Jurassic period

Types of fossils formed in this period

Time of the titans - dinosaurs of the Jurassic

By Dr Jo Wright

At the beginning of the Jurassic, dinosaurs started getting bigger. As vegetarians grew in size, so did the creatures that hunted them. Dr Jo Wright describes life in the time of the titans.

Late Jurassic Earth

The earliest dinosaurs were pretty small.Eoraptor was about one metre long. Its contemporary Herrerasaurus grew no more than four metres long and Coelophysis was about three metres long. And in all cases the length was mostly tail.

However the plant-eating prosauropod, Plateosaurus, that appeared at the end of the Triassic period, was a harbinger of things to come. At up to nine metres long it was the first really big dinosaur.

Prosauropods could walk either on all fours or just on their hind legs, leaving their hands free, perhaps to grasp branches and bring them within reach of their mouths. They disappeared at the end of the Early Jurassic period and their role was taken by the sauropods which thrived during from the Middle Jurassic to the Early Cretaceous period.

Sauropods had huge elephantine bodies coupled with very long necks and tails. They walked only on all fours. Some such as Diplodocus could probably have reared up on their hind legs using their tails as props.

Diplodocus skeletons show a number of features that support this: They have high vertebral spines over the hip region, showing the creatures had strong muscles and ligaments there. They also have skid-like bones underneath their tails, which would have protected delicate nerves and blood vessels when their tales were resting on the ground.

Keep on growing

In the Early Jurassic the maximum size of both herbivores and carnivores increased. This trend continued throughout the Jurassic culminating in the staggeringly large sauropods such as Apatosaurus and Brachiosaurus. Carnivores had also increased in size, although not by as much and the 12 metre long Allosaurus was dwarfed by its sauropod prey. Allosaurus would have had to pick on the young or weak or may have hunted in packs.

Evidence based on growth rings and bone texture indicates that sauropods reached their adult size in 10-20 years. If, as was once suggested, sauropods took 70 years to reach maturity, it would be unlikely that many would survive to reproduce. The data suggesting that they grew quickly also fits that from the bones of theropods and ornithopods, who were thought to reach maturity fast.

Impressive plates

Stegosaurus was an early armoured dinosaur and its defences were formidable. Later armoured dinosaurs were veritable living tanks - some even had armoured eyelids!

There are two main types of dinosaurs, named from the configuration of their pelvic bones: bird-hipped and lizard-hipped. Sauropods and theropods are lizard-hipped dinosaurs; Stegosaurus is a bird-hipped dinosaur, an early member of a group which became much more common and diverse in the Cretaceous period.

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.

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 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.