The evolution of dinosaurs and their environment

Australia: The Land Where Time Began

The evolution of dinosaurs and their environment

Triassic

As the Mesozoic opened the terrestrial vertebrates had been severely depleted but there was soon a bout of explosive radiation that saw the few therapsids that had survived the Permian Mass Extinction flourish once again, again becoming important parts of the terrestrial fauna, though now they had competition from the archosaurs that underwent an explosive radiation of their own. The first phase of the archosaur radiation saw a large number of thecodont forms, some reaching large size, possibly a tonne or more. One group of these thecodonts evolved into crocodile-like forms, while others evolved into armoured land herbivores. Many were forms carnivores with erect legs that had achieved their erect posture along a different path from the dinosaurs. Instead of the femur head turning inwards the hip socket expanded to envelop the femur head so that the femur shaft was directed downward. Of these erect-legged archosaurs some were almost bipedal, though others became herbivores lacking teeth. It seems the thecodonts of the Triassic filled the niches that would later be held by dinosaurs.

The crocodilians, the only group surviving to the present, arose at this time, giving a glimpse of the appearance of archosaurs from Triassic. The earliest crocodilians from the Triassic were small, digitigrade, land runners with long legs. The author³ suggests the evolution of their sophisticated liver-pump lung system may have evolved to power a highly energetic lifestyle.

Like many of the thecodonts the crocodilians had a feature that is considered very un-dinosaurian, in that their ankles were complex joints that have been compared to a door hinge, with the muscles on the foot having help with leverage from a tuber the projected from one of the ankle bones, in a manner similar to that of mammals. It was in the Triassic that the pterosaurs evolved. It has been suggested that the pterosaurs were related to dinosaurs because they had the same type of simple hinge-ankle that was present in the dinosaurs. The pterosaurs, being energetic animals, were insulated, though it is not yet known if other nondinosaurian archosaurs were also covered with thermal fibres.

A number of small predatory archosaurs appeared in the fossil record in the last stage of the Middle Triassic, the Landian that exhibited many of the features of dinosaurs. In these forms the head of the femur had been turned inwards, though the hip sock was still not open internally, that allowed the legs to function in a vertical plane. The had a simple hinge ankle, a skull that was lightly constructed. The lagosuchian protodinosaurs first appear in South America. It is not yet known if the group originated in South America or spread to there from elsewhere. By the Norian they had spread to at least North America. They were extinct by the Norian.

Herrerasaur theropods, that had fairly large bodies, small hips and 4 toes had arisen. The terrestrial faunas were dominated at that time by the archosaurs and they had gone by the early Norian, possibly because they lacked the aerobic capacity to compete with the competition.

The bird-footed avepod theropods appeared in the Norian, with large hips and the beginnings of a respiratory system that was of the avian type, imply an improved aerobic performance and thermoregulation. At about this time the herbivorous small-hipped prosauropods that were semibipedal made its first appearance on the fossil record. They were followed almost immediately by the quadrupedal sauropods that had bigger hips. The rapid expansion in diversity and size of these new forms gave the thecodonts increasing competition. Within 15-20 million years of their evolution of the first protodinosaurs there were prosauropods and sauropods up to 2 tonnes in mass had developed, and in another 10 million years there were sauropods as big as elephants, the first very large land animals. The first animals that could browse at high levels were these first long-necked herbivores. At this time dinosaurs were showing their potential for developing very large bulk, as well as dimensions, on land, the attributes that were also seen in the mammals. The first of the beaked herbivorous ornithischians arose in the Carnian. It has been suggested³ that these small semibipedal animals and the small prosauropods may possibly have dug burrows as protection from the many carnivores. The saurischians were becoming the dominant land animals by the last stage of the Triassic, though there were still thecodonts and some therapsids. The first mammals arose from these therapsids at this time, the mammals and dinosaurs have lived together for more than 200 million years, the mammals remaining small for the first 140 million years.

At this time, when the continents were fused together to form Pangaea, there was no part of the supercontinent that animals couldn’t roam to. The result was a tendency for faunas to vary little from one region to another. While the continents were all part of Pangaea the climate remained harsh over most of the supercontinent, described by the author³ as a greenhouse world, the prevailed throughout the Mesozoic.

