Carentonosaurus

Carentonosaurus had very thick, heavy bones in its back, a condition called pachyostosis, which helped it stay underwater without floating up.1

Its neck bones were not thickened, only the back bones were pachyostotic, which is unusual among marine lizards.1

This lizard lived about 100 to 94 million years ago, during the Cenomanian stage of the Cretaceous Period.3

Carentonosaurus fossils have been found in France, Portugal, and Spain, showing it lived in the warm Tethys Sea.2

It is part of a group called aigialosaurs, which are close relatives of the giant mosasaurs.3

The first fossils were discovered in western France, in rocks that were once a shallow, warm sea near a coast.1