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AMERICAN MUSEUM Novttates PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, N.Y. 10024 Number 2902, pp. 1-12, figs. 1-3, table 1 December 30, 1987

Trematops milleri Williston, 1909 Identified as a Junior Synonym of Acheloma cumminsi Cope, 1882, with a Revision of the Genus

DAVID W. DILKES1 AND ROBERT R. REISZ2

ABSTRACT The Early Permian temnospondyl amphibians shelf, an internal naris with an anteroposterior Acheloma cumminsi Cope, 1882 and Trematops length less than that of the posterior division of milleri Williston, 1909 of the family Trematop- the external naris, and a median septum of the sidae prove, upon reexamination ofthe holotypes, vomers that contacts the roof of the antorbital to be indistinguishable. Since the former name has region. priority, Trematops milleri is declared to be a sub- The slitlike, dorsally convex otic notch ofAche- jective junior synonym of Acheloma cumminsi. loma cumminsi is unique among labyrinthodont The family name Trematopsidae is retained. amphibians. The dimensions of this otic notch The generic diagnosis ofAcheloma cumminsi is suggest that the tympanum, if present, was simi- revised to emphasize the presence of a slitlike, larly reduced and could not have been part of an posteriorly closed otic notch, a supratympanic efficient impedance-matching hearing apparatus.

INTRODUCTION The family Trematopsidae is a small group Trematops. A third, Trematopsis (= Tre- of poorly known terrestrially adapted am- matopsoides Romer, 1966), erected by Olson phibians of the order Temnospondyli col- (1956) on the basis ofa fragmentary skeleton, lected from North American deposits of has been reduced recently to subjectivejunior Lower Permian age. At present, the-family is synonymy with Cacops cf. C. aspidephorus composed of the two genera Acheloma and by Milner (1985a). Actiobates peabodyi Ea-

I Department of Zoology, Erindale Campus, University of Toronto, Mississauga, Ontario L5L 1C6, Canada. 2 Associate Professor, Department of Zoology, Erindale Campus, University of Toronto.

Copyright © American Museum of Natural History 1987 ISSN 0003-0082 / Price $1.70 2 AMERICAN MUSEUM NOVITATES NO. 2902 ton (1973), a Late Pennsylvanian dissoro- argued that the unusually acute angle between phoid, has been placed provisionally within the scapula and coracoid was merely an ar- the family Trematopsidae by Berman et al. tifact of the poor postcranial ossification of (1985) and Milner (1985a, 1985b). The type the specimen and, thus, did not constitute a and only known specimen of Actiobates is valid diagnostic feature. Watson accepted thought by the former to exhibit features Cope's statement that Acheloma lacked otic suggestive of a juvenile and by the latter to notches and, recognizing the taxonomic sig- possess a combination of primitive dissoro- nificance of this character, placed Acheloma phoid and advanced trematopsid features. within a new family, the Achelomidae. Eaton (1973) did not comment on the on- It was not until a review of the family togenetic status ofActiobates but presumably Trematopsidae by E. C. Olson in 1941 that considered it to be an adult. Actiobates is the affinities of Acheloma were properly es- currently being studied by Ms. Eleanor Daly tablished. After a reexamination of the type of the Mississippi Museum of Natural Sci- material, Olson concluded that it possessed ence. the same elongate external naris and slitlike, E. D. Cope (1882) first describedAcheloma closed otic notch as Trematops and, accord- cumminsi and placed it within the family ingly,' expanded the family Trematopsidae to Eryopidae. Cope distinguished Acheloma include Acheloma. Olson interpreted the from Eryops primarily on the basis of a sca- Trematopsidae as carnivores that were closely pulocoracoid with a scapular blade tilted me- related to the family Dissorophidae, a mor- dially and posteriorly at a 300 angle to the phologically diverse group of terrestrially coracoid, the apparent absence ofotic notch- adapted temnospondyls that occupied a span es on the skull, and the absence of articular of time from the Middle Pennsylvanian condyles on the humeri. (Westphalian D) to Upper Permian. Trematops milleri, also from the Lower The possible synonymy of Acheloma and Permian red beds of Texas, was initially de- Trematops has not been studied in detail un- scribed by S. W. Williston in 1909. The