Jurassic placental mammal ancestor discovered


This video says about itself:

Jurassic Fossils and Mammalian Antiquity

11 aug. 2013

Virtual Reconstruction from CT scanning of the teeth of Megaconus mammaliaformis. Upper teeth P2-M3. Video of virtual reconstruction from the micro-CT scanning and the animation before and after the correction of the fracture through M1. Lower teeth m1-m2: video of virtual reconstruction from the micro CT scanning and the animation of the correction for the taphonomic compression of lower m1 and m2.

References:

Fossils throw mammalian family tree into disarray
Studies disagree on whether Jurassic animals were true mammals.
Nature doi:10.1038/nature.2013.13522

A new arboreal haramiyid shows the diversity of crown mammals in the Jurassic period
Nature 500, 199–202 (08 August 2013) doi:10.1038/nature12353

A Jurassic mammaliaform and the earliest mammalian evolutionary adaptations
Nature 500, 163–167 (08 August 2013) doi:10.1038/nature12429

From the Carnegie Museum of Natural History in the USA:

Discovery of a 160-million-year-old fossil represents a new milestone in early mammal evolution

Wednesday, August 24, 2011

A remarkably well-preserved fossil discovered in northeast China provides new information about the earliest ancestors of most of today’s mammal species—the placental mammals. According to a paper published August 25 in the prestigious journal Nature, this fossil represents a new milestone in mammal evolution that was reached 35 million years earlier than previously thought, filling an important gap in the fossil record and helping to calibrate modern, DNA-based methods of dating the evolution.

The paper by a team of scientists led by Carnegie Museum of Natural History paleontologist Zhe-Xi Luo 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 ancestral 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). 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 “Jurassic mother from China”

The fossil of Juramaia sinensis was discovered in the Liaoning Province in northeast China and examined in Beijing by Zhe-Xi Luo and his 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. Most importantly, Juramaia’s complete teeth and forepaw bones enable paleontologists to pin-point that it is closer to living placentals on the mammalian family tree than to the pouched marsupials, such as kangaroos.

Resetting the evolutionary clock

“Understanding the beginning point of placentals is a crucial issue in the study of all mammalian evolution,” says Luo. The date of an evolutionary divergence—when an ancestor species splits into two descendant lineages—is among the most important pieces of information an evolutionary scientist can have. 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 point of eutherians from metatherians posed a quandary for evolutionary historians: DNA evidence suggested that eutherians should have shown up earlier in the fossil record—around 160 million years ago. Yet, the oldest known eutherian, was Eomaia*, dated to 125 million years ago. The discovery of Juramaia gives 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.

Juramaia also reveals adaptive features that may have helped the eutherian newcomers to survive in a tough Jurassic environment. Juramaia’s forelimbs are adapted for climbing; since the majority of the 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 placental birth and reproduction that are so crucial for the evolutionary success of placentals. But it is their early adaptation to exploit niches on the tree that paved their way toward this success.”

###

*Eomaia was originally described in 2002 by a team of scientists led by Zhe-Xi Luo and Carnegie mammalogist John Wible.

See also here. And here.

A ‘Jurassic Park’ in China was once home to dinosaurs that lived among early mammals, amphibians and other strange creatures 160 million years ago: here.

The placenta evolved at least twice: once in mammals, and once in an obscure African lizard called Trachylepis ivensii: here.

The Y chromosome, which distinguishes males from females at the genetic level, appeared some 180 million years ago. It originated twice independently in all mammals. The team of professor Henrik Kaessmann at the Center for Integrative Genomics and the SIB Swiss Institute of Bioinformatics managed to date these events that are crucial for both mammalian evolution and our lives, because the Y chromosome determines whether we are born as a boy or girl. The results of this research have just been published in Nature. Read more here.

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9 thoughts on “Jurassic placental mammal ancestor discovered

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