Australia: The Land Where Time Began
Dinosaurs3 were archosaurs that were from small to gigantic that arise in the Triassic and survived until the close of the Cretaceous. They were present on all continents.
The leg posture was erect, the head of the femur fitting into a perforated hip socket and the ankle is a simple hinge joint. The hindlimbs are dominant in all theropods, whether the only legs are used for walking or if all 4 limbs are used the hindlimbs are built more strongly than the arms. When walking the wrists and ankles are held clear of the ground, the hands and feet being digitigrade. When quadrupedal the hands are always at least as far from the midline as the feet, or even further apart, as can be seen from the trackways. Also seen from the trackways they never hopped and their tails were usually carried clear of the ground. When scales are known they form a mosaic pattern that is not overlapping.
It is believed all dinosaurs laid hard-shelled eggs in pairs, nesting on the ground, growth rates were often moderate, though sometimes rapid, usually achieving sexual maturity before growth complete.
Habits & behaviour3
Strongly terrestrial though all being capable of swimming, none were marine. Apart from these characteristics they were highly variable.
Classification - based on the 3rd edition of Benton's Vertebrate Palaeontology
The opinion about the dinosaurs until relatively recently was that they were so slow and stupid that it was a wonder they had lasted as long as they did. The discovery of Deinonychus was a problem for the traditional view of dinosaurs, as it appeared to be a fleet-footed predator with a relatively large brain that gave the impression it was fast runner and a veritable killing machine that seemed to need more than a sluggish ectothermic reptilian metabolism. One researcher who thought there was more to dinosaurs than was assumed in the old ideas about their physiology and behaviour was Robert Bakker.
According the author2 Robert Bakker 'took up this theme by aggressively challenging the view that dinosaurs were dull, stupid creatures'. One of Bakker's arguments was that there was compelling evidence dinosaurs were the equivalent of mammals and birds. These ideas were first raised in comments by Richard Owen in 1842, when he first conceived the idea of dinosaurs.
The maintenance of high activity levels by their endothermic, warm-blooded, physiology is the feature that makes mammals and birds 'special'. Sustained aerobic activity levels, and the ability to be highly active regardless of the temperature of the environment are due to the high, constant body temperature of the living endotherms. These attributes distinguish birds and mammals from other vertebrates.
Bakker used anatomical observations made by Ostrum, and agreeing with Owen, he argued the following points:
The author2 says Bakker also studied dinosaur bone thin sections microscopically, finding evidence of complex structure and a rich supply of blood that would enable a rapid turnover between the bone and plasma of vital minerals, exactly the condition found in mammals of the present.
Bakker also studied the relative abundances of predators and the animals that have been assumed to be their prey species using fossils representing time-averaged communities both from the fossil record and the relative abundances of predators and prey of the present that have been studied directly. When he compared modern endotherm communities, cats, and modern ectotherm predators, predatory lizards, he estimated that the endotherms consumed an average of 10 times the volume of prey species during the same time period than did the ectotherms. In the ancient communities from the Permian, found in museum fossil collections, he found rather similar numbers or potential predators and prey. In the dinosaur communities from the Cretaceous he found the number of potential prey was much larger compared to the number of predators. Which is the same conclusion he came to after studying Tertiary mammal communities.
According to the author2 though he based his studies on simple proxies, he suggested that predatory dinosaurs must have had metabolisms that were more similar to mammals, and possibly this could apply to some or all of the other dinosaurs. If the communities were to stay in balance their needed to be enough prey to support the population of predators.
He also studied the macroevolutionary evidence - large scale patterns in fossil abundance based on the fossil record. He searched for evidence that might have a bearing on the putative physiology of dinosaurs using the ages of origin and extinction of dinosaurs. The dinosaurs arose during the Late Triassic, about 225 Ma, that coincided with the time at which some of the most mammal-like reptiles evolved, the first true mammals arising about 200 ma. Bakker suggested that the dinosaurs outcompeted the mammals because they evolved an endothermic metabolism slightly earlier than the mammals. He also suggested that if they had not been the first to achieve an endothermic physiology they would not have been able to outcompete the mammals. As support for this idea he noted that the mammals remained as small insectivores and scavengers that were probably nocturnal throughout the Mesozoic, when the terrestrial faunas were dominated by dinosaurs, only diversifying rapidly when the dinosaurs became extinct at the close of the Cretaceous about 65 Ma.
