A new microsaur (Tetrapoda: Lepospondyli) from the Lower Permian of Richards Spur (Fort Sill), Oklahoma

499 A new microsaur (Tetrapoda: Lepospondyli) from the Lower Permian of Richards Spur (Fort Sill), Oklahoma Jason S. Anderson and Robert R. Reisz Abstract: Bolterpeton carrolli n.gen. n.sp. is described from the Lower Permian fissure-fill deposits of Richards Spur, Oklahoma. Bolterpeton is united with the gymnarthrid microsaur Cardiocephalus by having teeth compressed labiolingually, a narrow anterior process of the prearticular penetrating the splenial, and the arrangement of the contacts among the splenial, prearticular, and coronoids. It is united with both Cardiocephalus and Euryodus by the size and distribution of the coronoid teeth and the presence of longitudinal striations of enamel on the lingual tooth surface. Unlike those two genera Bolterpeton has peg-like teeth, but it remains unknown whether this represents the primitive condition or a reversal of the massive conical teeth typical of gymnarthrids. Bolterpeton possesses a flat lamina that runs along the lingual surface of the tooth margin. Where two laminae meet at the point of the tooth a labiolingual ridge is formed, which is most pronounced at smaller sizes. Reexamination of Cardiocephalus shows it to have the same morphology on its incisors. Previous authors have defined teeth bearing this ridge as weakly bicuspid. If this ridge were homologous with the strongly bicuspid condition of lissamphibians, Bolterpeton would provide the first example of this tooth morphology in lepospondyls and would strengthen recent hypotheses suggesting lepospondyls gave rise to some, if not all, modern amphibians. Résumé : Bolterpeton carrolli, nouveau genre et nouvelle espèce, est décrit à partir des dépôts de remplissage de fissure permiens précoces de Richards Spur, en Oklahoma. Bolterpeton est relié au microsaure gymnarthridé Cardiocephalus du fait qu il a les dents comprimées labiolingualement, un processus préarticulaire antérieur mince pénétrant le splénial et selon l arrangement des contacts entre le splénial, le préarticulaire et les coronoïdes. Il est relié avec Cardiocephalus et Euryodus par la taille et la distribution des dents coronoïdes et la présence de stries longitudinales de l émail sur la face linguale des dents. Contrairement à ces deux genres, Bolterpeton possède des dents en forme de piquet, mais il demeure indéterminé si cela représente une condition primaire ou un renversement des dents coniques massives, typique des gymnarthridés. Bolterpeton possède une lamine plate qui parcourt la surface linguale de la marge des dents. À l endroit ou deux lamines se rencontrent à la pointe des dents, une crête labiolinguale est formée, laquelle est plus prononcée pour les tailles plus petites. Un nouvel examen de Cardiocephalus montre qu il possède la même morphologie sur ses «incisives». Des auteurs précédents ont défini les dents possédant cette crête comme «faiblement bicuspides». Si cette crête était homologuée à la condition «fortement bicuspide» des lissamphibiens, Bolterpeton deviendrait le premier exemple de cette morphologie dentaire chez les lépospondyles et viendrait renforcer les hypothèses récentes qui suggèrent que les lépospondyles ont donné naissance à certains, sinon à tous, les amphibiens modernes. [Traduit par la Rédaction] Anderson and Reisz 505 Dedication Robert Carroll is well known for his work on the earliest amniotes from Joggins, Nova Scotia, on changes associated with terrestrial vertebrates returning to an aquatic lifestyle, and his current work using the tools of developmental biology to inform the question of the origins of modern amphibians. However, if one had to choose a single exemplar to represent his work, it would have to be his big book of microsaurs, The Order Microsauria (Carroll and Gaskill 1978). It is an exhaustive monograph documenting what had been a poorly known collection of small amphibians. Through careful illustrations and many years of effort he and illustrator Pamela Gaskill brought microsaurs out of obscurity. They now seem to be critical to understanding the origin of caecilians (Carroll and Currie 1975; Anderson 2001; Carroll 2001), if not all (Laurin and Reisz 1997; Laurin 1998), modern amphibians. It is only appropriate that, in a festschrift for Bob, a new species of microsaur be described in his honour. Received 6 February 2002. Accepted 25 July 2002. Published on the NRC Research Press Web site at http://cjes.nrc.ca on 14 April 2003. Paper handled by Associate Editor B. Chatterton. J.S. Anderson 1 and R.R. Reisz. Department of Biology, University of Toronto at Mississauga, 3359 Mississauga Road, Mississauga, ON L5L 1C6, Canada. 1 Corresponding author (e-mail: jsanders@credit.erin.utoronto.ca). Can. J. Earth Sci. 40: 499 505 (2003) doi: 10.1139/E02-066

