Scholarly article on topic 'A new Lower Permian trematopid (Temnospondyli: Dissorophoidea) from Richards Spur, Oklahoma'

A new Lower Permian trematopid (Temnospondyli: Dissorophoidea) from Richards Spur, Oklahoma Academic research paper on "Biological sciences"

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Academic research paper on topic "A new Lower Permian trematopid (Temnospondyli: Dissorophoidea) from Richards Spur, Oklahoma"



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Zoological Journal of the Linnean Society, 2011, 161, 789-815. With 15 figures

A new Lower Permian trematopid (Temnospondyli: Dissorophoidea) from Richards Spur, Oklahoma


Department of Biology, University of Toronto, Mississauga Campus, 3359 Mississauga Road North, Mississauga, Ontario, Canada L5L 1C6

Received 31 January 2010; revised 16 March 2010; accepted for publication 23 March 2010

A new trematopid amphibian, Acheloma dunni, is reported based on excellently preserved cranial and postcranial elements recovered from the Lower Permian fissure fill deposits of the Dolese Brothers Co. limestone quarry near Richards Spur, Oklahoma. The new taxon is characterized by lateral exposures of the palatine (l.e.p.) and ectopterygoid (l.e.e.), which are clearly visible externally and completely enclosed within the suborbital elements. This large, terrestrial carnivore may represent the top predator of the Richards Spur assemblage. A phylogenetic analysis including 12 taxa and 53 cranial characters yielded a single most parsimonious tree, placing Ach. dunni within the monophyletic Trematopidae as the sister taxon to Acheloma cumminsi. Furthermore, the analysis includes the enigmatic Ecolsonia and Actiobates within Trematopidae, forming a clade with the Upper Pennsyl-vanian Anconastes and the Lower Permian Tambachia. The study comprehensively analyses all valid and aberrant forms of Trematopidae.

© 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 161, 789-815. doi: 10.1111/j.1096-3642.2010.00668.x

ADDITIONAL KEYWORDS: Amphibia - Olsoniformes - Tetrapoda - Trematopidae.


The Dolese limestone quarry, situated near Richards Spur, Oklahoma is home to the most diverse known assemblage of Palaeozoic terrestrial tetrapods. Regular excavation of Ordovician Arbuckle limestone has continually yielded fossil-rich clays and conglomerates (Olson, 1991). Although often disarticulated, fossil material is abundant and well preserved, representing a great variety of taxa (Reisz, 2007; Frobisch & Reisz, 2008). A large proportion of this fossil material (Daly, 1973; Olson, 1991) has been assigned to the small terrestrial eureptile, Captorhi-nus aguti. Olson (1991) interpreted the relative abundance of C. aguti material as indicative of an Early Permian age for the faunal assemblage. More specifically, he inferred that the fauna of Richards Spur was contemporaneous with the Leonardian Arroyo

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Formation of the Clear Fork Group of Texas (Olson, 1991). This assignment has been further supported by biostratigraphical evidence based on the fauna recovered from the South Grandfield locality of southern Oklahoma (Daly, 1973; Sullivan, Reisz & May, 2000; Maddin, Evans & Reisz, 2006).

The Richards Spur assemblage represents a dramatic departure from contemporary Early Permian faunas. Whereas the majority of North American localities typically yield faunal samples consistent with aquatic, semi-aquatic, and terrestrial forms of lowland, deltaic environments (Olson, 1956), the Richards Spur assemblage is comprised exclusively of small and medium-sized terrestrial forms. This unique faunal composition has led to the suggestion that the locality has preserved a predominantly arid, upland assemblage (Sullivan et al., 2000; Anderson & Reisz, 2003; Schoch, 2009). Skeletal elements of larger taxa have been reported from Richards Spur, corroborating the hypothesis that the distinct composition of the fossil assemblage may have been the

product of palaeoecological factors, rather than tapho-nomic biases (Sullivan et al., 2000). These large remains include isolated elements assignable to a varanopid (Maddin et al., 2006), the sphenacodontid Thrausmosaurus (Evans, Maddin & Reisz, 2009), the dissorophid Cacops (Sullivan et al., 2000; Reisz, Schoch & Anderson, 2009), a large trematopid suggested to be Acheloma (Bolt, 1974a; Schultze & Chorn, 1983; Sullivan et al, 2000), Seymouria (Sullivan & Reisz, 1999), and an unidentified eryopid, possibly Eryops (Olson, 1991).

Similar to the typical smaller taxa, the large forms recovered from Richards Spur appear to represent taxa that were primarily terrestrial. As early amniotes, there is little doubt that the varanopid and sphenacodontid were well suited for life on land. Dissorophids and trematopids, collectively forming the clade Olsoniformes (Anderson et al., 2008), are considered the most terrestrial representatives of late Palaeozoic temnospondyl amphibians (Bolt, 1969; Berman, Reisz & Eberth, 1987; Sumida, Berman & Martens, 1998; Dilkes & Brown, 2007; Markey & Marshall, 2007; Schoch, 2009). Likewise, authors agree that Seymouria was principally terrestrial, dependent on an aquatic habitat only during reproduction and very early growth stages (Sullivan & Reisz, 1999; Berman, 2000; Klembara et al., 2007; Schoch, 2009). Although it may be argued that ery-opids may have been aquatic or semi-aquatic, the identification of the skull fragment as Eryops is doubtful, and the fragment is not diagnostic. In fact, we propose that it is more appropriate to designate the skull fragment as Temnospondyli incertae sedis.

Described here is a new large trematopid skull (OMNH 73281) collected from Richards Spur in 2006. Although trematopids were previously known from Richards Spur on the basis of isolated, fragmentary elements, the newly recovered skull is nearly complete, showing little distortion. This material represents the largest recorded taxon of the Richards Spur assemblage, rivalling the varanopid described by Maddin et al. (2006) in size. Trematopids form a monophyletic clade of poorly understood temno-spondyl amphibians known from the Late Pennsylva-nian and Early Permian of North America and central Germany (Berman et al., 1987; Dilkes, 1990; Sumida et al., 1998). The taxonomic history of Trematopidae is long and complex. Whereas the monophyly of the clade has remained largely unquestioned, the group as a whole has yet to be subjected to rigorous phylo-genetic analysis.

Trematopidae as erected by Williston (1910) included only two taxa known from Lower Permian type specimens: Acheloma cumminsi (Cope, 1882) and Trematops milleri (Williston, 1909). Mehl (1926) described an additional species of Trematops, Trem-

atops thomasi; however, the family remained largely unexamined until Olson's (1941) comprehensive study. In his review, he erected several new species of Acheloma and Trematops: Acheloma pricei, Acheloma whitei, and Trematops willistoni. He later expanded the family by describing Trematopsis seltini (Olson, 1956) and Trematops stonei (Olson, 1970). The type specimen of Trematopsis was later found to be a junior synonym of Cacops (Milner, 1985b). Moreover, Olson was first to recognize the close affinities between trematopids and the diverse group of terrestrially adapted amphibians of the family Disso-rophidae. Dissorophids span a greater temporal and geographical range than trematopids and are distinguished from trematopids by the possession of armoured dermal plates associated with the neural spines and the absence of an elongated external naris (Olson, 1941; Carroll, 1964a; DeMar, 1966b; DeMar, 1968).

Subsequent authors also recognized the close relationship of trematopids and dissorophids (Vaughn, 1969; Berman, Reisz & Eberth, 1985, 1987; Dilkes, 1990; Daly, 1994; Sumida et al., 1998; Anderson et al., 2008). However, because ingroup relationships of both families remained largely unresolved, several taxo-nomic problems arose when forms seemingly possessing a combination of trematopid and dissorophid characters were discovered and described. Mordex calliprepes and Parioxys ferricolus were both originally described as trematopids (Romer, 1947). Mordex was later synonymized with Amphibamus (Carroll, 1964a) only to be resurrected as a dissorophid with a trematopid-like elongated external naris (Milner, 1986). Presently, Mordex is in the process of once again being reclassified as a basal trematopid (Milner, 2007). Parioxys is still considered closely related to dissorophids, but has been removed from Trematopi-dae and placed within its own family, Parioxyidae (Mustafa, 1955; Carroll, 1964b).

Longiscitula houghae (DeMar, 1966a) was originally described as possessing both a trematopid-like external naris and the distinctive dermal armour of dissorophids. Re-examination of the type specimen revealed the apparent elongated external naris was an artefact of preservation (Bolt, 1974c). Milner (2003) eventually synonymized L. houghae with Dis-sorophus multicinctus. The enigmatic Ecolsonia cut-lerensis was described as sharing affinities with dissorophids but was tentatively assigned to Trem-atopidae by Vaughn (1969) based on its elongated external naris and lack of exostoses and armour. Berman et al. (1985) argued the skull proportions and structure of the otic notch of Ecolsonia were in fact indicative of dissorophid affinities, and that its elongated external naris evolved convergently. Daly (1994) incorporated Ecolsonia into her study of disso-

rophoid relationships but was unable to resolve its assignment (Sumida etal, 1998). Eaton (1973) described the Late Pennsylvanian Actiobates pea-bodyi as a dissorophid; however, he stipulated that the distinction between trematopids and dissorophids was non-existent and all taxa assigned to both families should fall under Dissorophidae. Later authors that recognized both families agree Actiobates is a trematopid possibly exhibiting an early stage of development (Berman et al., 1985; Milner, 1985a; Daly, 1994). Partly for these reasons, both Ecolsonia and Actiobates have been excluded from most cladistic analyses of trematopid relationships (Berman et al., 1987; Dilkes, 1990; Sumida et al., 1998).

