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Fossil discovery represents new milestone in early mammal evolution

s on the mammalian family tree than to the pouched marsupials, such as kangaroos.

"Understanding the beginning point of placentals is a crucial issue in the study of all mammalian evolution," says Luo.

Modern molecular studies, such as DNA-based methods, can calculate the timing of evolution by a "molecular clock."

But the molecular clock needs to be cross-checked and tested by the fossil record.

Prior to the discovery of Juramaia, the divergence of eutherians from metatherians posed a quandary for evolutionary biologists: DNA evidence suggested that eutherians should have shown up earlier in the fossil record--around 160 million years ago.

The oldest known eutherian was Eomaia, dated to 125 million years ago. (Eomaia was originally described in 2002 by a team of scientists led by Luo and Carnegie mammalogist John Wible.)

The discovery of Juramaia provides much earlier fossil evidence to corroborate the DNA findings, filling an important gap in the fossil record of early mammal evolution and helping to establish a new milestone of evolutionary history.

"These scientists have used the rich fossil mammal record to test evolutionary hypotheses proposed by their colleagues studying living mammals using genetic data," says Chuck Lydeard, program director in the National Science Foundation's NSF) Division of Environmental Biology, which co-funded the research with NSF's Division of Earth Sciences.

Juramaia also reveals adaptive features that may have helped the eutherian newcomers survive in a tough Jurassic environment.

Juramaia's forelimbs are adapted for climbing. Since the majority of Jurassic mammals lived exclusively on the ground, the ability to escape to the trees and explore the canopy might have allowed eutherian mammals to exploit an untapped niche.

Luo supports this perspective: "The divergence of eutherian mammals from marsupials eventually led to the place

Contact: Cheryl Dybas
National Science Foundation

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