Dinosaurs habitat
Discussions by Yann Oliver
Up: Dinosaurs lifestyles
Lots of dinosaurs were long thought to live in water, sometimes only because their corpses were carried and fossilised in river beds. It is now clear that they were in fact completely terrestrial, and inhabited even very dry areas (such as middle Asia in the late Cretaceous). The cases for terrestrial sauropods and hadrosaurs are described here, these two groups being the ones for which an aquatic habitat was the most seriously suggested.
There were some other mistakes regarding the habitat of some dinosaurs. For example, Hypsilophodon was thought to be a tree climber, and is now thought of as a fast runner.
The case for terrestrial sauropods
The case for terrestrial hadrosaurs
The case for terrestrial sauropods
The most common argument in favour of aquatic sauropods was that they would have needed water to support their huge body weight. Moreover, the high-situated nostrils (on the top of the skull) led some scientists to the conclusion that they used their necks as a snorkel.
Yet if their chests were to be under more than roughly one metre water, the pressure would have prevented them from breathing (their lungs would have collapsed).
Moreover, the skeleton (especially the vertebrae) of sauropods was greatly lightened by numerous hollow bones.
The limbs do not show the slightest aquatic adaptation; in fact, sauropod limbs are rather similar in shape and functioning to those of elephants.
The second common argument used to conclude that sauropods were aquatic, or at least lived near ponds and lakes, is the small size of their head and mouth compared with their weight. They had to eat lots of plants to feed their body, and they supposedly had to browse nearly all the time, with even no time left to chew, hence they had to eat soft, aquatic plants.
It is highly probable that sauropods used gastroliths (gizzard stones) to help them digest. Gastroliths are a very efficient way of feeding since they allow to quickly ingest high amounts of food, leaving the digestion for later; the digestion time can be spent elsewhere, safe from predators. Sauropods could have housed a big gizzard in their huge belly, whose mastication potential would be impressive. Sauropods are also likely (but we have no evidence for that) to have had, like today\\\'s ruminants, some improved digestive system.
Gastroliths are usually found among crocodiles and birds, which are precisely the closest extant relatives of dinosaurs. The use of gastroliths is even more probable than heaps of polished stones have been found inside or around sauropod carcasses, from a kind of stone sometimes not found kilometres around. Gastroliths are certainly known from some dinosaurs (see Digesting).The finding of such gizzard stones is not very common, but we must keep in mind that birds\\\' and crocodiles\\\' gastroliths are seldom found as well.
Gastroliths were certainly not used to crush soft, aquatic plants. In fact, sauropod teeth, even if then do not seem especially adapted to tough food, show traces of wear (for which the explanation found by aquatic lifestyle supporters is that they sauropods shellfish...).
Furthermore, sauropods had a very little foot surface compared to their size, hence the moist grounds neighbouring lakes, far from helping sauropods support their weight, would have been dangerous to them: they would get easily bogged down.
Thus, we must conclude that sauropods could not have lived in or near water, and that they simply had every adaptation necessary to feed on land, like any other animal.
A sauropod able to walk on land:
Image: sauropod
The case for terrestrial hadrosaurs
Hadrosaurs have long been thought of as completely aquatic animals, for at least four reasons. 1) Their snout superficially resembles that of ducks (hence their nickname: \\\'duck-bills\\\'). 2) One hadrosaur has been found showing what looks like webbed fingers. 3) The hollow crests of some hadrosaurs, connected to the nostrils, could have been used for air storage during diving. 4) Their tails are laterally compressed, like those of other aquatic reptiles.
1) Their mouths are actually filled by huge teeth batteries (up to 2,000 teeth), continuously replaced one by another. This only is sufficient to show that hadrosaurs ate tough plants, not soft and aquatic ones: one hadrosaurs fossil show traces of seed and conifer needles in its belly. The apparently duck-like bills had sharp, cutting edges.
2) The webbed fingers appear in fact to be desiccated hoof pads, more indicating a terrestrial habitat. Furthermore, the fingers are very short and close to each other, contrary to what would be seen on an efficient flipper.
3) The crests are in fact rigid, so they cannot be used in breathing.
4) The tails are completely stiffened by numerous little bones that really prevent any lateral undulation. Moreover, the base of the tail seems too fragile to support the strong muscles needed to drive such heavy animals in water. And, unexpectedly for a swimmer, the tail is highest near the croup, not at the tip.
As well as sauropods, hadrosaurs have relatively narrow feet, preventing a walk on moist grounds. Hadrosaurs actually seem to have been even less adapted to swimming than their terrestrial ancestors.
Up: Dinosaurs lifestyles