Currently, Trematopidae consists of five known genera. Actiobates peabodyi is at least provisionally still a valid trematopid (Berman et al., 1987; Milner, 1985a; Dilkes, 1990; Daly, 1994; Sumida et al, 1998). Anconastes vesperus (Berman et al., 1987) represents a Late Pennsylvanian trematopid from Texas. Dilkes & Reisz (1987) found Trematops milleri to be a junior synonym of Acheloma cumminsi, and retained the name Acheloma for the genus. Dilkes (1990) synony-mized the small Early Permian trematopid taxa erected by Olson (1941) into a new genus, Phonerpeton pricei. Finally, the Early Permian Tambachia trogallas (Sumida et al., 1998) recovered from central Germany represents the most recent addition to the family. Here we describe a new species of trematopid and comprehensively re-evaluate the interrelationships of trematopids using a phylogenetic analysis. Furthermore, this study is the first to include the aberrant Ecolsonia and Actiobates in an analysis of trematopid ingroup relationships in an attempt to resolve their taxonomic positions and define Trematopidae.


The holotype, OMNH 73281, consists of an exceptionally preserved skull measuring 164 mm along its midline. The skull has experienced no crushing but has been slightly sheared to the right. All other damage appears to have occurred as a result of the blasting and transport operations of the quarry from which it was recovered. The antorbital region of the left side of the skull is substantially damaged and remains unprepared. The rest of the skull has been prepared and shows little sign of damage with the following exceptions: the posterior portion of the quadratojugals and tabulars are broken, whereas the quadrates are missing entirely. Also, a break is present along the roof of the skull between the frontal and nasal, extending onto a portion of the prefrontal. The palate has been extensively prepared; leaving only a small area along the medial edges of the right

premaxilla and maxilla obscured by matrix and a tightly associated captorhinid lower jaw. Only the most anterior portions of the lower jaws are present but remain disarticulated. The atlas-axis complex is present along with disarticulated elements comprising the third and fourth cervical vertebrae.

The referred specimens consist of well-preserved large postcranial elements, jaw joint, and two small partial skulls. The pelvic girdle OMNH 73514 is nearly complete and associated with a femur, tibia, fibula, and disarticulated elements of the pes. OMNH 52365 consists of partial elements comprising the jaw articulation. OMNH 52545 is a complete right humerus. BMRP2007.3.4 is a small partial skull measuring 56 mm along its dorsal midline. It appears to have been extensively acid prepared, leaving the dermal sculpturing severely eroded but all sutures clearly visible. The anterior-most half of the skull is absent along with the braincase, palate, and most of the lower jaw. BMRP2007.3.1 is a partial skull consisting only of the antorbital bar and its associated cheek region.

The holotype of Acheloma cumminsi (AMNH 4205) was examined for anatomical comparison. The material consists of a nearly complete but substantially crushed skull, a string of 22 articulated presacral vertebrae, both scapulocoracoids, and both humeri. Also examined was the type specimen of Acheloma stonei (CMNH 10969), represented by a distorted partial skull. The specimen was observed for comparative purposes but left out of the analysis because of its lack of informative characters. A cast of the only specimen of Acheloma thomasi (MU 501) was also made available for study.

Specimens of almost all other trematopid taxa were also available for study, including the holotype (CM 41711) and paratype (CM 28590) of Anconastes vesperus; the holotype (UCLA VP 1734) and paratypes (CM 38017; CM 41703) of Ecolsonia cutlerensis; and the holotype (MNG 7722) of Tambachia trogallas.

Observations of Phonerpeton pricei were based on a complete skull with partial postcranial skeleton (AMNH 7150) and published reconstructions (Dilkes, 1990).

Specimens of Actiobates peabodyi were not examined. Comparisons and character coding was accomplished using published illustrations and descriptions (Eaton, 1973; Milner, 1985a).


Institutional abbreviations

AMNH, American Museum of Natural History, New York, NY; BMRP, Burpee Museum of Natural History, Rockford, IL; CM, Carnegie Museum of Natural

History, Pittsburgh, PA; CMNH, Cleveland Museum of Natural History, Cleveland, OH; MCZ, Museum of Comparative Zoology, Harvard University, Cambridge, MA; MNG, Museum der Natur, Gotha, Germany; MU, University of Missouri, Columbia, MO; OMNH, Sam Noble Oklahoma Museum of Natural History, University of Oklahoma, Norman, OK; UCLA VP, University of California, Los Angeles, CA.

Anatomical abbreviations

a, angular; art, articular; at, atlas; axi, axial inter-centrum; axn, axial neural spine; bo, basioccipital; c, centrale; c3, third cervical vertebra; c4, fourth cervical vertebra; cr, cervical rib; ct, cultriform process; d, dentary; ect, ectopterygoid; eo, exoccipital; f, frontal; fe, femur; fi, fibula; i, intermedium; il, ilium; in.f, internarial fenestra; is, ischium; j, jugal; l, lacrimal; l.e.e., lateral exposure of the ectopterygoid; l.e.p., lateral exposure of the palatine; m, maxilla; n, nasal; obt, obturator foramen; op, opisthotic; p, parietal; paf, para-articular foramen; pal, palatine; pc, pleuro-centrum; pco, precoronoid; pf, postfrontal; pm, premaxilla; po, postorbital; pp, postparietal; pra, prearticular; prf, prefrontal; ps, parasphenoid; pt, pterygoid; ptf, post temporal fenestra; pu, pubis; q, quadrate; qj, quadratojugal; s, stapes; sa, surangular; se, sphenethmoid; sm, septomaxilla; so, supraoccipi-tal; sp, splenial; sq, squamosal; st, supratemporal; t, tabular; ti, tibia; tib, tibiale; v, vomer.


Temnospondyli Zittel, 1887-1890 Euskelia Yates & Warren, 2000 Dissorophoidea Bolt, 1969 Olsoniformes Anderson etal., 2008 Trematopidae Williston, 1910 Revised diagnosis: The clade consisting of Acheloma cumminsi and all other taxa that share a more recent common ancestor with Ach. cumminsi than with Dissorophus.

Acheloma Cope, 1882 Revised generic diagnosis: Large trematopid temno-spondyl characterized by the following autapomor-phies: toothed, raised crest running anteroposteriorly along the vomer, mesial to the choana; otic notch with a nearly horizontal ventral margin.

Acheloma dunni sp. nov. (Figs 1-14)

Type specimen: OMNH 73281, nearly complete skull with associated atlas-axis complex and partial lower jaw.

Referred specimens: BMRP2007.3.4, small trematopid skull; BMRP2007.3.1, trematopid snout; OMNH 52365, jaw articulation; OMNH 73514, pelvic girdle; OMNH 52545, right humerus.

Occurrence: Dolese Brothers Co. limestone quarry, near Richards Spur, Comanche County, Oklahoma; fissure fill deposits in Ordovician Arbuckle limestone probably equivalent to Leonardian Arroyo Formation of Clear Fork Group, Lower Permian.

Etymology: The specific name honours Brent Dunn, who has graciously collected and donated several specimens from the Dolese Brothers Co. limestone quarry for study.


General description

As observed in all other trematopids (Berman et al., 1987; Dilkes, 1990; Sumida et al, 1998), the skull of Acheloma dunni is tall, box-like, and roughly triangular in shape (Figs 1, 2). The surface of the skull roof is covered in deep, rounded dermal pitting. Overall, the skull most closely resembles Acheloma cumminsi in its large size, the presence of an enlarged, elongate key-hole shaped, external narial opening, and slit-like otic notch with broad, overhanging shelf (Dilkes & Reisz, 1987). However, whereas the skull roof of OMNH 73281 is slightly larger in both overall length and width than the holotype of Ach. cumminsi (AMNH 4205), the snout of Ach. dunni is narrower and considerably less robust. Descriptions are based primarily on OMNH 73281 and other specimens as cited.

