ABSTRACT
Placodonts are a clade of small to medium-sized marine reptiles from the Triassic that are characterized by crushing dentition (hence the name of the clade) and more or less extensive dermal armor in the postcranium. Both the dermal armor plates and endoskeletal elements have been sampled histologically. The dermal armor plates grow centrifugally around a growth center and usually comprise an intricate meshwork of structural fibers close to the center, surrounded by parallel-fibered bone. In rare cases, some cartilage-like tissue was found that raised questions about the homology between these placodont armor plates and osteoderms found in other extant and extinct reptiles. Long bone histology revealed a range of bone tissues from weakly vascularized lamellar-zonal bone to highly vascularized fibrolamellar bone. This variation reflects a spectrum of lifestyles, growth patterns, and swimming modes in placodonts, including sluggish near-shore dwellers to more active open-water inhabitants.
