The physiology of dinosaurs has historically been a controversial subject, particularly thermoregulation. Recently, many new lines of evidence have been brought to bear on dinosaur physiology generally, including not only metabolic systems and thermoregulation, but on respiratory and cardiovascular systems as well.
With the discovery of much more complete skeletons in the western United States, starting in the 1870s, scientists could make more informed interpretations of dinosaur biology. Edward Drinker Cope, opponent of Othniel Charles Marsh in the Bone Wars, propounded at least some dinosaurs as active and agile, as seen in the painting of two fighting "Laelaps" produced under his direction by Charles R. Knight. In parallel, the development of Darwinian evolution, and the discoveries of Archaeopteryx and Compsognathus, led Thomas Henry Huxley to proposed that dinosaurs were closely related to birds. Despite these considerations, the image of dinosaurs as large reptiles had taken root, and most aspects of their paleobiology were interpreted as being typically reptilian for the first half of the twentieth century.
These debates sparked interest in new methods for ascertaining the palaeobiology of extinct animals, such as bone histology, which have been successfully applied to determining the growth-rates of many dinosaurs.
Today, it is generally thought that many or perhaps all dinosaurs had higher metabolic rates than living reptiles, but also that the situation is more complex and varied than Bakker originally proposed. For example, while smaller dinosaurs may have been true endotherms, the larger forms could have been inertial homeotherms, or many dinosaurs could have had intermediate metabolic rates.
The feeding habits of ornithomimosaurs and oviraptorosaurs are a mystery: although they evolved from a predatory theropod lineage, they have small jaws and lack the blade-like teeth of typical predators, but there is no evidence of their diet or how they ate and digested it.
Features of other groups of dinosaurs indicate that they were vegetarians: jaws that opened to only a small extent and closed more like shutters, so that all the teeth met at the same time; large abdomens to acommodate the large amounts of vegetation they ate and store this food for a longer time, as vegetation is harder to digest than meat; these large digestive systems may also have contained endosymbiotic micro-organisms which can digest cellulose, as no known animal can digest this tough material directly.
Sauropods, which were vegetarians, did not chew their food, as their teeth and jaws appear suitable only for stripping leaves off plants. Ornithischians, also vegetarians, show a variety of approaches. The armored ankylosaurs and stegosaurs had small heads and weak jaws and teeth, and are thought to have fed in much the same way as sauropods. The pachycephalosaurs had small heads and weak jaws and teeth, but their lack of large digestive systems suggests a different diet, possibly fruits, seeds, or young shoots, which would have been more nutritious than leaves.
On the other hand ornithopods such as Hypsilophodon, Iguanodon and various hadrosaurs had horny beaks for snipping off vegetation and jaws and teeth that were well-adapted for chewing. The horned ceratopsians had similar mechanisms.
It has often been suggested that at least some dinosaurs used swallowed stones, known as gastroliths, to aid digestion by grinding their food in muscular gizzards, and that this was a feature they shared with birds. In 2007 Oliver Wings reviewed references to gastroliths in scientific literature and found considerable confusion, starting with the lack of an agreed and objective definition of "gastrolith". He found that swallowed hard stones or grit can assist digestion in birds that were mainly feeding on grain but may not be essential, and that birds which eat insects in summer and grain in winter usually get rid of the stones and grit in summer. Gastroliths have often been described as important for sauropod dinosaurs, whose diet of vegetation required very thorough digestion, but Wings conluded that this idea was incorrect: gastroliths are found with only a small percentage of sauropod fossils; where they have been found, the amounts are too small and in many cases the stones are too soft to have been effective in grinding food; most of these gastroliths are highly polished, but gastroliths used by modern animals to grind food are roughened by wear and corroded by stomach acids; hence the sauropod gastroliths were probably swallowed accidentally. On the other hand he concluded that gastroliths found with fossils of advanced theropod dinosaurs such as Sinornithomimus and Caudipteryx resemble those of birds, and that the use of gastroliths for grinding food may have appeared early in the group of dinosaurs from which these dinosaurs and birds both evolved.
Because the line of dinosaurs that includes Allosaurus and Tyrannosaurus diverged from the line that led to Tenontosaurus very early in the evolution of dinosaurs, the presence of medullary bone in both groups suggests that dinosaurs in general produced medullary tissue. On the other hand crocodilians, which are dinosaurs' second closest living relatives after birds, do not produce medullary bone. This tissue may have first appeared in ornithodires, the Triassic archosaur group from which dinosaurs are thought to have evolved.
