The evolution of the temporal region of placodonts (Diapsida: Placodontia) – a problematic issue of cranial osteology in fossil marine reptiles

The osteology of the temporal region of placodonts is re-investigated. The elements identified as quadrates by previous descriptions of the basal taxa Palatodonta bleekeri and Paraplacodus broilii are re-interpreted to represent entirely or in part the quadratojugal. A new interpretation and reconstruction of the best preserved skull of Paraplacodus broilii is presented. In Placodus, the squamosal and quadratojugal are fused into a compound bone, as proposed by some previous authors. The structure of the upper zygomatic arch in cyamodontoids, which possess a large quadratojugal that even enters the margin of the fenestra supratemporalis, cannot be satisfactorily explained by the current evolutionary scenario, which assumes a total loss of the quadratojugal in basal placodontiforms and basal placodonts and a re-appearance and unique enlargement in the highly derived cyamodontoids. With the new interpretation, this apparent disagreement between proposed phylogenetic relationships within placodonts and osteological observations is resolved. The quadratojugal was never lost in the ancestral line of placodonts or in early members of the group. It probably continuously increased in size in derived placodonts. This led to a strengthening of the single remaining zygomatic arch of these animals. As most peculiar features in the skull of these reptiles, this unusual condition also may represent an adaptation to the shift in diet to durophagy.