Carpus and tarsus of Temnospondyli

David Dilkes

doi:10.18435/b5mw2q

Abstract

The carpus of Eryops megacephalus and tarsus of Acheloma cumminsi known from complete and articulated individuals have provided the standard anatomy of these skeletal regions for temnospondyls. Restudy of the carpus of Eryops confirms the presence of only four digits, but refutes evidence for a prepollex, postminimus, and distal carpal 5. The supposed contact surface on centrale 1 for a prepollex is reinterpreted as part of the articulation for metacarpal 1 that includes distal carpal 1. Contrary to previous interpretations, a notch on the intermedium does not fit against the lateral corner of the radius. An articular surface on the distal end of the ulna thought previously to contact an absent postminimus fits against the ulnare. Preparation of the tarsus of the type specimen of Trematops milleri (junior synonym of Acheloma cumminsi) and a previously undescribed crus and pes of Eryops finds no evidence for a pretarsale in either genus. Centrale 4 of the tarsus shares a similar rectangular shape with a wide contact for the tibiale among several temnospondyls whether terrestrial or aquatic. Limited flexibility of the carpus of Eryops and a strong palmar arch are probably weight-bearing features. A proximal-distal line of flexibility is present along the tibial side of the tarsus between the tibiale and centrale 4 and between centrale 2 and centrale 1. A phylogenetic analysis of Temnospondyli including new characters of the carpus and tarsus reveals considerable instability, highlighting the significance of Dendrerpeton acadianum, Balanerpeton woodi, Capetus palustris, and Iberospondylus schultzei.

Highlights

The autopodium consisting of the carpus or tarsus and the digits is the hallmark feature of the tetrapod limb
A well-ossified tarsus consisting of the fibulare, intermedium, tibiale, four centralia, and the first distal tarsal is known for the stem-amniote Proterogyrinus scheelei, but only two distal carpals are ossified (Holmes 1984)
Specimens Examined: AMNH FARB 4186, articulated left forelimb of Eryops megacephalus consisting of the humerus, radius, ulna, carpus, and partial digits; FMNH UC 930, isolated carpal and tarsal elements of Cacops aspidephorus; FMNH UC 640, holotype of Trematops milleri consisting of an incomplete skull, vertebral column, and appendicular skeleton; FMNH UC 1756, incomplete skeleton of Acheloma cumminsi; MCZ 7555, right tibia, distal half of right fibula, most bones of the tarsus, portions of the first four metatarsals, and several phalanges; MCZ 4169, left hind limb of Dissorophus multicinctus; MCZ 4172 and 4173, incomplete skeletons of Dissorophus multicinctus with partial forelimbs

Summary

INTRODUCTION

The autopodium consisting of the carpus (wrist) or tarsus (ankle) and the digits is the hallmark feature of the tetrapod limb. Distal carpal 4: Distal carpal 4 shares a similar shape in Eryops, Dissorophus, and Acheloma It has a wider medial side contacting centrale 3 and centrale 4 and a tapering lateral end contacting the ulnare proximally and metacarpal 4 distally The shape of centrale 4 in Eryops (Fig. 6) matches that in Acheloma including the broadly rounded articular end for the tibiale (crushed in MCZ 7555), a ventral concavity that is continuous with the depression on the ventral side of the intermedium, narrower proximo-distal width along fibulare side than tibiale side, and a separation distally between a surface for distal tarsal 4 and centrale 3 and another surface for centrale 2. Centrale 4 in Dissorophus (Fig. 7) shares the same basic morphology of centrale 4 in Eryops and Acheloma It is a transversely rectangular bone with a concave ventral side formed by raised medial and lateral sides. A pair of smaller circular bones (Fig. 8E) is likely two smaller centralia of either the carpus or tarsus

DISCUSSION

Findings

CONCLUSIONS

References for Appendices