation of these elements suggests that of the unibranched and bibranched elements of R. andinus, but that the symmetries displayed by the Utah specimens are different from those of the South American conodonts on which Serpagli based that species. For this reason, they reported their specimens as ?Reutterodus sp. Subsequently Repetski (1982) coined the name Reutterodus borealis for elements with the same spectrum of morphologies as ?Reutterodus sp. of Ethington and Clark. Repetski’s collections came from the El Paso Group in western Texas, where they also occur stratigraphically beneath specimens resembling the conelike elements of R. andinus. More recently, Sweet and Tolbert (1997) recorded the same stratigraphic distribution of R. borealis and the coniform ?R. andinus in the Shingle Limestone in eastern Nevada (Fig. 1). In a study of Ordovician conodonts from the San Juan Limestone of Precordilleran Argentina, Lehnert (1995) included elements like those of Prioniodus n. sp. B of McTavish in an apparatus that he reported as Prioniodus oepiki (McTavish, 1973), commenting only that the multi-element descriptions of this latter species provided earlier by McTavish (1973) and by Stouge and Bagnoli (1988) are adequate. He found these specimens, which he interpreted as Pa, M, Sa and Sc elements, in only one sample in the Cerro La Silla section. Albanesi (1998b), echoing the earlier interpretation of Lehnert, also reported elements having the morphology of Prioniodus n. sp B from the Precordillera of Argentina as P. oepiki. He considered these elements to be morphologically close to the specimens from western Newfoundland that Stouge and

INTRODUCTION

of prioniodontid conodonts from the Emanuel Formation of the Canning Basin in Western Australia, McTavish (1973) named eight new species and reported others that were left in open nomenclature (Baltoniodus n. sp. A and Prioniodus n. sp. A through D). This study focuses on one of the latter, Prionodus n. sp. B of McTavish. A major factor in McTavish’s decision not to provide a name for this conodont was the paucity of specimens (four) in his collection. Nevertheless, he recognized three kinds of elements (prioniodiform, ramiform, and oistodiform) that he believed to constitute the apparatus of the species. His discussion of the elements cited flexing and twisting of the processes as a major departure from the morphologies shown by previously known species of Prioniodus Pander, 1856. Although not particularly emphasized as an attribute of the species, McTavish (1973, p. 46) observed that ‘‘White matter is present in cusps, denticles and processes of all elements.’’ He noted that the denticles of the processes are numerous, short, erect, and subequal in size and reported that they distinguish this species from P. elegans Pander, 1856, which he inferred to be its descendant. He envisioned a lineage from one of his named new species, Baltoniodus minutus, through a form he termed Baltoniodus n. sp. A to Prioniodus n. sp. B and eventually to P. elegans, the latter of which was not found in the Emanuel Formation. Furthermore, he concluded that this interpreted lineage is one branch in a postulated web of braided evolutionary relationships that he suggested as a ‘‘possible phylogeny of the Prioniodontidae’’ (see McTavish, 1973, fig. 7). Prioniodus n. sp. B has not been reported as such other than by McTavish, and it has not been listed in synonymies although seemingly identical elements have been assigned to species of other genera by various authors. Serpagli (1974) established the genus Reutterodus for a multielement species that he recognized among conodonts from the San Juan Formation in the Precordillera of Argentina. The three elements of this apparatus were described as conelike, unibranched, and bibranched, respectively. The branched elements display processes bearing stout, closely crowded denticles, whereas the conelike element is nondenticulate. The apparatus of the type species, R. andinus, has been reported without question only from Argentina (Serpagli, 1974; Lehnert, 1995; Albanesi, 1998b). Elements morphologically identical to the conelike elements that Serpagli included in the apparatus of R. andinus were reported by Ethington and Clark (1981) from the Pogonip Group (Fillmore, Wah Wah, and Juab Formations) of

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Silla Formation and the basal 8 m of the overlying San Juan Formation, both in the Portezuelo Yanso Section which is located about 60 km north of where Lehnert’s collection was made. DISCUSSION

FIGURE 1—Stratigraphic chart showing stratigraphic range of Stiptognathus borealis (Repetski) in the Lower Ordovician relative to conodont zonal schemes for Laurentia and Baltoscandia.

