The earliest ancestor of most of today’s mammals has been discovered in northeast China, according to a paper in the latest issue of the journal Nature.
Named Juramaia sinensis, which means the "Jurassic mother from China," the small, shrew-like animal spent some of its time in trees while dinosaurs thrived on land.
"Because it lived 160 million years ago, and nobody was there to sign the birth certificate of its descendants, Juramaia could be our great grandmother 160 million years removed or it could also be our great grand aunt that represents a relative on the side lines," lead author Zhe-Xi Luo told Discovery News.
Luo, a Carnegie Museum of Natural History paleontologist, and his colleagues Chong-Xi Yuan, Qing-Jin Meng and Qiang Ji analyzed the well-preserved remains of the prehistoric animal, which was discovered in Liaoning Province.
The fossils include an incomplete skull, part of the skeleton and even impressions of residual soft tissues, such as hair. Most importantly, they also include Juramaia’s complete set of teeth as well as its forepaw bones. These features led the paleontologists to conclude that the animal was closer to living placentals on the mammalian family tree than to pouched marsupials, such as kangaroos.
The placenta is a flattened circular organ in the uterus of pregnant eutherian (i.e. placental) mammals that nourishes and maintains the fetus through the umbilical cord.
"Marsupials, by contrast, make up for the short development of fetuses in the mother by having a longer pouch life before the fetuses become independent," Luo said. "Marsupials just optioned for a different reproductive strategy."
Before Juramaia’s discovery, the oldest known eutherian was Eomaia that lived 125 million years ago. DNA evidence has suggested the last common ancestor to eutherians and marsupials lived around 160 million years ago.
Its front limbs were adapted to grasp and scurry up trees, an ability that must have come in handy when so many large ground animals were stomping around below and looking for dinner. Juramaia weighed under a pound, according to the researchers. Its teeth were sharp and built for eating insects.
"So I imagine this animal to be a shrew-sized insectivorous mammal that hunted insects and was capable of being active in the trees," Luo said.
Robert Asher, a lecturer and curator at the University of Cambridge’s Museum of Zoology, told Discovery News that Luo and his team "have made a compelling case based on a good analysis," but he added that "it is very difficult at this point in the mammalian tree to identify early members of what are now extremely diverse groups."
Asher therefore said, "One possible alternative interpretation is that Juramaia represents an animal close to the common ancestor of marsupials and placentals, but one that is neither eutherian or metatherian (the stem group of marsupials). That would leave the previous eutherian record and calibration date of 125 million years ago intact."
Gregory Wilson, an assistant professor in the University of Washington’s Department of Biology, fully supports the new paper’s conclusions.
"This new specimen is a real jewel among the spectacular treasure chest of the Chinese fossil record," Wilson said. "The exquisitely preserved anatomical details leave little doubt that we’re looking at the earliest eutherian yet known," he continued, explaining that it was "not quite a placental yet but on the line to placentals."
Wilson concluded, "This significantly pulls several major branches of the mammalian phylogenetic tree even farther back into the Age of Dinosaurs, providing better understanding of the context of mammalian evolution. This work will grip the attention of paleontologists, molecular systematists, and anyone interested in their ancient forebears."
Juramaia is presently stored at the Beijing Museum of Natural History.