Discussions by Yann Oliver
Up: Dinosaurs lifestyles
Dinosaurs walked with an erect stance and did not left their tails dragging. The bipedal ones held their backs horizontally and some quadrupedal ones could stand up using their tails as a prop. Some but not all were able of very fast running. The armoured dinosaurs probably showed a very active defence pattern, hitting with their tails or charging.
Dinosaurs\\\' most striking anatomic feature when compared to the sprawling lizards is the nearly vertical position of their limbs, like that of mammals and birds (though there were early attempts to reconstruct some quadrupedal dinosaurs sprawling). This is unambiguous from fossil tracks and anatomical analysis.
It has also been suggested that ceratopians had vertical hind limbs and sprawling forelimbs since they were descended from bipedal animals, but they now seem to have had vertical forelimbs. See Ceratopian gait).
It now seems that, contrary to what has long been thought, no (or very few) dinosaurs left their tails dragging on the ground, were they two- or four-legged (even sauropods). This is clear from the fossil tracks, and from the stiffened tail of lots of species. In particular, the bipedal ones had a stance less similar to that of a standing kangaroo than to that of an ostrich, holding their backs horizontally, which is much more efficient for balancing.
However, the kangaroo-like posture seems to have been used by lots of quadrupedal forms while feeding. For instance, stegosaurs and certain sauropods seemingly had the ability to pivot up on their hind limbs, going from a quadrupedal stance with tail aloft to a bipedal stance with tail as a prop, in order to catch a higher vegetation, as is shown from the high apophyses above their hips. The short forelimbs of stegosaurs, previously often thought of as a misadaptation, would have helped this mechanism.
All ornithopods were traditionally thought to be two-legged. But the heaviest of them (iguanodonts, hadrosaurs) were more likely quadrupedal, with occasional rearing; the more than they had hoofs on the hands.
Last point regarding the posture: most dinosaurs had an S-shaped neck. In hadrosaurs however, the upper backbone was strongly incurved downwards, and their heads were hold quite low for browsing. The position, vertical or horizontal, of the sauropod necks has been highly controversial.
The speeds computed from fossil tracks are generally rather low (a few kilometres per hour); however, tracks generally indicate the usual speed of a calmly walking animal, and not its maximum running speed. Yet there are a few tracks showing very high speeds (those of a galloping horse), probably from a theropod.
The speed evaluation from anatomical parameters depends largely on the weight of the animal (which can triple depending on the restoration and assumed internal anatomy) and on the shape of the limbs. The heaviest dinosaurs efficiently used cartilage to cushion mechanical constraints. Some dinosaurs (for instance ceratopians) with longer hind- than forelimbs could have had mobile shoulder blades, used to increase their stride.
Theropods, even quite large, were lightly built and probably ran very fast (like most predators). The speed of the largest of them (particularly tyrannosaurids) is subject to caution: there are no extant bipedal animals we could use as a model.
Little ornithopods could run fast too, whereas the heavier ones like iguanodonts and hadrosaurs could not.
Sauropods, stegosaurs and ankylosaurs were quite slow (sauropods limbs are comparable to those of elephants and they could have walked the same way).
Ceratopians had short, very strong limbs, which allows rapid accelerations together with a reasonable top speed, hence they could charge their enemies (see also Ceratopian gait). The same may be true for some ankylosaurs.
In any case, all dinosaurs were able to move around at a medium speed without needing water to support their weight.
Other rapid movements include the ability of stegosaurs, ankylosaurs and diverse sauropods to use their tails as weapons, controlled by strong hip muscles (moreover, stegosaurs had a very flexible tail, a unique feature among ornithischians); stegosaurs were helped in quick turning round their hind limbs by the small size of their forelimbs and the back position of their centre of gravity.
Up: Dinosaurs lifestyles