til now. Although expressing some doubt, unique attributes of Trematops include an Williston (1909) accepted Cope's description elongate external naris occupying most ofthe ofAcheloma as accurate and concluded that antorbital region and a slitlike, posteriorly each was a valid genus. Romer (1935) stated closed otic notch. Williston (1910) subse- that Trematops was probably synonymous quently erected the monogeneric family with Acheloma but did not elaborate. Fol- Trematopsidae to house this unusual am- lowing Olson's review of the family, Romer phibian. (1947) apparently reversed his opinion and Following the discovery of Trematops mil- retained Acheloma and Trematops as distinct leri, the taxonomic position of Acheloma genera, a conclusion unquestioned to the cumminsi was reevaluated as part of inde- present. pendent studies of Williston (1916) on the tetrapods of the Permian and Carboniferous ACKNOWLEDGMENTS ofNorth America and Watson (1919) on the early evolution ofamphibians. Although nei- We wish to express our thanks to Dr. Eu- ther author was apparently able to examine gene Gaffney of the American Museum of firsthand the holotype of Acheloma, each Natural History for arranging the loan of the considered it to be sufficiently distinct from holotype of Acheloma cumminsi, to Ms. Eryops to warrant its removal from the fam- Charlotte Holton, also ofthe AMNH, for her ily Eryopidae. They differed, however, in their assistance to the first author during his visit final taxonomic conclusions. Williston chose to the museum, and to Dr. John Bolt of the to assign Acheloma to the family Trematop- Field Museum of Natural History for allow- sidae, but made this decision with extreme ing us to examine the holotype of Trematops reservations because he was unsure of the milleri. Support for this study was provided true nature ofthe external naris and otic notch. by a grant from the Natural Sciences and En- Watson, on the basis of illustrations of the gineering Research Council of Canada pectoral girdles of Acheloma (Case, 1911), (A0077). 1987 DILKES AND REISZ: ACHELOMA AND TREMATOPS 3

ABBREVLATIONS The left quadratojugal is complete, although slightly damaged, whereas the right quadra- Institutional tojugal is fragmentary. The left quadrate is AMNH American Museum ofNatural His- complete. Both lower jaws are in place and tory lack the section beneath the external naris. FMNH UC Field Museum of Natural History The dermal sculpturing exhibited by each ele- Anatomical ment consists of a uniform pattern of round pits at the ossification center and slightly oval a angular pp postparietal d dentary prf prefrontal pits at the periphery. The prominent ridges ect ectopterygoid pt pterygoid and protuberances characteristic of many f frontal q quadrate dissorophids (Carroll, 1964; DeMar, 1968; in.f. internarial fenestra qj quadratojugal Berman et al., 1981) and commonly found mI. jugal sa surangular along the margins ofthe skull table, the cheeks, lacrimal sm septomaxilla or the medial rim of the orbit are m maxilla sp splenial absent in n nasal spp postsplenial Acheloma. There are no lateral line grooves p parietal sq squamosal or pits. pal palatine st supratemporal A large internarial fenestra is situated near pf postfrontal t tabular the tip of the snout, opening below into a pm premaxilla v vomer Po postorbital small, median, internarial pit. The position of the partial posteromedial rim of the right REDESCRIPTION OF external naris indicates that the external naris HOLOTYPES is greatly elongated posteriorly. An elongate external naris is a feature found in Descriptive statements are derived given below the limited to those characters that are relevant trematopsids, dissorophid Ecolsonia to the taxonomic reevaluation of the holo- (Vaughn, 1969; Berman et al., 1985), and types. Actiobates (Eaton, 1973). Medial to the rim of the right external naris is a partial narial Acheloma cumminsi (AMNH 4205) flange consisting of a laterally concave sheet of bone projecting down and inwards from Many ofthe details ofcranial morphology, the roofofthe snout. As preserved, this flange in particular the pattern ofskull roof sutures, is composed ofseparate sheets from the nasal were unknown to earlier workers because and prefrontal that abut against each other AMNH 4205 had been only partially pre- with no sign of overlap. Although the narial pared. Extensive mechanical and chemical flange