Bakker argued that if the dinosaurs had not been endotherms the endothermic mammals that were 'supposedly' superior would have replaced the dinosaurs as the dominant animals of the terrestrial ecosystems by the Early Jurassic. Bakker suggested that a cooling period at the close of the Cretaceous was the factor that finally led to the extinction of the dinosaurs. He believed that the dinosaurs were large and endothermic but lacked a covering of hair or feathers to keep them from losing too much heat from the skin in cooler conditions they were unable to survive long enough until the climate warmed again. The mammals and birds had no such problems with cool weather so they thrived in a world without dinosaurs. He also believed the dinosaurs were too big to do what modern reptiles do and overwinter in a burrow or other place with some protection from the worst of the weather.
Based on all the above Bakker proposed that dinosaurs were intelligent, highly active animals that 'had stolen the world from the traditionally superior mammals for the remaining 160 million years of the Mesozoic'. He suggested that instead of being forced to extinction by evolutionary superior mammals they had succumbed to a freakish climatic event that occurred 65 Ma.
Were dinosaurs warm-blooded
At the time Richard Owen invented the term 'dinosaur' he stated in the last sentence of his paper 'The Dinosaurs ... may be concluded to have ... [a] superior adaptation to terrestrial life ... approaching that which now characterises the warm-blooded Vertebrata'.
Almost 50 years later Thomas Huxley proposed that dinosaurs and birds should be considered close relatives, based on the similarities of their anatomy that could be demonstrated between Archaeopteryx, the earliest known fossil bird at that time, living birds and Compsognathus, a theropod that was newly discovered at that time, concluding
... it is by no means difficult to imagine a creature completely intermediate between Dromaeus [an emu] and Compsognathus [a dinosaur] ... and the hypothesis that the ... Class Aves has its root in the Dinosaurian reptiles; ...
(Huxley 1868: 365)
According to the author2 all things being equal it could be expected that the appearance of the true mammals about 220 Ma at the Triassic-Jurassic boundary, a time when the land faunas of the world were dominated by reptiles, the mammals would rapidly diversify and push the then dominant reptiles to extinction, or at least to a diminished role in the faunas. According to this expectation there should have been a rapid rise of the mammals in the Early Jurassic and eventual dominance of the ecosystems of the Mesozoic. What is seen from the fossil record is that it was the dinosaurs that rose to dominance in the Late Triassic, about 220 Ma, while the mammals only increasing in size and diversity after the dinosaurs had gone extinct. According to Bakker's proposal the dinosaurs required an endothermic metabolism to achieve this dominance.
Dinosaurs had their legs beneath their bodies. The birds and mammals are the only living vertebrates to do this, the others all sprawl with their legs pointing away from the bodies. The legs of many dinosaurs were slender, a feature associated with fast movement, reflecting the fact that in nature structures usually tend to be as they are for a reason, nature doesn't waste resources. The author2 suggests it might therefore be expected that an animal with slender legs would need an energetic 'motor', endothermic physiology to make use of its potentially fast-running legs. Speed of movement alone is not the exclusive domain of endotherms, humans can be run down by ectotherms such as crocodiles, the difference in the speed between endotherms and that of ectotherms is that the length of time the speed can be maintained. If the gap between a human and a crocodile or komodo dragon is large enough the ectothermic reptiles would tire much sooner than the endothermic human, as the muscles of ectotherms build up a large oxygen debt so they need to rest until their muscles have recovered. Endotherms have the advantage in sustained running because their more efficient and high-pressure blood system quickly replenish the oxygen in their muscles.
Bipedal walking has been suggested as a refinement of this argument, as it linked exclusively to endothermy, being used by many mammals, all birds and some dinosaurs. This argument is related to the maintenance of the posture as well as the posture itself. Considerable stability when walking is a feature of quadrupeds. Bipeds need a sophisticated system of sensors monitoring balance to walk successfully, as well as a coordinating system that is rapid, brain and central nervous system, as well as muscles that are of a rapid-response type to correct and maintain balance as bipedalism is inherently unstable.
The brain needs to have a constant capacity to work quickly and efficiently as it is central to the whole dynamic 'problem'. To allow the brain chemistry to function optimally at all times it is necessary that oxygen, food and heat are supplied constantly. According to the author2 for this type of stability it is essential to have a 'steady' endothermic physiology. In situations such as when cold the activity levels of ectotherms shut down periodically, reduce the nutrient supply to the brain, which is consequently less sophisticated and integrated closely to overall body functions.
