Classification
The closest related predecessor of plesiosaurs were the pistosaurids, which had small heads and long necks. They evolved approximately 220 million years ago during the Upper Triassic, and became extinct 175 million years ago at the beginning of the Jurassic period.
The next group of plesiosaurs were characterized by a large head and a short neck, and they are know as the pliosaurs, the largest specimens in this group included the kronosaurs, the megalneusaurs, plesiopleurodons, and pliosaurs. They had jaws up to 3 m long and could be more than 12–15 m long, weighing 10,000 kg. Isolated vertebrae and teeth found in England could belong to specimens that are thought to have been up to 22 m long. The pliosaurs had large, conical teeth and were the dominant carnivores of their time, pliosaur teeth marks have been discovered on other plesiosaurs, such as cryptoclidus. Pliosaurs evolved about 200 million years ago, during the Lower Jurassic and they became extinct approximately 80 million years ago in the Cretaceous period.
The third group also had a long neck and a small head and they are known as the cryptoclidus. They were shorter and more graceful that the plesiosaurs, but they had necks that were longer than their bodies. Their teeth were small and thin and were possible used to filter their food out of sediment in shallow coastal seas. This group first appeared approximately 160 million years ago at the end of the Jurassic period and survived up until the extinction event at the end of the Cretaceous period, approximately 66 million years ago.
The last group, the elasmosaurus also had a long neck and small head. They were the largest plesiosaurs reaching 13–17 m in length but the majority of this was the neck, they weighed less than the largest pliosaurs. They had more than 72 vertebrae in their necks, more than any other animal. They lived at the same time as the cryptoclidus, so they must have occupied a different ecological niche.
Read more about this topic: Plesiosaur