The heyday of the reptiles

Despite the cold conditions, there were thriving faunas and floras in Gondwana, the latter producing extensive coal deposits. The dominant element in the Gondwanan floras was Glossopteris and associated plants, providing evidence for the existence of this supercontinent. As the Permian continued, the glaciers disappeared. This would be the last glacial episode for a quarter of a billion years. It would not be until the last few million years that the world would again see glaciations on the same scale. This latter is the Ice Age of popular lore. The concept of Gondwana was first recognised because of the similarity of this flora and the cold climatic indicators on all the southern continents at that time. It is ironic that we now know that most of the world's continents were then joined together in a single land mass, Pangea, the 'Gondwana' continents at this time being those clustered around the south pole of the day. Only earlier, at the Cambrian-Precambrian boundary and again at the end of the Triassic, did the modern southern continents plus India cluster together as a single land mass to the exclusion of the northern continents; i.e. only at those times did Gondwana exist, yet evidence of it was originally recognised from a geological period when it didn't have a separate existence.

In both the northern and southern hemispheres, as the Permian continued and into the Triassic, the world became steadily warmer. As a consequence, in Asia, Europe and North America during the Permian, vast deserts appeared with salt deposits being formed and a totally different flora lacking Glossopteris. Today, the greatest extremes of temperature and places of lowest rainfall typically occur in the centres of the world's largest land masses at intermediate latitudes, far from the moderating influence of the ocean. In many parts of central Asia, for example, the annual range of temperatures is 80oC, with winter minima plunging to -40oC and summer maxima near 40oC. In the Triassic, all the continents were linked together in a single land mass that stretched from pole-to-pole so that there were vast land areas at intermediate latitudes which therefore favoured the development of harsh deserts. In addition, the ocean currents were fundamentally different from those of today for there could be no east-west flow across this land mass. All over the world, the Triassic is typified by redbeds formed in a desert environment. Conditions in the Early Triassic were so harsh that nowhere in the world is coal of that age known. This time immediately followed the greatest episode of extinction known on earth, that which marked the end of the Palaeozoic Era.

In response to this climatic deterioration, the coal swamp floras of the Carboniferous in which trees were giant hollow reeds gave way to the more pithy confers, cycads and cycadeoids that prevailed during the Permian and Triassic.

As the widespread dry conditions of the Triassic came to an end, a major revolution in the terrestrial vertebrate community took place. The mammal-like reptiles which had dominated the land during the Permian and Triassic declined sharply at the end of the Triassic and finally became extinct by the end of the Jurassic. Their disappearance coincided with the appearance of dinosaurs and many other groups which were to dominate the rest of the Mesozoic.

During the Jurassic period, cycads and gymnosperms such as araucarians, ginkgoes and pines came to dominate the flora.

The first terrestrial vertebrate community: the mammal-like reptiles

The earliest of the synapsids are the pelycosaurs which are found primarily in North America although a few specimens are recorded from elsewhere. Best known of these is the sail-backed Dimetrodon, a carnivore which is often mistaken for a dinosaur. Alongside it lived the herbivorous Edaphosaurus which also had a row of tall spines on its back. These peculiar structures were evidently devices for regulating body heat in the tropical environment where these animals lived. Similar structures are found on dinosaurs that also lived in equatorial regions.

Not all pelycosaurs, however, lived in the equatorial regions. There, they lacked the sail-back structure. An example of this is the herbivorous Ennatosaurus from Russia.

Pelycosaurs were not only the earliest synapsids, but as might be expected, the most primitive ones. They had a sprawling stance in sharp contrast to the more upright posture of their successors, the therapsids. Also unlike the therapsids, all their teeth were much alike instead of being quite different from front to back along the length of the tooth row.

It was out of the carnivorous pelycosaurs that the therapsids arose in the Middle Permian. This was to be the typical pattern of synapsid and later mammalian evolution: carnivores give rise to more advanced carnivores and also advanced herbivores while herbivore lineages became extinct.

The therapsids are first known from the Late Permian of Russia where there is an abundance of them. These include large carnivores such as Eotitanosuchus and Inostrancevia, together with a variety of herbivorous therapsids such as the moose-like Estemmenosuchus and deep-skulled, almost cow-like Ulemosaurus, plus omnivorous forms such as Dvinia.