There is also also a requirement for an efficiently operating heart if the ability to pump at high pressure to the brain which is often appreciably higher than the heart in many birds, mammals and dinosaurs that have adopted an upright posture. There are important hydrostatic consequences of this head-hart height difference. As the head is higher than the heart blood must be pumped at high pressure up to the brain, though blood being pumped at this pressure to the lungs would burst the delicate capillaries of the lungs. This problem has been circumvented by the heart of such animals being divided in to separate sections that pump blood to different circulatory systems, the left side to the head and body that run at high pressure, while the right half of the heart pumps blood to the pulmonary circulation that takes blood to the lungs at low pressure.
The head of all living reptiles is close to the same level as the heart. As a consequence of this their hearts are not divided in 2 separate sections for the systemic and pulmonary circuits. The heart and circulatory system of reptiles can also have advantages for ectotherms as they are capable of shunting blood around their bodies in ways that mammals and birds cannot. One such advantage is that when an ectotherm is basking in the sun to raise it body temperature it can shunt blood preferentially to the skin where it can absorb heat and carry it to other parts of the body. With this system there is also a major disadvantage, the blood cannot be pumped at high pressure, a feature that is essential for an active, fast moving animal that needs to get oxygen and food to hard-working muscles.
The author2 suggests all this implies that because of their posture dinosaurs had a high-pressure blood circulatory system that allowed them to have a lifestyle that was active and that these high levels of activity could be sustained, such as a system that in the modern world is present in the endothermic mammals. He also states that Richard Owen's speculation is strongly supported by these observations.
If it is to allow the efficient heart and circulatory system to have maximum effect it also requires that dinosaurs must have been able to keep the muscles supplied with sufficient oxygen to allow the high aerobic activity levels to be sustained. In some saurischian dinosaurs such as theropods and sauropodomorphs, though not in the ornithischians, traces have been found of pleurocoels, pouches or cavities in the sides of the vertebrae. In the living birds similar structures are equated with the presence of extensive air sacs. The very efficient pulmonary systems of birds involves just such air sacs that are indicated in the saurischian dinosaurs, that are part of a bellows-like system that makes the breathing so efficient. The author2 suggests it is very likely they had the very efficient lungs that are found in birds of the present.
According to the author2 the suggestion shows that the theropods and sauropodomorphs could maintain high aerobic levels of activity, though it also shows that it should not be presumed that all dinosaurs were the same in all aspects of their physiology, as there are no known traces of air sacs in the ornithischians.
Brain size and dinosaur sophistication
Deinonychus was a visual predator with large eyes that is indicated by its build and the proportions of its legs to have been a fast runner with an unusually stiff tail, gaff-like toes on its inner hind feet and long grasping hands with large claws. The author2 suggests that the build of this dinosaur indicates it was probably a pursuit predator that used its stiff tail to assist in maintaining its balance while making sharp turns as it ran at high speed. The tail would have allowed it to make rapid direction changes, as well as probably when it leapt onto a prey animal, that it possibly disabled with the claws on its hind feet.
Some support for this scenario was found in Mongolia where a Velociraptor, closely related to Deinonychus, was found grasping the head of a Protoceratops and was apparently caught in the act of kicking at the throat of the Protoceratops. Both animals have been suggested to have died by suffocation during a dust storm.
Activity levels seen in endotherms of the present are indicated to have also applied to dinosaurs by design 'sophistication', inferred function and way of life.
Mammals and birds have large brains and exhibit what appears to be intelligence. The reptiles, ectotherms, have much smaller brains and are not normally perceived as being as intelligent as mammals or birds. The author2 suggests there appears to be a link between endothermy and brain size. A constant supply of oxygen and food and a stable temperature are requirements of large, complex brains. The supply of oxygen and food to the brains of reptiles is not a problem, but maintaining a constant temperature is, their body temperature varying across a normal 24-hour cycle, that prevents them from developing a large, sophisticated brain.
Comparison of the ratio of brain volume to body volume of a range of animals, that included dinosaurs, was carried out by Jim Hopson of the University of Chicago. His results demonstrate that while most dinosaurs had the typical reptilian ratio there were some that didn't. These more 'brainy' dinosaurs were the theropods that were highly active and bipedal.
