Another problematic group of early marine reptiles are the pachypleurosaurs – the lizards with thick ribs, fhey were once thought to have been a part of the nothosaur group but arc currently believed to be separate from them, and more primitive. The heads were short and the necks were long.
Dactylosaurus was the smallest of this group of small reptiles.
Features: The distinctive feature of Dactylosaurus is the shape of the humerus, indicating that the muscle attachment would mean a different swimming action from its contemporaries. There is a reduced number of bones in the flippers, showing a specialization to a swimming mode of life. However the primitive nature of the rest of the skeleton indicates that it was at best semaquatic. Like the other pachy pleurosaurs. the ribs are thick (hence the name of the group), to help to control the buoyancy while submerged.
Originally thought to have been a pachypleurosaur Corosaurus is now regarded as the most advanced nothosaur so far known. It lived in quiet waters, as shown by the presence of stromatolites (mineral mounds built up by algae and bacteria) in the same shallow marine strata. It is possible that these mounds may have made suitable nesting areas for these reptiles.
Features: The shoulder and hip girdles of Corosaurus are very similar to those of the early plesiosaurs. The paddles are also highly adapted, although not quite in the plesiosaur manner.
They would have been ideal for paddling through quiet waters, but less suited for clambering about on land.
Nevertheless, Corosaurus probably spent some time on land, basking or laying eggs.