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Caudal autotomy in Mesosaurus tenuidens Gervais, 1865 under scrutiny and a surprising new pattern of vertebral organization in the mesosaur tail

Graciela PIÑEIRO, Jorge FERIGOLO, Brodsky Dantas Macedo de FARIAS, Pablo NÚÑEZ DEMARCO & Michel LAURIN

en Geodiversitas 47 (2) - Pages 17-38

Published on 23 January 2025

Mesosaurs are basal amniotes that lived in an extended epicontinental sea resulting from Devonian and Carboniferous glaciations reported in Southern Gondwana. Previously considered to be marine animals, this interpretation is not supported by their skeletal anatomy, and was updated to a more semiaquatic style, prompting increased interest for mesosaur studies. Recently, transverse fractures and weak ossified areas at the subcentral surface of some caudal vertebrae have been interpreted as fracture planes related to a putative capability of autotomy in mesosaurs. We reassess the data used in support of caudal autotomy and identify the constraining morphological features of the involved vertebrae that contradict the development of such ability in mesosaurs. Moreover, we analyze the physiological and behavioral negative consequences that the loss of part of the tail would represent for aquatic and semiaquatic animals. The fact that no extant taxa living in these environments developed caudal autotomy supports our interpretations. We present an alternative hypothesis that suggests the presence of multipartite caudal centra represented by pleurocentrum and intercentrum arranged in the way expected for a reverse embolomerous pattern, previously undescribed for early stegocephalians or amniotes. However, a rapid revision of specialized literature suggests that the pattern could have been present in other basal amniotes and the possibility deserves additional studies, mainly in juvenile individuals. Our reinterpretation of the structure of mesosaur caudal vertebrae supports the proposed morphological plasticity observed in many clades of basal stegocephalians and amniotes and would match the recently suggested phylogenetic affinities of mesosaurs with respect to taxa that are close to, or part of the amniote stem under some phylogenies.


Keywords:

Mesosauridae, vertebral patterns, caudal vertebrae, autotomy, Gondwana

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