European origin of placodont marine reptiles and the evolution of crushing dentition in Placodontia

Sauropterygia was the most successful marine reptile radiation in history, spanning almost the entire Mesozoic and exploiting a wide range of habitats and ecological niches. Here we report a new, exceptionally preserved skull of a juvenile stem placodont from the early Middle Triassic of the Netherlands, thus indicating a western Tethyan (European) origin for Placodontia, the most basal group of sauropterygians. A single row of teeth on an enlarged palatine supports this close relationship, although these are small and pointed instead of broad and flat, as is the case in placodonts, which demonstrate the strongest adaptation to a durophagous diet known in any reptile. Peg-like, slightly procumbent premaxillary teeth and an ‘L-shaped’ jugal also confirm a close relationship to basal placodonts. The new taxon provides insight into the evolution of placodont dentition, representing a transitional morphology between the plesiomorphic diapsid condition of palatal denticles and the specialized crushing teeth of placodonts.