Medullary bone has been found in specimens of sub-adult size, which suggests that dinosaurs reached sexual maturity before they were fully-grown. Sexual maturity at sub-adult size is also found in reptiles and in medium- to large-sized mammals, but birds and small mammals reach sexual maturity only after they are fully-grown – which happens within their first year. Early sexual maturity is also associated with specific features of animals' life cycles: the young are born relatively well-developed rather than helpless; and the death-rate among adults is high.
From about 1870 onwards scientists have generally agreed that the post-cranial skeletons of many dinosaurs contained many air-filled cavities (pleurocoels), especially in the vertebrae. For a long time these cavities were regarded simply as weight-saving devices, but Bakker proposed that they contained air sacs like those which make birds' respiratory systems the most efficient of all animals'.
John Ruben et al (1997, 1999, 2003, 2004) disputed this and suggested that dinosaurs had a "tidal" respiratory system (in and out) powered by a crocodile-like hepatic piston mechanism - muscles attached mainly to the pubis pull the liver backwards, which makes the lungs expand to inhale; when these muscles relax, the lungs return to their previous size and shape, and the animal exhales. They also presented this as a reason for doubting that birds descended from dinosaurs.
Critics have claimed that, without avian air sacs, modest improvements in a few aspects of a modern reptile's circulatory and respiratory systems would enable the reptile to achieve 50% to 70% of the oxygen flow of a mammal of similar size, and that lack of avian air sacs would not prevent the development of endothermy. Very few formal rebuttals have been published in scientific journals of Ruben et al’s claim that dinosaurs could not have had avian-style air sacs; but one points out that the Sinosauropteryx fossil on which they based much of their argument was severely flattened and therefore it was impossible to tell whether the liver was the right shape to act as part of a hepatic piston mechanism. Some recent papers simply note without further comment that Ruben et al argued against the presence of air sacs in dinosaurs.
Researchers have presented evidence and arguments for air sacs in:
So far no evidence of air sacs has been found in ornithischian dinosaurs. But this does not imply that ornithischians could not have had metabolic rates comparable to those of mammals, since mammals also do not have air sacs.
Three explanations have been suggested for the development of air sacs in dinosaurs:
Calculations of the volumes of various parts of the sauropod Apatosaurus’ respiratory system support the evidence of bird-like air sacs in sauropods:
On this basis, Apatosaurus could not have had a reptilian respiratory system, as its tidal volume would have been less than its dead-space volume, so that stale air was not expelled but was sucked back into the lungs. Likewise, a mammalian system would only provide to the lungs about 225 − 184 = 41 liters of fresh, oxygenated air on each breath. Apatosaurus must therefore have had either a system unknown in the modern world or one like birds', with multiple air sacs and a flow-through lung. Furthermore, an avian system would only need a lung volume of about 600 liters while a mammalian one would have required about 2,950 liters, which would exceed the estimated 1,700 liters of space available in a 30-ton Apatosaurus′ chest.
Dinosaur respiratory systems with bird-like air sacs may have been capable of sustaining higher activity levels than mammals of similar size and build can sustain. In addition to providing a very efficient supply of oxygen, the rapid airflow would have been an effective cooling mechanism, which is essential for animals that are active but too large to get rid of all the excess heat through their skins.
Plates which may have functioned as in the same way as uncinate processes have been reported in fossils of the ornithischian dinosaur Thescelosaurus, and have been interpreted as evidence of high oxygen consumption and therefore high metabolic rate.
Ruben et al have argued in several papers that:
However, objections have been raised against this argument:
In principle one would expect dinosaurs to have had two-part circulations driven by four-chambered hearts, since many would have needed high blood pressure to deliver blood to their heads, which were high off the ground, but vertebrate lungs can only tolerate fairly low blood pressure. In 2000, a skeleton of Thescelosaurus, now on display at the North Carolina Museum of Natural Sciences, was described as including the remnants of a four-chambered heart and an aorta. The authors interpreted the structure of the heart as indicating an elevated metabolic rate for Thescelosaurus, not reptilian cold-bloodedness. Their conclusions have been disputed; other researchers published a paper where they assert that the heart is really a concretion of entirely mineral "cement". As they note: the anatomy given for the object is incorrect, for example the alleged "aorta" is narrowest where it meets the "heart" and lacks arteries branching from it; the "heart" partially engulfs one of the ribs and has an internal structure of concentric layers in some places; and another concretion is preserved behind the right leg. The original authors defended their position; they agreed that the chest did contain a type of concretion, but one that had formed around and partially preserved the more muscular portions of the heart and aorta.