Bagnoli (1988) described in the apparatus of P. oepiki (McTavish). Albanesi observed that the specimens from Argentina are characterized by large bases with deep cavities. His collections came from samples collected in the upper 17 m of the La

We have examined collections (provided to Ethington by Robert Nicoll of the Australian Geological Survey Organization) from the Emanuel Formation of the Canning Basin, Australia, that justify reconsideration of the interpretation of some of the Argentine specimens as conspecific with Baltoniodus oepiki McTavish, 1973. The conodonts from the Emanuel Formation also clarify the apparatus and the affinities of Reutterodus borealis. McTavish’s collections from the Emanuel Formation show that P. oepiki [which he described as a new species of Baltoniodus Lindstro¨m, 1971] is restricted to a thin stratigraphic interval that terminates about 30 m below the lowest occurrence of Prioniodus n. sp. B. If the geniculate specimen McTavish reported as part of Prioniodus n. sp. A is, as we believe, actually an M element of Prioniodus n. sp. B, the lowest occurrence of the latter in the Canning Basin is somewhat below the top of the range of P. oepiki. The apparatus of P. oepiki comprises elements that are robust, with strongly developed lateral costae or processes and prominent cusps. Denticles on the posterior processes of ramiform and P elements are short and basally fused; lateral processes are nondenticulate or have a few stubby denticles. The denticles are cloudy to albid in their interiors but hyaline at their surfaces. The M (geniculate) element is slender with a drawn-out anterobasal region that bears low, fused denticles; the whole element, including cusp and all parts of the base, occupies a common plane. Specimens from the Factory Cove Member of the Shallow Bay Formation (Cow Head Group) of western Newfoundland illustrated by Stouge and Bagnoli (1988) as Prioniodus oepiki are consistent with this description. In contrast, specimens from the Emanuel Formation that conform to the illustrations and descriptions that McTavish provided for Prioniodus n. sp. B are strongly albid except for a narrow basal region on the processes, tend to have flexed lateral processes, and have denticles that are closely crowded, needlelike, and fused except at their tips. Basal cavities consist of narrow, shallow slits beneath processes; they coalesce into moderately deep, sharp-tipped maxima where the processes merge. Cusps, although present, are not prominent. The geniculate (M) elements are longitudinally flexed rather than planar as in P. oepiki; stubby denticles ahead of the cusp are fused virtually to their tips, and the portion of the base posterior to the cusp is short and has a markedly arcuate oral margin. Elements in the Emanuel collections examined by us are assigned easily to P. oepiki or to Prioniodus n. sp. B; no transitional morphologies that might be difficult to identify are present. We conclude that some of the specimens from Argentina should have been compared with Prioniodus n. sp. B of McTavish and not P. oepiki. This conclusion is buttressed by an affirmation from G. Bagnoli and S. Stouge (1998, personal commun. with Repetski and with Lehnert) that they had not seen such elements in their collections from western Newfoundland. Careful comparison of Prioniodus n. sp. B from the Emanuel Formation directly with specimens of ?Reutterodus of Ethington and Clark (1981) and of Reutterodus borealis demonstrates to our satisfaction that they are conspecific. Ethington and Clark (1981) and Repetski (1982) tentatively identified these elements with Reutterodus Serpagli, 1974, because of the perceived similarity of their mode of denticulation to that of the unibranched and bibranched elements of the type species. Albanesi (1998b)

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reinterpreted R. andinus as having an apparatus with five kinds of elements (P, M, Sa, Sb, Sc); he assigned it to the Rhipidognathidae. His interpreted Sb and Sc elements seem to be minor variations that depart slightly from the bilateral symmetry of the Sa elements. None of the elements has a posterior process, but all of the ramiform elements have a strong medial posterior carina. The M element is the cone-shaped element of Serpagli (1974) and is morphologically like the elements from Laurentia that have been reported not to be associated with ramiform elements (Ethington and Clark, 1981; Repetski, 1982; Stouge and Bagnoli, 1988; Pohler and Orchard, 1990; Smith, 1991). If an element of this morphology is part of the apparatus of R. andinus, it is markedly different in morphology from the typical geniculate M element of the apparatus of Prioniodus n. sp. B McTavish. These differences in the individual elements of the apparatuses of R. andinus and Prioniodus n. sp. B preclude including them in the same genus, and a new generic assignment for R. borealis is dictated. The elements of Reutterodus borealis Repetski, 1982, [5Prioniodus n. sp. B McTavish] conform morphologically to the locational elements generally accepted for the apparatus of members of the Prioniodontidae. The details of their morphogy and their highly albid structure are atypical of any of the genera recognized within that family, and we conclude that they cannot be accommodated within Prioniodus, Baltoniodus, or Oepikodus. Accordingly, we propose a new genus, Stiptognathus, with Reutterodus borealis Repetski, 1982, as the type species. In addition to the collections of Ethington and Clark (1981) and Repetski (1982) and the specimens from the Emanuel Formation available to us, we were assisted in this study by the recovery of more than 450 elements of Stiptognathus borealis from a sample collected in the Fillmore Formation in Skull Rock Pass Section 6 of Hintze (1973), and by several tens of additional specimens recovered subsequently from a sample near the topotype horizon in the El Paso Group at the Scenic Drive section near El Paso, Texas; these collections allowed us to evaluate the degree of variation displayed by elements of this species. BIOSTRATIGRAPHY