Distribution by latitude
Fossils of dinosaurs have now been found in Yukon area of North America and in the Southern Hemisphere, in Australia and Antarctica. During the Cretaceous all these areas would have been in their respective polar regions, which suggests that dinosaurs needed to be endothermic to survive in such cold conditions, as is the case at the present, ectotherms don't live in areas where the temperature is too low for them to remain active.
The author2 suggests there is a problem with this conclusion. evidence from the plant fossil record suggests that during the Cretaceous Mediterranean and subtropical plants were living in these polar regions. A feature of these plants is the leaf dropping that occurs in winter when the light levels and temperatures are low. No evidence has been found of polar ice caps in the Cretaceous, suggesting that even at high latitudes the summer temperatures were extremely mild. It has been suggested that in the winter the herbivorous dinosaurs migrated north or south away from their respective poles to find sufficient food, returning to the poles during the summer.
According to the author2 the world at the Triassic/Jurassic boundary was not particularly suited to endotherms, with generally arid, though seasonal conditions affecting much of Pangaea, deserts were becoming widespread around the globe. Selective pressures on endotherms and ectotherms that were exerted by these conditions of high temperatures but low rainfall affected the 2 groups differently.
Less food is required by ectotherms than endotherms. Ectotherms are better able to survive such difficult conditions of low biological productivity than are endotherms. Water loss is reduced by the scaly skin of reptiles in such arid conditions, and they also further reduce water loss by excreting material that is dry and pasty, not unlike bird droppings, instead of urinating. Ectotherms are suited to high ambient temperatures because their internal chemistry can operate at optimum temperatures under such conditions with few problems. As ectotherms are built in the classic reptilian mould they can be expected to cope well in arid, desert-like conditions..
Mammals, on the other hand, being endotherms can be subjected to physiological stress under such conditions as high temperatures. Mammals, being endotherms, have evolved to lose excess heat to the environment, the internal thermostats maintaining the body at temperatures above that of normal environmental conditions, adjusting their physiology accordingly. The fur of animals can be raised, the insulating air layer becoming thicker than normal, to reduce heat loss in cold conditions, and generate extra muscular heat by shivering, or raising their metabolic rate. It becomes vital to lose heat to the environment to avoid lethal overheating under high ambient temperature conditions. Mammals have several options for losing heat, one of which is evaporative cooling that can be achieved by either by panting, as in dogs, or sweating through pores in the skin, sweat glands, as in humans. The problem with this method of losing heat is that large volumes of water are lost to the air in the process, with the potential for a fatal amount of water loss. Mammals also lose water in urine, urinating being necessary to flush out wastes. Some, such as the Spinifex Hopping Mouse, that lives in very arid parts of Australia, have reduced the water in its urine to an absolute minimum, though it still needs to urinate. Mammals also require much more food than ectotherms to maintain their physiology. The low productivity of deserts cannot support large endotherm populations.
The author2 suggests that environmentally the Late Triassic/Early Jurassic was possibly an unusual time, the climate possibly being such that it favoured the ectotherms and suppressed the endotherms such as the early mammals, that were restricted to small size and probably to nocturnal niches. Nearly all mammals inhabiting deserts of the present are small nocturnal insectivores and rodents, apart from animals such as the camel. They dig burrows beneath the sand to escape the extreme heat of the day because the deeper levels of the sand are much cooler than the surface, and they can also conserve water better because the burrows become more humid than the dry air at the surface, emerging at night to feed when the temperature is much lower.
Following the beginning of the breakup of Pangaea continental seas began to spread where the continents would eventually separate with the result that the arid conditions of the Late Triassic/Early Jurassic began to ameliorate. Once this began to happen the climate is believed to have changed to one that was extremely warm and humid across very broad bands of latitude. From the time the dinosaurs arose until their demise their doesn't seem to have been any glaciers at the poles. According to the author2 the climate of the present, with ice caps at both poles, appears to be an unusual condition when compared to much of the history of the Earth, and as a result unusually narrow climatic bands. Productivity is believed to have risen dramatically under the changed conditions of the Jurassic and in areas that had vegetation cover of forests that were long-lived and densely forested, major coal deposits were laid down. These conditions have been suggested to have led to a wide range and diversity of dinosaurs during the Jurassic.