Despite the Sun being slightly cooler than at present the temperatures were high enough to prevent even the poles were considered to have been fairly warm in winter, kept high by the atmospheric CO₂ levels that were 2-10 higher than at present.

As with Australia at the present there were not many tall mountain ranges, as a result of low levels of tectonic activity, to capture rain from any passing clouds, or seaways in the interior to provide moisture. During this time there were large expanses of desert and most vegetated lands were seasonally semiarid. Forests did exist in the few regions that received heavy rainfall and groundwater created by climatic zones and rising uplands. In many ways the flora was fairly modern. Rushes and horsetails grew along watercourses, and some ferns grew in wet areas and on shaded situations on the floors of forests. Some ferns grew in open areas that were seasonally arid. The author³ suggests that fern prairies may have existed over large parts of the world that was comparable to the grassland and shrubland of the present. In wetter areas it was common for tree ferns to be abundant, and the cycadeoids, similar to the extant cycads of the tropics, were even more abundant. The ginkoids, that prefer wetter areas, were among the taller trees. The maidenhair tree is the only survivor of this line. Conifers were the dominant plant type of the this time, most having broad leaves than the needles of the modern species. The conifers could grow into very large trees, and it was these trees that formed the petrified forest of Arizona. Angiosperms were completely absent.

There was another mass extinction about 200 Ma at the close of the Triassic. Millions of years before the extinction a giant impact occurred in south-eastern Canada. During this mass extinction of the land vertebrates the thecodonts and the therapsids were most affected, the thecodonts being exterminated and few of the therapsids survived along with their mammal relatives.

By the end of this mass extinction event crocodilians, pterosaurs and especially the dinosaurs survived very well, carrying on into the Jurassic with few problems.

Jurassic

The prosauropods appear to have been outcompeted by the related sauropods that were more sophisticated, and were extinct by the end of the Early Jurassic. According to the author³ the sauropods had some superior features, such as hip muscles that were larger and had begun developing a respiratory system that was of the bird type, 2 features that allowed them to evolve the high pressure circulatory system and the more efficient respiration that was necessary before they could grow to very large size. At this time the gigantic sauropods were not affected by the theropods that were only getting to moderate sizes. Ornithischians were still uncommon at this time, and one group was the first dinosaurs to develop armour as protection. Another group of ornithischians were small heterodontosaurs with chisel teeth that were semibipedal, are believed to probably be the first dinosaurs to have evolved fibre coverings to maintain a constant body temperature. Crocodilians remained small and were either fully or partially terrestrial at this time, though other groups evolved into marine giants.

The Tethys tropical ocean partially split Pangaea into Laurasia in the north and Gondwana in the south. The Mediterranean Sea is all that remains of this once huge ocean. African style rift valleys began forming further west along the east coast of what is now the eastern seaboard of North America as Pangaea began breaking up, the end result was the opening of the Atlantic Ocean. The increased tectonic activity that was occurring in various parts of Pangaea at this time resulted in a rising of the ocean floor in a number of places to such an extent that shallow epicontinental seaways formed in a number of places on the various constituent continents of Pangaea and this greatly affected the dinosaurs in the various regions that were that were being isolated by the marine incursions, the dinosaurs on each isolated region evolving and diversifying indifferent directions as a result of their isolation from each other.

As the surface area of water on the continents expanded it had another effect, the humidity levels of the atmosphere over the continents increased leading to increased rainfall, though in most habitats would still have a seasonal climate with yearly dry seasons. The rainfall was also increased by the formation of more mountain ranges by the tectonic activity.

The Middle Jurassic began about 175 Ma and was a time when the sauropods flourished, as their increasingly sophisticated circulatory and respiratory systems allowed them to grow even bigger. The author³ suggests they thrived in dry habitats by feeding on the forests that lined the river courses, and in wetter areas, by feeding on the fern prairies.