The synapsids did not have the world to themselves, however. Living alongside them were other reptiles as well as amphibians. These included pariasaurs such as Scutosaurus, structurally primitive reptiles that may have given rise to turtles, labyrinthodont amphibians such as Platyoposaurus, a reptile that mimicked labyrinthodonts, Lanthanosuchus, together with primitive diapsid reptiles such as Mesenosaurus which would ultimately give rise to the dinosaurs.

From the Early Triassic has come one therapsid that was long thought to be strong evidence for the existence of Gondwana. This was Lystrosaurus, a herbivore with two prominent tusks but no other teeth which occurs in abundance in South Africa and is also quite common from India. When it was discovered in Antarctica in 1969, it was seen as confirmation for the concept of continental drift as well as that Gondwana had existed. However, Lystrosaurus is now also known from Thailand, Russia, China, and Mongolia. Its distribution supports not just union of the southern continents, Gondwana, but all the major land masses as Pangea. The continents where it does not occur, Australia, North and South America, are those without Earliest Triassic continental rocks. If rocks of that age and type are ever found in any of those places, quite likely the wide ranging Lystrosaurus will be collected from them.

The later part of the Russian record of therapsids overlaps with the South African where it continues until the end of the Triassic. At that time, there was a widespread extinction of terrestrial vertebrates together with the appearance of several new groups. Therapsids did not quite become extinct at that time but after having declined markedly, one small group managed to hang on until the Middle Jurassic when they, too, disappeared.

Dinosaurs

Classifying dinosaurs. Dinosaurs are divided into two groups, the ornithischians and the saurischians. The latter are in turn divided into the theropods and sauropods. The theropods or 'beast-footed' include all the carnivorous dinosaurs, ranging in size from the huge Tyrannosaurus in North America and its cousin Tarbosaurus in Asia, to the smaller, more agile and more intelligent Deinonychus, Gallimimus, and Coelophysis. The brain size of the large theropods is what is to be expected of a modern reptile if they grew that large. But some of the smaller ones like Deinonychus have brains about ten times as large as a modern reptile that size would have. This is the size to be expected in a living mammal or bird of the same body weight. These smaller theropods were far more intelligent than their larger cousins and probably some of them had elaborate hunting strategies, perhaps attacking in groups like wolves.

Sauropods were the most massive of dinosaurs, and included such forms as the well known Apatosaurus, commonly but incorrectly called Brontosaurus, and Mamenchisaurus. The earliest sauropods were much smaller and some were even two-legged and carnivorous. But they soon became so large that only four-footed locomotion was possible. All of these larger ones were strictly herbivorous. Eventually, they grew so large that they reached the limits of what was mechanically possible for an animal living on land. If they ran at all, they would have had to have been very cautious, for otherwise they might well have broken a bone. But being so large, they probably had little reason to run, for an adult would have been as difficult to attack as an adult elephant is today.

As the names imply, pelvis structure distinguishes saurischians from ornithischians. In addition, unlike ornithischians, saurischians have hollow bones much like birds. It is thus not surprising that most palaeontologists now agree that saurischian dinosaurs, more particularly small theropods, gave rise to early birds such as Archaeopteryx. When the names were first given to these two major groups of dinosaurs, this was not understood. The scientist who named them certainly would have picked a word other than 'saurischian' which means 'lizard-hipped' for the ancestor of birds and 'ornithischian' which means 'bird-hipped' for the other dinosaurs if he had known this at the time. However, the names cannot be switched now because it would cause endless confusion.

Types of dinosaur fossils. When people think of dinosaurs, the first idea that comes to mind is a skeleton of one of the common forms such as the carnivorous Tyrannosaurus rex or the herbivorous Stegosaurus. Certainly, that is the evidence of their former existence that is most spectacular and persuasive. But their footprints give information about their behaviour that the bones alone cannot provide. Footprints, for example, suggest that sauropod dinosaurs travelled in herds with the young in the centre and the adults on the flanks, much like elephants behave today. A brief glimpse of the interaction between one large carnivorous dinosaur and a mixed group of small theropods and ornithischians can be seen in the 'dinosaur trackways' at Lark Quarry, near Winton, Queensland. There, a lone large carnivore, perhaps similar to Allosaurus or Tyrannosaurus, advanced towards this group of more than 100 small dinosaurs and moments later, they ran in the opposite direction, their tiny footprints crossing the large footprints of the big flesheater that had passed moments before.

