Advance of the Mammals

There are quite a lot of fossils showing how the mammal-like reptiles slowly became mammals. Yet we have among living animals some creatures that do not fit neatly into either group. This suggests they may be intermediate between the two. These animals which do not 'fit' are the duck-billed platypus, and echidna or spiny anteater, both of which are found in Australia.

Egg-laying mammals

When they were discovered at the end of the 18th century and in the early 19th century, the scientific world was shaken. The platypus is a furry animal with fairly warm blood, and yet it lays eggs. It feeds its young from simple milk glands, not through teats. To confuse things further, the platypus has a flattened tail like a beaver's, but webbed feet and a beak like a duck's. Its reproductive and excretory organs are more like those of the reptiles and birds than a mammal's.

Duck-billed Platypus

The echidna or spiny anteater

When the echidna turned up, it also was found to be warm-blooded and egg-laying. It does, however, hatch its eggs inside its pouch and not in a nest. The echidna has a small beak-like snout, but its fur is replaced by spines, and it feeds on termites or white ants. To catch these insects, the echidna has powerful clawed feet for tearing termite mounds apart, and a very long, sticky tongue.

Echidna or Spiny Anteater

Australia breaks away

Australia seems to have broken away from the other continents about 45 million years ago, and so her mammals are able to develop free from the competition with other animals that occurred in the rest of the world. In a sense the platypus and echidna are as much 'living fossils' as the lungfishes and the coelocanth.

It is believed that the first mammals all laid eggs, and so the platypus and the echidna could be a link between the mammal-like reptiles and the mammals. On the other hand, they have some reptile features. The platypus and the echidna must have evolved at some time in the Triassic, but there is no record of them as fossils over the intervening 200 million years, except for a few teeth. These are not enough by themselves to show exactly where the platypus and anteater fit into the pattern of mammal development.