Osteoderm microstructure indicates the presence of a crocodylian‐like trunk bracing system in a group of armoured basal tetrapods

Abstract

Buchwitz, M., Witzmann, F., Voigt, S. and Golubev, V. 2012. Osteoderm microstructure indicates the presence of a crocodylian-like trunk bracing system in a group of armoured basal tetrapods. -Acta Zoologica (Stockholm) 93: 260-280.The microstructure of dorsal osteoderms referred to the chroniosuchid taxa Chroniosuchus, Chroniosaurus, Madygenerpeton and cf. Uralerpeton is compared to existing data on the bystrowianid chroniosuchian Bystrowiella and further tetrapods. Chroniosuchid osteoderms are marked by thin internal and relatively thick external cortices that consist of lowly vascularised parallel-fibred bone. They are structured by growth marks and, in case of Madygenerpeton, by lines of arrested growth. The cancellous middle region is marked by a high degree of remodelling and a primary bone matrix of parallel-fibred bone that may include domains of interwoven structural fibres. Whereas the convergence of Bystrowiella and chroniosuchid osteoderms is not confirmed by our observations, the internal cortex of the latter displays a significant peculiarity: It contains distinct bundles of shallowly dipping Sharpey's fibres with a cranio-or caudoventral orientation. We interpret this feature as indicative for the attachment of epaxial muscles which spanned several vertebral segments between the medioventral surface of the osteoderms and the transversal processes of the thoracic vertebrae. This finding endorses the hypothesis that the chroniosuchid osteoderm series was part of a crocodylian-like trunk bracing system that supported terrestrial locomotion. According to the measured range of osteoderm bone compactness, some chroniosuchian species may have had a more aquatic lifestyle than others.

Morphology, microanatomy, and histology of titanosaur (Dinosauria, Sauropoda) osteoderms from the Upper Cretaceous of Patagonia

“…Another possible mode of osteoderm formation is intramembranous ossification, in which the newly formed osseous tissue displaces the preformed soft tissue structures instead of incorporating them. Based on the absence of mineralized structural fibers, this kind of ossification has been proposed for different extinct taxa, including some basal tetrapods (Buchwitz et al, 2012), pareiasaurs (Scheyer and Sander, 2009), and 'aetosaurine' aetosaurs (Cerda and Desojo, 2011). Given that, in the titanosaur osteoderms studied here, the primary bone tissue is composed of coarse bundles of collagenous structural fibers, and considering that there is no evidence of periosteal bone deposition, we interpret that these structures have a metaplastic origin.…”

Novel data on aetosaur (Archosauria, Pseudosuchia) osteoderm microanatomy and histology: palaeobiological implications

“…In this regard, the compactness of osteoderms of aquatic and semiaquatic vertebrates is usually higher (pachyostosis sensu lato , Houssaye ) than those of the taxa adapted to a terrestrial habitat (Hua & de Buffrénil ; Scheyer & Sander ; Witzmann ; Buchwitz et al . ). This increase in compactness (i.e.…”

“…Scheyer & Sander ; Witzmann & Soler‐Gijón ; Buchwitz et al . ). According to Scheyer & Sander (), the association of osteoderm compactness and lifestyle is still inconclusive in vertebrates.…”

“…Bystrowiella schumanni , Buchwitz et al . ). Microanatomical analysis from different taxa reveals that high values of compactness are the plesiomorphic condition in Aetosauria.…”

“…Besides the axial and appendicular elements, some authors have also used osteoderm microanatomy to make inferences about lifestyles in different vertebrate groups (Hua & de Buffrénil ; Scheyer & Sander ; Witzmann ; Buchwitz et al . ; Cerda et al . ).…”