The traditional view of dinosaurs

Throughout the earlier part of the 20th century, it was widely (and perfectly reasonably) assumed that dinosaurs were a group of extinct reptiles. Admittedly, some were dramatically large or rather outlandish-looking compared to modern reptiles, but they were crucially still reptiles. Richard Owen (and Georges Cuvier before him) had confirmed that dinosaurs were anatomically most similar to living reptiles, creatures such as lizards and crocodiles. On this basis it was inferred, logically, that most of their biological attributes would have been similar, if not identical, to those of these living reptiles: they laid shelled eggs, had scaly skins, and had a 'cold-blooded', or ectothermic, physiology.

To help demonstrate that this view was correct, Roy Chapman Andrews had discovered that Mongolian dinosaurs laid shelled eggs, and Louis Dollo (among others) had identified impressions of their scaly skins; so their overall physiology would be expected to resemble that of living reptiles. This combination of attributes created an entirely unexceptional view of dinosaurs: they were large, scaly, but crucially slow-witted and sluggish creatures. Their habits were assumed to be similar to those of lizards, snakes, s

and crocodiles, which most biologists had only ever seen in zoos. | The only puzzle was that dinosaurs were mostly built on a far S

grander scale compared to even the very biggest of known s crocodiles. n

There were many depictions of dinosaurs in popular books, and scientific ones, wallowing in swamps, or squatting as if barely able to support their gargantuan bodies. Some particularly memorable examples, such as O. C. Marsh's Stegosaurus and Brontosaurus, reinforced these conceptions. Both had enormous bodies and the tiniest of brains (even Marsh remarked in disbelief at the 'walnut-sized' brain cavity of his Stegosaurus). So lacking in brainpower was Stegosaurus that it was deemed necessary to invent a 'second brain', in its hip region, to act as a sort of back-up or relay station for information from distant parts of its body, thus confirming the 'stupid' and 'lowly' status of dinosaurs beyond reasonable doubt.

While the weight of comparative evidence undoubtedly sustained this particular perception of the dinosaur, it ignored, or simply glossed over, contradictory observations: many dinosaurs, such as little Compsognathus (Figure 14), were known to be lightly built and designed for rapid movement. By implication they should have had rather un-reptile-like levels of activity.

Armed with this battery of prevailing opinion and Ostrom's observations and interpretations based on Deinonychus, it is easier to appreciate how this creature must have been challenging his mind. Deinonychus was a relatively large-brained, fast-moving predator capable of sprinting on its hind legs and attacking its prey - common sense said that this was no ordinary reptile.

One of Ostrom's students, Robert Bakker, took up this theme by aggressively challenging the view that dinosaurs were dull, stupid creatures. Bakker argued that there was compelling evidence that dinosaurs were more similar to today's mammals and birds. It M should not be forgotten that this argument echoes the incredibly | far-sighted comments made by Richard Owen in 1842, when he £ first conceived the idea of the dinosaur. Mammals and birds are

regarded as 'special' because they can maintain high activity levels that are attributed to their 'warm-blooded', or endothermic, physiology. Living endotherms maintain a high and constant body temperature, have highly efficient lungs to maintain sustained aerobic activity levels, are capable of being highly active whatever the ambient temperature, and are able to maintain large and sophisticated brains; all these attributes distinguish birds and mammals from the other vertebrates on Earth.

The range of evidence Bakker used is interesting when considered from our now slightly more 'tuned' palaeobiological perspective. Using the anatomical observations made by Ostrom, he argued, in agreement with Owen before him, that:

i) Dinosaurs had legs arranged pillar-like beneath the body (as do mammals and birds), rather than legs that sprawl out sideways from the body, as seen in lizards and crocodiles.

ii) Some dinosaurs had complex, bird-like lungs, which would have permitted them to breathe more efficiently - as would be necessary for a highly energetic creature.

iii) Dinosaurs could, based on the proportions of their limbs, run at speed (unlike lizards and crocodiles).

However, borrowing from the fields of histology, pathology, and microscopy, Bakker reported that thin sections of dinosaur bone, when viewed under a microscope, showed evidence of a complex structure and rich blood supply that would have allowed a rapid turnover of vital minerals between bone and blood plasma - exactly paralleling that seen in modern mammals.

Turning to the field of ecology, Bakker analysed the relative abundances of predators and their supposed prey among samples of fossils representing time-averaged communities from the fossil s record and the present day. By comparing modern communities |

of endotherms (cats) and ectotherms (predatory lizards), he S

estimated that endotherms consume, on average, ten times the 5.

volume of prey during the same time interval. When he surveyed S

ancient (Permian) communities, by counting fossils of this age in museum collections, he observed rather similar numbers of potential predators and prey. When he examined some dinosaur communities from the Cretaceous period, he noticed that there was a considerably larger number of potential prey compared to the number of predators. He came to a similar conclusion after studying Tertiary mammal communities.

Using these admittedly simple proxies, he suggested that dinosaurs (or at least the predators) must have had metabolic requirements more similar to mammals; for the communities to stay in some degree of balance, there needed to be sufficient prey items to support the appetites of the predators.

Within the fields of geology and the 'new' palaeobiology, he also looked for macroevolutionary evidence (large-scale patterns of change in fossil abundance) taken from the fossil record. Bakker examined the times of origin and extinction of the dinosaurs for evidence that might have had a bearing on their putative physiology. The time of origin of the dinosaurs, during the Late Triassic (225 Ma), coincided with the time of the evolution of some of the most mammal-like creatures, with the first true mammals appearing about 200 Ma. Bakker suggested that dinosaurs evolved into a successful group simply because they developed an endothermic metabolism slightly earlier than mammals. If not, or so he argued, dinosaurs would never have been able to compete with the first truly endothermic mammals. In further support of this idea, he noted that true early mammals were small, probably nocturnal insectivores and scavengers during the entirety of the Mesozoic, when the dinosaurs ruled on land, and only diversified into the bewildering variety that we know today once the dinosaurs became extinct at the end of the Cretaceous. On that basis, so M Bakker argued, dinosaurs simply had to be endotherms, otherwise | the supposedly 'superior' endothermic mammals would have £ conquered the land and replaced the dinosaurs in the Early

Jurassic. Moreover, when he considered the time of extinction of the dinosaurs at the close of the Cretaceous (65 Ma), Bakker believed that there was evidence that the world had been subjected to a temporary period of low global temperatures. Since dinosaurs were, in his opinion, large, endothermic, and 'naked' (that is, they were scale-covered and had neither hair nor feathers to keep their bodies warm), they were unable to survive a period of rapid climatic cooling and therefore died out. This left the mammals and birds to survive to the present day. Dinosaurs were too big to shelter in burrows, as do the modern reptiles that evidently survived the Cretaceous catastrophe.

Combining all these lines of argument, Bakker was able to propose that far from being slow and dull, dinosaurs were intelligent, highly active creatures that had stolen the world from the traditionally superior mammals for the remaining 160 million years of the Mesozoic. Rather than being ousted from the world by the evolutionary rise of superior mammals, they had only given up their dominance because of some freakish climatic event 65 million years ago.

It should now be obvious that the palaeobiological agenda for research is rather more intellectually broad-based. The 'expert' can no longer rely upon specialist knowledge in his or her own narrow area of expertise. However, this part of the story does not end here. John Ostrom had another important part to play in this saga.

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