Aquatic Dinosaurs

Feature / Palaeontology A prehistoric revolution Many dinosaurs were big – very big. So just how did they manage to support themselves and get around? Brian J Ford thinks he knows – and it could change the way we think of these marvellous prehistoric beasts for ever inosaurs don’t work. They pose a huge problem – the big- gest problem that we have ever encountered on land. The This interpretation of Spinosaurus immense size of the largest genera places impossible load- aegyptiacus, posted on the juras- ings on their extremities. A single limb would have to sup- sicpark forum, is a typical recon- port many tonnes – this would not be compatible with the struction of a large carnivorous Dagility we associate with dinosaurs. We accept the remains of their dinosaur. Each limb would have footprints without demur, although for such gigantic creatures the had to support up to 15 tonnes. It imprints that we observe in rocky strata make no sense. The prints seems incongruous on dry land are roughly as deep in the layers of Liassic mud as ours might have been, although the high mass of an adult dinosaur would cause it to sink up to its knees. The footprints seem to be those of an altogether lighter organism. Children were traditionally fascinated by dinosaurs, and in recent years, new dinosaur discoveries have refocused popular attention. Had you said ‘raptor’ to a teenager a generation ago it would have sig- nified birds of prey; try it later and you’ll find it has become a clawed and vicious creation from Jurassic Park. Once we celebrated Tyrannosaurus rex; now youngsters are as like- ly to know about Spinosaurus and Velociraptor. Books on dinosaurs have flourished like never before, from the great National Geographic Dinosaurs to the Dorling Kindersley Dinosaurs Eye to Eye. Television, always eager to capitalise on any commercial trend, was soon to fol- low. The BBC transmitted Walking with Dinosaurs, and followed that with Planet Dinosaur. There has been an upsurge of enduring interest with the discoveries in North America by Professor Jack McIntosh In this revised image, I have and now in China by Dong Zhiming, so publishers and programme shown it immersed in water to producers are sinking millions into the subject. the shoulders. The tail becomes Animatronic dinosaurs were not just manufactured, but went on buoyant and the mass of the tour like rock stars. The first such exhibition was of concrete dino- body is supported by the water. saurs made by Benjamin Waterhouse in 1854 for the Crystal Palace The loading per limb would be exhibition. Waterhouse was personally instructed by Sir Richard less than 1 tonne – and would Owen, the greatest dinosaur expert of the Victorian era. And now be reduced to zero if the dinosaur floated there are computer games like Dino Hunt – there is no end to the in water like a present-day crocodile current fascination for the great dinosaurs 24 / April 2012 / labnews.co.uk Palaeontology / Feature A prehistoric revolution Yet dinosaurs make no sense. The most obvious problem to me so each limb must support some 2.6 tonnes. Of the rhinoceros spe- is the sheer mass of the dinosaur and the modest plantar area of the cies, Ceratotherium simum is the largest and weighs up to 4.5 tonnes. THE AUTHOR lower limbs. Compounding this is the bipedal stance that is typical Each limb thus has to support 1.5 tonnes. The adult hippopotamus Professor Brian J Ford of these gigantic creatures. Today’s heaviest land creatures are the (Hippopotamus amphibious) weighs some seven tonnes and thus elephant, the hippopotamus and rhinoceros; they sensibly spread the each limb comfortably supports a weight up to 2.3 tonnes. load across four limbs rather than two. This is an obvious evolution- The effects of immersion in water are dramatic. Animal tissues, ary necessity. And it is worse – dinosaurs seemingly compound the being predominately composed of water, may be regarded as of problem by possessing a tail that is proportionately massive. It is not neutral density for this discussion and a large mammal that is 90% clear to me how one would calculate the expenditure of metabolic en- below the water level will exert proportionately reduced loading on ergy required to hold such a huge member horizontal and clear of the the limbs – from about eight tonnes to 800 kg for the elephant, from ground, but the lack of tail drag-marks associated with footprints in seven tonnes down to 700 kg for the hippo. Similarly, a large croco- fossiliferous strata substantiates my view that little ground contact for dile that is scarcely able to stand on its own feet on land becomes the tail was normal for dinosaurs. In every reconstruction, they hold effectively weightless in