Although the line of argument that follows is not universal to dinosaurs, it is instructive in the sense that it shows what some dinosaurs were capable of doing. The classic example is John Ostrom's dromaeosaur Deinonychus (Figure 29). As was summarized in Chapter 2, this dinosaur was a large-eyed visual predator that could clearly run fast, judging by its limb proportions and general build. In addition, it had an unusual stiff, narrow tail, extraordinary gaff-like inner toes on its hind feet, and long, sharply clawed, grasping arms. It is not unreasonable to suggest that this animal was built as a pursuit predator, was capable of using its narrow tail as a dynamic balancing aid (flicking the tail to one side or the other would allow this animal to change direction extremely quickly), and very probably leapt at its prey, which it then disabled using the claws on its feet. We have never seen a Deinonychus in action, but this scenario is based upon observable features of the skeleton, and is partially supported by one remarkable fossil discovered in Mongolia.
The latter comprises two dinosaurs, the small herbivorous ceratopian Protoceratops and a close relative of Deinonychus known as Velociraptor. This extraordinary fossil shows the two creatures caught in a death struggle; they probably choked to death in a dust storm while fighting with each other. The Velociraptor is preserved clinging to the head of its prey using its long arms, and in the very act of kicking at the throat of its unfortunate victim.
Such overall 'sophistication' in design, inferred function, and way of life strongly suggests activity levels that are more similar to those
M exhibited by modern endotherms.
£ Echoing some of the argument seen in the discussion concerning dinosaurs' ability to move bipedally, the brains of both mammals and birds are large and both groups exhibit what appears to be intelligent behaviour. In contrast, ectothermic reptiles possess smaller brains and are not usually renowned for their intellectual prowess (though this is in part a fiction that we have propagated). There does, however, appear to be a general link between overall brain size and endothermy. Large brains are highly complex structures that demand constant supplies of oxygen and food, as well as a stable temperature in order to function efficiently. Ectothermic reptiles clearly can supply both food and oxygen to their brains effectively, but their body temperature does vary across a normal 24-hour cycle, and as a consequence they are unable to supply the needs of a large and sophisticated brain.
Tradition has it that dinosaurs were notoriously lacking in brain power (the walnut-sized Stegosaurus brain is often cited as a classic example). However, Jim Hopson at the University of Chicago has done much to rectify this somewhat erroneous view. Comparing the ratio of brain volume to body volume across a range of animals, including dinosaurs, Hopson was able to demonstrate that most dinosaurs had fairly typically reptile-sized brains. Some, however, were unexpectedly well endowed in the 'brains department' - not surprisingly perhaps, these were the highly active, bipedal theropods.
Earlier in this chapter it was mentioned that charting distributional data had been one of the spurs to pursuing the physiological status of dinosaurs. Recently, reports have shown numbers of dinosaurs in the Yukon area of North America as well as in Australia and 0
Antarctica. These areas would have fallen within their respective 0 polar regions in Cretaceous times, and have been used to support 5 the idea that dinosaurs must have been endothermic to have =
survived. It is, after all, clearly the case today that ectothermic land 0
vertebrates are incapable of living at such high latitudes. b
However, upon careful consideration, these observations are not as persuasive as they seem at first sight. Evidence from the plant fossil record suggests that Mediterranean and subtropical styles of vegetation existed in these polar regions in Cretaceous times. Unusually, these plants share the habit of seasonal leaf loss, probably in response to low winter light levels and temperatures. The Cretaceous world shows no evidence of polar ice caps and it seems probable that even at high latitudes, during the summer season at least, temperatures were extremely mild. Under such circumstances, it is highly likely that herbivorous dinosaurs migrated north or south, depending upon the season, to take advantage of rich pastures. As a result, discovery of their fossil remains at very high Mesozoic latitudes may reflect their migratory range rather than polar residency.
Measuring Mesozoic community structure was one of Bakker's most innovative suggestions in his search for proxies for dinosaurian physiology. The idea is beguilingly simple: endothermic and ectothermic animals require differing amounts of food in order to survive - these amounts reflect the basic 'running costs' associated with being either an endotherm or ectotherm. Endotherms, such as mammals and birds, have high running costs because much of the food that they eat (in excess of 80%) is burned to produce body heat. By contrast, ectotherms need far less food because very little is used to generate body heat. As a rough guide, ectotherms need about 10%, sometimes much less, of the food requirements of similarly sized endotherms.
