Physiology of dinosaurs

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Note: In this article "dinosaur" means "non-avian dinosaur", since most experts regard birds as an advanced group of dinosaurs.

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.

Contents

[edit] History of study

[edit] Early interpretations of dinosaurs: 1820s to early 1900s

The study of dinosaurs began in the 1820s in England. Pioneers in the field, such as William Buckland, Gideon Mantell, and Richard Owen, interpreted the first, very fragmentary remains as belonging to large quadrupedal beasts.[1] Their early work can be seen today in the Crystal Palace Dinosaurs, constructed in the 1850s and which present known dinosaurs as elephantine lizard-like reptiles.[2] Despite these reptilian appearances, Owen speculated that dinosaur heart and respiratory systems were more mammal-like than reptile-like.[1]

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.

The 1897 painting of "Laelaps" (now Dryptosaurus) by Charles R. Knight.
The 1897 painting of "Laelaps" (now Dryptosaurus) by Charles R. Knight.

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.[3] 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.[4] Despite these considerations, the image of dinosaurs as large reptiles had taken root,[3] and most aspects of their paleobiology were interpreted as being typically reptilian for the first half of the twentieth century.

[edit] Changing views and the Dinosaur Renaissance

However, in the late 1960s views began to change, beginning with John Ostrom's work on Deinonychus and bird evolution.[5] His student, Bob Bakker, popularized the changing thought in a series of papers beginning with The superiority of dinosaurs in 1968.[6] In these publications, he argued strenuously that dinosaurs were warm-blooded and active animals, capable of sustained periods of high activity. In most of his writings Bakker framed his arguments as new evidence leading to a revival of ideas popular the late 19th century, frequently referring to an ongoing dinosaur renaissance. He used a variety of anatomical and statistical arguments to defend his case,[7][8] the methodology of which was fiercely debated among scientists.[9]

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,[10][11] or many dinosaurs could have had intermediate metabolic rates.[12]

[edit] Reproductive biology

When laying eggs, female birds grow a special type of bone in their limbs between the hard outer bone and the marrow. This medullary bone, which is rich in calcium, is used to make eggshells. Medullary bone has been found in fossils of the theropods Tyrannosaurus and Allosaurus and of the ornithopod Tenontosaurus. 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, this suggests that dinosaurs in general produced medullary tissue. Medullary bone has been found in specimens of sub-adult size, which suggests that dinosaurs reached sexual maturity rather quickly for such large animals.[13][14]

[edit] Respiratory System

This subject is currently the subject of intense and sometimes acrimonious debate (see for example A Reply to Ruben on Theropod Physiology).

[edit] Air sacs

Birds lungs obtain fresh air during both exhalation and inhalation, because the air sacs do all the "pumping" and the lungs simply absorb oxygen.
Birds lungs obtain fresh air during both exhalation and inhalation, because the air sacs do all the "pumping" and the lungs simply absorb oxygen.

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'.[15]

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.[16][17][18][19][20]

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,[21] and that lack of avian air sacs would not prevent the development of endothermy.[22] 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.[23] Some recent papers simply note without further comment that Ruben et al argued against the presence of air sacs in dinosaurs.[24]

Researchers have presented evidence and arguments for air sacs in:

  • sauropods (the group that includes Brontosaurus) (2003, 2006). In early sauropods only the cervical (neck) vertebrae show these features; in advanced sauropods ("neosaoropods") the vertebrae of the lower back and hip regions also show signs of air sacs. If the developmental sequence found in bird embryos is a guide, air sacs actually evolved before the channels in the skeleton that accommodate them in later forms.[25][26]
  • coelurosaurs (fairly advanced theropods, most fairly small but including the tyrannosauroids) (2004)[27]
  • ceratosaurs (a theropod group that is not regarded as particularly advanced) and probably all theropods (2005)[24]
  • Coelophysis (an early theropod) (2006), in which only the cervical (neck) vertebrae show evidence of air sacs. The sauropodomorph Thecodontosaurus and theropod Coelophysis, both from the late Triassic, are the earliest dinosaurs whose skeletons show evidence of channels for air sacs.[26]

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.[28]

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.[28]