The processes involved in finding features of dinosaur brains can be seen in the DVD of the National Geographic series Jurassic CSI, part 2.
Many dinosaurs grew to very large sizes, a condition that could have advantages under certain conditions. An advantage of large size, when temperature are not too high, is the heat exchange between the body and the environment occurs very much more slowly than it does in small animals, thus allowing the large animal to maintain a constant body temperature whatever the ambient temperature of its environment. Even in ectotherms such as large crocodiles they are able to maintain a very stable internal body temperature day and night. Hatchlings, on the other hand, have a body temperature range matching exactly the night and day temperatures. Therefore it is believed large dinosaurs' internal body temperature changed little over time. Another result of being large was that postural muscles had to work hard to stop the animal collapsing under its own weight. Significant quantities of heat are generated by muscular activity that can assist in keeping the internal body temperature constant.
The author2 suggests that because saurischian dinosaurs probably had a bird-like lung system suggests they were capable of providing enough oxygen to their tissues to sustain aerobic activity.
Based on these factors is seem very likely that dinosaurs had many of the attributes that are associated with endothermy at the present. Dinosaurs would have been relatively thermally inert, as they were typically large. They also lived at a time when the global climate was constantly warm and non-seasonal. The author3 states that though the argument is convincing it fails to take the close relationship between dinosaurs and birds into consideration.
These are fossilised dung. They were first recognised by William Buckland of Oxford University. When he examined polished sections of coprolites he could identify fish bones and large concentrations of the sharp hooks from belemnite tentacles. As predatory marine reptiles were also found in the same rocks he concluded that these coprolites were probably produced by these reptiles. Coprolites have been useful in determining what various animals ate, the main problem being determining which animals produced them.
Coprolites, and sometimes the gut contents, have been found inside the fossils of vertebrates, notably fish. The connection of coprolites to specific dinosaurs has proven difficult. In 1998 Karen Chin et al. of the USGS reported their discovery of a large coprolite in an article entitled 'A King-sized theropod coprolite'. They had discovered the coprolite in sediments from the latest Cretaceous (Maastrichtian) from Saskatchewan. It was in the form of a knobbly lump of material that was more than 40 cm long and had a volume of about 2.5 litres. Broken bone fragments and bone material was found around and immediately inside the specimen and finer, sand-like powder of bone material was present throughout the mass.
High concentrations of bone material were confirmed by chemical analysis that found high levels of calcium and phosphorus. Thin sections of the fragments were studied histologically. This study confirmed the cellular structure of bone, and that the most likely prey were dinosaurian, indicating that the coprolite was most likely produced by a large carnivore. The only animal found in the rocks of the area that was large enough to produce such a coprolite was the large theropod T. rex. The bone material in the coprolite indicated that the bones of the prey animal had been pulvrised in the mouth of the carnivore. From the bone structure in the histological thin sections it is believed a juvenile ceratopsian ornithischian was the most likely prey. As not all the bone in the coprolite had been digested indicates that the material had moved through the gut rapidly, that could suggest to some that T. rex was an endotherm.
While dinosaurs were present in Australia during the Mesozoic, like anywhere else in the world at the time, their discovery came much later than in other places. The first meagre finds were made in the mid-1800s, but the records of the earliest discovered ones were not good. It is only comparatively recently that more significant finds were made.
Most of the sediments deposited in the Australian Triassic are river and lake deposits, with some minor marine deposits. At this time Australia was in fairly low latitudes, still part of Gondwana, and the climate was humid and temperate, with seasonal variation in rainfall. The vast tracts of arid land hadn't yet developed, and the continent was well-watered and covered with vegetation. The Glossopteris flora had given way to the Dicroidium Flora, pre-adapting many components of this flora for the coming aridity.
Many fish and the large amphibian labyrinthodonts lived in rivers and lakes. Among the fish were many osteichthyans (bony fish) - the deep-bodied Cleithrolepsis, primitive sharks, some with neck spines (xenacanths). Fossil fish from this time are well represented in fossil deposits in the St Peters, Gosford and Brookvale areas of New South Wales and the Knocklofty Ranges near Hobart in Tasmania.
Amphibians and reptiles from this age are best known from the sites near Rolleston and Bluff in outcrops of the Arcadia Formation and Rowan Formation. In New South Wales, the sandstones near Sydney - Narrabeen Group and Wianamatta Group, and the Hawkesbury Sandstone. In the Erskine Ranges of Western Australia, in outcrops of the Blina Shale, near Blina Station. In Tasmania, the Knocklofty Formation sites around Hobart, some rare early reptiles have been found only at Knocklofty Formation sites and the Queensland sites. The rest of the sites contain only fish and labyrinthodonts.