500 Can. J. Earth Sci. Vol. 40, 2003 Introduction Richards Spur, near Fort Sill, Oklahoma, is renowned for its fauna of exquisitely preserved small to medium sized terrestrial tetrapods. The dominant constituent is the reptile Captorhinus (Fox and Bowman 1966; Holmes 1977; Modesto 1996, 1998), but a wide range of taxa are known. Among amphibians present at Richards Spur are the dissorophoids Cacops (Bolt 1977a; Sullivan et al. 2000), Doleserpeton (Bolt 1969), and an unidentified trematopid (Bolt 1974; Sullivan et al. 2000), an unidentified erypoid (Olson 1991), the aistopod Sillerpeton (McGinnis 1967; Lund 1978; Anderson 2002), and the microsaurs Llistrofus, Euryodus, and Cardiocephalus (Gregory et al. 1956; Carroll and Gaskill 1978). In 1980, Bolt described two enigmatic species (which he did not name), possessing what he termed weakly bicuspid teeth. Most specimens belonged to a smaller, relatively gracile morphotype that Bolt called Species X while a few larger elements he felt were morphologically distinct from Species X. Because the only specimens available to Bolt were disarticulated jaw fragments, he could make no determination of relationships beyond suggesting they were either microsaurs or reptiles. Here we describe the most complete specimen from the larger morphotype that, while only a fragmentary jaw, possesses articulated medial bones, revealing it to be a microsaur with close affinities to Cardiocephalus and Euryodus. The Linnean classification of Carroll and Gaskill (1978) is used throughout this paper, despite recent phylogenetic hypotheses (Laurin and Reisz 1997; Laurin 1998; Anderson 2001; Ruta et al. 2003) suggesting that many of these groups are paraphyletic. This is done for ease of communication and because the relationships among lepospondyls are, in our opinion, too unstable at this time to provide a meaningful phylogeny-based classification. Materials and methods All specimens were collected by screen washing fissure-fill material from the Dolese Brothers Quarry. Specimens were examined and drawn using a Leica (Wild) dissecting microscope with a camera lucida and Nikon digital camera. Measurements were taken using an eyepiece reticle to the closest 0.0625 mm. Catalogued specimens examined Institutional abbreviations FMNH, Field Museum of Natural History, Chicago, Illinois, U.S.A.; KUMNH, University of Kansas Museum of Natural History, Lawrence, Kansas, U.S.A.; OMNH, Oklahoma Museum of Natural History, Norman, Oklahoma, U.S.A. Bolterpeton carrolli n.gen. n.sp. OMNH 52364 Holotype. Right dentary with articulated medial bones. OMNH 71111 Small anterior right dentary, including symphysis. Unidentified Species X OMNH 71112 Large fragmentary right dentary, including symphysis. FMNH PR1085 Right dentary, including complete tooth row. FMNH PR1086 Left dentary, including symphysis. Cardiocephalus peabodyi Carroll and Gaskill, 1978. KUMNH 9923 Articulated left lower jaw. Systematic Paleontology Tetrapoda Goodrich, 1930 Lepospondyli Zittel, 1888 Microsauria Dawson, 1863 Gymnarthridae Case, 1910 Bolterpeton n.gen. TYPE SPECIES: Bolterpeton carrolli n.sp. TYPE MATERIAL: OMNH 52364, incomplete right dentary and articulated medial bones. HORIZON AND LOCALITY: Richards Spur, near Fort Sill, Oklahoma. Fissure-fill deposits probably equivalent with the Arroyo Formation, Lower Clear Fork Group, Early Permian, in Ordovician Arbuckle Limestone. Specimens were collected from the Dolese Brothers limestone quarry. DIAGNOSIS: Gymnarthrid with narrow, peg-like teeth. Single row of marginal teeth compressed lingually and recurved. Enamel with longitudinal striations on the lingual surface in all specimens. Teeth with flat lamina along lingual