Regardless of the object's identity, it may have little relevance to dinosaurs' internal anatomy and metabolic rate. Both modern crocodilians and birds, the closest living relatives of dinosaurs, have four-chambered hearts, although modified in crocodilians, and so dinosaurs probably had them as well. However such hearts are not necessarily tied to metabolic rate.
|Weight (kg)||Comparative growth rate of dinosaurs||Modern animals in this size range|
|0.22||Slower than marsupials||Rat|
|1 - 20||Similar to marsupials, slower than precocial birds (those that are born capable of running)||From guinea pig to Andean Condor|
|100 - 1000||Faster than marsupials, similar to precocial birds, slower than placental mammals||From Red Kangaroo to Polar Bear|
|1500 – 3500||Similar to most placental mammals||From American Bison to rhinoceros|
|25000 and over||Very fast, similar to modern whales; but about half that of a scaled-up altricial bird (one that is born helpless) - if one could scale up a bird to||Whales|
Tyrannosaurus rex showed a "teenage growth spurt":
A 2008 study of one skeleton of the hadrosaur Hypacrosaurus concluded that this dinosaur grew even faster, reaching its full size at the age of about 15; the main evidence was the number and spacing of growth rings in its bones. The authors found this consistent with a life-cycle theory that prey species should grow faster than their predators if they lose a lot of juveniles to predators and the local environment provides enough resources for rapid growth.
It appears that individual dinosaurs were rather short-lived, e.g. the oldest (at death) Tyrannosaurus found so far was 28 and the oldest sauropod was 38. Predation was probably responsible for the high death rate of very young dinosaurs and sexual competition for the high death rate of sexually mature dinosaurs.
Large dinosaurs may also have maintained their temperatures by inertial homeothermy, also known as "bulk homeothermy" or "mass homeothermy". In other words, the thermal capacity of such large animals was so high that that it would take two days or more for their temperatures to change significantly, and this would have smoothed out variations caused by daily temperature cycles. This smoothing effect has been observed in large turtles and crocodilians, but Plateosaurus, which weighed about , may have been the smallest dinosaur in which it would have been effective. Inertial homeothermy would not have been possible for small species nor for the young of larger species. Vegetation fermenting in the guts of large herbivores can also produce considerable heat, but this method of maintaining a high and stable temperature would not have been possible for carnivores nor for small herbivores or the young of larger herbivores.
Since the internal mechanisms of extinct creatures are unknowable, most discussion focuses on homeothermy and tachymetabolism.
Assessment of metabolic rates is complicated by the distinction between the rates while resting and while active. In all modern reptiles and most mammals and birds the maximum rates during all-out activity are 10 to 20 times higher than minimum rates while at rest. However in a few mammals these rates differ by a factor of 70. Theoretically it would be possible for a land vetebrate to have a reptilian metabolic rate at rest and a bird-like rate while working flat out. However an animal with such a low resting rate would be unable to grow quickly. The huge herbivorous sauropods may have been on the move so constantly in search of food that their energy expenditure would have been much the same irrespective of whether their resting metabolic rates were high or low.
Dinosaurs were around for about 150 million years, so it is very likely that different groups evolved different metabolisms and thermoregulatory regimes, and that some developed different physiologies from the first dinosaurs.
If all or some dinosaurs had intermediate metabolisms, they may have had the following features:
Robert Reid has suggested that such animals could be regarded as "failed endotherms". He envisaged both dinosaurs and the Triassic ancestors of mammals passing through a stage with these features. Mammals were forced to become smaller as archosaurs came to dominate ecological niches for medium to large animals. Their decreasing size made them more vulnerable to heat loss because it increased their ratios of surface ares to mass, and thus forced them to increase internal heat generation and thus become full endotherms. On the other hand dinosaurs became medium to very large animals and thus were able to retain the "intermediate" type of metabolism.
However as a result of other, mainly later research, bone structure is not considered a reliable indicator of metabolism in dinosaurs, mammals or reptiles:
Nevertheless de Ricqlès persevered with studies of the bone structure of dinosaurs and archosaurs. In mid-2008 he co-authored a paper which examined bone samples from a wide range of archosaurs, including early dinosaurs, and which conluded that:
But a preliminary study of the relationship between adult size, growth rate, and body temperature concluded that: larger dinosaurs had higher body temperatures than smaller ones had; Apatosaurus, the largest dinosaur in the sample, was estimated to have a body temperature exceeding , whereas smaller dinosaurs were estimated to have body temperatures around – for comparison, normal humal body temperature is about . Based on these estimations, the study concluded that large dinosaurs were inertial homeotherms (their temperatures were stabilized by their sheer bulk) and that dinosaurs were ectothermic (in colloquial terms, "cold-blooded", because they did not generate as much heat as mammals when not moving or digesting food). These results are consistent with the relationship between dinosaurs' sizes and growth rates (described above}.