Skull roof

The premaxilla of Ach. dunni has a well-developed posterodorsal alary process, overlapping the antero-lateral area of the nasal. An internarial fenestra measuring 3 mm in diameter is situated along the midline suture between the premaxillae. The same structure is present in the type specimen of Ach. cumminsi but is proportionately larger, measuring 7 mm in diameter. A proportionately large internarial fenestra is also present in Phonerpeton (Dilkes, 1990), whereas the structure appears to be completely absent in all other trematopids (Berman et al., 1985, 1987; Dilkes, 1990; Sumida et al., 1998). The presence of an internarial fenestra is not exclusive to trem-atopids as it has been identified in a variety of other dissorophoids (Carroll, 1964a; Bolt, 1977a; Reisz et al., 2009). Mehl (1926) described the partial snout of the Lower Permian trematopid Acheloma thomasi (as Trematops thomasi) from Snyder, Oklahoma that

Figure 1. Reconstruction of skull of Acheloma dunni in A, dorsal view; B, ventral view; C, right lateral view. Abbreviations: ct, cultriform process; ect, ectopterygoid; f, frontal; in.f, internarial fenestra; j, jugal; l, lacrimal; l.e.e., lateral exposure of the ectopterygoid; l.e.p., lateral exposure of the palatine; m, maxilla; n, nasal; op, opisthotic; p, parietal; pal, palatine; pf, postfrontal; pm, premaxilla; po, postorbital; pp, postparietal; prf, prefrontal; ps, parasphenoid; pt, pterygoid; q, quadrate; qj, quadratojugal; s, stapes; se, sphenethmoid; sm, septomaxilla; sq, squamosal; st, supratemporal; t, tabular; v, vomer. Scale bar = 50 mm.

possessed a reduced or perhaps absent internarial fenestra. Authors have generally accepted Olson's (1941) suggestion that Ach. thomasi is a junior synonym of Ach. cumminsi (as T. milleri), although

the assertion was made only under the condition that the internarial fenestra was in fact present. Mehl did not list a specimen number in his description and it appears the only record of Ach. thomasi material is of

Figure 2. Skull of Acheloma dunni, holotype (OMNH 73281) in dorsal view. Abbreviations: f, frontal; in.f, internarial fenestra; j, jugal; l, lacrimal; l.e.e., lateral exposure of the ectopterygoid; l.e.p., lateral exposure of the palatine; m, maxilla; n, nasal; pf, postfrontal; pm, premaxilla; po, postorbital; pp, postparietal; prf, prefrontal; qj, quadratojugal; sq, squamosal; st, supratemporal; t, tabular. Scale bar = 50 mm.

a cast of the type specimen (MU 501) in the University of Missouri at Columbia collections (Katz, 2008). The cast is in poor condition; however, it is informative enough to indicate that the internarial fenestra is absent.

Seven teeth are preserved on each premaxilla, along with another six empty alveoli. Initially, there is a steady increase in size of teeth posteriorly, with the largest being in the ninth or tenth tooth position. The last three teeth in the premaxillary series decrease in size posteriorly. The preserved teeth on the maxilla exhibit a similar pattern, at first increasing in size posteriorly and reaching their maximum size at about the seventh or eighth tooth position, beneath the posterior margin of the external naris. The remaining teeth gradually decrease in size posteriorly. There are 18 preserved teeth on the right maxilla with spaces for at least ten more. The maxilla is a long and slender element, stretching posteriorly past the level of the anterior border of the otic notch. The maxilla has the greatest dorsal expansion at the level of the largest teeth, beneath the enlarged external narial opening. Its dorsal edge makes up the

majority of the ventral border of the external naris where the maxilla also has a well-developed medial shelf that forms the floor of the external naris.

The conspicuous elongated external narial opening, often cited as the defining character of trematopids (Olson, 1941, 1970; Vaughn, 1969; Eaton, 1973; Daly, 1994), is bordered by the premaxilla, maxilla, nasal, prefrontal, and lacrimal. The increase in the height of the maxilla in conjunction with a lateral expansion of the nasal along the margin of the external naris, partitions the external naris into distinct anterior and posterior sections. This gives the narial opening an overall keyhole shape (Fig. 3). The dorsal expansion of the maxilla occurs just posterior to the area where the septomaxilla is located in the opening in all other trematopids (Sumida et al., 1998), including Ecolso-nia, delineating the posterior extent of the functional external naris (Bolt, 1974c; Berman et al., 1987; Dilkes, 1993). Although the septomaxilla is not preserved in Ach. dunni, it probably was in a similar position. A laterally concave, smooth lamina known as the narial flange (Dilkes, 1990, 1993) descends from the ventral surface of the skull roof and lies in close

Figure 3. Skull of Acheloma dunni, holotype (OMNH 73281) in right lateral view. Abbreviations: ct, cultriform process; f, frontal; j, jugal; l, lacrimal; l.e.e., lateral exposure of the ectopterygoid; l.e.p., lateral exposure of the palatine; m, maxilla; n, nasal; pf, postfrontal; pm, premaxilla; po, postorbital; prf, prefrontal; pt, pterygoid; qj, quadratojugal; se, sphenethmoid; sq, squamosal; st, supratemporal; t, tabular; v, vomer. Scale bar = 50 mm.

association with the external naris. Comprised of contributions of the nasal, prefrontal, and lacrimal, the narial flange makes broad contact with the antor-bital bar. This condition is common amongst other trematopid taxa (Eaton, 1973; Berman et al., 1987; Dilkes & Reisz, 1987; Dilkes, 1990; Sumida et al., 1998). A narial flange has been found in other disso-rophoids including Doleserpeton (Carroll, 1964a; Bolt, 1974c; Reisz et al. 2009); however, the structure does not appear to contact the antorbital bar in any of these taxa. A narial flange is visible in the holotype of Ecolsonia in cross section; however, the configuration of the narial flange cannot be confirmed because the area surrounding the antorbital bar remains absent or unprepared in all specimens.

The nasal capsule of Ach. dunni is floored by the vomer and palatine. The choana lies well anterior to the posterior margin of the external naris as in Ach. cumminsi (Dilkes & Reisz, 1987). The posterior border of the choana is in line with the posterior margin of the external naris in all other trematopids (Eaton, 1973; Berman et al, 1985, 1987; Dilkes, 1993; Sumida et al., 1998). A partially prepared sheet of bone is exposed medial to the narial flange. It appears to be the dorsally directed laminar process running along the midline of the rostrum, referred to as the median vomerine septum by Dilkes (1990). This structure is present in Ach. cumminsi, Phonerpeton

(Dilkes, 1990), and was also revealed in Anconastes and Tambachia (Sumida et al., 1998). Examination of the holotype of Ecolsonia (UCLA VP 1734) indicates a median vomerine septum is present. Contact between the skull roof and the median vomerine septum can be confirmed in only Ach. cumminsi (Dilkes & Reisz, 1987); however, the same structure approaches the skull roof but does not contact it in Phonerpeton (Dilkes, 1990), and in Ecolsonia. The dorsal extent of the median vomerine septum is unknown in either specimen of Anconastes (CM 41711; CM 28590) or Tambachia (MNG 7722). If the median vomerine septum made even cartilaginous contact with the skull roof, the strut-like structure may be an adaptation to withstand compressive and shearing forces acting on the rostrum and vomers, respectively, during feeding. Anteriorly, the vomerine walls bifurcate, contacting the ventrolateral surfaces of the premaxillae.

The lacrimal makes up the ventral half of the antorbital bar, contributing to both the posteroventral margin of the external narial opening and the anteroventral margin of the orbit. Anteriorly, the bone has a relatively short subnarial process, suturing bluntly with the dorsally expanded portion of the maxilla (Fig. 3). A short subnarial process is also present in Tambachia and Actiobates (Sumida et al., 1998). In other trematopids, the lacrimal extends

Figure 4. Acheloma dunni, referred specimen (BMRP2007.3.4) in left lateral view. Abbreviations: f, frontal; j, jugal; l, lacrimal; l.e.e., lateral exposure of the ectopterygoid; l.e.p., lateral exposure of the palatine; m, maxilla; n, nasal; pf, postfrontal; po, postorbital; prf, prefrontal; sq, squamosal; st, supratemporal; t, tabular. Scale bar = 10 mm.

anteriorly to a point about level with the subdivision of the external narial opening.

The prefrontal is a triangular bone that makes up the dorsal half of the antorbital bar. Anteriorly, it contributes to the posterodorsal margin of the external narial opening. The posterior process of the pre-frontal forms the anterodorsal margin of the orbit and makes contact with the frontal. The prefrontal does not contact the postfrontal; instead, the frontal separates the two elements and contributes to the dorsal border of the orbit.

The jugal is tall and broad, spanning most of the length of the cheek region. Dorsally, it contributes to the circumorbital series by forming the ventral margin of the orbit. It extends ventrally to meet the maxilla near the posterior limit of the latter. The posterior-most portions of the jugal form a tall and slightly curved wedge, contacting the squamosal dor-sally, and the quadratojugal posteroventrally.

The most intriguing feature of Ach. dunni is the presence of several distinct exposures of sculptured bone visible on the lateral surface of the cheek region. The roughly oval-shaped elements lie adjacent to one another, bordered ventrally by the maxilla and dorsally by the lacrimal and jugal. Three separate elements can be seen in OMNH 73281 and BMRP2007.3.4 (Fig. 4). The most anterior element appears to be a l.e.p. It is comprised of a sculptured lateral expansion of a portion of the palatine lying just posterior to the palatal fang and replacement pit. Posterior to the l.e.p. is a small l.e.e., consisting of only the most anterior portion of the ectopterygoid. The third and largest exposure is formed by a secondary lateral expansion of the ectopterygoid associated with the lateral margin of the socket housing the ectopterygoid fang and replacement pit. A similar pattern is observed in BMRP2007.3.1 although it would appear that the

l.e.e. l.e.p

Figure 5. Acheloma dunni, referred specimen (BMRP2007.3.1) in right lateral view. Abbreviations: j, jugal; l, lacrimal; l.e.e., lateral exposure of the ectopterygoid; l.e.p., lateral exposure of the palatine; m, maxilla; n, nasal; prf, prefrontal. Scale bar = 10 mm.

lateral exposures of the ectopterygoid are fused (Fig. 5).