Based on this observation, and an understanding that the general M economy of Nature tends to keep supply and demand more or less | in balance, Bakker suggested that censuses of fossil communities £ might indicate the balance between predator and prey, and by implication the physiology of these animals. He combed through museum collections to gather the data he needed. This included data from ancient (Palaeozoic) reptile, dinosaur (Mesozoic), and relatively more recent (Cenozoic) mammal communities. His results seemed encouraging: Palaeozoic reptile communities indicated a rough equivalence of predator and prey numbers; by contrast, dinosaur and Cenozoic mammal communities indicated a preponderance of prey animals and very small numbers of predators.
At first the scientific community was impressed with these results; however, considerable doubt now exists about the value of the original data. Using museum collections to estimate numbers of predators or prey is an exceedingly dubious exercise: there is no proof that the animals being counted lived together in the first instance; there are enormous biases in terms of what was (or was not) collected at the time; and all manner of assumptions are being made about what a predator will or will not eat; and, even if there was some sort of biological signal, it would surely only apply to the predator. Additionally, work on communities of living ectotherm predators and their prey has revealed that the predators may be as few as 10% of their potential prey numbers, mimicking the proportions seen in Bakker's supposedly endotherm communities.
This is an excellent example of a brilliant idea that sadly cannot be supported because the data simply will not yield results that are in any way meaningful scientifically.
Considerable attention has been directed toward understanding fine details of the internal structure of dinosaur bone. The mineral 0 structure of dinosaur bone is generally unaffected by fossilization. 0 As a result, it is often possible to create thin sections of bone that 5 reveal the internal structure (histology) of the bone in amazing § detail. Preliminary observations suggested that the bones of 0
dinosaurs were closely similar in internal structure to those seen b in living endothermic mammals, rather than those of modern §
In general terms, the mammal and dinosaur bones revealed high levels of vascularization (they were very porous), while the ectotherm bones were poorly vascularized. The highly vascularized type of bone structure can arise in different ways. For example, one pattern of vascularization (fibrolamellar) reflects very rapid phases of bone growth. Another pattern (Haversian) represents a phase of strengthening of bone by remodelling that occurs later in the life of an individual.
What can be said is that many dinosaur remains show evidence of them having been able to grow quickly, and an ability to strengthen their bones by internal remodelling. Dinosaurs sometimes exhibit periodic interruptions in their pattern of growth (which mimics the intermittent pattern seen in the bones of living reptiles), but this style of growth is by no means uniform. Equally, and less probably, some endotherms (both bird and mammal) exhibit a style of bone structure (zonal) that displays very little vascularization, while living ectotherms can exhibit highly vascularized bone in parts of their skeletons. There are, surprisingly, no clear correlates between an animal's physiology and its internal bone structure.
Dinosaur physiology: an overview
The discussion above illustrates the range and variety of approaches that have been used in an attempt to investigate dinosaur metabolism.
Robert Bakker took an unquestioning stance when assessing the significance of the mammalian replacement by dinosaurs on land in M the Early Jurassic. This pattern, he argued, could only be explained | if dinosaurs were able to compete with his model of the 'superior' £ endothermic mammals: to do so, they simply had to be endothermic. Is this true? The answer is actually: no ... not necessarily.
At the close of the Triassic and very beginning of the Jurassic, the world was one that we mammalian humans would not find particularly hospitable. Much of Pangaea at the time was affected by seasonal, but generally arid, conditions in which deserts became widespread globally. Such conditions of high temperatures and low rainfall exert selective pressures on endothermic and ectothermic metabolisms in very different ways.
Ectotherms, as argued above, need to eat less than endotherms and are therefore better able to survive times of low biological productivity. Reptiles have scaly skin that greatly resists water loss in dry, desert conditions; they also do not urinate but instead excrete a dry, pasty material (similar to bird droppings). High ambient temperatures suit ectotherms well because their internal chemistry can be maintained at optimum temperatures with relative ease. All in all, ectotherms, built in the classic reptilian mould, can be predicted to cope well with desert-like conditions.
