Skeletal histology of the dermal armor of Placodontia: the occurrence of 'postcranial fibro-cartilaginous bone' and its developmental implications.

Placodontia (Reptilia: Sauropterygia) is a group of enigmatic armored marine reptiles restricted to the Triassic time period. Only a single row of osteoderms dorsal to the spine is present in the basal placodontoid Placodus gigas, whereas derived cyamodontoids superficially resemble turtles in enclosing their body in an armor shell. Despite the extensive occurrence of the dermal armor in the derived cyamodontoid group, little research has focused on its bone histology and development. Here, I present an overview of the bone microstructures that reveals the unique presence of cartilaginous tissue in the postcranial armor plates. Placodont armor plates stand in contrast to osteoderms of other tetrapods that develop intramembraneously or through metaplastic ossification without cartilaginous preformation. The different developmental pathways leading to this 'postcranial fibro-cartilaginous bone' tissue found in placodont plates compared to the dermal bone tissues of most other tetrapod osteoderms indicate the non-homology of these structures. A resulting morphogenetic model of histogenesis is given to exemplify how the derived armor morphologies (i.e. spiked, flat polygonal and hexagonal, and rhomboidal shapes) together with the peculiar bone histologies could have developed through differential growth. In accordance with the pachyostotic limb bones of placodonts, the presence of the compact 'postcranial fibro-cartilaginous bone' is interpreted as an osteosclerotic trend in the armor plates which aids in buoyancy control and affects maneuverability and swimming speed.