cutting edge forming a labiolingual ridge at the tip, more prominent in smaller specimens. Three coronoids bearing small teeth on broad patches in two or three rows. Teeth on second coronoid absent posteriorly. Splenial broad, but prevented from contacting second coronoid by prearticular. Prearticular with a long anterior process into the posterior splenial just ventral to the coronoids. ETYMOLOGY: Named for Dr. John Bolt, who has spent much of his career studying the fossil amphibians from this locality, and who first recognized the distinctiveness of this species. Bolterpeton carrolli n.sp. (Figs. 1, 2a, 3A, 3B) DIAGNOSIS: As for genus. ETYMOLOGY: Named in honor of Dr. Robert Carroll. DESCRIPTION: The holotype specimen OMNH 52364 (Fig. 1) is a right jaw preserving the dentary, first two coronoids, the splenial, and a portion of the prearticular. It is large for specimens attributed to this tooth type by Bolt (1980), having a basal tooth diameter (measured per Bolt 1980) ranging from 0.625 mm to 1.0625 mm. The tooth row is incomplete on both ends. A second specimen (OMNH 71111) preserving the symphyseal area is referable to Bolterpeton based upon identical tooth morphology and size, arrangement of coronoid scars, and placement of a large foramen with a posteriorly oriented sulcus. It holds an additional nine teeth anterior to the first coronoid, judging from the location of articulation scars on the lingual surface. The dentary in lateral view is straight, relatively deep

Anderson and Reisz 501 Fig. 1. Bolterpeton carrolli, n.gen. n.sp., holotype OMNH 52364, in (a) labial, (b) lingual, and (c) occlusial views. ap, anterior process of prearticular; c1 c2, first and second coronoids; d, dentary; pa, prearticular; s, splenial. posteriorly, and rapidly narrows and rotates medially as it approaches the symphyseal area, seen in the referred jaw (Fig. 2a). No dermal ornamentation is present, but mental foramina are abundant at the symphysis (Fig. 2a). The symphysis is primarily composed of the dentary, but it seems likely that the splenial had limited participation, as in Cardiocephalus. The lower jaws would have formed a V-shape when articulated. The dentary forms the dorsal and ventral surfaces of the jaw, encircling Meckel s canal on three sides. In OMNH 52364, two complete coronoids are present, with the surface for articulation with the third coronoid present on the posterior of the second. Sixteen teeth were present on the dentary of the holotype (OMNH 52364) in various stages of replacement. Tooth bases are nonpedicellate, bulbous so as to prevent close articulation between adjacent tooth crowns, and base diameters are smallest posteriorly. Teeth are compressed lingually and gently recurved. Enamel is present beginning half way up each tooth, and is indicated by the commencement of the fluting. Striations formed by enamel ridges are present on the medial surface and run to near the distal tooth margin. Striations are faintly evident on the labial surfaces of the largest teeth only. Along the anterior and posterior edges of the lingual surface is a flat lamina that forms a ridge where they meet at the tooth s apex (Figs. 3a, 3b). Bolt (1980) called teeth possessing this ridge weakly bicuspid, which is distinguished from strongly bicuspid teeth by the absence of a sulcus separating the lingual from the labial cuspules, as in Doleserpeton and Tersomius (Bolt 1977b; Figs. 3e, 3f). In OMNH 52364, the bicuspid morphology is only present in the smallest teeth (Figs. 3A, 3B). Larger teeth have poorly defined laminae that are usually worn away. Reexamination of a complete lower jaw of Cardiocephalus from Richards Spur (KUMNH 9923; Fig. 4) shows tooth morphology identical to Bolterpeton to be present on the incisors (Fig. 3C). Two complete coronoids are present on OMNH 52364 Figs. 1b, 1c). The two coronoids articulate along an oblique suture with a slight labial interdigitation. The first coronoid is a little more than twice the length of the second, while the second is broader labiolingually. The first coronoid has a long, low lying process that extends anteriorly along the bases of three marginal teeth. Both coronoids are ventrally