is incomplete anteriorly and poste- preparation allows, for the first time, a critical riorly, the nasal portion probably continued comparison of the two holotypes. forward underneath the nasal. The prefrontal The holotypic skull ofAcheloma cumminsi portion of the flange, curving laterally to fol- (fig. 1) has suffered significant dorsoventral low the rim of the narial opening, probably crushing with a slight increase in width ofthe reached the narrow antorbital bar that sep- temporal region. The majority of the dermal arates the orbit and external naris. The height skull elements are preserved. The left tabular, of the narial flange increases posteriorly, re- left supratemporal, left postfrontal, and both ducing the space between the flange and pal- parietals and postparietals are complete. Ac- atal surface. There is, however, no evidence companying these elements are a partial right of osseous contact between the preserved supratemporal, right postfrontal, and right portion of the narial flange and palate. The tabular. Portions of the right frontal, pre- morphology of the narial flange ofAcheloma frontal, and nasal that form the dorsal margin conforms with that of the smaller trematop- of the orbit and posteromedial margin of the sids as described by Bolt (1974a) and differs external naris are present. The premaxillae, significantly from the narial flange of disso- maxillae, lacrimals, and jugals are partially rophids (Bolt, 1974a, 1974b). preserved. Neither squamosal is complete; Dissorophoid amphibians and several oth- the left, however, is missing only that portion er groups of labyrinthodont amphibians and bordering the anterior corner ofthe otic notch. primitive reptiles possess a ventral process 4 AMERICAN MUSEUM NOVITATES NO. 2902

Fig. 1. Acheloma cumminsi Cope 1882. Skull of holotype (AMNH 4205) in (A) dorsal view, and (B) ventral view. Scale = 2 cm. of the prefrontal (VPP) inside the anterior Posterior closure ofthe otic notch is also pres- rim of the orbit either in combination with ent in the North American dissorophids Ca- or separate from a lateral exposure of the cops (Williston, 1910; Bolt, 1977); Dissoro- palatine (LEP) along the suborbital bar (Bolt, phus (DeMar, 1968); Longiscitula (DeMar, 1974b). These features may have evolved 1966); Ecolsonia (Berman et al., 1985); and primarily as a consequence of the common the Russian dissorophids Kamacops, Iratu- possession of large orbits and, in the case of saurus (Gubin, 1980), and Zygosaurus (Ef- labyrinthodonts, palatal fangs beneath the or- remov, 1940). The manner of otic closure in bit. It cannot be determined if a VPP is pres- these dissorophids differs, however, from that ent on AMNH 4205 because both orbits are of Acheloma. When the dissorophid skull is incomplete anteriorly. The suborbital por- viewed laterally, the tabular process is bent tion of the left orbit is poorly preserved and down sharply at an approximate right angle sutures are difficult to establish. There ap- to the dorsal edge ofthe skull table and fused pears to be no LEP. to the quadrate process, thus enclosing a large, The otic notch of Acheloma cumminsi is triangular otic notch. greatly reduced in height and closed poste- The dorsal surface ofthe left quadrate pro- riorly by a long extension of the tabular that cess of AMNH 4205 is unfinished bone in- curves gradually down to meet a robust quad- dicating a probable cartilaginous continua- rate process. There is no indication of any tion. A deep groove that extends dorsally and fusion between the tabular and quadrate pro- medially along the posteromedial surface of cesses; instead the tabular process rests along the tabular process and ends in a shallow, the lateral face of the quadrate process and oval depression may have received the car- inserts into a shallow depression at its base. tilaginous portion of the quadrate process. 1 987 DILKES AND REISZ: ACHELOMA AND TREMA TOPS 5