Dinosaur eggs were first found in abundance in Mongolia in the 1920s. Since then, there have been major dinosaur egg finds in Europe, Asia, and North and South America. It is now possible to work out some of the reproductive patterns of dinosaurs from the study of eggs and the nests and bones associated with them. For example, some dinosaurs such as hypsilophodontids appear to have been born at an advanced stage and had little or no parental care, while others such as the duckbilled hadrosaurs were less advanced when born and the parents attended to them for some time. What is difficult to establish is whether all dinosaurs laid eggs. Did some of them bear their young alive as some living reptiles do, carrying the eggs internally until they hatch? We do know that the porpoise-like icthyosaurs did this because there are fossils of females with the young half expelled from the body of the mother. (Icthyosaurs, while extinct reptiles, are not dinosaurs.) There are no dinosaur fossils known which are comparable to the icthyosaurs in this way but then there are no fossil placental mammals known either which died and were fossilised during the birth process.

Small theropod dinosaurs gave rise to the birds we see flying around us. Therefore, at some stage between these dinosaurs and birds, feathers developed. But so far, despite many artists' renditions, there is no evidence for dinosaur skin being anything but scaly - like living reptiles. Some dinosaurs had bones embedded in their skins like living crocodilians. In 1996, there was a flurry of publicity about 'feathered dinosaurs' from China. Subsequently, a group of foreign vertebrate palaeontologists visited China and examined the specimens in question. They concluded that there was an odd structure associated with these specimens but that they were not feathers. What skin impressions of dinosaurs are known are just that: a natural mould or cast of the external surface of the skin. As such, they do not show the colour. This can be seen in the Museum of Victoria specimen of duckbilled dinosaur skin. Where skin is actually fossilised, sometimes lighter and darker areas are seen, suggestive of the colour pattern of the living animal. However, in those rare cases, the actual colours are not preserved.

Unlike mammals, reptiles use their teeth primarily to grasp their food and tear it off their prey whether that prey is another animal or a plant. They do not masticate their food with their teeth. Rather, swallowing it in large lumps, it is first broken down mechanically in the gizzard. Gastroliths, the rocks in the gizzards of these animals, are often found in well preserved dinosaurs in that part of the body where they are to be expected. Contrary to popular belief, they are not shiny but generally dull in texture, the acids of the digestive tract having etched the surface. In this regard, dinosaurs were like living reptiles and unlike mammals. The teeth of the carnivores typically had a sharp, dagger-like form, while those of sauropods were little more than weak pegs. All ornithischians were herbivorous. Their teeth were complicated, serrated structures, clearly suitable for cutting plant material.

A major clue to dinosaur food preferences is fossilised dung or coprolites. Frequently traces of bone or plant matter is found in such fossils. It is difficult if not impossible in most cases to associate the fossilised dung with particular dinosaurs.

'Dinosaur plants'. Until the Late Cretaceous, the dinosaurs lived in a world without flowering plants. The herbivorous ones ate primarily the conifers, cycads and ferns that predominated. Two groups of dinosaurs which were almost unknown before the Late Cretaceous were the horned ceratopsians and duckbilled hadrosaurs. Their great expansion during the Late Cretaceous has long been associated with the appearance and rapid expansion of the flowering plants during that time. But fossilised gut contents show that at least some of these dinosaurs preferred the conifers instead of the flowering plants, so whether in fact either or both groups had a preference for flowering plants that appeared at about the same time they did is not established.

Warm or cold blooded? Although the hadrosaurs and ceratopsians had a dentition that could easily have handled coarse vegetation like conifers, ferns or cycads, the sauropods did not. Their proportionally tiny skulls were equipped with almost preposterously small, weak, simple peg-like teeth. What an animal with a head no more than an eighth the mass of that of an elephant could have eaten with such weak teeth to have kept a body going that could be ten times as massive as an elephant is a puzzle. With a much more powerful dental battery, a modern elephant must eat for 16 hours per day in order to get enough food to sustain itself. This factor is a major piece of evidence that the sauropods were probably not warm blooded dinosaurs. A cold blooded reptile of a given body weight needs to eat only about 10% as much food as a warm-blooded mammal of the same mass.

