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Ancient Chinese Mini-mammal

A small skeleton of an ancient mammal from China is posing more questions than answers about how mammals evolved.

According to three palaeontologists, the beautifully preserved skeleton of the mouse-sized Jeholodens jenkinsi has an unexpected mixture of primitive and advanced features.

Although Jeholodens is new to science, the extinct group that it belongs to (the triconodonts) have been known for over a century. However, all previous finds have consisted only of isolated teeth or parts of jaws. From this meagre evidence it was apparent that the triconodonts were some of the earliest true mammals, being more primitive than any living mammals including the egg-laying monotremes (the platypus and echidna). The lack of more complete material left many questions unanswered about this pivotal group of early mammals.

Jeholodens is between 145 and 120 million years old, from the middle of the age of dinosaurs. It is the first complete triconodont skeleton known to science. It is thought to be a ground-dwelling animal with poor climbing abilities.

While some features, such as a sprawling gait and a primitive spine are seen in ancestors of the triconodonts, other features are more advanced, including the way the shoulder girdle is built.

The puzzle is that the shoulder girdle is more advanced than those of some descendants such as the monotremes. This suggests a phenomenon known as mosaic evolution where similar or identical features are evolved independently in closely related groups.