The unsculptured ventral border of the otic angular septomaxilla, its damaged posterior notch, formed by the squamosal and qua- edge raised slightly, is situated within the right dratojugal, is highly convex. The quadrato- external naris of AMNH 4205 along the an- jugal sheathes the quadrate lateral and pos- terolateral rim of the internal naris. terior to the quadrate process. Fragments of a partial braincase, recog- A pronounced supratympanic shelf over- nized for the first time, are present on AMNH hangs the otic notch. Medial to this shelf, a 4205 and consist ofthe left exoccipital, most vertical supratympanic flange projects down of the left opisthotic, portions of the body of into the notch space. Dermal sculpturing is the parasphenoid, the base of the cultriform absent from the ventral surface of the supra- process, both basipterygoid processes, and a tympanic shelfand the supratympanic flange. small section of sphenethmoid immediately The squamosal lacks the semilunar curvature anterior to the right basipterygoid process. found on most dissorophids (DeMar, 1968). The morphology of the braincase of Ache- The sutures delineating the right supratem- loma follows in most details the expected poral can be traced from the skull roofto the rhachitome pattern (Sawin, 1941; Romer and underside of the shelf but disappear before Witter, 1942; Berman et al., 1981). The left reaching the supratympanic flange. Along the paroccipital process is present but incomplete posterior edge of the skull table is a well- anteriorly. Its ventral surface is slightly con- developed groove lacking sculpture and a cave. There is a deep, transverse groove along posteroventrally oriented sharp ridge that the posteroventral edge of the paroccipital probably served for the insertion of cranio- process continuing onto the occipital process cervical muscles. This ridge is more pro- of the tabular and the posteromedial surface nounced at the common suture of the post- of the tabular. A craniocervical muscle may parietals above the foramen magnum and have inserted along this groove. The circular along the homlike tabular processes. fenestra ovalis has a diameter ofabout 5 mm. A significant portion of the palate of The parasphenoid portion of the basiptery- AMNH 4205 is preserved; sutures, unfor- goid process is fused to the pterygoid, pre- tunately, cannot be established with confi- venting any movement between palate and dence. The interpterygoid vacuities are braincase. smaller than in dissorophids and the ptery- The dentition ofAcheloma consists of ap- goids probably reached the vomers, a prim- proximately 9 premaxillary teeth, 23 to 26 itive condition for dissorophoids. Anteriorly, maxillary teeth, and the expected paired pal- the vomers are deflected dorsomedially to atal fangs on the vomer, palatine, and ecto- form a deep internarial pit. Posterior to the pterygoid. The second to last premaxillary internarial pit, each vomer has a median sheet tooth, located below the anterior division of ofbone that probably contacts the roofofthe the external naris, is greatly enlarged. The snout. These two vertical sheets ofbone, sep- nature of the maxillary teeth beneath the ex- arated only by a slight gap, form a structure ternal naris is unknown because this region here referred to as a median vomerine sep- is not preserved. tum. The median vomerine septum extends The postcranial skeleton consists ofa string posteriorly beyond the rim of the interpter- of 22 articulated presacral vertebrae past the ygoid vacuities perhaps to contact the sphen- atlas-axis complex, ribs associated with the ethmoid. Olson (1941) described a similar first six vertebrae, both scapulocoracoids, and structure on an isolated snout ofa large trem- both humeri. A radius and ulna described by atopsid. Case (1911) but not mentioned by Cope The internal naris is situated directly below (1882) are not among the holotypic material the posterior expansion ofthe external naris. and may be specimens that Case mistakenly The anteroposterior length of the internal included with the holotype. The vertebrae are naris is considerably less than that of the ex- rhachitomous in design resembling those of pansion ofthe external naris. In contrast, the Eryops (Moulton, 1974). The ribs are short, posterior margins ofthe internal and external flat, and expanded distally. Both scapulocor- nares of smaller trematopsids and the dis- acoids suffer from poor ossification and pres- sorophid Ecolsonia are equal. A large, rect- ervation. The portion of the coracoid below I-