It may be that sauropods were unique among dinosaurs in being cold-blooded. In the Late Cretaceous dinosaur deposits of northern Alaska are found all the other groups of dinosaurs of that age. The smaller terrestrial vertebrates that lived alongside them which today have close living relatives are all warm-blooded: the mammals. Further south in Wyoming, and therefore in a warmer environment, this same assemblage of dinosaurs and small mammals is joined by sauropods and small terrestrial vertebrates with close living relatives that are cold-blooded: lizards and snakes.

Dinosaur hunters. The first dinosaurs were named from fragments found in England in the 1820s, and it was on the basis of those and additional, similar scrappy specimens recovered from there during the following decade that Sir Richard Owen coined the word 'dinosaur' ('terrible lizard') in 1841. However, it was elsewhere that the complete skeletons of dinosaurs were found that that would capture the public's imagination in the late 19th century. The year 1877 was a watershed for dinosaur discovery. On both sides of the Atlantic that year, the first great concentrations of dinosaur skeletons were found. Deep in a Belgian coal mine, more than a dozen skeletons of Iguanodon were excavated while at an obscure bluff in southeastern Wyoming, skeletons of many different dinosaurs were recovered.

In the following decades numerous dinosaur skeletons were collected from the Rocky Mountain region of North America. This was driven in part by the scientific feud between Professor Othenial Charles Marsh of Yale University and Professor Edward Drinker Cope of the Philadelphia Academy of Sciences. Both wanted to name and describe as many fossils as possible. Legend would have it that their field parties actually fought over fossil sites. Certainly there were some tense moments but no gunfire! Soon thereafter, the desire for large dinosaurs as centrepieces for new, major museums, led to the exploration of other continents. Between the 1890s and the outbreak of World War I, major dinosaur excavations were carried out in South America and Africa. In the 1920s, the Central Asiatic Expeditions of the American Museum of Natural History, New York, startled the world with the discovery in southern Mongolia not only of dinosaurs but their eggs as well. Soon thereafter, the rich dinosaur fields of China were discovered.

Antarctica was not to yield dinosaurs until the 1980s when specimens were found first on the Antarctic Peninsula which is south of South America and somewhat later when specimens were discovered in the heart of the continent in the Transantarctic Mountains.

Australia's dinosaurs. Australia's first dinosaur was a single, partial claw of a carnivorous dinosaur found near Cape Paterson, Victoria, at the turn-of-the-century. It was to be Victoria's only dinosaur for nearly eighty years. Beginning in the 1920s, occasional dinosaurs started to reach the Queensland Museum. This has continued to the present, as the bulk of dinosaur skeletons from this continent have come from Queensland. The first dinosaurs from the opal field at Lightning Ridge were described in 1932 by a German worker examining material held in the British Museum (Natural History). Only in the past two decades has a systematic attempt been made to recover more of these unique specimens for they are the only opalised dinosaur fossils in the world.

The study of dinosaur footprints has played a disproportionately important role in Australia. This is because they are preserved in many rock types where bones cannot be found. Footprints found in coal mines in Queensland give the only evidence in Australia for the presence of dinosaurs during the Triassic and except for one skeleton about 20% complete, the only record for the Jurassic as well. From what preliminary study of these footprints indicates, there was as diverse an assemblage of dinosaurs in Australia during those periods as on other continents. And in Western Australia, apart from a few scraps of fossil bone, it is the Early Cretaceous fossil footprints found near Broome that provides what little knowledge there is of dinosaurs in the western half of the continent.

Renewed interest in Victoria's dinosaurs was sparked by the discovery of a number of specimens by systematic prospecting of the coastline between 1978 and 1980. From that work and the excavations that followed has come the most diverse assemblage of dinosaurs that lived within the polar circles of either hemisphere. Study of it has shown that the hypsilophodontids were adapted for year-round activity and had an enhanced ability to see under low light conditions. Other dinosaurs in the assemblage evidently hibernated during the Winter. Conditions were so cold that at times permafrost developed when Victoria's dinosaurs were alive. Despite this, the flora was far more diverse and luxuriant than living permafrost floras today.

Victoria's polar dinosaurs. About a dozen different 'polar' dinosaurs are now known from Victoria. Of those, only four are adequately known to have been identified to genus. Four aspects of this fauna immediately strike the observer. First, in stark contrast to the modern terrestrial mammalian fauna of this continent, it is not uniquely Australian. While new genera have been recognised, they clearly belong to families known elsewhere. This has made their reconstruction easy. In most cases, if all that was known of these dinosaurs was what has been found in Victoria, there would be little basis for reconstructing the whole animals. When reconstructions of Victorian dinosaurs have been made from initially one or two bones and subsequently partial skeletons have been found, to date we have never found any discrepancies.

