August 25, 2011
Split Of Mammals, Marsupials Began Earlier Than Thought
A well-preserved fossil discovered in China provides new evidence that the split between placental mammals and marsupials may have occurred 35 million years earlier than previously believed, according to a new study published Wednesday in the journal Nature.
The scientists, led by Carnegie Museum of Natural History paleontologist Zhe-Xi Luo, said the discovery fills an important gap in the fossil record, and helps to calibrate modern, DNA-based methods of dating evolution.
The paper describes Juramaia sinensis, a small shrew-like mammal that lived in China 160 million years ago during the Jurassic period. Juramaia is the earliest known fossil of eutherians--the group that evolved to include all placental mammals, which provide nourishment to unborn young via a placenta.
As the earliest known fossil ancestor to placental mammals, 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).
“Juramaia, from 160 million years ago, is either a great-grand-aunt or a great-grandmother of all placental mammals that are thriving today,” said Luo.
The fossil of Juramaia sinensis, which means "Jurassic mother from China”, was discovered in the Liaoning Province in northeast China. The fossil has an incomplete skull, part of the skeleton, and, remarkably, impressions of residual soft tissues such as hair.
Juramaia's complete teeth and forepaw bones enable paleontologists to pinpoint that it is closer to living placentals 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," said Luo.
Modern molecular studies, such as DNA-based methods, can calculate the timing of evolution by a "molecular clock."
However, the molecular clock needs to be cross-checked and tested by the fossil record, the scientists said.
Prior to the discovery of Juramaia, the divergence of eutherians from metatherians posed a quandary for evolutionary biologists, in that 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.
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," said Chuck Lydeard, program director in the National Science Foundation's Division of Environmental Biology, which co-funded the research.
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.
"The divergence of eutherian mammals from marsupials eventually led to the placental birth and reproduction that are so crucial for the evolutionary success of placentals,” said Luo.
"But it is their early adaptation to exploit niches on trees that paved their way toward this success."
The paper was published online August 24 in the journal Nature.
Image Caption: Life restoration of the nocturnal mammal Juramaia, hunting insects on a tree fern. Credit: Mark A. Klinger, Carnegie Museum of Natural History
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