Digit and Ungual Morphology Suggest Adaptations for Scansoriality in the Late Carboniferous Eureptile Anthracodromeus longipes

Arjan Mann,David S Berman,Amy C Henrici,Thomas W Dudgeon,Stephanie E Pierce

doi:10.3389/feart.2021.675337

Abstract

A new skeleton of the exceedingly rare, late Carboniferous eureptileAnthracodromeus longipes(Carroll and Baird, 1972), reveals the presence of a reduced phalangeal count in the manus and pedes and uniquely recurved unguals. With these data, we quantitatively evaluate the locomotor ecology ofAnthracodromeususing morphometric analyses of the phalangeal proportions, ungual curvature, and ungual shape. Our findings indicate that the anatomy ofAnthracodromeuslikely facilitated scansorial clinging to some degreeviadistally recurved unguals and increased surface area of the large manus and pes. This suggests thatAnthracodromeuswas among the earliest amniotes to show climbing abilities, pushing back the origins of scansoriality by at least 17 million years. It further suggests that scansoriality arose soon after the origin of amniotes, allowing them to exploit a wide range of novel terrestrial niches.

Highlights

Scansoriality, or the capability to climb, is a common adaptation in living tetrapod groups
In addition to revising the diagnosis for Anthracodromeus longipes, we provide a thorough comparative description of the new material that includes a well-preserved, ungual-bearing manus and pedes that reveal reduced phalangeal counts compared to other early amniotes
Specimens were studied at the following institutions: American Museum of Natural History (AMNH FARB), New York; Carnegie Museum of Natural History (CM), Pittsburgh; Field Museum of Natural History (FMNH), Chicago; Canadian Museum of Nature (CMN), Ottawa; Museum of Comparative Zoology at Harvard University (MCZ), Cambridge; Redpath Museum (RM), Montreal; and Yale Peabody Museum (YPM), New Haven

Summary

Introduction

Scansoriality, or the capability to climb, is a common adaptation in living tetrapod groups. One of the most common characteristics of scansorial forms is the possession of recurved claws This morphology has been noted in the fossil record as early as the mid-Carboniferous. While once considered part of the paraphyletic Protorothyrididae, Anthracodromeus is currently considered a basal eureptile with close relationships to the -aged Cephalerpeton and later Early Permian Protorothyris (Mann et al, 2019; Ford and Benson, 2020). In their original description, Carroll and Baird recognized the elongated manus and pes of Anthracodromeus but did not elaborate on their ecological significance. Spindler et al (2018) proposed that Anthracodromeus was adapted for scansoriality through phalangeal grasping; that is, the animal used

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Discussion

Conclusion