The second aspect is the age of some of the dinosaurs. By what is known of them in other parts of the world, the specimen of Allosaurus from here is 25 million years too young. Allosaurus was a large carnivorous dinosaur well known from the Late Jurassic of North America that probably also occurred in rocks of the same age in East Africa. Even more surprising is that a protoceratopsian found near Kilcunda is at least 25 million years older than any other ceratopsian. Ceratopsians include the well-known Triceratops and were one of the last groups of dinosaurs to appear. They thrived in western North America and east Asia for the last 20 million years that dinosaurs existed. Thus the Kilcunda specimen if correctly identified doubles the range of the group. These are just two examples that polar southeastern Australia was both a 'nursery' for new forms and a refuge for other dinosaurs that had become extinct else where. And it is not just dinosaurs from here that show this for other groups including fish, amphibians, and mammals also display these patterns of late survival and early appearance in Early Cretaceous polar Victoria.

The third aspect is the pronounced size bias. Almost all the dinosaurs found are the bones and teeth of animals that ranged in size from chickens to adult humans. In two cases, larger dinosaurs are represented by the smallest fragments that could possibly be identified of them. In each case, the specimens in question are no larger than your clenched fist. But they are sufficient to show that carnivorous dinosaurs standing 5 metres high were here along with the more abundant, smaller forms. This bias is owing primarily to the way in which the fossils were preserved. For the most part, they accumulated in the beds of small streams as individually transported bones which behaved like rocks of comparable size. In modern terms, such streams would have been capable of transporting the bones of rats and potoroos but not elephants except for the smallest fragments.

The fourth aspect is the preponderance of small, bipedal ornithischian dinosaurs. Although more distantly related to potoroos than you and I are, such dinosaurs had an appearance similar to them. Like them, they were herbivorous and had a prominent tail which served as a counter-balance much like that of modern potoroos. Unlike potoroos, such dinosaurs did not have external ears or hair. Elsewhere in the world such dinosaurs, although commonly present, are generally represented by one or two species in an assemblage. For example, there are about 100 named dinosaurs from the Late Jurassic of North America. Of these, two are small, bipedal ornithischians. In contrast, in Victoria, half the dinosaurs belong to this one group. They may have been particularly well suited to the polar conditions that occurred here.

The extinction of the dinosaurs. In 1980, the question of what caused the extinction of the dinosaurs took a dramatic shift. In that year, evidence was first presented that the impact of an asteroid may have been the cause. In the ensuing years, an impressive mass of evidence has accumulated which persuasively shows that such an impact did take place at just the time the dinosaurs became extinct. What is still to be understood is why such an event would cause extinction. Obviously, those individuals in the impact area would not survive. However, others halfway around the world would not be affected in that way. That all living things around us today had ancestors that survived the event is a reflection of the fact that the effect of the extinction event was quite selective. Presumably, the impact triggered a series of consequences which brought about the extinctions but what those consequences were is still a hotly debated topic. The realisation that such an impact did happen and had the consequences it did has led to preliminary discussions as to what the human race could do, (if anything), to prevent a similar catastrophe.

Other Mesozoic vertebrates

In addition, a number of other reptilian groups appeared at this time that would thrive for much or all of the remainder of the Mesozoic and then become extinct along with the dinosaurs. In the sea, the plesiosaurs and dolphin-like ichthyosaurs appeared. Plesiosaurs have been described as looking like a snake drawn through the body of a turtle. In the sky were the pterosaurs and, from near the end of the Jurassic, the birds as well.

The sea and the air would never be the environment of the dinosaurs - they remained strictly terrestrial during their entire existence. One group that arose at the same time as them on land were our ancestors, the mammals. Although we did in fact have ancestors that lived alongside the dinosaurs, they did not look anything like Fred Flintstone! For one thing they were quite small. Most were no larger than a marsupial mouse and would easily be mistaken for one if seen alive. The largest of the mammals that lived alongside the dinosaurs were about the size of a small dog, and they were much rarer than the smaller forms. Mammals were to remain small for 130 million years while the dinosaurs flourished. However, once the dinosaurs were gone, within 5 million years there were mammals as large as a present rhinoceros.