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L.; 1987 DILKES AND REISZ: ACHELOMA AND TREMATOPS 7 the supraglenoid foramen is missing and the interpreted as an LEP that had been excluded scapular blade is incomplete distally due to from the orbit by the contact ofthe jugal and imperfect ossification and loss ofmost ofthe lacrimal. The existence of this rectangular anterior region. Proximal and distal articular piece of bone cannot be confirmed. Bone is facets of the humeri are absent, further evi- present within the anterior portion ofthe right dence ofpoor postcranial ossification. A large orbit but it is heavily fractured and no VPP perforation of the entepicondyle of the right can be identified. humerus, interpreted by Cope (1882) as an The rim ofsmooth, finished bone along the entepicondylar foramen, was reinterpreted by left side of the skull table was correctly in- Case (191 1) and Olson (1941) as postmortem terpreted by Williston (1909) as the dorsal damage. The absence ofa corresponding hole margin of a posteriorly closed otic notch. A on the left humerus leaves little doubt that it well-developed supratympanic shelf over- is not a natural feature. hangs the otic notch and a supratympanic flange is situated medially. No sutures are Trematops milleri (FMNH UC 640) visible on the underside ofthe shelf or on the flange. The smooth ventral margin of the The skull (fig. 2) is missing the entire left notch appears to be strongly convex. Neither side below the orbit and portions ofthe right quadrate process is preserved; a region of side. Most of the skull table is preserved but broken bone on the dorsal surface ofthe right the dorsal rim ofthe right otic notch, includ- quadrate marks the former presence of a ing nearly the entire tabular and supratem- quadrate process. poral and small portions of the parietal and There is a groove along the point of union postparietal, are lost. Only a small portion of of the skull roof and occiput, similar to but the contributions of the squamosal and qua- slightly shallower than the one observed on dratojugal to the smooth ventral margin of Acheloma. It cannot be determined ifthe pos- the right otic notch are present. Dermal sible recess for the quadrate process found on sculpturing is uniform and the skull lacks lat- the tabular of Acheloma is present because eral line grooves or pits. both tabulars of Trematops are broken proxi- A large internarial fenestra is present at the mally. tip of the snout. The fenestra presumably The braincase of Trematops resembles that opened ventrally into an internarial pit; the ofEryops (Sawin, 1941), Edops (Romer and lower jaws, unfortunately, completely cover Witter, 1942), and Platyhystrix (Berman et that region of the palate. al., 1981). As pointed out by Olson (1941), The external naris of Trematops is elongate the parasphenoid, thought by Williston (1909) and oval in outline. A spur from the nasal to be absent, is actually present and covers partially divides the narial opening into a the ventral surface of most of the braincase. circular anterior portion and a slightly larger The ventral surface of the paroccipital pro- oval posterior portion. This spur probably cess of Trematops is deeply concave with the marks the boundary between the true exter- anterior and posterior edges developed as nal naris near the tip of the snout and the ventrally projecting flanges. The anterior derived posterior expansion. Williston (1909) flange is confluent with the supratympanic observed a flattened or concave bone in the flange and the posterior flange, unfortunately right naris which he interpreted as turbinated incomplete, occupies the position ofthe deep, bone. This bone is probably the septomaxilla posteroventral paroccipital groove observed (contra Olson, 1941). Unfortunately, little can on Acheloma. be said of the narial flange beyond confir- Only those portions of the palate anterior mation of its presence. to the orbits are present. No sutures are vis- It is uncertain whether or not FMNH UC ible. The interpterygoid vacuities are reduced 640 possesses an LEP along the ventral rim and the inner edge ofthe palatal shelfis thick- of the orbit or a VPP inside the anterior rim ened, presumably representing the anterior of the orbit. Bolt (1974b) noted the possible extension ofthe pterygoid to the vomers. The presence of a rectangular piece of bone ven- pterygoid and the parasphenoid are indistin- tral and anterior to the right orbit which he guishably fused at the basicranial joint. 8 AMERICAN MUSEUM NOVITATES NO. 2902