Our evidence indicates that the distribution of Stiptognathus borealis is restricted both geographically and stratigraphically. Albanesi and others (1998) reported the species to be present in Argentina in a 15-meter interval that begins just below the top of the La Silla Formation in the Potrerillo-Perico Range, about 40 km (ca 25 mi) north of the town of Jaćhal. These rocks within the Paroistodus proteus Zone indicate that the bottom of the species’ range in that section is in the Tremadocian/Arenigian boundary interval. The single occurrence recorded by Lehnert (1995) is in the San Juan Formation in a section at Sierra La Silla, located about 8 km (4.5 mi) south-east of Jaćhal. He interpreted the associated conodont fauna in this part of the San Juan to be indicative of the Prioniodus elegans Zone. McTavish also reported Stiptognathus borealis (5his Prioniodus n. sp. B) from just one sample collected at 488 m (1600 feet) in the type section of the Emanuel Formation on Emanuel Creek in the Canning Basin, northern Western Australia. As noted above, the range is extended downward by an added 40 m (130 feet) if a geniculate element he assigned to Prioniodus n. sp. A belongs in S. borealis. He observed that precise correlation of the faunas was difficult but reasoned that the most likely age assignment for the Emanuel is middle Arenigian. This conclusion was reaffirmed in a subsequent review by McTavish and Legg (1974). Webby and others (1981) summarized more detailed analyses of the faunas from the Canning Basin by Legg (1976, 1978) to correlate the Emanuel Formation with Bendigonian strata in other parts of Australia and considered it to be

of early (but not earliest) Arenigian age. In a discussion of the stratigraphy of the Prices Creek Group in the Canning Basin, Shergold and others (1995) assigned the interval in the Emanual containing this taxon to their conodont Assemblage C. Ethington and Clark (1981) recovered Stiptognathus borealis, which they reported as Reutterodus? sp., in a 40-meter interval in the brown slope and ledge member of the Fillmore Formation in the Ibex area of western Millard County, Utah (see Hintze, 1973, for details of section). The occurrence reported herein of abundant Stiptognathus borealis in a sample from Hintze’s (1973, p. 22) Skull Rock Pass 6 section extends the range downward into the slope forming shaly siltstone member of the Fillmore. Hintze’s correlation of this outcrop with the continuous Fillmore exposures to the south suggests that the stratigraphic range of the species in the Ibex area may be as much as 125 meters. Ross and others (1997) reported this part of the Fillmore Formation to be in the Protopliomerella contracta Zone [5traditional Zone G-2 of authors] of their newly established Tulean Stage in the Ibexian Series. This is consistent with the early (but not earliest) Arenigian age inferred for the occurrences in the Emanuel Formation. Sweet and Tolbert (1997) found Stiptognathus borealis in an interval 12 m thick in the lower part of the Shingle Limestone in the vicinity of Shingle Pass in the Egan Range south of Ely, Nevada; they considered this interval to represent part of the Tulean Stage. Repetski (1982) reported Stiptognathus borealis through a stratigraphic interval of 107 m that encompasses much of the upper one-fourth of the El Paso Group in the Franklin Mountains, near El Paso, Texas. Re-examination of his original collections established that the highest specimen that he reported (at 375 meters in his measured section) probably is a coniform element like those he identified as R. ?andinus, but, in any case, it is not a specimen assignable to Stiptognathus borealis. Furthermore, as discussed in the systematic section below, we now believe that some of the elements originally assigned to Drepanodus n. sp. 7 by Repetski (1982; e.g., the specimen illustrated as pl. 8, fig. 4, in that paper) are recognizable as Pa elements of Stiptognathus borealis. We also no longer consider the conelike element that Repetski (1982) included in R. borealis to be part of the apparatus of this species. This reinterpretation reduces the recognized range of Stiptognathus borealis in the Franklin Mountains to an interval from Repetski’s sample SD/861 to SD/ 900 (probable) or SD/920 (doubtful), a range of approximately 13 or 20 meters. This interval is within the lowest part of the Oepikodus communis Zone locally, and is within the uppermost part of the range of Acodus deltatus there. Recently, Repetski identified specimens of Stiptognathus borealis in two samples from the Spring Mountains, southern Nevada (USGS fossil localities 11473-CO and 11544-CO; A. G. Harris, unpublished data); the section is approximately the same as that reported by Miller and Zilinski (1981). These occurrences are in adjacent samples collected 124 m and 132 m below the top of the Mountain Springs Formation in the La Madre Spring 7½ minute quadrangle, Clark County, Nevada. They are constrained within the lowermost to lower Oepikodus communis Zone by samples at 117 m (most likely Reutterodus andinus Zone) and at 147 m below the top of the formation (lowermost Oe. communis Zone). Stiptognathus borealis also has been found in the upper member of the Marathon Limestone in west Texas (Izold, 1993). The consistent ranges of this species suggest that it is of value for correlation. It represents an interval high in the Tulean Stage of the North American Ibexian Series that encompasses an upper part of the Protopliomerella contracta Trilobite Zone as well as the upper part of the Acodus deltatus/Oneotodus costatus Conodont Zone and the lower part of the Oepikodus communis

