In the early days of palaeontology, the dinosaurs were a clear anomaly compared to other known animals (both alive and extinct). Just how could such huge creatures have found enough to eat, or even supported themselves when on land? Misunderstanding about their limbs (many people had them as lizard-like sprawlers, rather than the upright posture we now know they possessed) didn’t help, but discoveries of giants such as Diplodocus and Apatosaurus led to speculative ideas on what such bulk would mean for their biology.

One popular idea was that the larger dinosaurs lived primarily in rivers, lakes and swamps, using the water to buoy them up and quite literally take the weight off their feet. Various aspects of the anatomy of some were supposed to support this – the nostrils at the top of the head of Brachiosaurus were thought to let it breathe while it was otherwise submerged, and the deep tails of the hadrosaurs were supposed to help them swim. This general hypothesis for aquatic living was actually rather quickly dismissed in some quarters but has dragged on interminably, and even now one can still buy books that show big dinosaurs lolling in deep water and this idea also comes back around in the media periodically as well.

Certainly some dinosaurs spent quite some time in and around water. Spinosaurus and its relatives ate fish, and show isotopic signatures in their teeth that indicate they spent a lot of time in water. Plenty of dinosaurs are preserved in the sediments from ancient rivers and lakes, and while this might well have been at least in part from floods, or animals dying near water (to be preserved you generally have to be buried and that often means water), some must have been hanging around bodies of water regularly to be buried. The armoured ankylosaurs especially seem to have favoured estuaries and coastlines given how often their bones are found in deposits laid down in the sea, and others appear to have died on a shoreline or may even have drowned when crossing a river.

Aside from the obvious problem of finding enough 50 foot deep lakes for big dinosaurs to live in (in those old artworks every lake is conveniently just deep enough for them to have their heads on the surface), the biggest problem with the aquatic dinosaurs idea is that they would float! It is hard to walk around on the bottom of a lake if you are on the surface. Pretty much all tetrapods are less dense than water and you can only get so deep before your feet lift off the bottom and dinosaurs were no different. In fact the largest dinosaurs, the sauropods (these include things like Diplodocus) has numerous air-filled bones and might have floated quite high in the water.



Dinosaur footprints can tell us about their locomotion both on land, and occasionally, in water as well as revealing details about the anatomy of the foot. Photograph: Douglas C Pizac/AP Photograph: DOUGLAS C. PIZAC/AP

Even so, this would not inhibit their ability to move around when in water and it is safe to assume most, if not all, dinosaurs could swim. Very few animals are incapable of propelling themselves through the water (even if very poorly) and dinosaurs presumably did so as well. Although a number are highly controversial, there are at least some fossil tracks that show dinosaurs moving around in water. Footprints left in water show clear tracks and every toe is clearly in evidence, and then over a number of steps the prints become less and less deeply impressed in the substrate until finally there are just the tips of the claws showing, and then a return of the toes and finally whole feet. This would occur as the animal waded out, swum a few strokes with the toes barely touching down, and then on reaching the other side or perhaps a sandbar, the water was shallow enough for it to walk again and the tracks return.

Dinosaurs did then hang around water, and could swim, but how about becoming specialised as aquatic animals, such as hippos, otters or even crocodiles? Here there are few credible candidates (the spinosaurs might be one, but this is controversial), since there are a number of consistent characteristics we would expect to see in water-living animals, and they simply are not present in dinosaurs. Swimming and diving animals generally have their nostrils and eyes high up on their heads so they can see and breathe while still otherwise submerged, they generally have a streamlined body to reduce drag, a flattened and flexible tail for propulsion, and broad and spreading toes (generally webbed too) to help them swim.

Aquatic and semi-aquatic animals also tend to have more dense bones than other animals so that they do not float so high in the water, and as noted already, most dinosaurs have less dense bones than other vertebrates, so that is immediately a big strike against them being aquatic. Some dinosaurs do appear to have had retracted nostrils, but in the absence of any other features, (including raised eyes) this is hardly a great argument. Similarly, many did have deep tails, but these were not the flexible tails of swimmers, but bound together with strong tendons that would have limited lateral movement. Some also have well-spread toes, but there is no good evidence of webbing between them, and animals like hadrosaurs had hooves that would be dreadful in water or on soft ground, but ideal for walking on relatively hard surfaces.

Plenty of aquatic reptiles were in the water during the time of the non-avian dinosaurs such as this dolphin-like icthyosaur Photograph: Alamy Photograph: Alamy

The waters of the Mesozoic were teeming with reptile life though, both in the seas and freshwater environments. In addition to crocodilians and turtles and terrapins of various kinds, and more familiar animals like plesiosaurs and ichthyosaurs, at different times a huge raft of other reptiles took to the water including placodonts, phytosaurus, thalattosaurs and nothosaurs. That the non-avian dinosaurs were not aquatic animals, and do not appeared to have ever produced a real aquatic form, does seem like an oddity given their long period of life on Earth (mammals have produced the whales and their relatives, dugongs and manatees, and semi-aquatic forms like seals, otters, tapir, shrews, hippos and more all in the last 50 or so million years) though perhaps they faced strong competition at various times and simply never made it.

In short, dinosaurs were like the vast majority of terrestrial animals around today. Their habitats would have included water in the forms of rivers, lakes, and the sea, and numerous species would have needed to enter or cross bodies of water regularly. While many would have likely been capable swimmers, they were not aquatic animals spending the vast majority of their time in water, didn’t primarily eat water plants (as is seen by their strong teeth and stomach contents), and it is unlikely they used the water as a safe haven from predators. Until the birds took to the air (and indeed ventured into the water again), dinosaurs were very much creatures of dry land.