Field of Science

Unique Carapace Structure in a Triassic Tetrapod

Wow, and I thought that aetosaur carapaces were complex. Osteoderms provide more than defense and display.  They also provide support and stabilization for the axial skeleton. Choniosuchians seem to take this to a unique level.

Buchwitz, M. and S. Voigt. 2010. Peculiar carapace structure of a Triassic chroniosuchian implies evolutionary shift in trunk flexibility. Journal of Vertebrate Paleontology 30:1697-1708. DOI: 10.1080/02724634.2010.521685
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.

3 comments:

  1. It's a very cool paper, I enjoyed the poster at SVP so I'm more than happy to see the paper is out so soon. Bill, do you think aetosaur carapaces have structural significance in locomotion?

    Cheers,

    Nick

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  2. Nick,

    Aetosaur carapaces almost certainly provided additional support for the axial skeleton in aetosaurs as it does in modern crocs. Interestingly; however, in some taxa such as desmatosuchines the cervical armor is thickened dorsoventrally and often fuses together. This probably reduced lateral movement in this area. Desmatosuchus also has well developed hypantra and hyposphenes in the vertebrae of the dorsal region, which also reduced movement. Someone needs to do a functional study of aetosaur armor.

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  3. @Nick: It's nice that someone appreciates the paper. The histological + phylogenetic parts will (hopefully) follow soon.

    I agree that defining the initial occurrence of trunk bracing or of other kinds of osteoderm support in the crocodile stem would be a worthwile task...

    ReplyDelete

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