ETHINGTON ET AL.—NEW EARLY ORDOVICIAN CONODONT GENUS conodont zone. Stiptognathus borealis is known to occur in the Paroistodus proteus Zone in in the Precordillera of Argentina (Albanesi et al., 1998) and perhaps ranges upward into the lower part of the Prioniodus elegans Zone. Equivalent rocks and faunas are in the Bendigonian Stage in Australia and low in the Arenigian Series elsewhere. PALEOBIOGEOGRAPHY

To date, Stiptognathus borealis has been found in only four areas: eastern to southern Great Basin (Ethington and Clark, 1981; Sweet and Tolbert, 1997; new herein), western Texas (Repetski, 1982; Izold, 1993), the Precordillera of Argentina (Lehnert, 1995; Albanesi, 1998a), and Western Australia (McTavish, 1973). Extensive studies of conodonts from thick Lower Ordovician successions of the Baltic Platform (Lindstro¨m, 1955; Lo¨fgren, 1985, 1993a, 1993b, 1994), China (An, Zhang, and others, 1983; An, Du, and Gao, 1985; An, 1987), western Newfoundland (Ji and Barnes, 1994; Stouge and Bagnoli, 1988), Greenland (Smith, 1991), the Canadian Arctic Archipelago (Nowlan, 1976), the Arbuckle Group of southern Oklahoma (Dresbach, 1998), and the Beekmantown and Knox Groups in the Appalachian region (Repetski, 1979, 1985, and unpublished USGS collections; Collamer, 1985; Shaw, 1987) have not yielded specimens. Apparently this was not a cosmopolitan species. Its known distribution does not correspond to that of the cold-/deep-water North Atlantic population mentioned in studies of Ordovician conodonts (e.g., Lindstro¨m, 1955; Lo¨fgren, 1985, 1993a,b), nor does it conform to that of the shallow-/warm-water Midcontinent faunas. Each of the regions from which it is known was occupied by open-marine, shallow-shelf conditions at the time Stiptognathus borealis was living there. The rocks from which it has been recovered in the Great Basin, west Texas and Argentina are thick, essentially uninterrupted successions of limestones. By contrast, the section where McTavish found it in the Canning Basin consists largely of shales containing widely separated and thin limestone bands. The absence of Stiptognathus borealis from equivalent strata across much of Laurentia may be explained by deposition in water that was shallower, warmer, and perhaps more saline than were the shelf waters of the places where it has been found. Interpretations of the paleogeographic positions of these localities in Early Ordovician time (Scotese and McKerrow, 1991; Dalziel et al., 1994) place the Great Basin and west Texas in the southern-hemisphere tropics, and interpretations of the origin of the Ordovician rocks of Precordilleran Argentina postulate their accumulation in a depositional basin flanking the southeastern part of Laurentia, also in southern tropical to subtropical latitudes. In contrast, the Canning Basin region of Western Australia is considered to have been in the northern subtropics, and about half way around the world from western and southern Laurentia (see Fig. 2). Perhaps, a westerly current pattern influenced the Laurentian distribution of Stiptognathus borealis; its occurrences are on what would have been the ‘‘leading edge’’ of that paleocontinent. We see no definitive connections, other than paleolatitude, between the Laurentian and Australian occurrences. Lehnert et al. (1997) reported similar difficulty in accounting for the paleogeographic distribution of Clavohamulus hintzei and Clavohamulus elongatus, whose known occurrences are restricted to Laurentia and Australia. SYSTEMATIC PALEONTOLOGY

The specimens illustrated in this report are deposited in the collections of the United States National Museum (USNM), Washington, D.C., and in the collections of the Australian Geological Survey Organization in Canberra (CPC).

