Dinosaurs - Chronology - 152 million years ago

The earliest dinosaurs were pretty small. Eoraptor was about 1 m long, its contemporary Herrerasaurus grew no more than 4 m long and Coelophysis was about 3 m 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 9 m 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: it has high vertebral spines over its hip region showing it had strong muscles and ligaments there and it has skid-like bones underneath its tail which would have protected the delicate nerves and blood vessels there when its tail was resting on the ground.

The Morrison Formation
The dinosaurs featured in this episode were all found in the Morrison Formation, except Anurognathus. The Morrison Formation was deposited approximately 155-145 million years ago and covers a large area of the western US, from Montana to Arizona and Utah to Colorado. In different areas and through time Morrison environments were diverse ranging from arid deserts in the southwest through rivers and flood plains to swamps in the north. Fossil plants indicate a rather wetter environment than the rock record and this may mean that the climate was seasonal.

Keep on Growing
In the Early Jurassic the maximum size of both herbiviores and carnivores increased, and 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 12m long Allosaurus was dwarfed by its sauropod prey. They would have had to pick on the young or weak or maybe they hunted in packs.
Evidence based on growth rings and bone texture indicates that sauropods reached their adult size in 10-20 years. Also if, as has been suggested, sauropods took 70 years to reach maturity, it would be unlikley that many would survive to reproduce. The data indicating that they grew quickly also fits that from the bones of theropods and ornithopods, who were thought to reach maturity quickly.

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.