|Unit: Reptiles|| 5 Subclass Parapsida|
Subclass ParapsidaThese are reptiles with one temporal fossa, placed high up on the skull (see figure
A number of different forms of aquatic reptile showed this form of skull organisation, such as the Protosaurs, Nothosaurs and Placodonts; but we will only deal with the two largest groups - the ICHTHYOSAURS
and the PLESIOSAURS.
These two lines of reptiles became modified for aquatic life in quite different ways, and it is possible that they are not at all closely related. They share the same basic type of skull organisation, but there are a number of minor differences in skull construction which suggest that they may have developed from separate CAPTORHINIDA
ancestors, and accidentally developed the same general type of skull.
Both the ichthyosaurs and the plesiosaurs became extinct at the end of the Cretaceous*
, when many other lines of terrestrial reptiles died out. There were some changes in the fish and invertebrate faunas at the end of the Cretaceous, which may have contributed to their extinction. But the sea is a more stable environment than the land, and it is not at all clear what factors were responsible for the extinction of such very successful reptiles.