Milestone in Mammalian Evolution

The Liaoning Province in China is famous for its amazing feathered dinosaur and early bird fossils, however, a team of scientists have published a paper on the discovery of a prehistoric mammal.  A tiny creature that scurried through the undergrowth – the fossils suggests that this may be one of the earliest ancestors of the placental mammals.

This well-preserved fossil discovered in north-east China provides new information about the earliest ancestors of most of today’s mammal species, including ourselves as we to are  placental mammals.  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.

Our Arboreal Ancestor – Juramaia sinensis

Picture Credit: Mark A. Klinger

It fills an important gap in the fossil record and helps to calibrate modern, DNA-based methods of dating evolution.

The paper, by a team of scientists led by Carnegie Museum of Natural History palaeontologist Zhe-Xi Luo, describes Juramaia sinensis, a small shrew-like mammal that lived in China 160 million years ago during the Mid Jurassic.

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; the Metatherians ( whose descendants include marsupials such as kangaroos ) and monotremes ( such as the duck-billed  platypus ).

As Luo explains,

“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 fossil of Juramaia sinensis was discovered in the Liaoning Province in northeast China and examined in Beijing by Luo and collaborators: Chong-Xi Yuan and Qiang Ji from the Chinese Academy of Geological Sciences and Qing-Jin Meng from the Beijing Museum of Natural History, where the fossil is stored.

The name Juramaia sinensis means “Jurassic mother from China.”

The fossil has an incomplete skull, part of the skeleton, and, remarkably, impressions of residual soft tissues such as hair.  The Liaoning Province has a reputation for producing the most exquisitely preserved fossils of animals and plants.

Juramaia’s complete teeth and forepaw bones enable palaeontologists to pinpoint that it is closer to living placentals on the mammalian family tree than to the pouched marsupials, such as kangaroos.

Luo commented:

“Understanding the beginning point of placentals is a crucial issue in the study of all mammalian evolution.”.

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.   The idea of a “molecular clock” is that evolutionary changes occur at regular time intervals.  It is assumed that the rate of genetic change (mutation) in an organism’s DNA has not changed over time, or at least can be averaged.  The molecular genetic difference or “distance” between two species can be measured and their rate of genetic change estimated.

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 (Early Cretaceous).  The primitive mammal Eomaia was originally described in 2002 by a team of scientists led by Luo and Carnegie mammal specialist, 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.

Chuck Lydeard, Programme Director in the National Science Foundation’s Division of Environmental Biology, which co-funded the research stated:

“These scientists have used the rich fossil mammal record to test evolutionary hypotheses proposed by their colleagues studying living mammals using genetic data.”

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 and to escape from all those cursorial, feathered dinosaur predators that shared their forest home.

Luo supports this perspective:  He stated:

“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.  But it is their early adaptation to exploit niches on trees that paved their way toward this success.”

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