Endotherms, such as mammals, are physiologically stressed in high-temperature conditions. Mammals are 'geared' to being able to lose heat to the environment from their bodies (their bodily thermostats maintain their temperature on average higher than normal environmental conditions) and adjust their physiology accordingly. When cold, mammals are able to reduce heat loss from the body by raising their fur to trap air and increase its insulatory efficiency, use 'shivering' to quickly generate extra muscular heat, or raise their basal metabolic rate. However, under conditions of high ambient temperature, the need to lose heat to the environment to prevent lethal overheating becomes vital. Evaporative cooling is one 0 of the few options available; this is achieved either by panting or o sweating through the skin surface. Both of these processes remove S large volumes of water from the body. In desert conditions, losing § water, which is in short supply, can prove fatal. To compound |
matters further, mammals remove the breakdown products of their b metabolism from the body by urinating, which flushes wastes out of § the body in a watery solution. In addition to the problems of heat load and water loss, mammals require large quantities of food to maintain their endothermic physiology. Deserts are areas of low productivity, so food supplies are restricted and not capable of sustaining large populations of endotherms.
Looked at from this purely environmental perspective, perhaps the Late Triassic/Early Jurassic world was unusual. It was a time when the environment probably favoured ectotherms and restricted early mammals to small size and primarily nocturnal niches. In deserts today, nearly all mammals (with the exception of those truly remarkable creatures known as camels) are small, exclusively nocturnal rodents and insectivores. They survive the extreme heat of the day by burrowing under the sand surface, where conditions are cooler and more damp, and they come out at night once the temperature has dropped and they can use their acute senses to find insect prey.
The striking aridity of the Late Triassic/Early Jurassic eventually ameliorated, as Pangaea began to disintegrate and shallow epicontinental seas spread across and between areas of land. The general climatic regime appears to have become extremely warm and wet, and these conditions prevailed across very broad latitudinal bands. It should be emphasized that there were no ice-covered polar regions throughout the time of the dinosaurs. The type of world we inhabit today is very unusual, when compared to much of the history of the Earth, in that it has both north and south poles covered in ice and consequently unusually narrowly confined latitudinal climatic bands. Under these relatively lush Jurassic conditions, productivity rose dramatically; major Jurassic coal deposits were laid down in areas where long-lived and M densely forested areas existed. So it is perhaps not surprising to | discover that the range and variety of dinosaurs surged during £ Jurassic times.
Dinosaur physiology: was it unique?
Dinosaurs are noteworthy as being large creatures; even medium-sized ones ranged between 5 and 10 metres in length, which is still very big by most standards - the average size of all mammals is probably about the size of a cat or small dog today. It is certainly true that no dinosaurs were mouse-sized (except as hatchlings).
Under some conditions, being large has advantages. Most notably, larger animals tend to lose heat to and gain heat from the environment very much more slowly than small ones. For example, adult crocodiles maintain a very stable internal body temperature day and night, whereas hatchlings exhibit a body temperature range that exactly mirrors the day and night changes. So, being dinosaur-sized means that your internal body temperature changes little over time. Being large also means that postural muscles need to work hard to prevent the body from collapsing under its own weight. This constant muscular 'work' generates significant quantities of energy (in the same way that we become 'flushed' with heat after muscular exercise), and this heat can assist in maintaining internal body temperature.
In addition to these advantages of size, we have seen that the probable agility as well as posture of dinosaurs, many with heads raised significantly above chest level, indicates the strong likelihood that they had highly efficient, fully divided hearts that were capable of rapidly circulating oxygen, food, and heat around the body, as well as removing harmful metabolism by-products. The fact that saurischian dinosaurs probably possessed a bird-like lung system further emphasizes their ability to provide the oxygen that their tissues needed during energetic, aerobic exercise. d
Considering these factors alone, it seems very likely that dinosaurs S possessed many of the attributes that we associate today with §
endothermy as seen in living mammals and birds. In addition, |
dinosaurs were typically large and therefore relatively thermally b inert. They also lived during a time of constantly warm, §
non-seasonal, global climate.
It could be the case that dinosaurs were the happy inheritors of an ideal type of biology that enabled them to prosper in the unique climatic conditions that prevailed in the Mesozoic Era. But, however convincing this argument might seem at this point, it does not take into account one other crucial line of evidence that has emerged over the last few years: the intimacy of dinosaur-bird relationships.
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