Before the dinosaurs

Dinosaurs were not the first big animals on land, and mammals did not come after them. Both lines are far older than that, and they split from each other long before the first dinosaur.

Two family trees

In the Carboniferous, the first animals that could lay eggs on dry land appeared. About 315 million years ago, they split into two lines. One line, the reptiles, would lead to lizards, snakes, crocodiles, dinosaurs and birds. The other, the synapsids, would lead to mammals, including us.

Early synapsids are often called “mammal-like reptiles”, but that name is misleading. They are all more closely related to mammals than to any reptile. A better name is non-mammalian synapsids: members of our own line that were not yet mammals.

The Great Dying

The Permian ended 252 million years ago with the largest mass extinction in the history of life. It hit sea life hardest, wiping out most of the sea invertebrates of the time, including the last trilobites. On land, a smaller extinction of reptiles and synapsids cleared the way for new kinds of animals, and the age of the dinosaurs began. You can read about what may have caused it in Mass extinctions.

The first mammals

Tiny teeth tell the story of the first mammals. The oldest known mammal is Brasilodon, from Brazil, dated to about 225 million years ago. Like us, it grew two sets of teeth, milk teeth and then adult teeth, which goes with suckling young and is linked with warm blood and fur. It lived at the same time as the oldest known dinosaurs and probably lived in burrows, as shrews do today.

Wales has some of the most important early mammal fossils in the world. In quarries in south Wales, many tiny bones and teeth of shrew-like animals have been found. One is named Morganucodon, the Glamorgan tooth. Its oldest teeth are about 205 million years old.