The first mammals

The story

Small and hidden

When you think of the age of the dinosaurs, you imagine giants walking through forests. But if you could look down at the ground, among the roots and dry leaves, you would see other animals, small and quick: the mammals. They appeared at almost the same time as the dinosaurs, about 225 million years ago, and lived for more than 150 million years in the shadow of those great reptiles. The first ones were the size of a shrew or a little mouse and probably came out mostly at night, to eat insects.

What “mammal” means

What makes an animal a mammal? People usually think of fur and milk. Mammals have hair or fur on the body, produce milk with mammary glands to feed their young (that is where the name comes from), are warm-blooded with a steady body temperature, have a relatively large brain and a middle ear with three little bones: hammer, anvil and stirrup. The teeth are distinctive too: they come in different shapes, incisors, canines and molars, and are replaced only once in a lifetime, from milk teeth to permanent ones. Reptiles, by contrast, have teeth that are all alike and are constantly renewed.

But almost all of these features, such as fur and milk, leave no trace in the bones. That is why palaeontologists rely on what is preserved: teeth, jaws, ear bones. Fur and milk are worked out indirectly, or seen in the very rare exceptional fossils in which the imprints of fur survive.

Where they come from

Mammals do not descend from the dinosaurs. They belong to another line, that of the synapsids, which split from the reptiles very long ago, in the Carboniferous. The synapsids of the Permian, such as Dimetrodon, and then the cynodonts, a group of animals more and more like mammals, are their ancestors. If you look at the fossil of a cynodont, you see a dog-like animal with a tail, still partly a reptile.

The change from cynodonts to mammals was not a sudden event, but a series of small changes. The lower jaw, which in the ancestors was made of many bones, shrank to a single bone, the dentary. The other bones that used to form the jaw joint moved into the ear and became the little bones that carry sounds. It is one of the best-documented cases of evolutionary change and is confirmed by fossils and by the embryos of mammals today.

The oldest mammal?

Saying which was the first mammal depends on how you define the word. One candidate is Brasilodon, found in southern Brazil, in rocks of about 225 million years ago: a little animal about 20 centimetres long, with two sets of teeth, milk teeth and permanent ones. But its remains are few, mostly teeth and jaws, and scholars disagree: some see it as a cynodont very close to mammals, others as a “mammaliaform”, that is, “mammal-shaped”, leaving open whether it is already a true mammal.

Well-documented animals come from slightly more recent fossils. Morganucodon, which lived about 205 million years ago and was found in Wales, China and elsewhere, was about 10 centimetres long and fed on insects. Hadrocodium, from China, about 195 million years old, had a skull less than 2 centimetres long and perhaps weighed two grams: among the smallest mammals of its time.

A life in the background, but not a dull one

For a long time it was thought that the mammals of the Mesozoic were all alike, small insect-eaters. Recent fossils, mainly from China, show that this was not so. Castorocauda, from about 164 million years ago, had a flat tail and webbed feet like an otter or a beaver, and caught fish; its fossils even preserve imprints of the fur. Repenomamus, from the Cretaceous, was as big as an opossum or a small badger and the remains of a small dinosaur have been found in its fossil stomach. There were mammals that glided, that dug, that climbed. After the end of the dinosaurs, that variety exploded.