The combined premaxillary and maxillary ton (1909) as being from the stratigraphically dentition of FMNH UC 640, according to higher Clear Fork Group. Thus, according to Williston (1909), numbers 25 or 26 teeth of Olson's interpretation, the boundary between which 6 are borne by the premaxilla. This the Wichita and Clear Fork groups separates estimate is probably too low because the tooth Acheloma and Trematops. The former genus row is damaged at its anterior and posterior would presumably be the more primitive of ends. A more accurate tooth count, based on the two because ofits lower stratigraphic po- the length of the alveolar ridge and the ob- sition. All other specimens were then as- served size of preserved teeth, is 9 premax- signed to either Acheloma or Trematops illary teeth and 23-26 maxillary teeth. The principally upon stratigraphic location. Ro- maxilla is swollen laterally beneath the pos- mer (1947) noted locality information plac- terior division of the external naris to hold a ing Acheloma in the Clear Fork but consid- pair of enlarged teeth. Although the premax- ered it to be erroneous and accepted Olson's illary teeth are damaged, the base ofa single, argument for a Wichita age. large tooth is present near the premaxilla- A locality card, yellowed with age and des- maxilla suture and below the functional nar- ignating the specimen as part of the Cope ial opening. Collection, was found with the holotype of According to Olson (1941), each presacral Acheloma cumminsi and may be the source intercentrum of Trematops milleri is closed oflocality information mentioned by Romer dorsally, presumably by inward curvature and (1947). The specific location given is Coffee union of the dorsal tips of the intercentra, Creek, Baylor County, Texas, clearly from thus producing a ring of bone to completely the classic Lake Kemp collecting region of surround the notochord. No mention of ring the Arroyo Formation and near the locality intercentra was made in Williston's (1909) where Trematops milleri was found. If this original description. Reexamination of the information is correct, then any theory of preserved presacral vertebrae of FMNH UC stratigraphic separation of Acheloma and 640 has failed to reveal any evidence of ring Trematops must be discarded. Regardless of intercentra. the accuracy ofthe A. cumminsi locality data, Presacral ribs are short in length through- stratigraphic position is not, on its own, suf- out the series and expanded proximally and ficient to warrant taxonomic separation of distally, the proximal expansion becoming specimens; the holotypes must possess de- reduced on the posterior ribs. monstrable morphological differences. The remainder of the postcranial skeleton Olson (1941, 1970) has provided a list of of Trematops milleri, as described by Willis- morphological differences to strengthen his ton, is similar to Eryops and displays a slight- argument ofthe generic validity ofAcheloma ly higher level of ossification than that ob- and Trematops. According to Olson, the fol- served in the holotype ofAcheloma cumminsi. lowing features may distinguish Trematops from Acheloma: (1) the external naris of COMPARISON AND Trematops is farther from the tip ofthe snout SYSTEMATIC RESULTS than in Acheloma; (2) the posterior tip ofthe In his review ofthe family Trematopsidae, maxilla of Trematops lies directly below the Olson (194 1) retained Acheloma and Trem- anterior corner of the otic notch while the atops as distinct genera because of perceived maxilla of Acheloma ends anterior to the morphological and stratigraphic differences. notch; (3) the nasal of Trematops is relatively Although he was unable to find a locality card longer than that ofAcheloma; (4) the ratio of with the holotype of Acheloma, and Cope the length ofthe frontals to parietals of Trem- (1882) failed to include specific locality data, atops is smaller than that of Acheloma; (5) Olson (1941: 15 1) concluded that "the nature the squamosal of Trematops is relatively of preservation, the type of matrix, and the smaller than the squamosal ofAcheloma; (6) similarity to referred specimens from the the tabular of Trematops is relatively larger Wichita formation suggest that the genotype than the tabular of Acheloma; (7) the inter- comes from the Wichita." The holotype of centra of Trematops are closed dorsally while Trematops milleri is documented by Willis- the intercentra of Acheloma are open dor- 1987 DILKES AND REISZ: ACHELOMA AND TREMA TOPS 9 sally; and (8) the scapulocoracoid of Trem- TABLE 1 atops displays a more advanced state of os- Selected Cranial Measurements (in millimeters) sification