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Family PRIONIODONTIDAE Bassler, 1925 Genus STIPTOGNATHUS Ethington, Lehnert, and Repetski new genus Type species.—Reutterodus borealis Repetski, 1982. Diagnosis.—Conodonts with prioniodontid apparatuses; elements strongly albid except for shallow hyaline regions along their basal margins. Denticles of S elements are slender, uniform in size, closely crowded, fused throughout most of their length, and oriented nearly normal to oral margins of processes. Denticles of Pa and Pb elements low, stubby, blunt, widely separated along posterior processes. Denticles of geniculate M elements extend along anterior margin from anterior basal corner through lower one-third of cusp; base foreshortened posterior to junction with cusp; basal outline markedly arcuate in lateral view. Processes of S elements and M elements flexed laterally. Base of M element strongly flared inwardly; flaring is asymmetrical, opening posteroaborally to almost posteriorly. Posterior flange of M element deflects downward and somewhat outward distally. Basal cavities of M and S elements shallow, those of Pa and Pb elements deep and flaring. Etymology.—Greek stiptos, crowded, and gnathos, jaw; in recognition of the closely crowded denticles on S and M elements of the apparatus. Occurrence.—Lower Ordovician (Ibexian Series); El Paso Group, Franklin Mountains, El Paso, Texas (Repetski, 1982); Upper Member, Marathon Limestone, west Texas (Izold, 1993); Fillmore Formation, House Range, Millard County, Utah (Ethington and Clark, 1981); Egan Range, White Pine County, Nevada (Sweet and Tolbert, 1997); La Silla and San Juan Formations in Precordillera of Argentina (Lehnert, 1995; Albanesi, 1998b); and Emanuel Formation, Canning Basin, Western; Australia (McTavish, 1973). STIPTOGNATHUS BOREALIS (Repetski, 1982) Figure 3.1–3.23 Baltoniodus sp. nov. A MCTAVISH, 1973, p. 44, pl. 3, figs. 10, 18, 19; ? figs. 15, 25; text-figs. 5e, f. ?Prioniodus sp. nov. A MCTAVISH, 1973, p. 46, pl. 2, fig. 11 [non figs. 3–5], text-fig. 6s [non text-figs. 6p–r]. Prioniodus sp. nov. B MCTAVISH, 1973, p. 46, pl. 3, figs. 1, 2, 7, 9, 13, text-figs. 5a–d. ?Reutterodus sp. ETHINGTON AND CLARK, 1981, p. 91–92, l. 10, figs. 14–16, 19. Reutterodus borealis REPETSKI, 1982, p. 41–42, pl. 19, figs. 4, 5,7 [non fig. 6]. Drepanodus n. sp. 7 s. f. REPETSKI, 1982, p. 25, pl. 8, fig. 4. Prioniodus oepiki McTavish, 1973. LEHNERT, 1995, p. 116–117, pl. 4, figs. 21, 24–28; [non figs. 19, 20; 5Prioniodus oepiki McTavish sensu Stouge and Bagnoli, 1988]; ALBANESI, 1998b, p. 156, pl. 10, figs. 11, 15–18 [non figs. 12–14; 5Prioniodus oepiki McTavish sensu Stouge and Bagnoli, 1988]. Periodon selenopsis (Serpagli, 1974). ALBANESI, 1998b, p. 174, pl. 9, figs. 29–30 [non fig. 28].

The elements that we include in the apparatus of Stiptognathus borealis conform to the general outline listed above in the generic diagnosis. Following are specific characters. Pa elements.—Elements have modified coniform shapes in that no processes are differentiated; cusps dominant elements of morphology. Anterior margin gently and continuously curved throughout length. Posterior margin of cusp straight or slightly curved over most of length but bent basally through an angle of 90–100 degrees to produce rudimentary posterior process. Specimens flexed in the anterior-posterior direction and longitudinally as well. Outer surface of cusp shallowly convex, inner surface somewhat more strongly so. Keel present basally at the anterior margin and may continue onto lower part of cusp; this region is deflected inward. Basal region strongly inflated inward

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FIGURE 2—Paleogeographic distribution of Stiptognathus borealis (Repetski). Upper left; occurrences in western United States: 1, Ibex Area, Utah; 2, eastern Great Basin, Nevada; 3, Scenic Drive Section, Franklin Mountains, west Texas. Upper right; occurrences in North America (1–3) and Precordilleran Argentina (4); reconstruction after Dalziel, 1997. Bottom; global distribution during the Tulean Stage including occurrence in Canning Basin (5), reconstruction after Scotese and McKerrow, 1991.

but not externally, enclosing spacious basal cavity whose triangular outline terminates near anterior margin at bottom of cusp. Cusp albid, base hyaline. A small number of widely spaced, low denticles (actually little more than nodes) on posterior basal margin; spaces approximately same width as denticles. Pb elements.—Pb elements are variations on themes displayed by Pa elements. They are less robust, have clearly defined denticles on posterior margin; most specimens show low, crowded

denticles on anterior margin of cusp and base which is flexed inward. Denticles of posterior margin are slender, fused except at apices; inclined at an angle of about 60 degrees to trend of margin where they are seated. Tips of denticles along posterior margin define nearly straight course, becoming progressively longer from those proximal to cusp through those of maximum length near middle of series and declining progressively from there.

