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.
New Jurassic eutherian mammal Juramaia sinensis
The original fossil (type specimen) is preserved on a shale slab from the Jurassic Tiaojishan Formation. The fossil belongs to the Beijing Museum of Natural History (BMNH PM1143) and is being jointly studied by Chinese and American scientists.
Etymology: “Jura” represents the Jurassic Period of the geological time scale; “-maia” means “mother;” sinensis means “from China.” The full name means “Jurassic mother from China.”
Photo: Zhe-Xi Luo/Carnegie Museum of Natural History
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.
Skeletal and fur reconstructions of the Jurassic eutherian Juramaia sinensis
The new Jurassic mammal Juramaia is a shrew-sized mammal with a skull 22 mm long (just over 3/4 of an inch). It weighed around 15 grams (about half an ounce), fed on insects and worms, and had a great ability to climb.
Artwork by Mark A. Klingler of Carnegie Museum of Natural History
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.”
Life restoration of Juramaia sinensis: a nocturnal mammal hunting insects on a tree fern.
Artwork by Mark A. Klingler of Carnegie Museum of Natural History
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 pinpoint 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.
Composition by Mark A. Klingler of Carnegie Museum of Natural History
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 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.”
The fossil site of Juramaia sinensis is in Jianchang County of Liaoning Province, China, northeast of the Chinese capital, Beijing. The Tiaojishan Formation that yielded the fossil is dated to be 160 million years in age in other parts of Liaoning Province.
*Eomaia was originally described in 2002 by a team of scientists led by Zhe-Xi Luo and Carnegie mammalogist John Wible.
Contacts and sources:
Leigh Kish
Carnegie Museum of Natural History
New Jurassic eutherian mammal Juramaia sinensis
The original fossil (type specimen) is preserved on a shale slab from the Jurassic Tiaojishan Formation. The fossil belongs to the Beijing Museum of Natural History (BMNH PM1143) and is being jointly studied by Chinese and American scientists.
Etymology: “Jura” represents the Jurassic Period of the geological time scale; “-maia” means “mother;” sinensis means “from China.” The full name means “Jurassic mother from China.”
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.
Skeletal and fur reconstructions of the Jurassic eutherian Juramaia sinensis
The new Jurassic mammal Juramaia is a shrew-sized mammal with a skull 22 mm long (just over 3/4 of an inch). It weighed around 15 grams (about half an ounce), fed on insects and worms, and had a great ability to climb.
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.”
Life restoration of Juramaia sinensis: a nocturnal mammal hunting insects on a tree fern.
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 pinpoint 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.
Fossil photograph and restorations of Juramaia.
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 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.”
Discovery site of Juramaia sinensis
*Eomaia was originally described in 2002 by a team of scientists led by Zhe-Xi Luo and Carnegie mammalogist John Wible.
Contacts and sources:
Leigh Kish
Carnegie Museum of Natural History
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