[edit] Uncinate processes on the ribs

The uncinate processes are the small white spurs about half-way along the ribs. The rest of this diagram shows the air sacs and other parts of a bird's respiratory system:1 cervical air sac, 2 clavicular air sac, 3 cranial thoracal air sac, 4 caudal thoracal air sac, 5 abdominal air sac (5' diverticulus into pelvic girdle), 6 lung, 7 trachea
The uncinate processes are the small white spurs about half-way along the ribs. The rest of this diagram shows the air sacs and other parts of a bird's respiratory system:1 cervical air sac, 2 clavicular air sac, 3 cranial thoracal air sac, 4 caudal thoracal air sac, 5 abdominal air sac (5' diverticulus into pelvic girdle), 6 lung, 7 trachea

Birds have spurs called "uncinate processes" on the rear edges of their ribs, and these give the chest muscles more leverage when pumping the chest to improve oxygen supply. The size of the uncinate processes is related to the birds' lifestyle and its oxygen requirements: they are shortest in walking birds and longest in diving birds, which need to replenish their oxygen reserves quickly when they surface. Non-avian maniraptoran dinosaurs also had these uncinate processes and they were proportionately as long as in modern diving birds, a fact which indicates that they needed a very powerful oxygen supply.[29][30]

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.[31]

[edit] Nasal turbinates

Nasal turbinates (often referred to as "turbinals" or "conchae") are convoluted structures of thin bone in the nasal cavity. In most mammals and birds these are present and lined with mucous membranes which perform two functions: they improve the sense of smell by increasing the area available to absorb airborne chemicals; and they warm and moisten inhaled air and extract heat and moisture from exhaled air, to prevent desiccation of the lungs.

Human nasal turbinates / conchae are rather simple, but similar in position to those of other mammals.
Human nasal turbinates / conchae are rather simple, but similar in position to those of other mammals.

Ruben et al have argued in several papers that:[32][16][17][18][33]

  • No evidence of nasal turbinates has been found in dinosaurs (the papers focussed on coelurosaurs)
  • All the dinosaurs they examined had nasal passages that were too narrow and short to accommodate nasal turbinates.
  • Hence dinosaurs could not have sustained the breathing rate required for a mammal-like or bird-like metabolic rate while at rest, because their lungs would have dried out.

However, objections have been raised against this argument:

  • Nasal turbinates are absent or very small in some birds (e.g. ratites, Procellariiformes and Falconiformes) and mammals (e.g. whales, anteaters, bats, elephants, and most primates), but these animals are fully endothermic and in some cases very active.
  • Other studies conclude that nasal passages of these dinosaurs were long enough and wide enough to accommodate nasal turbinates or similar mechanisms to avoid desiccation of the lungs.[34]
  • Nasal turbinates are fragile and seldom found in fossils. In particular none have been found in fossil birds.

[edit] Cardiovascular system

The possible heart of "Willo" the thescelosaur (center).
The possible heart of "Willo" the thescelosaur (center).

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.[35] Their conclusions have been disputed; other researchers published a paper where they assert that the heart is really a concretion. As they note, the anatomy given for the object is incorrect (for example, the "aorta" narrows coming into the "heart" and lacks arteries coming from it), it 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.[36] The original authors defended their position; they agreed that it was a type of concretion, but one that had formed around and partially preserved the more muscular portions of the heart and aorta.[37]

The question of how this find reflects on metabolic rate and dinosaur internal anatomy may be moot, though, regardless of the object's identity. Both modern crocodilians and birds, the closest living relatives of dinosaurs, have four-chambered hearts (albeit modified in crocodilians), so dinosaurs probably had them as well; the structure is not necessarily tied to metabolic rate.[38]

[edit] Metabolism

Scientific opinion about the life-style, metabolism and temperature regulation of dinosaurs has varied over time since the discovery of dinosaurs in the mid-19th century. Scientists have broadly disagreed as to whether dinosaurs were capable of regulating their body temperatures at all. More recently, the warm-bloodedness of dinosaurs (more specifically, active lifestyle and at least fairly constant temperature) has become the consensus view,[citation needed] and debate has focused on the mechanisms of temperature regulation and how similar dinosaurs' metabolic rate was to that of birds and mammals.