Occurrences of an l.e.p. contributing the ventral margin of the orbit have been reported in several dissorophoids (Bolt, 1974b) and the basal dvinosau-roids Acroplous and Isodectes (Sequeira, 1998; Engle-horn, Small & Huttenlocker, 2008). An l.e.p. and l.e.e. have been recognized in the trematopid Phonerpeton, although the relationship between the two exposures varies between specimens (Dilkes, 1990). Neither an l.e.p. nor l.e.e. are present in Actiobates, Anconastes, Ecolsonia, or Tambachia (Eaton, 1973; Berman et al., 1985, 1987; Sumida et al., 1998). Although all four

Figure 6. Acheloma dunni, referred specimen (OMNH 52365). Partial right upper jaw articulation in dorsal view. Abbreviations: pt, pterygoid; q, quadrate; qj, quadratoju-gal; st, supratemporal. Scale bar = 10 mm.

taxa possess relatively large orbits and narrow suborbital bars, it is the maxilla and not the palatine or ectopterygoid that contributes to the ventral margin of the orbit. Examination of the holotype of Ach. cumminsi confirms previous accounts that neither an l.e.p. nor l.e.e. are present (Bolt, 1974b; Dilkes & Reisz, 1987).

Posteriorly, the orbit is bordered by the postfrontal and postorbital (Fig. 3). Both bones are roughly triangular in shape. The postfrontal contacts both the

Figure 7. Skull of Acheloma dunni, holotype (OMNH 73281) in ventral view. Abbreviations: bo, basioccipital; cf, cultriform process; ect, ectopterygoid; in.f, internarial fenestra; m, maxilla; op, opisthotic; pal, palatine; pm, premaxilla; ps, parasphenoid; pt, pterygoid; s, stapes; sm, septomaxilla; v, vomer. Scale bar = 50 mm.

Figure 8. Skull of Acheloma dunni, holotype (OMNH 73281) in occipital view. Abbreviations: bo, basioccipital; eo, exoccipital; op, opisthotic; pp, postparietal; ps, parasphenoid; pt, pterygoid; s, stapes; so, supraoccipital; sq, squamosal; t, tabular. Scale bar = 50 mm.

frontal and parietal medially, and the supratemporal posteriorly. The postorbital is a narrow element forming a broad, interdigitated suture with the squamosal. Only the dorsal and ventral-most points of the postorbital make constricted contact with the supratemporal and jugal, respectively. Together, the postfrontal, postorbital, and squamosal form a narrow area of bone separating the orbit and the otic notch.

The squamosal is a large element, forming a substantial portion of the otic notch. The sculptured lateral margin of the embayment of the squamosal appears horizontal in outline in its most anterior portion. Medially, the squamosal has an internally directed flange that makes up the ventral surface of the otic notch. The flange is deflected ventrally at an acute angle, and contacts the quadratojugal posteri-

Figure 9. Partial left lower jaw of Acheloma dunni, holotype (OMNH 73281) in A, lateral view; B, medial view; C, dorsal view; D, ventral view. Abbreviations: d, dentary; pco, precoronoid; sp, splenial. Scale bar = 10 mm.

orly. Together, the squamosal, supratemporal, and tabular contribute to the dorsal margin of the otic notch. The bones form a deep posterolaterally directed supratympanic shelf overhanging a well-defined, unsculptured supratympanic flange. Presence of a supratympanic shelf has been confirmed in Ach. cumminsi and Phonerpeton; however, the supratympanic shelf appears to be replaced by sculpturing covering the lateral area above the otic notch in both Ecolsonia and Tambachia. The area above the

otic notch is unpreserved and undescribed in Ancon-astes and Actiobates, respectively.

Dorsally, the squamosal forms the majority of the anteroposteriorly directed supratympanic flange. The slope of the squamosal constricts the otic notch dor-soventrally; however the bone lacks the distinct semilunar curvature observed in other dissorophoids. Amongst trematopids, a semilunar curvature of the squamosal has been recorded in Ecolsonia, Phonerpeton, and Tambachia (Berman et al., 1985; Dilkes,

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1990; Sumida et al., 1998). A semilunar curvature of the squamosal is absent in Ach. cumminsi, whereas the area is unpreserved in Anconastes (Berman et al., 1987). Posteriorly, the squamosal contacts the tabular, excluding the semilunar flange of the supratemporal (Bolt, 1974a) from the ventral margin of the supratympanic flange. The posterior extent of

the tabulars is unknown in all specimens of Ach. dunni.

OMNH 52365 preserves the posterior-most portion of the otic notch (Fig. 6). The posteroventral margin of the squamosal slopes ventrally, overlapping the quadratojugal. The quadratojugal forms the posterior end of the otic notch before curving sharply dorsally

Figure 11. Acheloma dunni, referred specimen (OMNH 52365). Partial right lower jaw articulation in A, lateral view; B, posterior view. Abbreviations: a, angular; art; articular; pra, prearticular; sa, surangular. Scale bar = 10 mm.

and contributing to the lateral surface of a robust, rounded jaw articulation. A medially projecting process of the quadratojugal contacts the quadrate. The quadrate is a narrow, unsculptured bone forming the posterolateral corner of the skull. The dorsal surface of OMNH 52365is slightly damaged and what would appear to be the dorsal process of the quadrate has broken off.


The vomers are densely covered with small, recurved teeth. Anteriorly, the vomers are smooth, deflected laterally and contact the premaxillae. The dorsal extent of the bones contacts the nasals, forming a deep internarial pit (Fig. 7). The ventral area of the internarial fenestra is visible in this vicinity and appears laterally expanded with respect to its dorsal compliment. A prominent shelf housing a large fang with a replacement pit on each vomer overhangs the posterior margin of the internarial pit and is level with the anterior margin of the choana. An additional set of accessory fang pairs are situated directly pos-

terior to the larger fangs. A distinct toothed, raised crest runs anteroposteriorly along the medial border of the choana. The same crest has been observed in some primitive temnospondyls (Ruta & Bolt, 2006). The crest continues onto a posterolateral process of the vomer that flanks the medial edge of the palatine and contacts the pterygoid.

The ventral surface of the palatine is mainly occupied by a massive fang and a large replacement pit. As mentioned above, the palatine forms the posterior border of the choana. Medially, the palatine is excluded from the interpterygoid vacuity by contact between the vomer and the palatal ramus of the pterygoid. Retention of the vomer-pterygoid contact and a rather narrow interpterygoid vacuity is common amongst trematopids but considered primitive (Berman et al., 1985, 1987; Dilkes & Reisz, 1987; Dilkes, 1990; Sumida et al., 1998). In most dissorophoids, contact between the vomer and pterygoid is lost and the palatine and/or ectopterygoid contribute to the lateral margins of an expanded interpterygoid vacuity. Posteromedially, the palatine has a small, toothed, ridge-like swelling that contacts the pterygoid.

Figure 12. Atlas-axis complex and associated cervical vertebrae of Acheloma dunni, holotype (OMNH 73281) in A, right lateral view; B, left lateral view; C, anterior view. Abbreviations: at, atlas; axi, axial intercentrum; axn, axial neural spine; c3, third cervical vertebra; c4, fourth cervical vertebra; cr, cervical rib; pc, pleurocentrum. Scale bar = 10 mm.

Figure 13. Acheloma dunni, referred specimen (OMNH 52545). Right humerus in A, posterior view; B, extensor view; C, anterior view; D, flexor view; E, proximal view; F, distal view. Scale bar = 50 mm.

The ectopterygoid is a smaller palatal element than either the palatine or the pterygoid, but still houses a large fang and replacement pit. The overall size of the ectopterygoid teeth does not exceed that of the marginal teeth in Ach. cumminsi (Dilkes & Reisz, 1987). Although the tips of the fangs are broken in OMNH 73281, the teeth of Ach. dunni are clearly larger than any of the preserved marginal teeth. Posteriorly, the ectopterygoid forms the rounded anterior border of the adductor fossa. Similar to the palatine, a raised toothed ridge runs posterolaterally along the ectop-terygoid onto the pterygoid.

The right pterygoid of OMNH 73281 is nearly complete, missing only the posterior-most portion of the quadrate ramus. The entire palatal ramus and basip-terygoid region of the pterygoid are covered in a dense shagreen of small teeth, whereas the quadrate ramus appears smooth. Although slightly damaged, a curved transverse flange runs along the posterior margin of the palatal ramus into the adductor fossa. The dorsomedially directed internal process of the

basipterygoid region makes broad contact with the basipterygoid process of the parasphenoid. The basi-cranial joint is firmly sutured and immobile. Examination of AMNH 4205 reveals that the basicranial joint of Ach. cumminsi is also sutured, and not indis-tinguishably fused as described by Dilkes & Reisz (1987). The basicranial joint is mobile in Anconastes and Tambachia (Sumida et al., 1998).


