Bone histology of osteoderms of archosauriform diapsid reptiles (Sauropsida: Archosauriformes)

TORSTEN M. SCHEYER; JULIA B. DESOJO; IGNACIO A. CERDA

Washington

Congreso; The 11th International Congress of Vertebrate Morphology (ICVM)?; 2016

The presence of osteoderms (dermal armor) within the integument is widespread in many anamnian and amniote lineages, hence the skeletogenic potential of the integument has been proposed to represent a case of deep homology of vertebrates. The presence of osteoderms can be seen as a plesiomorphic trait in archosauriform archosauromorphs, constituting a very important (exo)skeletal component, besides the endoskeleton, which is useful for taxonomy and systematics and is known to serve multiple functional aspects. Among archosauriforms, some stem group members (e.g., Doswelliidae, Proterochampsia, and Phytosauria) and many crown group archosaurs (Pseudosuchia, Avemetatarsalia) show often stunning examples of osteoderm formation, such as bucket-sized osteoderms of titanosaurian sauropods, stegosaur plates and spikes, ankylosaur tail clubs, or hand-sized paravertebral shield osteoderms of giant crocodylians. It is surprising that until recently, only few studies (mostly focusing on Avemetatarsalia) on the development, microanatomy, and histology of osteoderms were available, especially when compared to histological studies focusing on long bones. Here, we compare and review the osteoderm histology of several Archosauriformes, including doswelliids and proterochampsians, as well as crown group members (aetosaurs, ?rauisuchians?, and few crocodylians). Results reveal a diversity of histological structures, but the mode of growth of all osteoderms is due to intramembraneous or metaplastic ossification. Besides crocodylian osteoderms, at least some members of the each group presents woven or fibrolamellar bone tissue, indicating faster bone deposition rates, whereas lamellar-zonal tissue in crocodylian osteoderms indicates a reversal to lower growth rates. Further, doswelliid osteoderm histology bears closer resemblance to phytosaur and pseudosuchian than to proterochampsid osteoderms, supporting the hypothesis of closer relationships of the former three taxa.