"Warm-bloodedness" is a complex and rather ambiguous term, because it includes some or all of:

  • Endothermy, i.e. the ability to generate heat internally rather than via behaviors such as basking or muscular activity.
  • Homeothermy, i.e. maintaining a fairly constant body temperature.
  • Tachymetabolism, i.e. maintaining a high metabolic rate, particularly when at rest. This requires a fairly high and stable body temperature, since: biochemical processes run about half as fast if an animal's temperature drops by 10C°; most enzymes have an optimum operating temperature and their efficiency drops rapidly outside the preferred range.[39]

Since the internal mechanisms of extinct creatures are unknowable, most discussion focuses on homeothermy and tachymetabolism.

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.

[edit] Evidence

Several lines of investigation have been used to ascertain the metabolic rates of dinosaurs, including anatomical, ecological and molecular evidence.

[edit] Growth rates

No dinosaur egg has been found that is larger than a basketball and embryos of large dinosaurs have been found in relatively small eggs, e.g. Maiasaura.[40] Like mammals, dinosaurs stopped growing when they reached the typical adult size of their species, while mature reptiles continue to grow slowly if they have enough food. Dinosaurs of all sizes grew faster than similarly-sized modern reptiles; but the results of comparisons with similarly-sized "warm-blooded" modern animals depend on their sizes:[41][42]

Weight (kg) Comparative rate 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 25,000kg Whales
A graph showing the hypothesized growth curves (body mass versus age) of four tyrannosaurids. Tyrannosaurus rex is drawn in black. Based on Erickson et al. 2004.
A graph showing the hypothesized growth curves (body mass versus age) of four tyrannosaurids. Tyrannosaurus rex is drawn in black. Based on Erickson et al. 2004.

Tyrannosaurus rex showed a spectacular "teenage growth spurt":[43][44]

  • 1 ton at age 10
  • very rapid growth to around 6 tons in the mid-teens (about 1 ton per year).
  • negligible growth after the mid-teens.

So dinosaurs grew from small eggs to several tons in weight very quickly. A natural interpretation of this is that that dinosaurs converted food into body weight very quickly, which requires a fairly fast metabolism both to forage actively and to assimilate the food quickly.[45]

But a statistical study of the relationship between adult size, growth rate and body temperature concluded that: larger dinosaurs were significantly warmer than smaller ones; the very large dinosaurs were inertial homeotherms (their temperatures were stable because of their sheer bulk); and dinosaur metabolism was ectothermic (in colloquial terms, "cold-blooded", because they did not generate as much heat as mammals when not moving or digesting food).[46]

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.[43] 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.[47]

[edit] Bone structure

Armand de Ricqlès discovered Haversian canals in dinosaur bones, and argued that they were evidence of endothermy in dinosaurs.[48] These canals are common in "warm-blooded" animals and are associated with fast growth and an active life style because they help to recycle bone in order to facilitate rapid growth and to repair damage caused by stress or injuries. Bakker argued that the presence of fibrolamellar bone (produced quickly and having a fibrous, woven appearance) in dinosaur fossils was evidence of endothermy.[8]

However as a result of other, mainly later research, bone structure is not considered a reliable indicator of metabolism in dinosaurs, mammals or reptiles:

  • Dinosaur bones often contain growth rings, formed by alternating periods of slow and fast growth; in fact many studies count growth rings to estimate the ages of dinosaurs.[42][43] The formation of growth rings is usually driven by seasonal changes in temperature, and this seasonal influence has sometimes been regarded as a sign of slow metabolism and ectothermy. But growth rings are found in polar bears and many hibernating mammals.[49][50]
  • Fibrolamellar bone is fairly common in young crocodilians and sometimes found in adults.[51][52]
  • Haversian bone has been found in turtles, crocodilians and tortoises,[53] but is often absent in small birds, bats, shrews and rodents.[52]

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:[54]

  • Even the earliest archosauriformes may have been capable of very fast growth, which suggests they had fairly high metabolic rates. Although drawing conclusions about the earliest archosauriformes from later forms is tricky, because species-specific variations in bone structure and growth rate are very likely, there are research strategies than can minimize the risk that such factors will cause errors in the analysis.
  • Archosaurs split into three main groups in the Triassic: ornithodirans, from which dinosaurs evolved, remained committed to rapid growth; crocodilians' ancestors adopted more typical "reptilian" slow growth rates; and most other Triassic archosaurs had intermediate growth rates.