502 Can. J. Earth Sci. Vol. 40, 2003 Fig. 2. (a) Bolterpeton carrolli, n.gen. n.sp., referred specimen OMNH 71111, in labial view. (b, c) Species X in labial view. (b) OMNH 71112, and (c) FMNH PR 1085. bevelled to receive the splenial or prearticular, which overlap them. Coronoid dentition is borne on raised patches. The tooth-bearing area on the first coronoid is twice the size of that of the second. The tooth-bearing area of the second coronoid is restricted to the anterior half; a second raised area is present on its posterior margin, but it is toothless. This margin has an interdigitation for articulation with the third coronoid that is identical to that between the second and first coronoid. Bolt and Lombard (2001) formally distinguished between denticles (basal diameters < 10% of the marginal teeth), teeth, (basal diameters = 10% and = 125% of the marginal dentition), and fangs (basal diameters 25% greater than the marginal dentition). Following these authors, Bolterpeton bears small teeth in two irregular rows on the first coronoid, and three rows on the second coronoid. These teeth have diameters 30 60% less than the marginal teeth, but are otherwise morphologically very similar, being compressed lingually and bearing enamel fluting. The splenial is a large, sheet-like bone covering the medial surface of much of the jaw. The medial surface of the holotype has suffered some compression so that the splenial is slightly crushed inwards. A large anteriorly directed foramen is present on the anteroventral surface of the splenial. The splenial articulates with the first coronoid, which was overlapped on its ventral surface. The splenial is prevented from articulating with the second coronoid by the prearticular, so the posterior extent of the spenial on its dorsal margin ends with the first coronoid. Ventrally, the splenial continues posteriorly an unknown distance below the prearticular. The prearticular has a narrow anterior process into the splenial. This pattern of interdigitation was also described by Gregory et al. (1956) in the gymnarthrid Cardiocephalus (Fig. 4) and is unique to it and Bolterpeton. DISCUSSION: Bolterpeton does not have the dermal ornamentation on the dentary typical of temnospondyls. It is lacking coronoid fangs and denticles, and labyrinthine in-folding of tooth dentine, which are common features in temnospondyls. Dissorophids such as Doleserpeton also lack in-folding of the tooth dentine at small sizes (Bolt 1977b); however, Bolterpeton has much larger basal tooth diameters than Doleserpeton, so presumably any labyrinthine in-folding of tooth dentine it might have possessed would be evident. It seems unlikely that Bolterpeton is closely related to temnospondyls. Neither does Bolterpeton seem to be a reptile. Most reptiles have one coronoid (if any) that may be covered with a shagreen of denticles, not teeth (e.g., Romer 1946; Laurin 1993). Some reptiles do possess enamel striations similar to Bolterpeton (e.g., Peyer 1968); of particular note is Haptodus, known from the Late Pennsylvanian of Garnett, Kansas (Currie 1977; Laurin 1993). However, the teeth of Haptodus tend to be much larger, strongly recurved at the tips and lack the flattened lamina at the tooth s cutting edge that forms the bicuspid ridge at the tip. It is also unknown at Richards Spur. Another synapsid, Mycterosaurus, is known from Richards Spur (Reisz et al. 1997), but it shows morphology quite unlike Bolterpeton in having large, strongly recurved teeth with no enamel fluting. The pattern of coronoid tooth distribution of Bolterpeton is shared with pantylid and gymnarthrid microsaurs. The striations on the enamel, or fluting, are shared with the gymnarthrids Cardiocephalus and Euryodus alone among microsaurs. Bolterpeton and Cardiocephalus share uniquely: labiolingual compression of the teeth, which is restricted to the tips in Cardiocephalus (Carroll and Gaskill 1978), the restriction of the dorsal splenial to the first coronoid by the contact of the prearticular with the second coronoid, and the anterior process of the prearticular into the splenial. Bolterpeton differs from Cardiocephalus in a couple of respects. In Cardiocephalus the second coronoid is the same length as the first, while in Bolterpeton the first coronoid is much longer, due to the long anterior extension of the first