Time of the titans - dinosaurs of the Jurassic

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

	Diplodocus walked on four legs, but may have reared up onto two

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.

	Stegosaurus had display plates its back

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.

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