Jurassic ichthyosaurs looked very like dolphins. They even had a dorsal fin and a big vertical tail fluke - we know this from some fossils in Germany which have the body outline preserved as a carbonised film. Ichthyosaurs propelled themselves through the water with strong side to side movements of their tails, steering with their flippers. The rise of Plesiosaurs Plesiosaurs arose in the Late Triassic but became very numerous in the Middle and Late Jurassic. Plesiosaurs used their flippers for their main propulsive force.
Ammonites are very common fossils in Jurassic rocks.
In life these molluscs looked as if a small squid had been stuffed into a spiral shell. When Mesozoic marine reptiles are first found in the fossil record they are already so specialised that it is difficult to trace their ancestry. We have very little idea from which groups of land reptiles ichthyosaurs and plesiosaurs were descended, but this only makes them all the more intriguing. As well as large ichthyosaurs and giant pliosaurs, other reptilian denizens of the Middle-Late Jurassic seas were the marine crocodiles. There were a couple of different types of marine crocodiles one of which was so adapted to life in the seas that their limbs had turned into flippers and they even had a fluke on their tails to help them push themselves more efficiently through the water.