The parasphenoid is complete and visible in ventral view of the skull (Fig. 8). The long, narrow cultriform process arches along the midline of the skull, reaching its maximum height at a point about level with the centre of the orbit. Although the tapered anterior end of the cultriform process undoubtedly reached the vomers, the area remains damaged and details of attachment between the elements are unknown. Dorsally, the cultriform process articulates with an ossified sphenethmoid. The sphenethmoid contacts

Figure 14. Acheloma dunni, referred specimen (OMNH 73514). Partial pelvic girdle and hindlimb. Abbreviations: c, centrale; fe, femur; fi, fibula; i, intermedium; il, ilium; is, ischium; obt, obturator foramen; pu, pubis; ti, tibia; tib, tibiale. Scale bar = 5 mm. Illustration by Heidi Richter.

tightly the ventral surface of the frontal and postfrontal. The body of the parasphenoid is rectangular, extending thin laterally expanding wings posteriorly. A deep anteroposterior depression runs medially along the ventral surface. A distinct triangular patch of tiny teeth sits on a transverse, tall ridge medial to the basipterygoid processes. The denticles extend only onto the posterior tip of the cultriform process. The absence of parasphenoidal dentition was used to unite Acheloma and Phonerpeton (Dilkes, 1990); however, re-examination of AMNH 4205 indicates that small teeth are in fact present on the parasphenoid of Ach. cumminsi but only the most proximal borders of the dentition have been preserved. Amongst dissoro-phoids in which the area is known, only Broiliellus and Dissorophus lack parasphenoidal dentition (Sumida et al., 1998).

The basioccipital is tightly associated with the parasphenoid and only enough of the bone has been prepared to confirm its presence. The basioccipital joins the exoccipitals to form the paired occipital condyles; however, the suture between the two elements is not visible. The exoccipitals are narrow ventrally and form the lateral borders of the foramen magnum. Each exoccipital makes contact with the posterolateral edge of the occipital flange of the postparietals. Interestingly, dorsal to this contact, the exoccipitals appear robust, and medially expanded,

forming the majority of the dorsal margin of the foramen magnum. In the only specimen of Ach. stonei (CMNH 10969) and Phonerpeton, this area consists of a gap, partially separating the exoccipitals and postparietals. The same gap is present in the aberrant temnospondyl Platyhystrix and is interpreted as evidence of a cartilaginous supraoccipital (Berman, 2000).

It is possible that the robust dorsal expansion of the exoccipitals of Ach. dunni represents an ossified supraoccipital fused to the exoccipitals. Within tem-nospondyls, similar ossifications thought to be homologous to the supraoccipital have been recorded in Eryops and Edops (Berman, 2000). Originally, Carroll's (1964a) description of Dissorophus angustus included an account of a supraoccipital similar to that of Ach. dunni; however, according to the character coding of Laurin & Reisz (1997), the supraoccipital is absent. The only other recorded instance of an ossified supraoccipital within Temnospondyli is in Schoch's (1999) description of the braincase of Kamacops acer-valis. However, the structure of the supraoccipital of K. acervalis differs greatly from that of Ach. dunni and other temnospondyls. Whereas the supraoccipital appears fused to the exoccipitals in Ach. dunni, in K. acervalis the bone fuses with the opisthotics, separating the exoccipitals from the postparietals in a manner similar to its homologue in microsaurs,

lysorophids, amniotes, and diadectomorphs (Schoch, 1999; Berman, 2000).

Both stapes are complete and preserved in place (Figs 1, 8). The thin, anteroposteriorly compressed shaft is curved with its distal end fitting into an open posteroventral notch along the supratympanic flange. A distinct stapedial foramen is visible on the posterior surface of the footplate (Figs 7, 8). The foramen is considered primitive in temnospondyls (Daly, 1994), but is retained in all trematopids. The footplate is roughly tetrahedral in shape, ventrally associated with the basal plate of the parasphenoid. The rest of the footplate contacts the fenestra ovalis. This configuration is common within Temnospondyli and may have been mobile in a pump-handle fashion (Bolt & Lombard, 1984).

Lower jaw

The lower jaw is represented by fragmentary material (OMNH 73281) consisting of the anterior-most portions of the dentary, precoronoid, and splenial (Figs 9, 10). The dorsal surface of the dentary is smooth, housing a large fang and a replacement pit medially. The posterior extent of the bone is missing, making an exact count of dentary teeth impossible. The dentary sutures with the precoronoid along a distinct dorsal depression, where a large foramen is present posterior to the dentary fang. A dense patch of small teeth occupies a narrow band along the dorsal surface of the precoronoid. Ventromedially, the precoronoid contacts the splenial along a deep, anteroposteriorly directed groove. The groove continues anteriorly, deflecting dorsally and creating a medially situated gap in the sutural surface of the symphysis. The symphysis consists mainly of the dentary with smaller posterior contributions from the splenial and precoronoid.

OMNH 52365 preserves the posterior corner of the lower jaw (Fig. 11). The surangular and articular are robust, forming the articulating surface of the lower jaw. Medially, a greatly interdigitated suture separates the surangular from prearticular and angular. The para-articular foramen is visible on the postero-medial surface of the prearticular.

Axial skeleton

Slightly disarticulated anterior vertebral elements were recovered in close association with OMNH 73281 (Fig. 12), including the atlas-axis complex and portions of the third and fourth vertebrae. Several fragments of tall, wing-like cervical ribs flank the vertebral column on either side. Although each structure appears to be in its correct relational position, each vertebral element has been rotated counter-

clockwise. The atlas-axis complex is a massive structure exhibiting an interesting configuration. The atlantal neural arches are present but only the right half appears to be complete. Both elements are orientated dorsally and are greatly separated from their counterpart medially by a single robust anterior expansion of the axial neural spine. Williston (1909) described a similar orientation of the neural arches in Ach. cumminsi (as Trematops milleri). The prezyga-pophyses are orientated almost vertically, probably articulating with paired proatlantal elements. The postzygapophyses are comparably more prominent and horizontally directed. The atlantal neural arches are fused to the centrum with no distinguishable suture, as observed in Doleserpeton (Bolt, 1969, 1977b) and Amphibamus (Carroll, 1964a; Daly, 1994). The body of the centrum is unipartite, consisting of one or more fused elements. A distinct, laterally constricted body and ventral keel are typical features of rachitomous intercentra and suggest that the atlantal intercentrum is at least one of the contributing elements to the centrum. The centrum appears subrect-angular in lateral view and dorsolaterally expanded beneath the neural arches. This expanded area may consist of the intercentrum and neural arches fusing to the atlantal pleurocentra that are otherwise absent. Anteriorly, the bicondylar articulating surface of the centrum aligns almost perfectly with the occipital condyle of the skull. The tight fit between the atlas and the occipital would have probably restricted the range of movement about this joint, if not prevented it altogether.

The axis is a tall, broad multipartite element. The stout axial neural arches contact the broad, postero-laterally protruding transverse processes but remain unfused to the centrum. The prezygapophyses are directed anterolaterally to articulate with the atlantal neural arches. The postzygapophyses are notably elongate in comparison to the prezygapophyses and directed posterolaterally. The axial neural arches fuse dorsally, forming the broad, rugous axial neural spine. The lateral surfaces of the neural spine are concave, narrowing anteriorly to wedge between the atlantal neural arches. Dorsally, the neural spine thickens greatly before tapering to a pyramidal point. The wedge-like axial intercentrum is present but displaced posteriorly. As in Ach. cumminsi the axial intercentrum is considerably smaller than those of the following cervical vertebrae (Williston, 1909). No pleurocentra are visible.

The third cervical vertebra appears mostly complete but disarticulated. The slightly bulbous dorsal tip of the neural spine is damaged. A dorsoventrally directed groove runs along the anterior face of the spine. Ventrally, the groove bifurcates to meet the strongly anteromedially directed prezygapohyses.

The postzygapophyses remain only partially prepared, with articulating surfaces orientated postero-laterally. A single, ventrolaterally directed, transverse process is visible. Posteriorly, the distal surface of the transverse process is broad and flat, and would have articulated with the wide cervical ribs. The intercen-trum is disarticulated but lies in close association with the rest of the third cervical vertebra. It is roughly crescentic and wedge-shaped in lateral view. Another intercentrum, presumably belonging to the fourth cervical vertebra is pressed against the posterior surface of the third cervical intercentrum. Only a single unpaired and disarticulated pleurocentrum is visible. It is unclear what particular vertebra this element contributed to.

Appendicular skeleton OMNH 52545 represents a complete right humerus (Fig. 13). The humerus is a massive element with flared proximal and distal ends orientated at approximately right angles to each other. The shaft is laterally constricted and appears somewhat subrect-angular in cross-section. Distally, a well-developed ectepicodyle and entepicondyle dominate the extensor surface. A convex radial condyle is positioned on the anteroventral surface of the flexor side. Proximal to this area, a prominent supinator process comes to a blunt point. A supinator process has been observed not only in other trematopids where the area is preserved (Williston, 1909; Olson, 1941; Dilkes & Reisz, 1987; Dilkes, 1990), but is also present in Eryops, seymouramorphs, and diadectimorphs (Pawley & Warren, 2006). Moreover, the development of a supi-nator process has been correlated with the degree of ossification of the humerus and is at least suggestive of a terrestrial lifestyle (Pawley & Warren, 2006). A posteriorly directed, anterior humeral keel joins the supinator process to a roughly oval attachment area for the pectoralis muscle alongside a distinct deltoid crest. Proximally, the humeral articulating surfaces are well developed and slightly convex.