Rise of the flowering plants

The rise to dominance of the flowering plants was swift. By the time the dinosaurs became extinct at the end of the Cretaceous, angiosperms were the dominant land plants. The earliest and most primitive ones were adapted to moist tropical conditions such as the Magnoliaceae, the living magnolias which are found today only in such climes. The more xeric and cold adapted flowering plants appear subsequently, adapting to harsher conditions.

The animals responded to the presence of these new plants in their environment. Numerous insects such as bees, specialised for pollinating flowers, appeared.

The many paths to flight

Convergent evolution is the phenomenon that groups which are quite unrelated often have a strikingly similar appearance because their modes of life are closely comparable. Commonly such groups evolved from totally different ancestors that may not have looked at all similar. The need to be able to fly imposes many mechanical constraints on vertebrates so that they all have light, hollow bones and an aerodynamic shape. Thus it is no surprise that pterosaurs, bats, and birds look superficially alike. However, when one looks beneath the skin, one sees that different bones do the same thing. For example, the wings of bats are supported by four fingers, those of pterosaurs one and those of birds, two that are fused together. Each does or did its job effectively but in a different way. The different ways that various groups evolved to accomplish the same task have consequences that seriously affected their subsequent evolution. For example, there are many flightless birds but no flightless bats. This is because the wing membrane of the bat extends onto the hind leg making them almost helpless on the ground whereas the wings of birds are restricted to the forelimb. Evolution does not occur as a result of deliberate, long-range planning but by maximising the survival chances of each generation of a species in the environmental circumstances it finds itself in.

Pterosaurs or flying reptiles are commonly referred to as "flying dinosaurs". However, their ancestors were clearly not dinosaurs but much less specialised, more primitive reptiles. On the other hand, the birds did evolve from small, active carnivorous dinosaurs similar to the Jurassic Compsognathus. From the same German Quarry where that dinosaur is found has come Archaeopteryx, the oldest animal that is clearly a bird. If feathers had not been found on Archaeopteryx, it would have been classified as a small dinosaur, a close cousin of Compsognathus. Unlike modern birds, Archaeopteryx's relationship to dinosaurs is less obscure. The tail is long and formed of a number of separate vertebrae instead of being shortened with the same bones fused together into one element. Teeth were present in the skull and jaws and on the wings there were claws.

That birds are warm blooded and descended from dinosaurs is one of the arguments that at least some dinosaurs, too, were warm blooded. It is difficult to imagine how the small, carnivorous dinosaurs whose anatomy suggests they were active predators, some having brains the same weight as birds with a similar body mass, could have been cold blooded and lived such a life.

During the Cretaceous Period, there was much evolutionary experimentation among birds. Most belonged to a group which was hardly known at all twenty years ago, the enantornithines. They flourished until the end of the period and then disappeared. The ancestors of the modern birds were much less conspicuous at that time.

The pterosaurs or flying reptiles were almost as diverse in their time as the birds are today. There were sparrow-sized ones and others with a wingspan of nearly 10 metres which would rival a private airplane in size. Unlike the private airplane, however, this flying giant, from Texas naturally, would have weighed only about 25 kg, the walls of the hollow bones being incredibly thin. Most pterosaurs were either toothless or had conical teeth suitable for catching fish. However, the peculiar looking South American Pterodaustro has hundreds of long needle-like teeth which served to strain out microorganisms much like flamingoes do today. The one thing that birds do that no pterosaur ever did is to evolve into flightless forms. Because there were many bipedal dinosaurs when they lived, perhaps that niche was simply not available. It was easier for birds to later evolve flightlessness because, except for kangaroos and some higher apes, mammals generally have remained quadrapedal, unlike the dinosaurs.

Time and again, a return to the sea

We tend to think that there are directions in evolution and one of them is towards increasing size. In fact, in the 19th century when there was a custom to giving the name 'Laws' to any such perceived regularity in nature, that trend was dubbed "Cope's Law", named for the man who proposed it, Edward Drinker Cope, one of the two protagonists in the notorious fossil war of the American West.

Icthyosaurs, however, are a group which belies "Copes Law". Some of the earliest icthyosaurs which lived in the Late Triassic of North America, were giants that were more than 10 metres in length. Never again in their entire history which continued until near the end of the Cretaceous, did these marine reptiles reach such dimensions.

