Early taxonomy of Plesiosaurs was
based on body proportions such as the relative size of the skull and the
relative length of the neck. The term Pliosaurus was intoduced in 1841
to include Plesiosaurs with larger heads and shorter necks, as opposed
to Plesiosaurus with a longer neck and smaller head.  These
two genera now stand as types for two superfamilies, the Plesiosauroidea
and the Pliosauroidea. The Plesiosauroids are divided into two groups,
the Plesiosauridae with up to 28 cervical vertebrae, and the Elasmosauridae
with 32 to 76 cervical vertebrae (Callaway and Nicholls,
1997). |