[edit] Oxygen isotope ratios

The ratio of 16O and 18O in bone depends on the temperature at which the bone was formed - the higher the temperature, the more 16O.

Barrick and Showers (1999) analyzed the isotope ratios in two theropods that lived in temperate regions with seasonal variation in temperature, Tyrannosaurus (USA) and Giganotosaurus (Argentina):

  • dorsal vertebrae from both dinosaurs showed no sign of seasonal variation, indicating that both maintained a constant core temperature despite seasonal variations in air temperature.
  • ribs and leg bones from both dinosaurs showed greater variability in temperature and a lower average temperature as the distance from the vertebrae increased.

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.

[edit] Predator-prey ratios

Bakker argued that:[55]

  • cold-blooded predators need much less food than warm-blooded ones, so a given mass of prey can support far more cold-blooded predators than warm-blooded ones.
  • the ratio of the total mass of predators to prey in dinosaur communities was much more like that of modern and recent warm-blooded communities than that of recent or fossil cold-blooded communities.
  • hence predatory dinosaurs were warm-blooded. And since the earliest dinosaurs (e.g. Staurikosaurus, Herrerasaurus) were predators, all dinosaurs must have been warm-blooded.

This argument was criticized on several grounds and is no longer taken seriously (the following list of criticisms is far from exhaustive):[56][57]

  • Estimates of dinosaur weights vary widely, and even a small variation can make a large difference to the calculated predator-prey ratio.
  • Fossil beds may not accurately represent the actual populations, e.g. smaller and younger animals have less robust bones and are therefore less likely to be preserved.
  • Bakker obtained his numbers by counting museum specimens, but these have a bias towards rare or especially well-preserved specimens, so they do not even represent what exists in fossil beds.
  • There are no published predator-prey ratios for large ectothermic predators, because such predators are very rare and mostly occur only on fairly small islands. Large ectothermic herbivores are equally rare. So Bakker was forced to compare mammalian predator-prey ratios with those of fish and invertebrate communities, where life expectancies are much shorter and other differences also distort the comparison.
  • The concept assumes that predator populations are limited only by the availability of prey. But many other factors can hold predator populations below the limit imposed by prey biomass, and this would misleadingly reduce the predator-prey ratio. Other possible limiting factors include shortage of nesting sites, cannibalism or predation of one predator on another.
  • A predator might prey on only some of the "prey" species present, and this would misleadingly reduce the predator-prey ratio.
  • Disease, parasites and starvation might kill some of the prey animals before the predators get a chance to hunt them.
  • It is very difficult to state precisely what preys on what. For example the young of herbivores may preyed upon by lizards and snakes while the adults are preyed on by mammals. Conversely the young of many predators live largely on invertrebrates and switch to vertrebrate as they grow.

[edit] Posture and gait

Hip joints and limb postures.
Hip joints and limb postures.

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.[58]

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.[59]

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.[8][60]

[edit] Feathers

Main article : Feathered dinosaurs

Skin impression of the hadrosaurEdmontosaurus
Skin impression of the hadrosaurEdmontosaurus

There is now no doubt that many theropod dinosaur species had feathers, including Shuvuuia, Sinosauropteryx and Dilong (an early tyrannosaur).[61][23][62] 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.[63] 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.[64] The skin of ceratopsians consisted of large polygonal scales, sometimes with scattered circular plates.[65] "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.[66] Likewise there is no evidence of insulation in the stegosaurs.

[edit] Polar dinosaurs

Dinosaur fossils have been found in regions that were close to the poles at the relevant times, notably in southeastern Australia, Antarctica and the North Slope of Alaska. There is no evidence of major changes in the angle of the Earth's axis, so polar dinosaurs and the rest of these ecosystems would have had to cope with the same extreme variation of day length through the year that occurs at similar latitudes today (up to 24 hours of daylight in summer, up to 24 hours of darkness in winter).[67]

Studies of fossilized vegetation suggest that the Alaska North Slope had a maximum temperature of 13°C and a minimum temperature of 2º to 8°C in the last 35M 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.[67] 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.[68] 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 dinsaurs would have had to be warm-blooded irrespective of whether they migrated or stayed for the winter.[45]