Some sauropods in China that was partially isolated by seaways evolved long, slender necks that allowed them feed 10 m (more than 30 ft.) above the ground. Some developed clubs and spikes on the tails for defence from the predators. The small armoured stegosaur ornithischians with tail spikes made their appearance at this time. The author³ suggests that a group of small ornithopods that were the beginnings of a group of ornithischians that had a respiratory system that possible paralleled that of mammals, and they had dental batteries that increased their evolutionary potential. Increasingly sophisticated theropods such as tetranurans, avetheropods and coelurosaurs evolved that had an avian type of respiratory system that was highly developed they didn’t produce any giant species at this time.

About 160 Ma, the Late Jurassic is suggested by the author³ to be the apogee of 2 herbivorous dinosaur groups, the sauropods and the stegosaurs. It was a time of the maximum diversity of the sauropods, with genera such as haplocanthosaurs, mamenchisaurs, dicraeosaurs, diplodocines, apatosaurines, camarasaurs and the first of the titanosaurs. Some neosauropods grew as large as 50-75 tonnes, with some reaching much more than 100 tonnes, around the size of the biggest of the baleen whales. The tallest of the sauropods could feed more than 20 m (70 ft.) into the trees.

By this time theropods first reached the size of hippos in the form of allosaurs and yangcuanosaurs that were the size of hippos. It was also a time when some of the sauropods that had become isolated on islands had been reduced to the size of rhinos as a result of dwarfing, a process that animals undergo when they are isolated with limited food resources. The anklylosaurs were diversifying. Large ornithopods with thumb spikes entered the fossil record for the first time. In Asia small semibipedal ceratopsians evolved. It was at this time that the stegosaurs were at their most diverse, with forms that were from rhino-sized to elephant-sized.

The ancestors of tyrannosaurs were still small are believed to have been developing at this time, as well as a number of maniraptor coelurosaurs that were gracile that were numerous. One theropod, Scansoriopteryx had a long thin finger similar to the aye-aye lemur indicating that some theropods were climbers. By the Late Jurassic the Alvarezsaurs had evolved. Their thick, short arms and hands are believed to have specialised for breaking into insect nests. A major occurrence at this time is the first appearance in the fossil record of the avepectorans that were probably at least partly arboreal. Archiornis is a Chinese dinosaur that is the first known dinosaur to have had large feathers on both its arms and legs. It has been suggested that this dinosaur, apparently a climber, could possibly the first example known of an animal that had lost an ability, in this case flying, as the feathers that were moderately long and symmetrical were not proper aerofoils in spite of the long arms. A few million years later what has been thought of as the first bird, Archaeopteryx, a deinonychosaur, evolved. Its status as the first bird has been challenged by a paper in Nature and some think it should be classified as a feathered dinosaur rather than an actual bird. See Xiaotingia zhengi. The deposits in which it was found were laid down in a lagoon that was at the time the northwest edge of the Tethys Ocean. The author³ suggests it was part of the process involved in the development of powered flight, as indicated by the combination of arms that were very large and wing feathers that were long and asymmetrical.

The first major increase in the mental powers of dinosaurs occurred as the brain size and complexity increased to the lower avian level, with the appearance of the small avepectorans. The pterosaurs retained the small brains and were of small size, most also retaining long tails. The crocodilians of types that were similar to the highly amphibious living members of the group were evolving at this time, though there were still some small terrestrial running types. The amphibious types readapted the liver-pump lung system to be buoyancy control devices. In the Jurassic the still small mammals were diversifying extensively. There were climbers that were insectivorous or herbivorous, some were burrowers, others were freshwater swimmers that reached a maximum of a few kilograms.

The CO₂ were very high during the Middle and Late Jurassic, reaching as high as -5-10 % of the atmosphere. At the close of the Jurassic the incipient Atlantic was of a similar size as the Mediterranean Sea of the present. As for the vegetation, it was much the same as it had been in the Triassic. According to the author³ there is a common impression that the monkey-puzzle tree, an umbrella-shaped member of the Araucariacae conifers from South America was a source of food for sauropods in the Jurassic, but he says this is wrong. There were conifers similar to cypress that dominated the wetter areas. He suggests the sauropods could be expected to have had a profound impact on the floral landscape by browsing heavily on the trees and probably also broke them as elephants do at the present, but to a much greater extent. There is disagreement about what happened to the fauna at the close of the Jurassic, some proposing a major extinction event, while others disagree with this scenario.

