110 Million years ago - when dinosaurs walked the earth
In the shadow of dinosaurs, the mammals' story
Mammals first appeared about 240 million years ago when all the Earth's continents were joined together. Soon after this, and particularly when the continents started to drift apart, they began to evolve into a range of very different groups. One group that evolved in the southern continent was the monotremes. Monotremes are different from other kinds of mammals because they lay eggs. At least four different kinds of monotremes that are 110 - 115 million years old are known from fossils found in Australia. Although monotreme fossils of this age have not been discovered on other continents, because of land connections it is almost certain that they lived in the other continents that made up Gondwana at this time. Northern continents and possibly Africa had a very different blend of mammal groups. These included the ancestors of placentals and marsupials, mammal kinds that don't appear to have reached Australia until much later.
Early Australian mammals
Some early Australian mammals were tiny, including a mouse-sized, platypus-like monotreme. Others were enormous by Cretaceous world standards, overlapping in size with some of the smaller dinosaurs.
Jewels for jaws: Steropodon
Steropodon is a monotreme, like the Platypus and echidnas. Unlike living monotremes, however, it had functional teeth and a powerful jaw. It may have been a semi-aquatic predator of small animals such as crustaceans. The name Steropodon means 'Lightning tooth' and refers to the flash of colour in this opalised fossil.
Steropodon.
Illustration: A Musser © Australian Museum.
Lower jaw (opalised) Steropodon galmani. Early Cretaceous. Lightning Ridge, New South Wales
Photo: J Fields © Australian Museum.
Molars of mystery: Kollikodon
Kollikodon had bizarre hot-cross-bun-shaped teeth, hence its nickname 'Hotcrossbunodon'. Its teeth are so different from those of Steropodon that their common monotreme ancestor must be much older. Although its diet is a mystery, it might have used its strange teeth to break the hard shells of molluscs.
Lower jaw (opalised) Kollikodon ritchiei. Early Cretaceous. Lightning Ridge, New South Wales
Photo: J Fields © Australian Museum.