It is more difficult to determine the climate of southeastern Australia when the dinosaur fossil beds were laid down 105 to 115 million years ago (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 -6°C and +3°C; oxygen isotope studies of these deposits give a mean annual temperature of -2º ± 5°C; but the diversity of fossil vegetation and the large size of some of fossil trees far exceed that 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 -2.9°C). [67] An annual migration from and to southeastern Australia would have been very difficult for fairly small dinosaurs in such as Leaellynasaura (a vegetarian about 60-90 centimetres [2.0-3.0 ft] long), because seaways to the north blocked the passage to warmer latitudes.[67] Bone samples from Leaellynasaura and Timimus (an ornithomimid about 3.5 metres [11.5 ft] long and 1.5 metres [5 ft] 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.[69]

[edit] Evidence for behavioural thermoregulation

Some dinosaurs, e.g. Spinosaurus and Ouranosaurus, had on their backs "sails" supported by spines growing up from the vertebrae. (This was also true, incidentally, for the synapsid Dimetrodon.) Such dinosaurs could have used these sails to:

  • take in heat by basking with the "sails" at right angles to the sun's rays.
  • to lose heat by using the "sails" as radiators while standing in the shade or while facing directly towards or away from the sun.

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.[70]
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.[71]

[edit] The crocodilian puzzle

It appears that the earliest dinosaurs had the features on which the arguments for warm-blooded dinosaurs are based - especially erect limbs. This raises the question "How did dinosaurs become warm-blooded?" The most obvious possible answers are:

  • "Their immediate ancestors (archosaurs) were cold-blooded, and dinosaurs began developing warm-bloodedness very early in their evolution." This would imply that dinosaurs developed a significant degree of warm-bloodedness in a very short time, possibly less than 20M years. But in mammals' ancestors the evolution of warm-bloodedness seems to have taken much longer, starting with the beginnings of a secondary palate around the beginning of the mid-Permian[72] and going on possibly until the appearance of hair about 164M years ago in the mid Jurassic[73]).
  • "Dinosaurs' immediate ancestors (archosaurs) were at least fairly warm-blooded, and dinosaurs evolved further in that direction." This answer raises 2 problems: (A) The early evolution of archosaurs is still very poorly understood - large numbers of individuals and species are found from the start of the Triassic but only 2 species are known from the very late Permian (Archosaurus rossicus and Protorosaurus speneri); (B) Crocodilians evolved shortly before dinosaurs and are closely related to them, but are cold-blooded (see below).

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:

  • If dinosaurs were to a large extent "warm-blooded", when and how fast did warm-bloodedness evolve in their lineage?
  • Modern crocodilians are cold-blooded but have several features associated with warm-bloodedness. How did they acquire these features?

Modern crocodilians are cold-blooded, but they have several features which are normally associated with warm-bloodedness because they improve the animal's oxygen supply:

  • 4-chambered hearts. Mammals and birds have 4-chambered hearts. Non-crocodilian reptiles have 3-chambered hearts, which are less efficient because they allow oxygenated and de-oxygenated blood to mix and therefore send some de-oxygenated blood out to the body instead of to the lungs. Modern crocodilians' hearts are 4-chambered, but are smaller relative to body size and run at lower pressure than those of modern mammals and birds. They also have a bypass which makes then functionally 3-chambered when under water, conserving oxygen.[74]
  • a secondary palate, which allows the animal to eat and breathe at the same time.
  • a hepatic piston mechanism for pumping the lungs. This is different from the lung-pumping mechanisms of mammals and birds but similar to what some researchers claim to have found in some dinosaurs.[16][18]

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?

Reconstruction of Terrestrisuchus, a very slim, leggy Triassic crocodylomorph.
Reconstruction of Terrestrisuchus, a very slim, leggy Triassic crocodylomorph.

It was suggested in the late 1980s that that crocodilians were originally active, warm-blooded predators and that their archosaur ancestors were warm-blooded.[59][39] 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.[75][76]

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.[54]

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 earliest crocodilians, e.g. Terrestrisuchus, were slim, leggy terrestrial predators.
  • Erect limbs appeared quite early in archosaurs' evolution, and those of rauisuchians are very poorly adapted for any other posture.[77]

[edit] References

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