Barrick and Showers concluded that both dinosaurs were endothermic but at lower metabolic levels than modern mammals, and that inertial homeothermy was an important part of their temperature regulation as adults. Their similar analysis of some Late Cretaceous ornithischians in 1996 concluded that these animals showed a similar pattern.
However this view has been challenged. The evidence indicates homeothermy, but by itself cannot prove endothermy. Secondly, the production of bone may not have been continuous in areas near the extremities of limbs – in allosaur skeketons lines of arrested growth ("LAGs"; rather like growth rings) are sparse or absent in large limb bones but common in the fingers and toes. While there is no absolute proof that LAGs are temperature-related, they could mark times when the extermities were so cool that the bones ceased to grow. If so, the data about oxygen isotope ratios would be incomplete, especially for times when the extremities were coolest. Oxygen isotope ratios may be an unreliable method of estimating temperatures if it cannot be shown that bone growth was equally continuous in all parts of the animal.
This argument was criticized on several grounds and is no longer taken seriously (the following list of criticisms is far from exhaustive):
Dinosaurs' limbs were erect and held under their bodies, rather than sprawling out to the sides like those of lizards and newts. The evidence for this is the angles of the joint surfaces and the locations of muscle and tendon attachments on the bones. Attempts to represent dinosaurs with sprawling limbs result in creatures with dislocated hips, knees, shoulders and elbows.
Carrier's constraint states that air-breathing vertebrates which have 2 lungs and flex their bodies sideways during locomotion find it very difficult to move and breathe at the same time. This severely limits their stamina and forces them to spend more time resting than moving.
Sprawling limbs require sideways flexing during locomotion (except for tortoises and turtles, which are very slow and whose armor keeps their bodies fairly rigid). But despite Carrier's constraint sprawling limbs are efficient for creatures which spend most of their time resting on their bellies and only move for a few seconds at a time, because this arrangement minimizes the energy costs of getting up and lying down.
Erect limbs increase the costs of getting up and lying down, but avoid Carrier's constraint. This indicates that dinosaurs were active animals because natural selection would have favored the retention of sprawling limbs if dinosaurs had been sluggish and spent most of their waking time resting. An active lifestyle requires a metabolism which quickly regenerates energy supplies and breaks down waste products which cause fatigue, i.e. it requires a fairly fast metabolism and a considerable degree of homeothermy.
Bakker and Ostrom both pointed out that all dinosaurs had erect hindlimbs and that all quadrupedal dinosaurs except the ceratopsians and ankylosaurs had erect forelimbs; and that among living animals only the endothermic ("warm-blooded") mammals and birds have erect limbs (Ostrom acknowledged that crocodilians' occasional "high walk" was a partial exception). Bakker claimed this was clear evidence of endothermy in dinosaurs, while Ostrom regarded it as persuasive but not conclusive.
There is now no doubt that many theropod dinosaur species had feathers, including Shuvuuia, Sinosauropteryx and Dilong (an early tyrannosaur). These have been interpreted as insulation and therefore evidence of warm-bloodedness.
But impressions of feathers have only been found in coelurosaurs (which includes the ancestors of both birds and tyrannosaurs), so at present feathers give us no information about the metabolisms of the other major dinosaur groups, e.g. coelophysids, ceratosaurs, carnosaurs, sauropods or ornithischians.
In fact the fossilised skin of Carnotaurus (an abelisaurid and therefore not a coelurosaur) shows an unfeathered, reptile-like skin with rows of bumps. But an adult Carnotaurus weighed about 1 ton, and mammals of this size and larger have either very short hair or naked skins, so perhaps the skin of Carnotaurus tells us nothing about whether smaller non-coelurosaurid theropods had feathers.
Skin-impressions of Pelorosaurus and other sauropods (dinosaurs with elephantine bodies and long necks) reveal large hexagonal scales, and some sauropods, such as Saltasaurus, had bony plates in their skin. The skin of ceratopsians consisted of large polygonal scales, sometimes with scattered circular plates. "Mummified" remains and skin impressions of hadrosaurids reveal pebbly scales. It is unlikely that the ankylosaurids, such as Euoplocephalus, had insulation, as most of their surface area was covered in bony knobs and plates. Likewise there is no evidence of insulation in the stegosaurs.