water. In the real world, a crocodile is most their massive tails aloft. This would have consumed a high propor- typically observed with only the top of the head exposed. In terms of tion of the energy input from their diet. It runs counter to evolution- metabolic efficiency, walking on land for a crocodilian can be seen as The physics of the brachiosaurs ary theory. a costly indulgence. makes far more sense in an aquatic We thus have a set of factors that makes the largest dinosaurs seem setting. This reconstruction by impracticable as a product of evolution. They are massive, whereas We now turn to dinosaurs. Volumetric analysis of scale models Richard Hubbert fits the current their surviving spoors suggest that they cannot have been so heavy or provides a helpful indication of the effects of an aquatic habitat on proposals well, though one hopes they would have sunk into the mud across which they walked. They the physics of a large extinct reptile like T. rex. The mass of an adult that readers will turn a blind eye to also developed colossal caudal structures which, in conventional is believed to be as much as much as seven tonnes. I find that the the angiosperms in this view portrayals, are held aloft and do not drag on the ground. Finally, and head and shoulders occupy roughly 15% of the volume of the crucial to my understanding of how dinosaurs are supposed to have entire animal. Partial immersion in the aquatic surroundings that functioned, they typically evolved to walk erect, thus forcing the hind I now postulate sets the figures into an interesting context; the limbs alone to bear the burden. An adult T. rex is reckoned to have loading per limb is reduced to 1050 kg in total, or 525 kg per had a mass of up to seven tonnes, which must have been supported limb when standing. The largest dinosaur yet discovered, Bru- by a single limb when the animal moved. hathkayosaurus, was a quadruped weighing some 120 tonnes. Each limb must have supported 60 tonnes when standing, 80 We may find substantiation of my views on the role of evolutionary tonnes when walking. Yet this creature, if immersed in water pressures in the development of load-bearing anatomy by looking so that only its head and neck are exposed, would have exerted at today’s largest land animals. The elephant and, to a lesser extent, less than five tonnes per limb when standing on all fours, ris- the hippopotamus conform exactly to the argument I propose. Their ing to 6.6 tonnes when walking. great mass is supported by four legs rather than two. They have tiny The anatomical adaptations we see in these species are con- tails that impose no metabolic burden. Crucially, the trunk of an sonant with that I propose: they have a large and elephant is an additional member whose weight has to be supported bulky body with a huge and muscular tail. The and you could analogise the elephant’s trunk to the tail of a dinosaur mass of the abdomen is immaterial when it is – yet it normally hangs downward and consumes little metabolic customarily submerged, whereas the nature energy for support. Indeed, the hippopotamus and (to a lesser extent) of the tail fits well with its use as a powerful the elephant find one practical answer to supporting their body mass, organ of propulsion and steering for a swim- for they often resort to partial immersion in lakes or rivers where the ming dinosaur. displacement of water can help to reduce the load on their limbs. The The diminished forelimbs are equally well hippo is now categorised as a semi-aquatic mammal. accounted for by this view. In land animals, Although both of these creatures have diminutive tails, there are like the elephant, hippo and rhinoceros, other large animals that are equipped with examples of dinosaur-like each of the four limbs is load-bearing proportions – present-day reptiles. Monitor lizards, culminating in and evolutionary pressure has been the Komodo dragon (Varanus komodoensis) have substantial tails against the reduction in size that though even the largest is far smaller than a dinosaur’s. Dinosaur we observe in flesh-eating dino- footprints are those of a walking creature and there is no sign of the saurs like Tyrannosaurus. If the tail touching the earth. I have observed monitor lizards in Borneo, large dinosaurs are conceived as Central America and elsewhere and they do not always hold the tail primarily aquatic, however, then free from the ground. The trail of the Komodo dragon is charac- the specialisation of the forelimbs terised by a deep groove left by the tail dragging along the ground. would be towards manipulative dex- Crocodilians, notably the salt-water crocodile (Crocodylus porosus) terity.

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