Jurassic Shrew is Earliest Known Mammal That Used a Placenta
Posted on August 25, 2011
A shrew fossil, discovered in Northeast China, provides new information about the earliest ancestors of most of today's mammal species - the placental mammals. The Juramaia sinensis fossil is the earliest known fossil of eutherians, a group that evolved to include all placental mammals, which provide nourishment to unborn young via a placenta. The fossil has an incomplete skull, part of the skeleton, and impressions of residual soft tissues such as hair.
According to a paper published today in the journal Nature, the fossil represents a new milestone in mammal evolution that was reached 35 million years earlier than previously thought. The paper was published by a team of scientists led by Carnegie Museum of Natural History paleontologist Zhe-Xi Luo.
Juramaia provides fossil evidence of the date when eutherian mammals diverged from other mammals: metatherians (whose descendants include marsupials such as kangaroos) and monotremes (such as the platypus).
Luo says, "Juramaia, from 160 million years ago, is either a great-grand-aunt or a great-grandmother of all placental mammals that are thriving today."
The researchers say Juramaia's forelimbs were adapted for climbing. Since most Jurassic mammals lived exclusively on the ground, the ability to escape to the trees and explore the canopy might have enabled eutherian mammals to exploit an untapped niche.
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