Studies of fossilized vegetation suggest that the Alaska North Slope had a maximum temperature of and a minimum temperature of to in the last 35 million years of the Cretaceous (slightly cooler than Portland, Oregon but slightly warmer than Calgary, Alberta). Even so, the Alaska North Slope has no fossils of large cold-blooded animals such as lizards and crocodilians, which were common at the same time in Alberta, Montana, and Wyoming. This suggests that at least some non-avian dinosaurs were warm-blooded. It has been proposed that North American polar dinosaurs may have migrated to warmer regions as winter approached, which would allow them to inhabit Alaska during the summers even if they were cold-blooded. But a roundtrip between there and Montana would probably have used more energy than a cold-blooded land vertebrate produces in a year, in other words the Alaskan dinosaurs would have had to be warm-blooded irrespective of whether they migrated or stayed for the winter.
It is more difficult to determine the climate of southeastern Australia when the dinosaur fossil beds were laid down , towards the end of the Early Cretaceous: these deposits contain evidence of permafrost, ice wedges, and hummocky ground formed by the movement of subterranean ice, which suggests mean annual temperatures ranged between and ; oxygen isotope studies of these deposits give a mean annual temperature of to . However the diversity of fossil vegetation and the large size of some of fossil trees exceed what is found in such cold environments today, and no-one has explained how such vegetation could have survived in the cold temperatures suggested by the physical indicators – for comparison Fairbanks, Alaska presently has a mean annual temperature of . An annual migration from and to southeastern Australia would have been very difficult for fairly small dinosaurs in such as Leaellynasaura, a vegetarian about to long, because seaways to the north blocked the passage to warmer latitudes. Bone samples from Leaellynasaura and Timimus, an ornithomimid about long and high at the hip, suggested these two dinosaurs had different ways of surviving the cold, dark winters: the Timimus sample had lines of arrested growth (LAGs for short; similar to growth rings), and it may have hibernated; but the Leaellynasaura sample showed no signs of LAGs, so it may have remained active throughout the winter.
But these were a very small minority of all the dinosaur species which are known.
One common interpretation of the plates on stegosaurs' backs is as heat exchangers for thermoregulation, as the plates are filled with blood vessels which could theoretically absorb and dissipate heat.
This might have worked for a stegosaur with large plates, such as Stegosaurus, but other stegosaurs, such as Wuerhosaurus, Tuojiangosaurus and Kentrosaurus possessed much smaller plates with a surface area of doubtful value for thermo-regulation. However the idea of stegosaurian plates as heat exchangers recently been questioned.
Crocodilians present some puzzles if one regards dinosaurs as active animals with fairly constant body temperatures. Crocodilians evolved shortly before dinosaurs and, second to birds, are dinosaurs' closest living relatives - but modern crocodilians are cold-blooded. This raises some questions:
Modern crocodilians are cold-blooded, but they can move with their limbs erect and they have several features which are normally associated with warm-bloodedness because they improve the animal's oxygen supply:
So why did natural selection favor the development of these features, which are very important for active warm-blooded creatures but of little apparent use to cold-blooded aquatic ambush predators which spend the vast majority of their time floating in water or lying on river banks?
It was suggested in the late 1980s that that crocodilians were originally active, warm-blooded predators and that their archosaur ancestors were warm-blooded.More recently, developmental studies have indicated that crocodilian embryos develop fully 4-chambered hearts first and then develop the modifications which make their hearts function as 3-chambered under water. Using the principle that ontogeny recapitulates phylogeny, the researchers concluded that the original crocodilians had fully 4-chambered hearts and were therefore warm-blooded and that later crocodilians developed the bypass as they reverted to being cold-blooded aquatic ambush predators.
More recent research on archosaur bone structures and their implications for growth rates also suggests that early archosaurs had fairly high metabolic rates and that the Triassic ancestors of crocodilians dropped back to more typically "reptilian" metabolic rates.
If this view is correct, the development of warm-bloodedness in archosaurs (reaching its peak in dinosaurs) and in mammals would have taken more similar amounts of time. It would also be consistent with the fossil evidence:
The physiology of Dinosaurs: circulatory and respiratory function in the largest animals ever to walk the earth.(Donald F Egan Scientific Memorial Lecture)(Report)
Jul 01, 2009; Introduction: Why Dinosaurs? How Can We Know Anything About the Physiology of Dinosaurs? Circulation: How Could the Tallest...