The discovery of ‘terrible claw’
In the summer of 1964 John Ostrom was prospecting for fossils in Cretaceous rocks near Bridger, Montana, and collected the fragmentary remains of a new and unusual predatory dinosaur. Further collecting yielded more complete remains, and by 1969 Ostrom was able to describe the new dinosaur in sufficient detail and to christen it Deinonychus (‘terrible claw’) in recognition of a wickedly hooked, gaff-like claw on its hind foot.
Deinonychus (Figure 16) was a medium-sized (2-3 metres in length), predatory dinosaur belonging to a group known as the theropods. Ostrom noted a number of unexpected anatomical features; these prepared the intellectual ground for a revolution that would shatter the then rather firmly held view of dinosaurs as archaic and outmoded creatures that plodded their way to extinction at the close of the Mesozoic world.
However, Ostrom was far more interested in understanding the biology of this puzzling animal than in simply listing its skeletal features. This approach is far removed from the pejorative epithet ‘stamp-collecting’ that palaeontology had attracted, and echoes the method of Louis Dollo in his earlier attempts to understand the biology of the first complete Iguanodon skeletons (topic 1). As an Bottom: Diagram of Archaeopteryx with feathers removed to show its basic theropod affinity.
16. Top: Three diagrams of a Deinonychus skeleton.
Features of Deinonychus
i) The animal was clearly bipedal (it ran on its hind legs alone) and its legs were long and slender.
ii) Its feet were unusual in that of the three large toes on each, only two were designed to be walked upon, the inner toe was held clear of the ground and ‘cocked’ as if ready for action (a bit like a huge version of the sharp retractile claws in a cat’s paw).
iii) The front part of the animal was counterbalanced at the hip by a long tail; however, this tail was not of the deep, muscular variety normally expected in these types of animal, but was flexible and muscled near the hips, becoming very narrow (almost pole-like) and stiffened by bundles of thin, bony rods along the rest of its length.
iv) The chest was short and compact, and supported very long arms that ended in sharply clawed (raptorial) three-fingered hands that swivelled on wrists that allowed the hands to be swung in a raking arc (like those of a praying mantis).
v) The neck was slender and curved (rather like that of a goose), but supported a very large head, which was equipped with long jaws, lined with sharp, curved, and saw-edged teeth; very large eye sockets that seem to point forward; and a much larger than expected braincase.
Deducing the biology and natural history of Deinonychus
Looking at Deinonychus using this type of ‘forensic’ perspective, what do these features tell us about the animal and its way of life?
The jaws and teeth (sharp, with curved and serrated edges) confirm that this was a predator capable of slicing up and swallowing its prey. The eyes were large, pointed forward, and would have offered a degree of stereoscopic vision, which would be ideal for judging distance accurately: very useful for catching fast-moving prey, as well as for monitoring athletic movements in three-dimensional space. This serves, in part at least, to explain the relatively large brain (implied from its large braincase): the optic lobes would need to be large to process lots of complex visual information so that the animal could respond quickly, and the motor areas of the brain would need to be large and elaborate to process the higher-brain commands and then coordinate the rapid muscular responses of the body.
The need for an elaborate brain is further emphasized by considering the light stature and slender proportions of its legs, which are similar to those of modern, fast-moving animals and suggest that Deinonychus was a sprinter. The narrowness of each foot (just two walking toes, rather than the more stable, and more usual, ‘tripod’ effect of three) suggests that its sense of balance must have been particularly well developed; this is further supported by the fact that this animal was bipedal, and clearly able to walk while balanced on two feet alone (a feat that, as toddlers prove daily, needs to be learned and perfected through feedback between the brain and musculoskeletal system).
Linked to this issue of balance and coordination, the ‘terrible claw’ on each foot was clearly an offensive weapon, evidence of the animal’s predatory lifestyle. But how, exactly, would it have been used? Two possibilities spring to mind: either it was capable of slashing at its prey with one foot at a time, as some large ground-dwelling birds such as ostriches and cassowaries do today (this implies that it could have balanced on one foot from time to time); alternatively, it may have attacked its prey using a two-footed kick, by jumping on its prey or by grasping its prey in its arms and giving a murderous double-kick – this latter style of fighting is employed by kangaroos when fighting rivals. We are unlikely to be able to decide which of these speculations might be nearest the truth.