Cretaceous

At the start of the Cretaceous about 145 Ma the continents continued the breakup, the South Atlantic began opening and on the continents marine incursions were snaking their way across their land surfaces. At this time the dinosaurs underwent their greatest burst of evolution. The very high CO₂ levels were dropping, but not to the levels of the present. In the early part of the Cretaceous the arctic regions were balmy even in winter, kept warm by the warm Arctic Ocean.

The continental climate of the South Pole had winters that were cold enough for permafrost to form. The general climate of the globe tended to be slightly wetter than earlier in the Mesozoic, though seasonal aridity continued to be the rule in most places, with true rainforest continuing to be scarce at best.

The sauropods, that were often very large, continued to be abundant though their diversity had declined. Some diplodocoids, that were small-bodied and with short necks, some having evolved broad mouths that were square-ended as an adaptation for grazing. The predominant types were the tall brachiosaurs and the titanosaurs that were broad-bellied.

The Ornithischians came into their own in the Cretaceous, all sizes of ornithopods flourishing, but especially the large ones. In the Northern Hemisphere the iguanodonts, that had thumb spikes, became common among the herbivores. The author³ suggests that a key to their success may have been their well-developed batteries of teeth. In a few their vertebral spines grew long and formed a tall sail on their backs. He also says that it was only recently that it was realised that the heterodontosaur clade had not gone extinct in the Jurassic, surviving with little change until the Early Cretaceous in Asia. Among the ceratopsians the small psittacosaurs, that were chisel-toothed, from Asia proliferated. The related protoceratopsids, that had big heads, arose in the same region, as did the first pachycephalosaurs that had domed heads. The stegosaurs, that soon went extinct, the last of the major groups of dinosaurs to go extinct since the prosauropods. The author³ suggests that the dinosaurs continued increasing their diversity through the Mesozoic by their tendency to evolve new groups without losing the older groups. The ankylosaurs soon replaced the stegosaurs becoming a major portion of the global fauna. The ankylosaurs were extremely fat-bellied armoured dinosaurs with bodies that were low-slung. The plates and spikes would have protected them from the large predators of Laurasia, large allosauroids and in Gondwana, the abelisaurs that were snub-nosed and had short arms. The spinosaurs were another group of giant theropods that had crocodile-like snout that appeared to be adapted for catching fish that probably formed at least part of their diet. Based on bone isotopes it seem the spinosaurs were semiaquatic as are hippos of the present, though they don’t appear to have any adaptations for swimming. Some evolved large sails on their backs.

In the Early Cretaceous theropods, especially the smaller ones, underwent a rapid diversification. The first of the ornithomimids, ostrich-mimics, appeared, as well as the first tyrannosaurs that were still small and with long running legs and arms that were reduced. Based on the lake deposits from north-eastern China, the author³ suggests the focus was on the avepectorans, such as deinonychosaurs, that evolved into a variety of flying and flightless forms, the flightless forms possibly being secondary flightless forms that descended from the flying forms. He also suggests that small aerialists with 2 pairs of wings, the hind legs and front legs forming equally sized pairs of wings, may have evolved into the dromaeosaurs that were famous for the sickle-shaped claws. The troodonts, another deinonychosaur group, that were more lightly built and are believed to have been able to run faster, also appeared at this time.

The author³ suggests the birds had descended from the deinonychosaurs, and as is shown by the deposits in China, by 125 Ma they had already undergone a spectacular evolutionary radiation. Some of these birds had teeth, though others had lost all their teeth. All the birds were small. Among the early beaked birds in the same formation were the omnivoropterygids that are very similar to caudipterygid and protoarchaeopterygid oviraptorosaurs. Another group of possible dinosaur-birds that were secondarily flightless may have been the short-tailed oviraptorosaurs that were more advanced than the archaeopterygians and the dromaeosaurs. The therizinosurs, pot-bellied theropods that were herbivorous, described as enigmatic by the author³, had appeared by the Early Cetaceous.