ETHINGTON ET AL.—NEW EARLY ORDOVICIAN CONODONT GENUS Sa elements.—Symmetrical ramiform elements have needlelike fused denticles oriented normal to and along entire length of paired lateral processes and posterior process. Denticles also present on lower reaches of lateral margins of cusp. Posterior process slender, long, inclined downward from base of cusp, so that, when oriented with cusp upward, element has shape of tripod. Two variants of this morphology are present. The more abundant ones have lateral processes diverging at an angle of approximately 45 degrees from the base of cusp; specimens whose lateral processes are separated by an angle of nearly 90 degrees are less common. Sb elements.—Tertiopedate, having three denticulate processes of which anterior and posterior ones are continuations of respective margins of cusp and third one extends laterally and downward from base of cusp. All processes are flexed as they extend from cusp so that entire element is concavo-convex with lateral process on concave side. Denticles of posterior process slender, fused, elongate, inclined posteriorly; progressively shorter with distance from cusp. Those of anterior process low, conical, crowded, turned somewhat toward concave side of element. Denticles of lateral process are similar to those of posterior process. Sc elements.—Cusp and posterior process similar to those of Sa and Sb elements. Anterior edge of cusp deflected strongly inwardly and continues aborally as laterally to posterolaterally directed, denticulate process. Anterior face of cusp broadly rounded aborally, with low, outer-anterolateral carina that produces slight flaring of the basal margin. M elements.—Tightly geniculate elements with stout cusp that is continuous with markedly extended and anteriorly denticulate anterobasal region. Denticles rotated toward one side of element; increase regularly in height from anterobasal corner to near juncture of base and cusp, then decline through lower part of cusp. Base foreshortened posterior to base of cusp, turns down sharply, and bears an oral keel. In lateral view, basal margin subtends an angle in the range of near 90 to as much as 120 degrees between longer anterior basal segment and much shorter posterior portion. Strong basal flare located beneath juncture of base and rear margin of cusp and directed laterally and posteriorly. Discussion.—Albanesi (1998b) recognized a septimembrate apparatus (Pa, Pb, M, Sa, Sb, Sc, Sd) from a collection (46 elements from five samples) taken in the uppermost La Silla and lowest San Juan Formations of Precordilleran Argentina. He compared this apparatus to the assemblage from the Cow Head Group of Newfoundland that Stouge and Bagnoli (1988) identified as Prioniodus oepiki (McTavish), and considered morphologic differences between the corresponding elements from the two localities to be the result of intraspecific variation. Our interpretation of S. borealis draws heavily on a sample from the Fillmore Formation of western Utah that yielded approximately 6,000 conodont elements, of which over 400 represent that species. Associated with S. borealis in that sample is a septimembrate species that we consider to be P. oepiki (McTavish) sensu Stouge and Bagnoli, of which we have over 800 elements. The M elements of that apparatus have a long, shallow basal region that extends far beyond the junction of the cusp and base, just as illustrated for P. oepiki by Stouge and Bagnoli (1988, Pl. 12, figs. 4, 5). In contrast, in the apparatus illustrated by Albanesi (1998b, pl. 10, figs. 15, 16), the basal region of the M elements extends only a short distance posterior to the bottom of the cusp and has a profoundly arcuate upper margin. This is the same morphology displayed by M elements of Prioniodus n. spp. A and B of McTavish and by the M elements we include in the apparatus of S. borealis. Of the other elements that Albanesi figured for P. oepiki, those identified as Pb, Sc, and Sd elements conform to the morphologies of elements in the sample from

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western Utah that we assign to that species. Those that he interpreted as Sa and Pa elements are closer in morphology to elements that we include in S. borealis than to corresponding elements of P. oepiki. We interpret this genus and species to possess a prioniodontid apparatus. Prioniodontid apparatuses typically display pastinate P elements (Sweet, 1988, p. 62) with each process denticulate, in contrast to the P elements of Stiptognathus borealis which lack lateral processes and have clearly developed denticles only on their posterior margins. McTavish (1973) considered nondenticulate Acodus deltatus Lindstro¨m to be the root stock from which the Prioniodontidae evolved. Sweet (1988, p. 60) also considered A. deltatus, which he suggested is better assigned to Tripodus Bradshaw, to be an early representative of the Prioniodontidae. Other authors have reassigned A. deltatus to Prioniodus Pander (van Wamel, 1974) or to Baltoniodus Lindstro¨m (Bagnoli et al., 1988; Bergstro¨m, 1988). Because Stiptognathus borealis appears near the top of the range of A. deltatus in the Great Basin of western United States and in the Emanuel Formation, it may be intermediate morphologically between nondenticulate early prioniodontid conodonts and the more typical denticulate forms; it is not on the primary axis of evolutionary development of the group as was inferred by McTavish (1973, fig. 7). The species also could be considered to have a periodontid apparatus. The asymmetrical S element has a marked lateral process that is suggestive of Sb elements of species of Periodon Hadding. The anteriorly denticulate M elements are consistent with the morphology of M elements of most periodontid species, but the M elements are nondenticulate in the earliest species of the evolutionary development within Periodon as interpreted by Stouge and Bagnoli (1988). The Pa and Pb elements of Stiptognathus borealis are modified coniform elements with denticles arranged along the lower posterior margins of their cusps; Pa elements lack denticles on their anterior margins but they are present anteriorly on the Pb elements. In contrast, P elements of periodontid conodonts are angulate pectiniform elements with prominent denticles on the posterior process and somewhat less highly developed anterior processes. In the original description of Stiptognathus borealis, Repetski (1982) included a cone-like element in the apparatus, inferring homology between those elements and the cone-like element of R. andinus Serpagli. Although these coniform elements share some secondary characters such as similar distribution of albid and hyaline matter with elements of Stiptognathus borealis, we no longer believe that the evidence is strong enough to include them in our reconstructed apparatus because such coniform elements have not been recognized from the other localities where elements of this species have been recovered. McTavish (1973) illustrated two M elements that he assigned to reconstructed apparatuses that he reported as Prioniodus n. sp. A and Prioniodus n. sp. B. He found each of these interpreted new species to be restricted in his collections to single samples in the upper part of the Emanuel Formation that are separated by a stratigraphic interval of somewhat more than 100 feet. Only one M element was recovered from each of these samples. The two M elements are markedly similar in morphology in all respects except that one has a nearly straight anterior margin and the other is moderately convex anteriorly. Both morphotypes are common in our collections from the Fillmore Formation in Skull Rock Pass in western Utah where they occur with individuals whose anterior profiles are intermediate between these morphologies. Accordingly, we conclude that both of McTavish’s figured specimens are morphologically identical to M elements we include in S. borealis. The apparatus of Prioniodus n. sp. B of McTavish is identical to that of S. borealis.