OMNH 73514 represents the pelvic girdle and extremity of Ach. dunni (Fig. 14). Both sides of the pelvic girdle are partly exposed in lateral view with no discernable sutures separating the ilium, ischium, and pubis. Most of the iliac blade is missing on the left pelvis, although the rest of the element appears intact. The right pelvis is missing portions of the ischium and pubis and has a large crack running along its surface. Any preserved structures dorsal to the ventral margin of the acetabulum remain obscured by matrix housing the hindlimb. Both pelves are broad, thin bones. The acetabulum is triangular in outline, bordered dorsally by a strongly developed transverse pelvic ridge and posterior supra-

acetabular notch. A large laterally directed obturator foramen is present. The right femur is complete and is in articulation with the right pelvis, and resembles the described femur of Ach. cumminsi (Williston, 1909; Olson, 1941). The proximal and distal heads are expanded, whereas the shaft is gently concave. The long fourth and internal trochanters of the adductor blade are robust, with a thin intertrochanteric ridge bordering a deep oval concavity. A deep adductor crest runs along the entire flexor surface of the femur, terminating near the fibular condyle. The distal head is all that remains of the left femur, which sits in close association with an articulated tibia, fibula, and elements of the pes.

The tibia (Fig. 14) has a greatly expanded femoral head with a well-developed cnemial crest and trough. The slender shaft appears circular in cross-section, broadening distally to meet the convex articular facet. The fibula is almost as long as the tibia. Its proximal and distal ends are about equal in width. Both ends are slightly directed toward the tibia by a medially concave curvature of the shaft. Following the description of the pes of Ach. cumminsi (Williston, 1909; Schaeffer, 1941), the tibia and fibula articulate with a proximodistally elongate intermedium (Fig. 14) along the medial surfaces of their distal ends. Distal to the intermedium is the wide, concave fourth centrale. The wedge-shaped tibiale flanks the tibia, rotated slightly out of position. A small bone possibly representing a centrale or tarsal sits out of place, displaced by the distal end of the right femur. Next to this bone are two broken unidentified phalangeal elements. A single element of the pes located next to the right pelvis has suffered pyrite damage and may represent the first centrale.


Phylogenetic analysis A review of previous studies involving trematopid relationships reveals that the family has typically been analysed in two ways. Although each approach affirms both the monophyly of Trematopidae and its position as the sister group of Dissorophidae, subtle differences in methodology have resulted in varying perceptions of trematopid interrelationships.

Firstly, cladistic analyses of trematopid ingroup relationships have been assessed using a single taxon as the outgroup while also excluding aberrant forms from the analysis altogether. Dilkes (1990) was first to show that Anconastes, Phonerpeton, and Acheloma formed a monophyletic sister-group to Amphibamus (Amphibamus (Anconastes (Phonerpeton, Acheloma))). Ecolsonia and Actiobates were not included in that study at the time because they were generally con-

sidered as a dissorophid or juvenile, respectively. Sumida et al. (1998) performed a similar cladistic analysis when describing Tambachia, heavily basing the choice of character states and outgroups on the work of Dilkes (1990) and Daly (1994). They concluded that the monophyletic Trematopidae consisted of two sister groups comprised of Tambachia and Anconastes in one clade, and Acheloma and Phonerpeton in the other (Amphibamus (Tambachia, Ancon-astes)(Acheloma, Phonerpeton)). As in earlier works, Sumida et al. (1998) used only Amphibamus for out-group comparisons and left Ecolsonia and Actiobates out of that analysis, despite growing evidence that neither form could be considered a dissorophid (Daly, 1994).

Secondly, trematopids have been included in larger scale phylogenetic studies of dissorophoid and temno-spondyl relationships. Although Ecolsonia and Actio-bates have also been included in most of these cases, character coding for trematopids has often been based primarily on only the best known forms, excluding more than half of the potential members of the family from analysis. Daly (1994) included Trematopidae in her analysis of amphibamid relationships along with Ecolsonia. The analysis placed Ecolsonia outside Trematopidae; however, although several different trematopid taxa were discussed, character coding for the group as a whole was based largely on Acheloma. Subsequent authors followed this trend, coding characters for trematopids using only Acheloma and Phonerpeton (Ruta, Jeffery & Coates, 2003; Schoch & Rubidge, 2005; Anderson et al., 2008; Frobisch & Reisz, 2008). The purposes of these studies were not to examine trematopid ingroup relationships; still, the approach maintained monophyly for trematopids, allowing the clade to be effectively utilized for out-group comparison. However, resolution within the group was lost through this method and the diversity existing within Trematopidae as highlighted by Sumida et al. (1998) remained unaccounted for. As a result, forms such as Ecolsonia and Actiobates failed to cluster with derived trematopid taxa like Ache-loma, and fell outside the clade.

Ruta et al. (2007) included all valid trematopid taxa, as well as Ecolsonia and Actiobates into a super-tree of temnospondyl relationships. The results showed that Actiobates grouped with the trematopids Anconastes and Tambachia, whereas Ecolsonia fell within Dissorophidae. Conversely, the analysis of dis-sorophoid relationships conducted by Huttenlocker, Pardo & Small (2007) indicated that Ecolsonia shared a more recent common ancestor with trematopids than dissorophids. Although the study did not resolve trematopid ingroup relationships, data from Ache-loma, Phonerpeton, Tambachia, and Actiobates were considered when coding characters for Trematopidae.

The present study undertakes the first comprehensive phylogenetic analysis of Trematopidae examining all valid and aberrant forms of the group along with the new taxon within the broader context of dissorophoid relationships. The data matrix used by Schoch & Rubidge (2005) provided the framework for our phylogenetic analysis. Twenty-four informative cranial characters were utilized from this matrix and another 30 were added from the literature (Appendix 2). Five taxa were selected for outgroup comparisons. Sclerocephalus haeuseri is a well described lower tetrapod known from abundant material (Schoch, 2003; Schoch & Rubidge, 2005). Micromelerpeton credneri was included as a representative of basal dissorophoids. Eoscopus lockardi (Daly, 1994) and Micropholis stowi were used to represent the two sister clades comprising Amphibami-dae (Schoch, 2003). Exquisite material of the recently described Cacops morrisi (Reisz et al., 2009) was available and used to represent Dissorophidae. The trematopids Ach. cumminsi, Anconastes vesper-sus, Phonerpeton pricei, Tambachia trogallas, Actio-bates peabodyi, and Ecolsonia cutlerensis formed the ingroup along with Ach. dunni. A total of 53 cranial characters was used in the analysis. Character coding for Ecolsonia, all trematopid taxa except Actiobates, and Cacops was based on first hand observations (Appendix 1). Coding for all other taxa was based primarily on the data matrices of Schoch & Rubidge (2005), Ruta & Bolt (2006), and published descriptions as cited. A parsimony analysis was performed using PAUP 4.0b10 (Swofford, 2003) and MacClade 4.08 (Maddison & Maddison, 1992). All analyses were performed using a branch-and-bound search with all characters equally weighted and unordered. Bremer support and bootstrap values were calculated to determine the robustness of nodes. The analysis resulted in one single most parsimonious tree with a tree length of 99 steps, consistency index of 0.66, and rescaled consistency index of 0.44 (Fig. 15).

The Olsoniformes (Dissorophidae, Trematopidae) forms a clade supported by three unambiguous syna-pomorphies (16, 19, 20). Two additional steps are required to collapse the node. It is supported by a bootstrap value of 66%.

The monophyly of Trematopidae is supported by four unambiguous synapomorphies (39, 41, 42, 44) and includes all previously valid trematopids along with Actiobates and Ecolsonia. The clade is strongly supported by a Bremer value of 5 and bootstrap value of 82%. Members of Trematopidae are defined by the presence of an elongated external naris (39); canini-form teeth (41); an inflection of the prearticular along the medial rim of the adductor fossa (42); and a median vomerine septum (44).

Figure 15. Single most parsimonious tree of dissorophoid relationships derived from cladistic analysis using PAUP 4.0b10 with A, relevant synapomorphies mapped on (* indicates a synapomorphy that supports the clade, but appears elsewhere in the tree); B, bootstrap (bold numbers) and Bremer decay (italic numbers) values calculated for the analysis; TL, tree length; CI, consistency index; RC, rescaled consistency index. Acheloma dunni is highlighted in bold.