The long arms and sharply clawed hands would be effective grapples for holding and ripping its prey in either of these prey-capture scenarios and the curious raking motion made possible by the wrist joints enhances their raptorial abilities considerably. In addition, the long, whip-like tail may well have served as a cantilever – the equivalent of a tightrope walker’s pole to aid balance when slashing with one foot – or it could have served as a dynamic stabilizer, which would prove useful when chasing fast-moving prey that were capable of changing direction very quickly or when leaping on prey.
While this is not an exhaustive analysis of Deinonychus as a living creature, it does provide an outline of some of the reasoning that led Ostrom to conclude that Deinonychus was an athletic, surprisingly well-coordinated, and probably intelligent predatory dinosaur. Why should the discovery of this creature be regarded as so important to the field of dinosaur palaeobiology? To answer that question, it is necessary to take a broader view of the dinosaurs as a whole.
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, and crocodiles, which most biologists had only ever seen in zoos. The only puzzle was that dinosaurs were mostly built on a far grander scale compared to even the very biggest of known crocodiles.
There were many depictions of dinosaurs in popular topics, 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 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 record and the present day. By comparing modern communities of endotherms (cats) and ectotherms (predatory lizards), he estimated that endotherms consume, on average, ten times the volume of prey during the same time interval. When he surveyed 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 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.
Ostrom and Archaeopteryx. the earliest bird
Having described Deinonychus, Ostrom continued to investigate the biological properties of dinosaurs. In the early 1970s a trifling discovery in a museum in Germany was to bring him right back to the centre of some heated discussions. While examining collections of flying reptiles, Ostrom noticed one specimen, collected from a quarry in Bavaria, that did not belong to a pterosaur, or flying reptile, as its label suggested. It was a section of a leg including the thigh, knee-joint, and shin. Its detailed anatomical shape reminded Ostrom of that of Deinonychus. On closer inspection, he could also make out the faintest impressions of feathers! This was clearly an unrecognized specimen of the fabled early bird Archaeopteryx (Figure 13). Excited by his new discovery, and naturally puzzled by its apparent similarity to Deinonychus, Ostrom began carefully restudying all the known Archaeopteryx specimens.
The more Ostrom studied Archaeopteryx, the more convinced he became of the extent of the anatomical similarity between this creature and his much larger predatory dinosaur Deinonychus (Figure 16). This led him to reassess the monumental and then authoritative work on bird origins that had been written by ornithologist and anatomist Gerhard Heilmann in 1926. The sheer number of anatomical similarities between carnivorous theropod dinosaurs and early birds drove Ostrom to question Heilmann’s conclusion in that work that the similarities could only have been due to evolutionary convergence.
17. Comparison of the clavicles of (a) early theropod dinosaurs, (b) Archaeopteryx (clavicles are fused together), and (c) modern birds
Armed with more recent discoveries of dinosaurs around the world, Ostrom was able to show that a number of dinosaurs did actually possess small clavicles, removing at a stroke Heilmann’s big stumbling block to a dinosaurian ancestry for birds. Encouraged by this discovery and his own detailed observations on theropods and Archaeopteryx, Ostrom launched a comprehensive assault on Heilmann’s theory in a series of articles in the early 1970s. This led to the gradual acceptance of a theropod dinosaur ancestry of birds by the great majority of palaeontologists, and would no doubt have pleased the far-sighted Huxley and deeply irritated Owen.
The close anatomical, and therefore biological, similarity between theropods and the earliest birds added fuel to the controversy concerning the metabolic status of dinosaurs. Birds are highly active, endothermic creatures; perhaps the theropod dinosaurs might also have possessed an elevated metabolism. The once clear dividing line between feathered birds, with their distinctive anatomy and biology which merited them being separated off from all other vertebrates as a discrete class, the Aves, and other more typical members of the class Reptilia (of which the dinosaurs were just one extinct group) became worryingly blurred. The extent of this blurred line has become even more pronounced in recent years (as we shall see in topic 6).