Dinosaurs - Chronology - 149 million years ago

The Mesozoic era truly was the age of giants. While there were huge dinosaurs dominating the land there were also large marine reptiles ruling the seas. Ichthyosaurs (fish reptiles) actually appeared much earlier than the dinosaurs; they are first found in the Early Triassic, and they are already very specialised, clearly recognisable as ichthyosaurs, with limbs modified into flippers. 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. There were two kinds of plesiosaurs - the large short-necked pliosaurs which were the top predators of Jurassic seas and the long-necked plesiosaurs which probably fed on small fish and other small prey; we known they ate belemnites (squid-like animals) - because parts of belemnites have been found in the fossilised stomach contents of plesiosaurs.

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. They were very successful in the Jurassic although they were probably rather slow moving; we know from from fossil stomach contents that they ate crinoids (sea lilies) - animals that are attached to the sea bed.

Still a mystery
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.

The animals in this episode are based mainly on fossil finds from the Oxford Clay. The Oxford Clay was deposited in a shallow marine environment when the British Isles and part of Western Europe was an archipelago.