How to recognise them

Vertebrae are thick. 

Fossil Info

Plesiosaurs are marine reptiles from 1.5 - 15 m long with long flattened bodies. They arose in the Late Triassic about 200 million years ago and went extinct at the end-Cretaceous mass extinction. Their limbs were modified into large flippers and they were carnivores. They fall into two groups with distinct body shapes: some had extremely long necks (and so look very different to ichthyosaurs) and were slower swimmers, but others had shorter necks and are thought to have been faster swimmers. 

Fun Fact

Plesiosaurs swam using their flippers like sea lions, undulating up and down, whereas ichthyosaurs swam by undulating from side to side like a shark.

Plesiosaur - photo by Ghedoghedo - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=28947499