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physical evidence for such features. However, given how | specialized creatures such as dinosaurs can be, when compared to living crocodiles and birds, this approach must be =

used cautiously. g design (shape and arrangement of the bones) of the skeleton or skull, and the influence that these would have on the distribution and functioning of the muscles. Such reconstructions also need to account for such factors as the proposed method of locomotion. For example, the details of the joints between the limb bones, a consideration of the simple mechanics associated with the positioning and range of movement of the limbs that was possible at each limb joint; and, in some cases, the real evidence left behind by dinosaurs in the form of fossilized tracks that indicate how they really did move around when alive.

While examining many bony fragments of Iguanodon in the collections of the Natural History Museum in London, an unusual specimen caught my eye. It consisted of the battered remains of a

Iguanodon Tracks

25. Left: oblique view of the natural cast of the brain cavity of Iguanodon. Right: line drawing of the brain cavity showing ear structures, nerves, blood vessels and olfactory lobes.

large, partial skull. A few teeth exposed in its upper jaw betrayed that it was indeed Iguanodon, but beyond that it seemed useless anatomically. For interest's sake, I decided to cut the specimen in half to see if any of its internal anatomy was better preserved. What was revealed proved to be unexpectedly interesting and exciting. Although the bones were battered and eroded, it was clear that this skull had been buried in soft, silty mud that had seeped into all the spaces. The mud had hardened (lithified) to a concrete-like consistency over millions of years. The lithification process was so complete that the mudstone had become impermeable so that ground-water containing minerals was unable to seep through the rock and mineralize the skull bones; as a result the bones were relatively soft and crumbly.

This peculiar preservation offered an unusual opportunity to N

explore skull anatomy. Careful removal of the crumbly skull bones I (rather than the hard mudstone matrix) revealed the shape of the t internal spaces in the skull as a natural mudstone cast (Figure 25). I It included the cavity where the brain had lain, the passages for the inner ear, and many of the blood vessels and nerve tracts that led to ¡^ and from the brain cavity. Given that this particular animal had died approximately 130 million years ago, it does seem remarkable that it should prove possible to reconstruct so much of its soft anatomy.

Iguanodon and dietary adaptation

The first recognizable fossils of Iguanodon were teeth, whose telltale features showed that it was a herbivorous animal; they were chisel-shaped to be able to slice and crush plants in the mouth before they were swallowed.

The need to cut and crush plant food hints at some important considerations concerning the diets of extinct creatures and some of the clues that their skeletons may contain.

Iguanodon's brain

The structure of the brain cavity shows large olfactory lobes at the front, suggesting that Iguanodon had a well-developed sense of smell. Large optic nerves passed through the braincase in the direction of the big eye sockets, apparently confirming that these animals had good vision. The large cerebral lobes indicate a well coordinated and active animal. The inner ear cast shows the looped semicircular canals that provided the animal's sense of balance, and a finger-like structure that was part of its hearing system. Beneath the brain cavity hangs a pod-like structure that housed the pituitary gland, which was responsible for regulating its hormone functions. Down either side of the cast are seen a series of large tubes, which represent the passages through the original braincase wall (chipped away here of course) for the twelve cranial nerves. Other smaller pipes and tubes passing through the braincase wall are also preserved, and these hint at the distribution of a set of blood vessels that carried blood into the floor of the brain from the heart (via the carotid artery) and, of course, drained the blood away from the brain through the large lateral head veins that lead back down the neck.

Carnivores have a diet largely comprising meat. From a biochemical and nutritional perspective, a diet of meat is one of the simplest and most obvious of options for any creature. Most of the other creatures in the world are made of roughly similar chemicals as the carnivores that eat them. Their flesh is therefore a ready and rapidly assimilated source of food, provided the prey can be caught, sliced into chunks in the mouth using simple knife-like teeth (or even swallowed whole), and then quickly digested in the stomach.

This whole process has the potential to be relatively quick and biochemically very efficient in that little is likely to be wasted.

Herbivores face a rather more challenging problem. Plants are neither particularly nutritious nor readily assimilable when compared to animal flesh. Plants are primarily built from large quantities of cellulose, a material that gives them strength and rigidity. The crucial, and extremely awkward, point about this unique chemical, so far as animals are concerned, is that it is completely indigestible: there is simply nothing in the armoury of chemicals in our guts that can actually dissolve cellulose. As a result, the cellulose portion of plants passes straight through animals' guts as what we call roughage. So, how do herbivores survive on what appears to be such an unpromising diet?

Plant-eaters have successfully adapted to this diet because they | exhibit a number of characteristic features. They have a good set ; of teeth with hard-wearing, durable, complex, and rough grinding o surfaces, and powerful jaws and muscles that can be used to grind §

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