than the scapulocoracoid of and Ratios of Acheloma cumminsi (AMNH 4205) Acheloma and the scapula of Acheloma is and Trematops milleri (FMNH UC 640) inflected posteriorly to a much greater extent Taxon than the scapula of Trematops. Relevant cra- Acheloma Trematops nial measurements of A. cumminsi and T. Character cumminsi milleri milleri are provided in table 1. Where it is possible to obtain accurate Total midline skull length (TL) 155 177 measurements, comparisons of element size Distance from anterior rim of orbit to tip of snout (SN) 80 94 and cranial proportions of Acheloma cum- Distance from anterior rim of minsi and Trematops milleri (table 1) reveal external naris to tip of snout no significant differences. Furthermore, a (SE) 17 16 thorough reexamination of the skull of each Length of nasal (NS) 52 59 holotype has failed to uncover any heretofore Length of frontal (FR) 40 50 hidden features that are not held in common, Length of parietal (PR) 32 43 provided the cranial regions in question are SN/TL 0.51 0.53 preserved and accessible in both skulls. The SE/TL 0.11 0.09 many similarities between the holotypic skulls NS/TL 0.34 0.33 that include a slitlike, posteriorly closed otic FR/PR 1.25 1.10 notch, an expanded external naris with a medially situated narial flange, an enlarged premaxillary tooth, a large internarial fenestra, loma was incompletely prepared and few su- reduced interpterygoid vacuities, and fused tures could be recognized. Combining the basicranial articulations are strongly sugges- morphologically distinct small and large tive of synonymy. trematopsids in the same genus is justified Olson (1941), in addition to determining only ifthe smaller ones are juveniles ofAche- the correct familial placement of Acheloma, loma and Trematops. This assumption is described a number oftrematopsid skulls col- implicit in Olson's review and explicit in his lected within the Lower Permian formations recent analysis of a larval trematopsid (Ol- of Texas by field parties from the Harvard son, 1985). Reexamination of those small Museum of Comparative Zoology and the trematopsids described by Olson (1941) and Walker Museum, University of Chicago in several collected after the publication of his 1934, 1936, and 1937. With the exception of review, indicates that the specimens repre- a single skull made a referred specimen ofA. sent individuals from a wide ontogenetic span. cumminsi, these skulls were assigned by Ol- Only the smallest may be convincingly son to Acheloma and Trematops as new thought of as juveniles; those at the opposite species. However, the cranial differences be- end ofthe size range display a series ofcranial tween this assemblage of trematopsids and and postcranial features consistent with an Acheloma and Trematops are far more strik- adult. Therefore, the inclusion of features ing than those between the two genera. Total found only on the smaller trematopsids in midline skull length of these trematopsids the diagnoses ofAcheloma and Trematops is seldom exceeds half that of Acheloma or incorrect. Trematops. The otic notch of the smaller Far greater emphasis has been placed on trematopsids is greatly enlarged, angular in the supposedly more distinct postcranial dif- outline, and open posteriorly. In this respect, ferences 7 and 8. However, these features are the smaller trematopsids bear a striking re- based on misinterpretation ofthe specimens. semblance to those small dissorophids with Ringlike presacral intercentra, as stated open otic notches (Carroll, 1964). The source above, cannot be established in any of the ofcranial differences 1-6 presented by Olson preserved vertebrae ofFMNH UC 640. Even (1941, 1970) to differentiate Acheloma and ifthe intercentra were closed dorsally, recent Trematops was undoubtedly the smaller work on the vertebral column of the classic trematopsids because the holotype of Ache- Early Permian rhachitome Eryops (Moulton, 10 AMERICAN MUSEUM NOVITATES NO. 2902