Within Trematopidae, Actiobates, Ecolsonia, Ancon-astes, and Tambachia form a clade based primarily on the contribution of the maxilla to the ventral margin of the orbit in the absence of an l.e.p. and l.e.e. The node collapses after an additional two steps and it has a bootstrap value of 75%. Anconastes and Tambachia are strongly united by a Bremer value of 6 and bootstrap value of 77%. Interestingly, both Ecolsonia and Actiobates may share a more recent ancestor with Anconastes and Tambachia (the two trematopids often left out of analyses) than either does with Acheloma or Phonerpeton. This may account for instances in which Ecolsonia fell outside Trematopidae when trematopid character coding was representative of only Acheloma or Phonerpeton. Furthermore, replacement of the supratympanic shelf by dermal sculpturing along the dorsal rim of the otic notch has been cited as a character uniting Dissorophidae (Daly, 1994), and was used to argue the assignment of Ecolsonia as a dissorophid (Berman et al., 1985). However, this character cannot be valid because the recently described dissorophid Cacops morrisi (Reisz et al., 2009) retains a supratympanic shelf along the dorsal border of its otic notch.

The new trematopid recovered from Richards Spur clusters with Acheloma and indicates that we can place it with that genus, but as a new species. The node uniting Ach. dunni and Ach. cumminsi is supported by two unambiguous synapomorphies: vomer with raised crest lying mesial to choana (33-0); and otic notch with nearly horizontal ventral margin (43). The node has a Bremer support value of 8 and bootstrap value of 98%. Acheloma dunni is distinguished from all other dissorophoids based on a single auta-pomorphy: the presence of sculptured lateral exposure of palatal elements that are excluded from the ventral margin of the orbit.

Phonerpeton does not form a clade with Acheloma as previous studies have suggested (Dilkes, 1990; Sumida et al., 1998; Ruta et al., 2007). This contradiction has resulted from changes in the character coding with respect to the presence of parasphenoid dentition in Acheloma. Originally, Acheloma and Phonerpeton were united based on the presence of an internarial fenestra and a lack of teeth on the paras-phenoid. The internarial fenestra appears in an array of other dissorophoids. Examination of the holotype of Ach. cumminsi along with the fact that dentition is present on the parasphenoid of Ach. dunni resulted in the collapse of the node uniting Phonerpeton and Acheloma.

Speculations on habits and lifestyle

Acheloma dunni appears to represent a large, terrestrial predator within the Richards Spur assemblage.

As an olsoniform, Ach. dunni falls within the group of Palaeozoic amphibians considered to exhibit morphological specializations for life on land (Bolt, 1969; Berman etal., 1987; Sumida et al., 1998; Dilkes & Brown, 2007; Markey & Marshall, 2007; Schoch, 2009). Its tall, box-like skull with laterally positioned orbits and lack of lateral line canals is comparable to that of Early Permian terrestrial amniotes and other olsoniforms (Dilkes & Brown, 2007; Reisz et al., 2009). Furthermore, well-ossified limb bones including a humerus with well-developed condyles and prominent supinator process suggest that Ach. dunni possessed large muscles capable of holding itself up on land (Pawley & Warren, 2006; Dilkes & Brown, 2007). What is known of the axial skeleton bears similarities to that of other olsoniforms characterized by a well-ossified atlas-axis complex and presacral vertebrae contributing to a relatively short trunk (Williston, 1909, 1910). Although many medium to large temnospondyls similar in size to the olsoniforms were aquatic (Schoch, 2009), their relatively short axial length and small tail would suggest that olsoni-forms were more suited for an amphibious to terrestrial lifestyle (Laurin, Girondot & Loth, 2004). Furthermore, the atlas-axis complex observed in Ach. dunni is robust like that of Diadectes (Sumida & Lombard, 1991). The atlas would have attached firmly to the occipital condyles, and probably would have provided support for the weight of the massive skull in terrestrial environments.

The function of the posterior expansion of the trematopid external narial opening has been the focus of limited inquiry (Olson, 1941; Bolt, 1974c; Dilkes, 1991, 1993). Bolt (1974c) proposed that the posterior portion of the external naris developed in response to the lateral expansion of a gland. Furthermore, as the external naris expanded to accommodate the gland, the narial flange may have developed its unique morphology to counteract stress concentrated on the narrow antorbital bar during feeding and avert the potential loss of structural integrity in the rostrum. He cited the lateral expansion of the glandula nasalis externa (salt gland) found in living reptiles as the most likely candidate, suggesting that it may have helped the amphibians cope with the demands of living in terrestrial environments. However, within extant taxa, salt glands are found primarily in birds and reptiles that are either marine or feed on vegetation with high potassium content; functioning primarily in ionic regulation rather than osmoregulation (Peaker & Linzell, 1975). Neither condition would appear to be met by the carnivorous, terrestrial trem-atopids. Regardless of whether or not the posterior expansion of the external naris was caused by the enlargement of a salt gland, Bolt (1974c) concluded that the typical trematopid elongated external naris

and narial flange could not be used as diagnostic characters for the family. He reasoned that in any labyrinthodont, the expansion of any structure lateral to the nasal cavity could result in an elongated external narial opening along with the subsequent development of a narial flange. The argument was later used to support removing Ecolsonia from Trematopi-dae (Berman et al., 1985).

Conversely, Dilkes' (1993) study of the cranial ontogeny of Phonerpton suggests that the narial flange developed early in the postmetamorphic ontogeny of trematopids. In turn, the posterior expansion of the external naris occurred successively, correlating with the growth of the premaxillary and maxillary caniniform teeth and probable changes in the areas of stress concentration in the skull. He argued that the narial flange evolved as a terrestrial adaptation in dissorophoids, enhancing water conservation and olfactory sensitivity by increasing the surface area of the nasal capsule. Additionally, the flange may have been modified in trematopids to also reinforce the skull. Furthermore, Dilkes (1993) stated that the presence of a salt gland is equivocal and as seen in living reptiles, an expansion of such gland would not necessarily result in the posterior expansion of the external naris. Although the true physiological function of the elongated external naris and narial flange is unknown, both Bolt (1974c) and Dilkes (1993) agree that changes in forces acting on the skull during feeding played a key role in their development. The advent of the use of finite-element analysis in assessing sutural stress responses to feeding forces (Ray-field, 2005) may provide promising insight into the purpose of these unique features.

Modifications in sutural morphology in response to changes in forces associated with feeding are also illustrated in the unique configuration of the l.e.p. and l.e.e. in Ach. dunni. Bolt (1974b) proposed that the l.e.p. (and probably the l.e.e.) evolved in early dissorophoids typically characterized by small skulls with large orbits. A thickening of the palatine and its subsequent migration into the circumorbital elements of a widening orbit would have provided mechanical support for compressive forces acting on the narrow suborbital bar during feeding. However, unlike other dissorophoids, Ach. dunni has small orbits and a tall suborbital bar comparable to that of Ach. cumminsi. A tall cheek region probably provided greater mechanical support than the typically thin suborbital bars of most dissorophoids, so the development of an l.e.p. and l.e.e. in Ach. dunni is surprising. In Ach. dunni, the ectopterygoid fang is relatively larger than that found in any other trematopid, including Ach. cum-minsi in which the ectopterygoid fangs are about the same size as the largest marginal tooth (Dilkes & Reisz, 1987). Moreover, the development of large

ectopterygoid fangs in Ach. dunni would have undoubtedly resulted in different forces acting on the palate and suborbital bar during feeding in comparison to Ach. cumminsi. If Bolt (1974b) was correct in asserting that the l.e.p. counteracted stress associated with feeding then the evolution of laterally exposed palatal elements in Ach. dunni may relate to forces ensued on the skull by the enlarged ectoptery-goid fangs.

Trematopids have generally been considered as terrestrial predators (Olson, 1941; Dilkes, 1990; Markey & Marshall, 2007). Recent fossil evidence indicates that large olsoniforms like Ach. dunni at least scavenged on top predators and played a significant role in their ecosystems (Reisz & Tsuji, 2006; Reisz et al., 2009). Acheloma dunni is the largest described species of the Richards Spur assemblage (Maddin et al., 2006) and displays a host of complex adaptations associated with specialized feeding habits. As such, Ach. dunni may represent the top predator of the Richards Spur ecosystem.


We are grateful to D. Scott for assisting in preparing and photographing specimens. Special thanks to B. Dunn and M. Feese for their continuing efforts to collect fossils from Richards Spur. This project, amongst countless others, would not have been possible without their generous donations to the Sam Noble Oklahoma Museum of Natural History. We are thankful to D. Berman of the Carnegie Museum of Natural History, Pittsburgh, PA; R. Ethington of the University of Missouri, Columbia, MO; M. Norell of the American Museum of Natural History, New York, NY; and M. Ryan of the Cleveland Museum of Natural History, Cleveland, OH for allowing us access to various trematopid specimens for study.


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Data matrix used for phylogenetic analysis including 12 taxa and 54 characters. 'a' represents polymorphic characters (0 and 1)


1111111112 1234567890

2222222223 1234567890

Acheloma cumminsi Acheloma dunni Actiobates peabodyi Anconastes vesperus Cacops morrisi Ecolsonia cutlerensis Eoscopus lockardi Micromelerpeton credneri Micropholis stowi Phonerpeton pricei Sclerocephalus haeuseri Tambachia trogallas

0110000701 3110000?01 2??000???0 2??001??0? 1111000011 2110001000 110000aa11 1?1000aa1a 1110111a11 a111000000 0000001000 21?0010?0?