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ETHINGTON ET AL.—NEW EARLY ORDOVICIAN CONODONT GENUS Only the M element of Prioniodus n. sp. A of McTavish is reassigned here; the other elements appear to belong to a species of Prioniodus. Repository.—Figured specimens from the Fillmore Formation of western Utah and from the El Paso Group of west Texas are deposited in the National Museum of Natural History, Washington, D. C., under catalogue numbers USNM 499,031 through USNM 499,046. Figured specimens from the Emanuel Formation of Western Australia are deposited in the collections of the Australian Geological Survey Organization, Canberra ACT, Australia, under catalogue numbers CPC 35002 to CPC 35007. Additional specimens on which this report was based are in the collections of the authors. Faunal associates.—[based primarily on a 2.5 kg sample from Skull Rock Pass 6 locality of Hintze (1973) that yielded approximately 6,000 elements including over 400 of Stiptognathus borealis]. Acodus deltatus Lindstro¨m, 1955; Acodus emanuelensis McTavish, 1973; Anodontus longus Stouge and Bagnoli, 1988; Colaptoconus quadraplicatus (Branson and Mehl, 1933); Drepanodus arcuatus Pander, 1856; Eucharodus toomeyi (Ethington and Clark, 1964); Eucharodus parallelus (Branson and Mehl, 1933); Jumudontus sp.; Oelandodus elongatus (Lindstro¨m, 1955); Oistodus bransoni Ethington and Clark, 1981; ‘‘Oistodus’’ sp. 3 of Ethington and Clark, 1981; Oneotodus costatus Ethington and Brand,1981; Parapanderodus striatus (Graves and Ellison, 1941); Paraserratognathus abruptus (Repetski, 1982); Protoprioniodus simplicissimus McTavish, 1973; ?Paltodus sp. or ?Drepanoistodus sp. [includes ‘‘Oistodus’’ inaequalis Pander s.f. of Ethington and Clark, 1981, as an element of apparatus]; Paroistodus proteus (Lindstro¨m, 1955); Prioniodus sp. aff. P. oepiki (McTavish, 1973) of Stouge and Bagnoli, 1988; Scalpellodus striatus Ethington and Clark, 1981; aff.‘‘Scandodus’’ flexuosus Barnes and Poplawski of Ethington and Clark, 1981; ‘‘Scandodus’’ sp. 1 of Ethington and Clark, 1981; Scolopodus krummi (Lehnert, 1995); Striatodontus prolificus Ji and Barnes,1994; Toxotodus carlae (Repetski, 1982); Tropodus comptus (Branson and Mehl, 1933): Ulrichodina simplex Ethington and Clark, 1981; Ulrichodina wisconsinensis Furnish, 1938; Ulrichodina aff. Ulrichodina sp. B of Smith, 1991. ACKNOWLEDGMENTS

The authors are indebted to R. Nicoll for providing samples from the Emanuel Formation of the Canning Basin which allowed us to demonstrate that the relatively more poorly-preserved material from Utah and Texas is conspecific with the forms reported by McTavish (1973) and to reconstruct the apparatus of Stiptognathus borealis. Gratitude also is due to A. Harris for calling our attention to the presence of this species in

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USGS collections from southern Nevada. The paper was improved significantly as a result of thorough reviews by G. Albanesi, S. Bergstro¨m, C. Barnes, D. L. Clark, and T. Dutro, Jr., for which we thank them. Research by O. Lehnert on CambroOrdovician faunas from Argentina, and their comparison with Laurentian conodonts during a visit to the United States, was supported by a grant of the Deutsche Forschungsgemeinschaft (LE 867/2), which is acknowledged gratefully. REFERENCES

