4. Life in the Mesozoic

Dinosaurs were the largest and most dominant land animals during the Mesozoic, but they were not the only animals from this time. Crocodilians, lizards, snakes, frogs and mammals were all found in the Mesozoic, and now-extinct reptile groups such as ichthyosaurs and pterosaurs dominated the sea and air. Invertebrate groups in the sea, and on land, continued to diversify, with some groups, such as ammonites, becoming prevalent throughout the Mesozoic.

Development of the Egg
The evolutionary development of an egg that could be laid on land and survive, was a key innovation in the history of vertebrates. Amphibian and fish eggs both need to be laid in water or a moist place on land, as they have no protecting skin against drying out. Reptile eggs were protected from drying out by a shell that could regulate the exchange of gases and moisture between the egg and the outside atmosphere, and by a membrane, the amnion, which held a fluid in which the developing young floated. This kind of egg, called the amniotic egg after the fluid-containing membrane, enabled reptiles to become non-dependent on water for reproduction, helping them to expand into new, drier habitats. This development occurred about 300 million years ago. Fossil dinosaur eggs have been found in Mongolia and many other parts of the world, but not yet in Australia or Antarctica.

Evolution of Birds
Archaeopteryx is the oldest known bird. It lived in Germany 150 million years ago, during the late Jurassic. Except for the presence of feathers, it is remarkably similar to a small carnivorous dinosaur found in the same deposits, Compsognathus. Like Compsognathus, Archaeopteryx had an elongated tail with bones along its whole length, and a mouth filled with teeth. However, one important difference was that the elongated tail of Archaeopteryx was covered with feathers while that of Compsognathus was bare. Because, among other things, both birds and small carnivorous dinosaurs have a wishbone, or furculum (essential for flight), it is thought that birds are descended from dinosaurs like Compsognathus. Modern birds differ from Archaeopteryx in several ways, including having an enlarged breastbone (the sternum) which anchors the flight muscles, and reduced bones in the tail.

Warm-bloodedness
Warm-bloodedness, or homeothermy is the maintenance of a high, constant body temperature by the use of food energy. Animals' bodies work best when they are at a certain temperature range; too hot or too cold, and their bodies cease to function. In the present day, only mammals, birds and some fish use homeothermy to keep their body temperature constant. Other animals (known as ectotherms) maintain body temperatures by using environmental sources of energy, through behavioural adaptations such as moving between sunny and shaded areas, or living in an environment of constant temperature, such as the polar sea. Being homeothermic can be an advantage for animals, as it allows them to be active at all times, unlike ectotherms which are less active and slower when their environmental source of energy is absent. However, to keep their body temperature constant, homeotherms need to eat around ten times as much food as an ectotherm of a similar weight.

Homeothermy developed at least twice in vertebrates – once among the ancestors of mammals about 280 million years ago and again, amongst the ancestors of birds about 160 million years ago. It may also have developed amongst the pterosaurs or flying reptiles, and amongst the small predatory dinosaurs such as Deinonychus, which, from their skeletons appear to have been quite agile and from their brains, quite intelligent. At the other extreme of dinosaurs, the huge sauropods like Mamenchisaurus were probably not homeothermic.

Dinosaurs

Early Ornithiscia. Source: Museum Victoria.

The dinosaurs evolved about 230 million years ago and became extinct 65 million years ago. Although they were reptiles, they had a number of quite unique characteristics which separated them from other reptiles like turtles, lizards, snakes and crocodilians. These characteristics included their stance, with their legs under their body rather than splayed, the ball-like head on the thighbone or femur, the presence of no more than three bones in the fourth finger, and the pattern of weight-saving gaps in the facial bones of their skulls. Dinosaurs have been divided into two groups based on the structure of the pelvis: the Ornithischia or 'Bird-Hipped' dinosaurs which were all herbivorous; and the Saurischia or 'Lizard-Hipped' dinosaurs which included all the carnivorous species such as Tyrannosaurus and Tarbosaurus, and the long-necked herbivorous species like Mamenchisaurus.

Late Ornithiscia. Source: Museum Victoria.

Saurischia. Source: Museum Victoria.

Ichthyosaurs

Ichthyosaur. Artist: Frey Micklethwait. Source: Museum Victoria.

Ichthyosaurs overlapped in time with the dinosaurs. They evolved slightly earlier and became extinct about 30 million years before the dinosaurs. They looked a lot like dolphins, yet we are more closely related to dolphins than they were! This is because ichthyosaurs were reptiles while dolphins and humans are both mammals. The ancestors of both ichthyosaurs and dolphins lived on land and returned to the sea—but one ancestor was a reptile and the other, a mammal. Ichthyosaurs and dolphins look so much alike because they both breathed air and became adapted to a way of life where a fish-shaped body was highly suitable. Cases in which two totally unrelated organisms evolve from quite different ancestors and yet look so much alike because their mode of life is so similar, is called convergent evolution.

Mammals
Mammals evolved at about the same time as the dinosaurs, 225 million years ago, and throughout the Mesozoic, were small, presumably nocturnal animals. Little is known about Mesozoic mammals within Australia. We know, from an early Cretaceous jaw found at Lightning Ridge, that the egg-laying monotremes were present. Marsupials, which dominate the living mammalian fauna of Australia, were not present, but have been found in Mesozoic rocks from North America. The partial jaw of a small mammal, found at Inverloch in Victoria and named Ausktribosphenos, may be from a placental mammal. Ausktribosphenos is unique in Australia's Mesozoic fossil record, as it is the only non-monotreme or non-marsupial mammal fossil known from this time.

Evolution of Flowering Plants
Many of the plants we see around us today are flowering plants or angiosperms. The first angiosperms appeared in the fossil record, in the Cretaceous over 100 million years ago. Angiosperms differ from other vascular plants in the specialized vascular tubes that flowering plants evolved to transport water and food around their bodies. Angiosperms also differ from other plants in the way they protect and nourish their seeds. The seeds of non-flowering plants are bare while those of flowering plants are enclosed in fruit. As they evolved, it is thought that angiosperms became more and more efficient at transporting food and water, and devoted more and more resources into producing successful seeds. These are some of the adaptations that may have helped angiosperms to become the most diverse and common plant group on the planet.

Non-Angiosperm. Artist: Kate Nolan. Source: Museum Victoria.

Angiosperm. Artist: Kate Nolan. Source: Museum Victoria.