Conflicting evidence for the use of caudal autotomy in mesosaurs

Mark J Macdougall,Vincent Fernandez,Antoine Verrière,Jörg Fröbisch,Tanja Wintrich,Aaron R H Leblanc

doi:10.1038/s41598-020-63625-0

Mark J Macdougall, Vincent Fernandez + Show 4 more

Open Access

https://doi.org/10.1038/s41598-020-63625-0

Copy DOI

Abstract

The early Permian mesosaurs were the first amniotes to re-invade aquatic environments. One of their most controversial and puzzling features is their distinctive caudal anatomy, which has been suggested as a mechanism to facilitate caudal autotomy. Several researchers have described putative fracture planes in mesosaur caudal vertebrae — unossified regions in the middle of caudal vertebral centra — that in many extant squamates allow the tail to separate and the animal to escape predation. However, the reports of fracture planes in mesosaurs have never been closely investigated beyond preliminary descriptions, which has prompted scepticism. Here, using numerous vertebral series, histology, and X-ray computed tomography, we provide a detailed account of fracture planes in all three species of mesosaurs. Given the importance of the tail for propulsion in many other aquatic reptiles, the identification of fracture planes in mesosaurs has important implications for their aquatic locomotion. Despite mesosaurs apparently having the ability to autotomize their tail, it is unlikely that they actually made use of this behaviour due to a lack of predation pressure and no record of autotomized tails in articulated specimens. We suggest that the presence of fracture planes in mesosaurs is an evolutionary relic and could represent a synapomorphy for an as-yet undetermined terrestrial clade of Palaeozoic amniotes that includes the earliest radiation of secondarily aquatic tetrapods.

Highlights

The early Permian mesosaurs were the first amniotes to re-invade aquatic environments
Mesosauridae is currently composed of three monotypic genera (Mesosaurus tenuidens Gervais, 18652, Stereosternum tumidum Cope, 18863, and Brazilosaurus sanpauloensis Shikama and Ozaki, 19664), all of which are only known from localities that would have been part of an inland Gondwanan sea during the early Permian
The extent and form of the observed splits is largely consistent with the fracture planes that have been previously identified in the Palaeozoic reptile Captorhinus[13], though there are some differences

Summary

Introduction

The early Permian mesosaurs were the first amniotes to re-invade aquatic environments One of their most controversial and puzzling features is their distinctive caudal anatomy, which has been suggested as a mechanism to facilitate caudal autotomy. Several researchers have briefly mentioned the presence of fracture planes on the centra of mesosaur caudal vertebrae, suggesting that mesosaurs may have exhibited intravertebral autotomy[14,15,16]. Using X-ray micro www.nature.com/scientificreports computed tomography (μCT), histological thin sections, and high-resolution photographs, we provide the first detailed account of fracture planes along the caudal vertebrae of several mesosaur specimens and demonstrate that mesosaurs would have been theoretically capable of caudal autotomy, but likely did not utilize the behaviour.

Methods

Results

Discussion

Conclusion