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Jurassic Mammal Moves Back Marsupial Divergence


Juramaia sinensis

Juramaia sinensis courtesy of Mark Klinger/Carnegie Museum of Natural History

A newly described pointy-nosed, rat-like animal did not just crawl out of some unsuspecting city's sewers. Rather, this now-extinct species spent its time scampering among prehistoric trees some 160 million years ago during China's Jurassic period. Its modern appearance might seem unremarkable, but its advanced anatomical features—both internal and external—are exactly what have drawn the attention of scientists.

Juramaia sinensis was a small (15- to 17-gram) insect-eater that likely used its short legs to climb trees in search of food and shelter. Unearthed in the Liaoning Province, much of its skeleton was found fossilized along with all of its teeth, part of its skull and even some hairs.

It also had a strange reproductive strategy—at least for that era. Instead of laying eggs (like a "monotreme" relative the platypus) or carrying tiny young to term in a pouch (like a "metatherian," a marsupial, such as a kangaroo), it was a "eutherian," which means that it bore full-term live young like most of today's mammals do, thanks to a nutrient-rich placenta.

This find "establishes a much older geological time for the split of the metatherian-marsupial and the eutherian-placental lineages than previously shown by the fossil record," by a good 35 million years, the researchers report in their paper, which was published online August 24 in Nature (Scientific American is part of Nature Publishing Group).

"Understanding the beginning point of placentals is a crucial issue in the study of all mammalian evolution," Zhe-Xi Luo, of the Carnegie Museum of Natural History in Pittsburgh and co-author of the new paper, said in a prepared statement. The establishment of when our early mammal ancestors first gained key capabilities, such as the placenta to nourish developing offspring, helps researchers obtain a better sense of the mammalian evolutionary timeframe—and its pacing. The new findings align closer to previous estimates that had been made using a DNA-based "molecular clock" model to arrive at divergence times than those based on an earlier find—the 125-million-year-old Eomaia.

Although the exact evolutionary path from the early eutherian mammals has yet to be traced, Luo noted that "Juramaia, from 160 million years ago, is either a great-great-aunt or a great-grandmother of all placental mammals that are thriving today."

The views expressed are those of the author and are not necessarily those of Scientific American.

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