1974) strongly indicates that coossification of inflection. Reinterpretation of the morphol- the separate cartilage embedded elements or ogy of the scapulocoracoid of A. cumminsi, portions of a single element is a size-related in the light of these observations, reveals no condition; larger and presumably older in- significant structural differences between it dividuals display a greater incidence ofcoos- and the scapulocoracoids of other rhachi- sification than do smaller ones. Dorsally tomes. closed intercentra, simulating the "stereo- In all observable aspects, no significant spondylous" condition ofmany Triassic tem- morphological differences exist between the nospondyls, have been reported in the pos- holotypes ofAcheloma cumminsi and Trem- terior presacrals and caudals of Parioxys atops milleri. In accordance with the law of (Shawki Moustafa, 1955). The dorsal tips of priority, Trematops milleri Williston, 1909 the presacral intercentra of the dissorophid is deemed to be the subjective junior syn- Ecolsonia (Berman et al., 1985) approach each onym of Acheloma cumminsi Cope, 1882. other but do not make contact. Thus, dorsal The family name, following Article 40 (syn- closure ofthe intercentra ofrhachitomes may onymy ofthe type-genus) ofthe International be variable both in position within the ver- Code of Zoological Nomenclature (Ride et tebral column and in the degree of enclosure al., 1985), remains Trematopsidae, Willis- of the notochord and is of little importance ton, 1910 (= Achelomidae Watson, 1919). taxonomically. As a consequence of the above proposed The scapulocoracoid of Acheloma cum- synonymy, the exclusion ofthe smaller trem- minsi, as originally described by Cope (1882) atopsids, and past misinterpretations ofmor- and repeated verbatim by Case (191 1), was phology, a revised diagnosis of Acheloma thought to be highly unusual for labyrintho- cumminsi is presented. donts. An area of poor ossification on the coracoidjust below the supraglenoid foramen SYSTEMATICS was interpreted as the glenoid fossa. When CLASS AMPHIBIA this "glenoid fossa" was directed laterally, the scapula was inflected back and over the ver- ORDER TEMNOSPONDYLI tebral column. The angle formed between the scapular blade and the coracoid was, accord- SUPERFAMILY DISSOROPHOIDEA BOLT, 1969 ing to this interpretation, considerably more FAMILY TREMATOPSIDAE WILLISTON, 1910 acute than in any other known labyrinthodont. Watson (1919), however, reinterpreted Genus Acheloma Cope, 1882 the unusual configuration of the scapulocor- TYPE SPECIES: Acheloma cumminsi Cope, acoid as the product of the presumed im- 1882. maturity of the animal. Olson (1941), al- REVISED GENERIC DIAGNOSIS: Large trem- th-ough probably aware ofWatson's argument, atopsid temnospondyl characterized by the followed Cope's original explanation. As dis- following derived features: a slitlike otic notch cussed in the previous section, the scapulo- closed posteriorly by the contact ofa homlike coracoids ofA. cumminsi are incomplete due tabular process and the base of a stout quad- to poor ossification and preservation. The ap- rate process; a groove on the posteromedial parent medial inflection of the scapula is the surface of the tabular; a long, broad shelf result of the misidentification of the region overhanging the otic notch; an internal naris of rough bone surface on the coracoid as the with a posterior margin well forward of the glenoid fossa. This surface is in fact an posterior margin of the external naris; an os- oblique, ventrolaterally facing section through sified median vomerine septum contacting the coracoid and is, therefore, not the glenoid the roof of the snout. fossa. Slight ventral distortion of the scapula A reconstruction of the skull ofAcheloma relative to the coracoid, particularly evident in dorsal and lateral views is given in figure in the left scapulocoracoid, has emphasized 3. Lack of information regarding the pattern the apparent inflection. Damage to the an- ofpalatal sutures and precise braincase mor- terior edge of the scapular blade is respon- phology prevents a reconstruction in ventral sible for the observed posterior aspect of the view. 1987 DILKES AND REISZ: ACHELOMA AND TREMA TOPS I1I

Neither holotype possesses a stapes. Olson (1941: fig. 9) described and illustrated the stapes of Trematops as a well-ossified and slender rodlike element which Would suggest the existence of a tympanum. The shape of the otic notch, however, argues against an impedence-matching hearing system because the vibrational properties of the postulated tympanum would be profoundly different from one with the same surface area but circular in outline. The reduced height of the tympanum would severely dampen vibrations, thereby limiting efficient sound detec- tion to lower frequencies.

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