Therapsids (mammal-like reptiles) were very abundant in Gondwana at this time, but not apparently in the Australian section. Fragments of a therapsid have been found in Queensland, but so far nowhere else in Australia.
In Australia there is a very diverse assemblage of Triassic amphibians, all of which belong to the temnospondyl group. No Triassic dinosaur bones have been positively identified in Australia. The only hint of their presence is some dinosaur bones that were collected somewhere in north Queensland in the mid 1800s, named Agrosaurus, that was thought to be of Late Triassic or possibly Early Jurassic age.
What is a dinosaur?3
The tetrapods, the vertebrates that have adapted for a terrestrial lifestyle – amphibians, reptiles, mammals and birds.
Amniotes are the tetrapod groups that lay hard-shelled eggs, though some have become livebearers.
The diapsida is the other group that includes the living tuataras that are lizard-like, true lizards and snakes, crocodilians and birds. Of the diapsida the Archosauria is the largest and most successful of the group, including dinosaurs and crocodilians. According to the author3 birds are literally flying dinosaurs.
Included among the Archosaurs are the basal forms that are known informally as the thecodonts, alluding to their socket teeth, a diverse group of aquatic and terrestrial forms that include the ancestors of crocodilians and pterosaurs that are not closely related to dinosaurs and birds.
There is general agreement among most researchers that the dinosaurs are a monophyletic group in that they shared a common ancestor, making then different from all other synapsids. Prior to 1970 it was widely believed that dinosaurs were of 2 distinct types that had arisen separately from thecodont ancestors. Birds were also believed to have evolved independently from another anther group of thecodonts. They are now the 2 major parts of the Dinosauria, the Saurischia and the Ornithischia, in a similar way to Mammalia being divided into the marsupials and the placentals. The formal definition of Dinosauria is the phylogenetic clade that includes the common ancestor of Triceratops and birds and all their descendants. The results differ to some extent between the various attempts to determine the exact relationships of the earliest dinosaurs there is some disagreement about whether the most primitive theropods, that had 4 toes, should be regarded as dinosaurs or just outside the group. In Paul’s book3 they are included in the Dinosauria.
One of the characteristics that most distinguishes dinosaurs anatomically is the hip socket. The femur head is a cylinder that is turned in at right angles to the femur shaft that fits into a hip socket that is internally open. This structure allows the legs to operate the characteristic near-vertical plane of the group, the feet being directly beneath the body. A vertical leg posture is also favoured by the ankle that is a simple fore-aft joint.. As a group dinosaurs were “hind-limb dominant”, even quadrupedal forms carrying most of their weight on their legs, the legs of dinosaurs being built more strongly than the arms. The hands and feet were generally digitigrade, with the wrist and ankle being clear of the ground.
Even when the birds are excluded dinosaurs were a group that were extremely diverse, that rivaled mammals in diversity. They ranged in form from the dromaeosaurs with their sickle-claws and a bird-like appearance to the ceratopsians with horns that had an appearance somewhat like a rhino, stegosaurs that were armour-plated to sauropods that were elephant-like and giraffe-like, and pachycephalosaurs that had domed heads, and birds that evolved the ability to fly. A limitation of dinosaurs was that they were persistently terrestrial. The author states that some birds were the only strongly aquatic dinosaurs, though some possibly spent time feeding in the water, in the fashion of moose or fishing cats. The most aquatic forms became strongly amphibious like hippos, though these remained much less than seals and whales. The marine reptiles of the Mesozoic were a number of forms of reptiles that took to the seas, but they weren’t dinosaurs.
The dinosaurs other than birds are sometimes referred to as non-avian dinosaurs, as birds are dinosaurs in the same way that bats are mammals, flying dinosaurs – flying mammals. In most literature that are simply referred to as dinosaurs.
During the dominance of the dinosaurs in the Mesozoic mammals were, according to the author3, small nocturnal animals that were as abundant and diverse as they are at the present. He also states that if the non-avian dinosaurs hadn’t succumbed during the mass extinction at the close of the Cretaceous they would probably still be dominant to mammals at the present.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|