By the Cretaceous most of the pterosaurs were short-tailed so were flying more dynamically, and the author³ suggests that they increased in size by competing with the increasing numbers of birds. The crocodilians of fresh water were also increasing in size, making them more of a threat to dinosaurs that came to the water to drink or entered the water. There were some semi-terrestrial crocodilians that could attack large dinosaurs both in the water and on the land, though there were also some small running crocodilians. The smallest of the dinosaurs and their young were also in danger from some of the bigger mammals that had reached a size of about 12 kg. There also gliding mammals at this time.

A major evolutionary event occurred in the latest part of the Early Cretaceous that the author believes aided the dinosaurs in their rapid evolution. Angiosperms evolved in the late Early Cretaceous. The first of these are believed to have been pioneer-type species that were small shrubs growing along the water courses, situations where they capitalised on their pioneer abilities to invade new territories when they became available. Other plant types, such as water lilies, the flowers of which were small and simple, were more fully aquatic. The author³ suggests that the rapid growth and the potential for strong recovery of angiosperms may possibly have been a factor in the development of ankylosaurs and ornithopods that were low browsing animals. He also suggests that it may have been the browsing pressure of the dinosaurs that was the driving force involved in the evolution of the angiosperms that were fast-spreading and fast growing new plants. In South America the conifers with monkey-puzzle foliage were also appearing.

About 100 Ma the Late Cretaceous began. At this time continental break was well under way and vast tracts of epicontinental shallow seas covered the interiors or land of the continents. As the atmospheric CO₂ levels continued declining the temperatures during the dark winters in the Arctic dropped to temperatures as low as in the high forests in the north, with glaciers creeping down high latitude mountains. The mammals were increasing similar to the modern types and were small. Both marine and terrestrial pterosaurs grew to gigantic size. The wings of oceanic pteranodonts had wings 8 m (25 ft) across. Towards the end of the Cretaceous he azhdarchids grew to even larger sizes with a wingspan of 11 m (more than 35 ft, and are estimated to have been heavier than ostriches. At this time there were still some small running crocodilians, a few of which had become herbivores. Of the freshwater crocodilians that continued to be carnivorous some grew to 12 m long and are estimated to have weighed up to 10 tonnes. This put them in the size range of the largest carnivorous theropods. It is believed these very large crocodilians fed mostly on fish and smaller tetrapods, the largest dinosaurs would appear to have been the only animals of the time that didn’t need to fear them. These giants don’t appear to have been very numerous in many places and they didn’t reach higher latitudes. The author³ suggests their presence in the water may have prevented the dinosaurs from evolving forms that were highly aquatic.

The sauropods became limited to the titanosaurs, but they diversified across most of the globe, greatly proliferating, though it was the Southern Hemisphere where they became especially diverse. After 150 million years they had become the most successful herbivore group of all time. For part of the Late Cretaceous the sauropods are not found in the fossil record of North America, though in drier regions that are again present in the fossil record towards the end of the Cretaceous.

There were some armoured forms of sauropods, that the author³ suggests may have been to protect the young of the group from the increasing diversity of predators. A few of the titanosaurs apparently adapted for grazing, having short necks and square, broad mouths. Some remained very large, growing to more than 50 tonnes, and possibly even more than 100 tonnes up to the end of the Cretaceous. The author³ suggests they may have been attacked by the largest fo the predators such as abelisaurs that sometimes reached the bulk of bull elephants. In the Late Cretaceous the spinosaurs, sail-backed dinosaurs from the Late Cretaceous of Africa, may possibly have been even larger. This group died out before the end of the Cretaceous. Big though these predators were it must still have been a hazardous venture tackling 100 plus tonne sauropods.

Especially in the Northern Hemisphere the ankylosaurs continued to thrive. One group developed tail clubs, though it is not known if they were mostly used for intraspecies battles or for defence. As the iguanodonts gradually disappeared from the fossil record their descendants, the hadrosaurs flourished, evolving the most complex grinding dental batteries seen in the dinosaurs. Many species had elaborate head crests that differed between species. In much of the Northern Hemisphere the hadrosaurs were the most common herbivores of their time. The author³ suggests they possibly were adapted to browse on the shrubs and ground cover that was replacing the fern prairies as well as to invade forest floors. Over large parts of the globe small ornithopods, that were not much different from the bipedal ornithischians that evolved in the early days of the Mesozoic, still survived.