0011010111 0011010111 0???1?111? 001???0111 0111010111 0011010011 111110110a 00?1001000 11110a0000 0011110111 0000000000 0011010111

10??001011 10?0001011 10?0010111 11?0000010 1100000011 1100000011 1011000011 1110000011 1a10000011 1000001011 0000101000 11??000?10

3333333334 1234567890

4444444445 555 1234567890 123

Acheloma cumminsi Acheloma dunni Actiobates peabodyi Anconastes vesperus Cacops morrisi Ecolsonia cutlerensis Eoscopus lockardi Micromelerpeton credneri Micropholis stowi Phonerpeton pricei Sclerocephalus haeuseri Tambachia trogallas

1100001011 1100001011 0??????11? 0??????010 00110??001 0110001010 101100?001 10111??100 10110??000 0010001011 10100??000 0?100??110

1111110000 002

1?11?10000 002

1???010100 111

1101?10100 111

0000201011 012

1101210111 110

0000000010 110

0000000000 000

0000000010 210

1101000010 011

0000000000 000

1101010100 111


Descriptions of characters used in phylogenetic analysis. Characters 1-24 are informative cranial characters based on Schoch & Rubidge (2005). Characters 25-38 are taken from Ruta & Bolt (2006). Characters 39-54 are from literature as cited.

1. Laterally exposed palatine: palatine overplated by jugal and lacrimal with no lateral exposure (0);

palatine wedging between lacrimal and jugal to make contribution to skull roof and orbital margin (1); maxilla contributes to orbital margin in the absence of lateral exposure of palatine (2) (based on Sumida et al., 1998); lateral exposure of palatine present and excluded from orbital margin by jugal and lacrimal contact (3).

2. Dorsal quadrate process: quadrate having smooth posterodorsal side in plesiomorphic state (0);

quadrate with prominent dorsoposterior outgrowth, the quadrate process (1).

3. Vomerine depression: ventral surface of vomers flat and element divided into anterior and posterior portion by transverse ridges that may or may not bear transverse tooth row (0); single unpaired depression in anterior portion of vomers that may or may not house an opening (1).

4. Parasphenoid dentition: basal plate of parasphe-noid bearing shagreen of small teeth (denticles) anteromedially (0); plate entirely smooth (1).

5. Parasphenoid denticle field: parasphenoid denticle field well established, with triangular outline and with apex reaching onto base of cultriform process (0); denticle field greatly expanded anteriorly to cover most of the cultri-form process (1).

6. Parasphenoid basal plate: basal plate roughly quadrangular, as long as wide (0); basal plate much shorter than wide, reaching about half the width (1).

7. Vomerine denticle field: vomer covered with more or less dense shagreen of teeth in addition to obligatory fang pair (0); shagreen confined to juvenile stages and /or absent throughout ontogeny (1).

8. Vomerine fangs: vomer lacking fangs in its medial portion, outside lateral tooth arcade, but having smaller accessory teeth in that region (0); vomer with additional fang pairs posterior to mid-vomerine depression (1).

9. Pterygoid-vomer: retention of suture between pterygoid (palatine ramus) and vomer (0); ptery-goid contacting only posterior-most portion of palatine and lacking suture with vomer (1).

10. Pterygoid flange: palatine ramus of pterygoid merging continuously into basipterygoid ramus

(0); palatine ramus broadening abruptly to form transverse flange (1).

11. Palatine, ectopterygoid: palatine and ectoptery-goid much wider than maxilla (0); palatine and ectopterygoid reduced to narrow struts not wider than adjoining maxilla (1).

12. Interpterygoid vacuity: interpterygoid vacuity roundish or oval in outline (0); interpterygoid vacuity greatly expanded laterally at mid-level

13. Narial flange: ventral (inner) side of prefrontal, lacrimal, and nasal smooth (0); inner side of these bones forming complicated bar-like structure (narial flange), permitting contact with antorbital bar (1).

14. Prefrontal process: prefrontal forming simple suture with lacrimal laterally (0); prefrontal underplating lacrimal widely by means of ventral prefrontal process contacting palatine (1).

15. Tabular size: Tabular narrower than postparietal, but reaching almost same size as latter (0); tabular minute and laterally constricted by unique enlargement of otic notch (1).

16. Tabular-squamosal: Tabular and squamosal widely separated by supratemporal (0); squamo-sal meeting tabular, excluding supratemporal from otic notch (1).

17. Postparietal length: postparietal forming transversely rectangular or quadrangular element (0); postparietal abbreviated and reduced to narrow, poorly ornamented strut at posterior margin of skull table (1).

18. Squamosal-supratemporal: suture between squa-mosal and supratemporal nearly as long as supratemporal itself (0); foreshortened squamoso-supratemporal suture reaching only one third or less of length of supratemporal (1).

19. Supratympanic flange (= semilunar flange in Schoch & Rubidge, 2005, terminology following Bolt, 1974a): squamosal continuously ornamented around margin of otic notch (0); squamo-sal having dorsally exposed and ornamented area (supratympanic flange) stepping abruptly into steeply aligned, poorly ornamented portion (1).

20. Semilunar flange (= supratemporal flange of Schoch & Rubidge, 2005): supratemporal without ventral projection into otic notch (0); supratemporal forming marked ventral flange participating in medial bordering of otic notch (1).

21. Prefrontal-postfrontal: prefrontal and postfrontal firmly sutured, excluding the frontal from orbital margin (0); both elements separated by frontal, at least dorsally (1).

22. Skull width: moderately wide skull with jugals, postorbitals, and medial skull roofing series usually longer than wide (0); skull table and cheek overall broadened, most elements being as wide as long or wider (1).

23. Palpebral ossifications: ossifications in orbit restricted to sclerotic ring (0); numerous palpe-bral ossicles at medial margin of sclerotic ring (1).

24. Stapes: stapes with pronounced dorsodistal curvature directed towards dorsally located otic notch (0); stapes abbreviated without dorsodistal curvature, directed laterally towards vertically aligned otic notch (1).

25. Absence (0) or presence (1) of a prefrontal-jugal contact.

26. Maxilla extending posterior to the level of the posterior margin of the orbit (0) or terminating at the level of such margin or anterior to it (1).

27. Absence (0) or presence (1) of inward inflection of skull outline in dorsal view at the level of the maxilla-premaxilla suture.

28. Presence (0) or absence (1) of a maxilla-quadratojugal contact.

29. Parietals more (0) or less (1) than two and a half times as long as wide.

30. Postparietal less than (0) or more than (1) four times wider than long.

31. Postorbital not narrowing (0) or narrowing (1) to an acute posterior point.

32. Absence (0) or presence (1) of condition: vomer with posterolateral ramus that extends posteriorly along the medial margin of the palatine.

33. Vomer with (0) or without (1) a toothed, raised crest running anteroposteriorly and lying mesial to the choana.

34. Palatine excluded from (0) or contributing to (1) the interpterygoid vacuity.

35. Absence (0) or presence (1) of condition: palatal ramus of the pterygoid forming a butt joint with the posterior margin of the palatine, thus producing a continuous sheet of bone with the latter.

36. Absence (0) or presence (1) of an exoccipital-tabular contact.

37. Absence (0) or presence (1) of an exoccipital-postparietal contact.

38. Jaw articulation lying posterior to (0), level with (1), or anterior to (2) the posterior facets of the exoccipitals.

39. External narial opening: uniform, oval shaped margin (0); posteriorly expanded with distinct anterior and posterior regions giving external naris an overall 'key-hole' shape (1) (Dilkes, 1990).

40. Internarial fenestra: present (0); reduced (1); absent (2) (Dilkes, 1990).

41. Marginal teeth: uniform in size (0); caniniform teeth on premaxilla and maxilla (1) (Dilkes, 1990).

42. Inflection of the prearticular along the medial rim of the adductor fossa: absent (0); present (1) (Dilkes, 1990).

43. Ventral border of otic notch: slopes posteroven-trally (0); nearly horizontal (1) (Dilkes & Reisz, 1987).

44. Median vomerine septum: absent (0); present (1) (Dilkes & Reisz, 1987; Dilkes, 1990).

45. Tabular process: short or absent (0); curves gradually to meet robust quadrate process (1); bent down sharply at approximately a right angle to the dorsal edge of the skull table and fused to the quadrate process (2) (Dilkes & Reisz, 1987).

46. Stapedial foramen: absent (0); present (1) (Daly, 1994).

47. Knobby exostoses ornamenting the skull roof: absent (0); present (1) (Daly, 1994).

48. Subnarial lacrimal process: long (0); short (1) (Sumida et al., 1998).

49. Semilunar curvature of the squamosal along ventral border of the supratympanic flange: absent (0); present (1) (Berman et al., 1985; Dilkes, 1990).

50. Dorsal rim of occiput: smooth (0); ornamented (1).

51. Ratio of preorbital length to postorbital length: preorbital length greater than postorbital length by greater than 10% (0); preorbital and postorbital lengths approximately equal (1); postorbital length greater than preorbital length by greater than 10% (2).

52. Suborbital bar height: greater than 10% of the total midline skull length (0); less than 10% of the total midline skull length (1).

53. Minimum distance between otic notch and orbital margin: greater than 25% of the total midline skull length (0); between 10 and 25% of the total midline skull length (1); less than 10% of the total midline skull length (2).