coronoid. Also, the second coronoid is dentigerous along its entire length in Cardiocephalus. In Bolterpeton, the teeth on the second coronoid are restricted to its anterior half. Bolterpeton differs from Cardiocephalus in having three, not two, rows of teeth on the second coronoid (Figs. 1, 4), and the much more dorsal location of the anterior process of the prearticular. We feel confident that Bolterpeton is a distinct new taxon of gymnarthrid microsaur, closely related to Cardiocephalus, despite the incomplete nature of the type specimen. The smaller specimens attributed to Species X by Bolt (1980) seem to be distinct, but closely related to Bolterpeton (Figs. 2b, 2c). The teeth of Species X have much smaller diameters and lack the bulbous bases of Bolterpeton, so crowns are more closely packed. More teeth are present anterior to the first coronoid and presumably on the entire tooth row. However, Species X is similar in possessing the same weakly bicuspid morphology as Bolterpeton, the same enamel striations, a similar distribution of foramina on the labial surface, and an almost identical arrangement of scars on the lingual surface of the dentary for articulation with the

Anderson and Reisz 503 Fig. 3. Bolterpeton carrolli, n.gen. n.sp., holotype OMNH 52364. (A) Photomicrograph and (B) line drawing of a posterior tooth cusp showing the weakly bicuspid morphology, in oblique view. (C) Cardiocephalus sp., KUMNH 9923, line drawing of incisor showing weakly bicuspid condition in oblique view. (D) cf. Broiliellus sp. FMNH PR 892, partial right vomerine fang in lingual view, showing the weakly bicuspid morphology in a temnospondyl. (E) Doleserpeton, FMNH 5253 and (F) Tersomius, FMNH UR 2381 showing grades of strongly bicuspid teeth, with a sulcus between the cuspules, in anteroposterior view. (D F) redrawn from Bolt (1977b). lam, lamina. three coronoids and the splenial. Because of the size difference between Bolterpeton and Species X, it cannot be said whether Species X is a different species or an ontogenetic variant; we await the discovery of new specimens to clarify this question. Anderson (2001) presented the most complete analysis of lepospondyl relationships to date (Fig. 5). In this study Anderson found that Microsauria was a paraphyletic assemblage approximately dividing the Tuditanimorpha and Microbrachomorpha of Carroll and Gaskill (1978). Of particular note for the present study is the close relationship among the pantylid, gymnarthrid, brachystelechid and goniorhynchid microsaurs. These microsaurs form a plausible transformational series to modern caecilians via the Jurassic Eocaecilia (Jenkins and Walsh 1993; Carroll 2000). Caecilians, according to Anderson (2001), are sister taxa to brachystelechid microsaurs. Brachystelechids have multiple tooth cusps; however, they are oriented anteroposteriorly rather than labiolingually as in caecilians. This initially suggests that the multicuspid tooth morphology of brachystelechids is not homologous with that of caecilians. Bolt (1977b; 1980) proposed a phyletic sequence within amphibamid dissorophids, in which the primitively monocuspid tooth of lower temnospondyls develops a labiolingual ridge (seen in Broiliellus; Fig. 3D), which then develops a shallow sulcus (seen in Tersomius, Fig. 3F) that increases in depth and produces an obviously bicuspid tooth (present in Doleserpeton and lissamphibians, Fig. 3E). The tooth bearing a labiolingual ridge with no sulcus was termed weakly bicuspid by Bolt (1980), while the tooth possessing the sulcus was termed strongly bicuspid. Bolterpeton has tooth morphology similar to that seen in Broiliellus (Bolt 1977b; compare Figs. 3B and 3D). Cardiocephalus also shares this morphology (Fig. 3C). Assuming that Bolt is correct in this homology, Bolterpeton and Cardiocephalus would be the first lepospondyls close to the origins of caecilians to have had bicuspid teeth with similarly oriented cuspules. This