Protoceratopsids with small bodies and large heads were common in many places in the Northern Hemisphere. They gave rise to the ceratopsids of rhino size and elephant size, that are some of the most spectacular dinosaurs that had horns on the oversized heads, as well as neck frills and great beaks that were parrot-like and slicing dental batteries. They flourished for the last 15 million years of the Mesozoic, largely on the part of North America that was on the western side of the interior seaway.

Some birds still had teeth, and both the toothed birds and the toothless birds continued to thrive. One bird group had evolved into marine divers that had lost the power of flight completely. The classic coelurosaurs with short arms had disappeared Of the small predatory theropods there were, according to the author3, troodonts, that were intelligent and sickle-clawed, and the leaping dromaeosaurs, some of which he suggests were still capable of flying. Other successful groups included the avepectorans with short tails that were non-predatory and oviraptorosaurs that were deep-headed, and the therizinosaurs that had small heads and large claws. Some species of both groups became large. The author3 suggests the ornithomimids with long, slender legs were possibly the fastest dinosaur to have evolved, though they are believed to have been closely matched by the alvarezsaurs.

The tyrannosaurids were the culmination of the 150 million years of theropod evolution, suggested by the author³ to be ‘the most sophisticated and powerful of the gigantic predators’. The classic tyrannosaurids evolved about 15 million years before the close of the Mesozoic and are found only in Asia and North America. The author³ suggests they appear to wave wandered, together with other theropods, hadrosaurs and ankylosaurs, across the subpolar Bering land bridge to North America. There was apparently a size-race in North America, tyrannosaurids, ceratopsids, ankylosaurids and pachycephalosaurids, in the last few million years of the Mesozoic all reaching the largest size they had reached since their first appearance in the fossil record. The author³ speculates that this may have been the result of an arms-race between the predators and their prey or as a result of the expansion of food resources that resulted from the gradual regression of the epicontinental seaway that allowed access to the full area of the continent of North America, or possibly a combination of both. The hadrosaurs, on the other hand, didn’t increase in size, in fact some of the earlier edmontosaurs were possibly larger than those near the end of the Mesozoic. Some of the hadrosaurs that were present at the close of the Mesozoic had adapted for grazing. The armoured nodosaurids didn’t enlarge at this time.

The author³ suggests that the ‘the pattern indicates that the enormous size and fire-power of the Merican Tyrannosaurus was a specialisation for hunting the equally oversized contemporary horned dinosaurs rather than just dispatching the easier-to-kill edmontosaurs’. A Triceratops fossil has been found that had a horn bitten off, the tooth marks indicating a tyrannosaur was responsible, but it had healed, so it seems the Triceratops won this encounter. Life must have been hard tyrannosaurs if they did indeed specialise in hunting such dangerous prey.

The pattern of continent spread around the world was beginning to appear similar to the modern configuration. A round of uplift and mountain building that occurred at the close of the period had helped drain the much of the epicontinental seaways. The angiosperms in the flora were becoming increasingly important. The first hardwood trees, among which is the plane tree, evolved near the close of the period, and were becoming the first large hardwood trees when the Period ended. The dominant trees were still conifers, such as the dawn redwood that barely survived until the present. The very tall classic redwoods were growing to great heights then as now. Classic rainforests still hadn’t come into existence. Grasses that tended to be water-loving forms had arisen, though they didn’t yet form dry grassland prairies as at present.

Everything was going so well then it suddenly all ended in a devastating mass extinction event.

Sources & Further reading

Long, John A, 1998, Dinosaurs of Australia and New Zealand, University of New South Wales Press.
Norman, David, 2005, Dinosaurs: A Very Short Introduction, Oxford University Press
Paul, Gregory S., 2010, The Princeton Field guide to Dinosaurs, Princeton University Press.