Plesiosaur Family Tree

I posted this at Lake Monsters and so I thought I should post it here. Darren Naish made the base chart of this BUT he threw some rather puzzling "Leftover" Longnecked Plesiosaur fossil genera in with the Elasmosaurs mostly because he did not seem to want another family. Those genera were formerly stated said to be a prolongation of the Cryptoclidid family by the standard references and the family would include the putitive genus of surviving post-Cretaceous Plesiosaurs. It is marked with an asterisk (*) on the chart above. The reason to draw attention to this group and to make a fuss is because the structure of the neck is entirely different from the Elasmosaurs, and lumping them in with the Elasmosaurs suggests their necks were similarly stiff and inflexible. The key point is that the anatomy of the vertebrae was different, the cross-section of the vertebral centrum was different, the neck was much shorter overall and very likely was considerably more flexible
http://en.wikipedia.org/wiki/Aristonectidae
Because the vertebrae of this group was known first, Edward Drinker Cope had at first assumed that the vertebrae of Elasmosaurus were reversed back to front in order to have a comparable anatomy. This soon became an obvious error. But it goes to show you that the anatomy of the vertebrae were really very much different in comparison