504 Can. J. Earth Sci. Vol. 40, 2003 Fig. 4. Cardiocephalus sp., KUMNH 9923, in (a) lingual and (b) occlusial views. Modified from Gregory et al. (1956). a, angular; ap, anterior process of prearticular; c1 c3 first through third coronoids; d, dentary; pa, prearticular; ps, posterior splenial; s, splenial; sy, symphysis. Fig. 5. Tuditanomorph microsaur phylogeny, from Anderson (2001). Bolterpeton is figured in two possible positions. Presence of multicuspid teeth (m) and cuspule orientation (ap, anteroposterior; l, labiolingual) is mapped onto this hypothesis. Character optimized using accelerated transformation. also would suggest that the brachystelechid multicusped teeth are homologous with those of at least caecilians given the phylogeny of Anderson (2001) but derived from the more primitive, labiolingual orientation (Fig. 5). This might be supported by the fact that albanerpetontids, widely considered lissamphibians, have anteroposteriorly oriented cuspules, like Batropetes and Carrolla. Further, the triple cusped tooth morphology of the brachystelechid microsaur Quasicaecilia (Carroll 1998) is placed in a derived, rather than basal, position on Anderson s (2001) tree (Fig. 5), suggesting that the double cusped tooth is primitive for brachystelechids. More anatomical and phylogenetic study is necessary to clarify the homology of these putative bicuspid teeth. Richards Spur is unusual for Lower Permian localities by preserving a fully terrestrial assemblage. While the reptile Captorhinus overwhelmingly dominates the fauna, the next most common taxa are terrestrial anamniotes such as dissorophoid temnospondyls and microsaurs (Sullivan et al. 2000). No taxa unambiguously over 1 m are known (Sullivan et al. 2000). We suspect that this size partitioning is not taphonomic because the material was briefly transported by water, and large sized specimens are known. If there were larger tetrapods present, we would expect to find identifiable fragments, yet none are known except for a putative eryopoid (Olson 1991). With the description of Bolterpeton, Richards Spur has at least four, and possibly five (if our interpretation of Species X is correct), recognized microsaurs. This large number of microsaurs at a single locality is only matched by the Pennsylvanian-aged deposits at Joggins, Nova Scotia, and Ný any, Czech Republic, each of which has five named microsaur taxa (Carroll and Gaskill 1978; Reisz and Modesto 1996). Joggins and Richards Spur, both preserving terrestrial faunas, share a remarkably similar collection of microsaurs given their separation in time, having primarily tuditanimorph microsaurs, including the terrestrial gymnarthrids. Ný any, on the other hand, has a microsaur fauna numerically dominated by the aquatic Microbrachis, which is completely unknown from Joggins and Richards Spur. The gymnarthrids common at Joggins and Richards Spur are absent at Ný any. Because of the unique depositional environments at Joggins and Richards Spur that preserve large numbers of terrestrial tetrapods from a restricted area, we believe that microsaur diversity, and the diversity of all smaller tetrapods, is probably much greater than currently appreciated. Conclusion Bolterpeton carrolli is a new species of microsaur with close affinities to Cardiocephalus. It shares patterning of coronoid dentition with pantylid and gymnarthrid microsaurs, and enamel fluting with the gymnarthrids Cardiocephalus and Euryodus. It shares uniquely with Cardiocephalus specializations of the splenial prearticular suture and tooth morphology. There is no evidence to link Bolterpeton with reptiles. Bolterpeton provides an interesting line of investigation into the acquisition of strongly bicuspid teeth and, possibly by extension, into the origins of tooth pedicelly, only known to date in lissamphibians and Doleserpeton (Bolt 1969). The presence of weakly bicuspid teeth in gymnarthrid microsaurs supports a recent phylogenetic hypothesis (Anderson 2001) that suggested that microsaurs gave rise to caecilians, a relationship previously supported only on anatomical grounds (Carroll and Currie 1975; Carroll and Gaskill 1978; Carroll 2001). Acknowledgments We thank Robert Carroll for his guidance and encouragement while we were students at McGill University, Montréal, Quebec. We thank John Bolt and William Simpson of the Field Museum of Natural History and Richard Cfelli of the Oklahoma Museum of Natural History for the loan of specimens. We are also grateful to Diane Scott for assistance with figure preparation.

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