Peculiar carapace structure of a Triassic chroniosuchian implies evolutionary shift in trunk flexibility

ABSTRACT

Dermal ossifications are widespread in Permian and Triassic tetrapods, but only members of the Chroniosuchia possess a series of dorsal osteoderms with a complex plate-to-plate articulation mechanism in addition to a contact between each osteoderm and its associated vertebral spine. The stratigraphically youngest chroniosuchid, Madygenerpeton pustulatus, from the Triassic of Kyrgyzstan provides new insight on the function of the chroniosuchian osteoderm system. Osteoderms of M. pustulatus are broad, peaked-roof-shaped to arched, with enlarged posterodorsal and anteroventral articulation facets bearing unique sets of concentric rail-like ridges and furrows. Supplementing the multiple-overlap chroniosuchian-type articulation, the interlocking ridges and furrows confined the relative motion of two neighboring osteoderms to a rotation in slightly oblique and curved contact planes. Given the significant lateral narrowing of the dorsal ornamented non-overlap area, the horizontal component of the plate-to-plate rotation angles could reach up to 7.5°, enabling more extensive lateral flexion of the trunk than in other chroniosuchids. Considering functional analogs, the chroniosuchian osteoderm system probably stabilized the vertebral column against shearing, torsion, tension, and compression loads and thus facilitated terrestrial locomotion at the expense of trunk flexibility. With its particular morphology, the carapace of M. pustulatus, however, was more suitable for locomotion styles featuring lateral body undulation than the carapaces of Permian chroniosuchids. We interpret this speciality as a secondary adaptation to an aquatic habitat.

ACKNOWLEDGMENTS

We are thankful to Robert Georgi for his effort in the recovery of the Madygenerpeton fossils, to Rainer Schoch and the preparators of the Stuttgart Museum of Natural History for their help and advice, to Valery Golubev for discussion and help with the chroniosuchian type material in Moscow, to Jozef Klembara for his support with literature, to Andrew Heckert, Allan J. Lerner, Jörg W. Schneider, Jan Fischer, and three anonymous reviewers for their comments on the manuscript, and to Ilja Kogan for the translation of Russian texts. This research has been financed by the German Research Foundation (DFG II—VO 1466/1–1) and by the State of Saxony (scholarship to M.B.).