In their history which spanned more than 100 million years, successive families arose, persisted for a time and then became extinct. To all but the specialist, there was little difference other than size between them. The later ones were somewhat more streamlined than the earliest icthyosaurs but other than that, there is little to separate them. As might be expected, they swam the world's oceans and the same genera are found in Australia and Kansas.

The plesiosaurs have been described as 'a snake drawn through the body of a turtle'. Except that plesiosaurs did not have a shell, this description is quite apt. As such, unlike the icthyosaurs, there is no modern counterpart to the plesiosaurs. Both were alike in one respect: they thrived on a diet of fish. Plesiosaurs probably favoured inshore habitats, unlike the icthyosaurs. This would explain why the plesiosaurs of Australia differ more from those elsewhere than the icthyosaurs that lived alongside them. However, the swimming mode of different plesiosaurs was not all the same. Those with long necks and small heads used their paddles to push their bodies through the water while they were able to dart their head about in order to grasp fish. Those with short necks and large heads, some having the largest skulls of any vertebrates other than whales, swam much more rapidly by using side-to-side body motion as well as their paddle-shaped limbs.

The terrestrial reptilian ancestors of these two groups are unknown. However those of the mosasaurs are quite evident: they were lizards that had become adapted to an aquatic environment. Because lizards did not arise until the Jurassic, it is not surpising that the mosasaurs do not appear until the Cretaceous. All of these marine reptiles either became extinct when the dinosaurs did or even preceeded them by a few million years.

However, one group of marine reptiles did persist through the time of the Cretaceous extinction event: the crocodilians. We think of crocodiles today as animals that typically divide their time between the land and water. However, during the Mesozoic, there were gigantic forms that were strictly marine and others that probably only rarely ventured into water at all. The sprawling posture of the living crocodiles, gavials, and alligators is far from typical of the group as a whole. Many of the more terrestrial forms had their legs drawn in under the body so that they were more effective at running on land than any of their modern relatives.

Marine turtles during the Mesozoic grew even larger than the living leatherback. None of them, however, gave rise to any of the turtles that dwell in the sea today. Rather, it was their small relatives who in the Cretaceous inhabited freshwater that would subsequently re-invade the oceans of the world after all the marine turtles then living perished simultaneously with the last of the dinosaurs.

Mesozoic invertebrate faunas

One of the most significant of groups to arise in the Mesozoic was the ammonites. These coiled cephalopods, descendants of the nutiloids, had shells with sealed chambers that enabled them to float in the water. They could also control their flotation and move up and down in the water column like a submarine. Because they were floating organisms, they were widely distributed throughout the seas of the time and serve as valuable index fossils for correlating from one part of the Earth to another. In the Late Cretaceous, the ammonites underwent changes which led to a bewildering variety of shell shapes. Many had shells which were partly uncoiled or even straight. Some were nearly two metres in diameter, but the majority were from 5 to 40 centimetres.

Unlike other continents, the record of the marine Mesozoic succession is incomplete in Australia. In the Triassic and Jurassic most of Australia was dry land and there was very little marine sedimentation preserved. There are some marine Triassic strata in Queensland near Maryborough and some marine middle Jurassic in Western Australia around Geraldton. During the Cretaceous there was an enormous inland sea, now represented by the Great Artesian Basin, in the centre of eastern Australia, but by the end of the Cretaceous marine sedimentation was restricted to the continental margins. Most sediments of Late Cretaceous age do not outcrop at the surface but are known in the subsurface from bores drilled for oil or water.

In Victoria, sediments of Triassic and Jurassic are absent or very limited in extent. In the Early Cretaceous sediments were deposited in lakes and river valleys, and some of these sediments are fossiliferous. The most famous of these fossil deposits is from Koonwarra in South Gippsland. This is composed of insects, crustaceans, a horseshoe crab, spiders, possible earthworms, and a bivalved mollusc. This fauna is of significance because of the exquisite preservation of the insects, including some colour banding, and also because immature stages of the freshwater insects are present in the fauna. Most of the insect fauna belongs to families with living representatives. The fauna lived in a backwater of a lake that seems to have periodically had an influx of fresh water from a river. The age of this fauna was measured by radiometric means as 115 to 118 million years old, which is in agreement with the correlation by fossil plants.

References:

Terrestrial ecosystems through time. Evolutionary paleoecology of terrestrial plants and animals. University of Chicago Press, Chicago.

Carroll, R.L. 1988. Vertebrate paleontology. W. H. Freeman & Co., New York.

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