Cimoliasaurus maccoyi (a small plesiosaur)
"Dave" (a plesiosaur)
Umoonasaurus ("Eric") (a small pliosaur)
Kronosaurus queenslandicus (a BIG pliosaur)
Leptocleidus clemai (a small pliosaur)
Platypterygius longmani (an ichthyosaur)
Woolungosaurus glendowerensis (a plesiosaur)
These marine reptiles were not dinosaurs, and were not as closely related to them as were pterosaurs (flying reptiles), crocodiles or birds. Mosasaurs were related to modern monitor lizards such as the Komodo dragon and the Australian goanna. A relative of the mosasaurs may have also been the ancestor of snakes, losing their paddle-like legs through evolution to make them more streamlined swimmers. Snakes and lizards are only very distantly related to the Archosaurs (dinosaurs, birds, pterosaurs, crocodiles) or the other marine reptiles (plesiosaurs, pliosaurs, ichthyosaurs).
The similarity between these extinct marine reptiles and modern marine animals is no accident. Some shapes are just so well adapted to marine living that they are repeated over and over again by creatures that may not be even closely related, a condition known as covergent evolution. All of these marine reptiles were air breathers that had to return to the surface periodically or else drown, like modern whales and dolphins, and most species seemed to have prefered shallow waters rather than deep ocean.