The earliest dinosaurs were all bipeds, walking on two legs that moved directly forwards and backwards under their bodies (akin to birds and mammals) rather than sprawling out to the sides like lizards and crocodiles.

This, and the fact they walked on their toes with a relatively long stride, would have allowed them to move faster and further without stopping. By contrast, lizards and crocodiles find it hard to breathe and run at the same time because their body undulates from side to side, meaning air has to be shunted between their lungs. So it has been deduced that bipedal dinosaurs possessed more stamina and could run for longer than many of today's reptiles.

While carnivorous dinosaurs (theropods) remained bipedal throughout their evolution - with smaller grasping forelimbs as epitomised by T. rex - semi-quadrupedal and quadrupedal herbivores began to emerge in the Early Jurassic epoch. This has caused some confusion: for example, the early plant-eating prosauropod Plateosaurus was traditionally depicted as quadrupedal, only for a recent anatomical study of its forelimbs to demonstrate that four-legged walking was impossible, since the palms of its "hands" could not rotate far enough to face the ground.

As for those that definitely walked on four legs - including large sauropods such as Apatosaurus (formerly known as Brontosaurus) and Diplodocus - one explanation for their posture is the large "fermenting" stomach they needed to digest tough plant material. This sat in front of the dinosaur's pelvis, and would have caused serious balance problems if they had continued to walk on two legs.

Great weight

In the case of armoured dinosaurs such as Stegosaurus, their heavy plates and spikes demanded a four-legged posture (with a highly arched back) to support the great weight of this armour. And ceratopsians such as Triceratops possessed the largest heads of any terrestrial animal - their extravagant horns and bony protective frill would have been impossible to support on two legs without toppling over, so these dinosaurs evolved a four-legged stance in the Late Cretaceous epoch.

Incidentally, it has been speculated that (on account of their small forelimbs) the largest bipedal carnivores such as T. rex might have struggled to get up if ever they fell over. This is not true - and anyway, their long, thick tails gave them very good balance. This almost three-limbed design made them very stable; much more so, in fact, than humans, who must react constantly to sensory stimulus to keep themselves upright.

Did you know?

The fastest dinosaurs are often said to be the bipedal ornithomimids, "ostrich mimics". However, a University of Manchester study calculated that Compsognathus, one of the smallest dinosaurs ever found, could have matched them by reaching speeds of 40mph (faster than a modern ostrich).