A cruel sea - when reptiles ruled the waves

The Mesozoic era truly was the age of giants. While huge dinosaurs dominated the land, large marine reptiles ruled the seas. Dr Jo Wright describes life in Jurassic seas.

Late Jurassic Earth

The Ichthyosaurs (fish reptiles) appeared much earlier than the dinosaurs. They are first found in the Early Triassic, and they are already very specialised, with limbs modified into flippers.

	Ophthalmosaurus - Large eyes help it hunt in deep, dark waters

By the 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. They used their flippers to move through the water. There were two kinds of plesiosaurs. The large short-necked pliosaurs were the top predators of Jurassic seas. Long-necked plesiosaurs probably fed on small fish and other small prey. We known they ate squid-like animals because parts of belemnites have been found in their fossilised stomachs.

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, 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.

	Ammonites - Large specimens weighed up to 100 kg

Ammonites are very common fossils in Jurassic rocks. When they were alive, these molluscs looked as if a small squid had been stuffed into a spiral shell. They were very successful in the Jurassic although they were probably rather slow moving; we know from fossil stomach contents that they ate crinoids (sea lilies) - animals that are attached to the seabed.

Still a mystery

Mesozoic marine reptiles are already so specialised when they are first found in the fossil record that it is difficult to trace their ancestry. We have very little idea which groups of land reptiles ichthyosaurs and plesiosaurs descended from, but this only makes them all the more intriguing.

This article was written to accompany episode three of Walking with Dinosaurs.