Acheloma dunni, Polley & Reisz, 2011

Polley, Brendan P. & Reisz, Robert R., 2011, A new Lower Permian trematopid (Temnospondyli: Dissorophoidea) from Richards Spur, Oklahoma, Zoological Journal of the Linnean Society 161 (4), pp. 789-815 : 792-806

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: BMRP 2007.3.4, small trematopid skull; BMRP 2007.3.1, 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.

DESCRIPTION

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 anterolateral 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 trematopids 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 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 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 Ecolsonia , 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 association with the external naris. Comprised of contributions of the nasal, prefrontal, and lacrimal, the narial flange makes broad contact with the antorbital 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 dissorophoids 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 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 prefrontal 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 dorsally, 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 BMRP 2007.3.4 ( 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 BMRP 2007.3.1 although it would appear that the 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 dvinosauroids Acroplous and Isodectes ( Sequeira, 1998; Englehorn, 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 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 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 posteriorly. 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 welldefined, 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 Anconastes and Actiobates , respectively.

Dorsally, the squamosal forms the majority of the anteroposteriorly directed supratympanic flange. The slope of the squamosal constricts the otic notch dorsoventrally; 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, 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 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.

PALATE

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 posterior 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.

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 ectopterygoid 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 basipterygoid 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 basicranial joint is firmly sutured and immobile. Examination of AMNH 4205 reveals that the basicranial joint of Ach. cumminsi is also sutured, and not indistinguishably fused as described by Dilkes & Reisz (1987). The basicranial joint is mobile in Anconastes and Tambachia ( Sumida et al., 1998) .

BRAINCASE

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 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 dissorophoids 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 temnospondyls, 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 acervalis . 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 posteromedial 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 prezygapophyses 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 subrectangular 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, posterolaterally 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 posterolaterally. 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 intercentrum 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 subrectangular 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 supinator 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.

Figures

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.

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.

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.

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.

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.

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

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.

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.

Figure 10. Partial right 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.

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.

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.

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.