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Evolution of Early Mammals

Describes evolution of early monotreme, marsupial, and placental mammals.

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Evolution of Early Mammals

Could you argue that the duckbilled platypus has some characteristics of other species?

Like a bird or a fish? You could. This might suggest that an ancestor of this species may have been one of the early mammals to evolve. 

Evolution of Early Mammals

The earliest mammals evolved from cynodonts. But the evolution of mammals didn’t end there. Mammals continued to evolve. Monotreme mammals probably split off from other mammals first. They were followed by marsupials. Placental mammals probably evolved last.

Evolution of Monotremes

The first monotremes may have evolved about 150 million years ago. Early monotreme fossils have been found in Australia. An example is a genus called Steropodon, shown in Figure below. It may have been the ancestor of the platypus. Early monotremes retained some of the traits of their therapsid ancestors. For example, they laid eggs and had a cloaca. These traits are still found in modern monotremes.

Steropodon illustration: monotreme ancestor

Probable Monotreme Ancestor: Steropodon. Like the platypus, Steropodon probably had a bill.

Evolution of Marsupials

The first marsupials may have evolved about 130 million years ago. One of the earliest was the extinct genus Sinodelphys. A fossil of this mammal is shown in. It is a remarkable fossil find. It represents a nearly complete animal. Even tufts of hair and imprints of soft tissues were preserved.

Sinodelphys was about 15 centimeters (6 inches) long. Its limb structure suggests that it was a climbing animal. It could escape from predators by climbing into trees. It probably lived on a diet of insects and worms.

Evolution of Placental Mammals

The earliest placental mammals may have evolved about 110 million years ago. The ancestor of placental mammals may be the extinct genus Eomaia. Fossils of Eomaia have been found in what is now China. It was only about 10 centimeters (4 inches) long. It was a tree climber and probably ate insects and worms. Eomaia had several traits of placental mammals. Figure below shows how Eomaia may have looked.

Eomaia illustration: Placental Mammal ancestor

Probable Ancestor of Placental Mammals: Eomaia. Eomaia lived a little over 100 million years ago.

The placental mammal descendants of Eomaia were generally more successful than marsupials and monotremes. On most continents, placental mammals became the dominant mammals, while marsupials and monotremes died out. Marsupials remained the most common and diverse mammals in Australia. The reason for their success there is not yet resolved.


  • Monotremes evolved about 150 million years ago. Like modern monotremes, they had a cloaca and laid eggs.
  • Marsupials evolved about 130 million years ago. They were very small and ate insects and worms.
  • Placental mammals evolved about 110 million years ago. They were also small and climbed trees.
  • Placental mammals became the dominant land mammals. Most marsupials and monotremes died out, except in Australia.


  1. Outline the evolution of monotreme, marsupial, and placental mammals.
  2. Describe Sinodelphys.
  3. Describe Eomaia.

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cynodont Therapsids that gave rise to modern mammals and their extinct close relatives; one of the most diverse groups of therapsids; named after their dog-like teeth.
Eomaia Extinct genus of early placental mammals.
Sinodelphys Extinct genus of early marsupials.
Steropodon Genus of monotremes; may have been the ancestor of the platypus.

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