Eutherian Mammals Are Older Than Previously Thought

The holotype fossil of  Eomaia

In 2002, an early mammal fossil was discovered in the early Cretaceous Yixian Formation of the Liaoning Province, China. Called Eomaia it was dated to 125 million years old and despite being mildly deformed during fossilisation, there were still traces of cartilage, fur and internal organs as well as the bones. A close study revealed that it held a great place of evolutionary science. It was the oldest known eutherian mammal on the planet.

Eutherians are the most diverse mammal group on the planet and include humans, carnivores and proboscideans (elephants). The other group of mammals, the metatherians, comprise the monotremes (such as platypuses and echidnas) and marsupials (such as kangaroos and wallabies). Eomaia was heralded as the basal (first) form of eutherian and quickly became a palaeontological treasure. However its special place has now been overshadowed by a new creature from the same region.

The holotype fossil of Juramaia sinensis
the new oldest known eutherian

Juramaia sinensis was discovered by a team of palaeontologists at the Tiaojishan Formation in the Liaoning Province. The specimen displayed a partial skull, fully articulated skeleton and a complete set of teeth as well as impressions of soft tissue. It was quickly identified as an early Eutherian. However the true shock came when the date was established. It turned out that Juramaia  was 160 million years old, during the Middle Cretaceous. Eutherian mammals are actually 35 million years older than previously thought, huge in evolutionary terms.

A look at its morphology shows several interesting features. It has long arms and fingers, suggesting that it was a climber. Such a skill would have been very useful in the tough world of the Jurassic. The Jurassic was a forest world of large pines. The fringes would have been frequented by large sauropod dinosaurs. The inner forest floor would have been home to large, predatory, allosaurid dinosaurs. The lower levels would have housed four winged pterosaurs and bird ancestors.

It is possible that Juramaia was exploiting the untapped habitat of the forest canopy. Such a lifestyle may have allowed mammals to survive during the age of the dinosaurs. Mammals conquering the arboreal world was perhaps one of the biggest steps in their evolution. This set the foundations for their success and ultimately the evolution of humans from the apes that left the trees for the ground.