ALBANESI, G. A. 1998a. Biofacies de conodontes de las secuencias Ordovicicas del Cerro Potrerillo, Precordillera Central de San Juan, R. Argentina. Actas de la Academia Nacional Repu´blica Argentina, 12: 75–98. . 1998b. Taxonomia de conodontes de las secuencias Ordovicicas del Cerro Potrerillo, Precordillera Central de San Juan, R. Argentina. Actas de la Academia Nacional de Ciencias, Repu´blica Argentina, 12:102–253. , M. A. HU¨NICKEN, AND C. R. BARNES. 1998. Bioestratigrafia de conodontes de las secuencias Ordovicicas del Cerro Potrerillo, Precordillera Central de San Juan, R. Argentina. Actas de la Academia Nacional de Ciencias, Repu´blica Argentina, 12:7–72. ALBERSTADT, L., AND J. E. REPETSKI. 1989. A Lower Ordovician sponge/algal facies in the southern United States and its counterparts elsewhere in North America. Palaios, 4:225–242. AN TAI-XIANG. 1987. The Lower Paleozoic Conodonts of South China. Peking University Publishing House, Beijing, 238 p. (In Chinese) , DU GUIO-QING, AND GAO QIN-PIN. 1985. Ordovician Conodonts from Hubei, China. Geological Publication House, Beijing. 64 p. (In Chinese) , ZHANG FANG, XIANG WEIDA, ZHANG YONGQUI, XU WENHAO, ZHANG HUIJUAN, JIANG DEBIAO, YANG CHANGSHENG, LIN LIANDI, CUI ZHANGTANG, AND YANG XINCHAN. 1983. The Conodonts of North China and the Adjacent Regions. Science Press of China, Beijing, 223 p. (In Chinese) BAGNOLI, G., S. STOUGE, AND M. TONGIORGI. 1988. Acritarchs and conodonts from the Cambro-Ordovician Furuha¨ll (Ko¨pingsklint) sec¨ land, Sweden). Rivista Italiana di Paleontologia e Stratigrafia, tion (O 94:163–248. BASSLER, R. S. 1925. Classification and stratigraphic use of the conodonts. Geological Society of America Bulletin, 36:218–220. BERGSTRO¨M, S. M. 1988. On Pander’s Ordovician conodonts: Distribution and significance of the Prioniodus elegans fauna in Baltoscandia. Senckenbergiana Lethaea, 69:217–251. BRANSON, E. B., AND M. G. MEHL. 1933. Conodonts from the Jefferson City (Lower Ordovician) of Missouri. University of Missouri Studies, 8:53–64. COLLAMER, J. B. 1985. Environmental Distribution of Conodonts from the Axemann Formation (Lower Ordovician), Bellefonte, Pennsylvania. Unpublished M.S. thesis, University of Maryland, College Park, 257 p.

← FIGURE 3—SEM photomicrographs of Stiptognathus borealis. 1, 3, 6, 9, 10, 14, and 15 are from a sample from the Skull Rock Pass 6 locality of Hintze (1973), Utah; 2, 4, 5, 7, 8, 11, 12, 13, and 20 are from sample JR 3-23-97H, from the El Paso Group in the Scenic Drive section near El Paso, Franklin Mountains, Texas; 16–19 and 21–23 are from sample 705/212 in the type section of the Emanuel Formation, Canning Basin, Australia. Note the strong diagenetic alteration in elements from Skull Rock Pass, Utah where denticles on the processes are overgrown by blocky apatite crystals (e.g., 14). The extension of the deep slit-like basal cavities to the end of the processes in Stiptognathus borealis is visible in 11, 15, 16, 18–20. 1, Pa element, 390, inner lateral view, USNM 499,031. 2, Sa element, 3125, posterior view, USNM 499,032. 3, Sb element, 3150, oral view, USNM 499,033. 4, Pb element, 390, lateral view, USNM 499,034. 5, Sb element, 390, lateral view, USNM 499,035. 6, Pb element, 3165, lateral view, USNM 499,036. 7, Sb element, 365, lateral view, USNM 499,037. 8, Sc element, 3130, postero-lateral view, USNM 499,038. 9, Sb element, 3143, lateral view, USNM 499,039. 10, Sa element, 375, posterior view, USNM 499,040. 11, Sa element, 3143, lateral view, USNM 499,041 12, Sb element, 3190, lateral view, USNM 499,042. 13, M element, 3120, inner lateral view, USNM 499,043. 14, Sa element, 3133, lateral view, USNM 499,044. 15, M element, 3123, lateral view, USNM 449,045. 16, Sb element, 3127, antero-basal view, CPC 350032. 17, Sa element, 3127, antero-lateral view, CPC 35003. 18, Sa element, 3150, postero-basal view, CPC 35004. 19, Pb element, 3140, lateral view, CPC 35005. 20, Sb element, 3165, lateral view, USNM 449,046. 21, Pa element, 3135, inner lateral view, CPC 35006. 22, M element, 398, inner lateral view, CPC 35007. 23,, close up of striation developed on the Pa element ilustrated in 21, 3485.

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