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Jurassic Mammal Moves Back Marsupial Divergence

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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.”

Katherine Harmon Courage About the Author: Katherine Harmon Courage is a freelance writer and contributing editor for Scientific American. Her book Octopus! The Most Mysterious Creature In the Sea is out now from Penguin/Current. Follow on Twitter @KHCourage.

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

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  1. 1. chris y 8:04 pm 08/24/2011

    So this article doesn’t say what characteristics of the specimen identify it as a eutherian and why. This is very disappointing. I could subscribe to Nature to find out, but it seems as though the main facts of the matter are left out here. Also, if the hard parts are sufficient to identify it as a eutherian relative, is there anything to suggest whether it had in fact evolved a typically eutherian reproductive strategy by this early date or is that speculation?

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  2. 2. jtdwyer 3:34 am 08/25/2011

    I generally agree with you about the links to Nature articles (seemingly self promotional – $35), but by reviewing the abstract I did find one statement that seemed to shed some light on your very good question about the general unavailability fossil soft-body parts:
    “This mammal has scansorial forelimb features, and provides the ancestral condition for dental and other anatomical features of eutherians.”

    It would have been helpful if this derivation methodology had been at least mentioned in this SA article. Thanks.

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  3. 3. mudphud 7:27 am 08/25/2011

    chris y – You can google “Juramaia sinensis” and get an interesting link to National Geographic and a few other sources with more detailed articles. The actual Nature article has some actually easy to understand illustrations (for Nature) that basically show what jtdwyer said- it’s all based on bone features and the shapes of the teeth. The figures nicely show how they have evolved over time and how they differ between the lineages, but not $35 nice. Theres nothing spectacular like preserved soft tissue showing a uterus.

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  4. 4. JamesDavis 7:36 am 08/25/2011

    “all placental mammals”. That is a really broad statement. Humans are “placental mammals”. Is Katherine saying that humans now evolved from a rat like mammal instead of a monkey, and even the monkey evolved from this rat mammal? I know that is probably not what she meant, but that is the way she wrote it. Maybe she should’ve said that the modern rat evolved from this placental mammal…that would’ve made more sense. I know she copied the article from Nature, but she should’ve thought a little before she made such a broad statement.

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  5. 5. ASHIK 7:41 am 08/25/2011

    Behavior of food gathering hasnt changed in todays rats

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  6. 6. jtdwyer 10:46 am 08/25/2011

    Interestingly, these researcher might not have actually identified a placental mammal.

    “Eutheria… is a group of mammals consisting of placental mammals plus all extinct mammals that are more closely related to living placentals (such as humans) than to living marsupials (such as kangaroos). They are distinguished from noneutherians by various features of the feet, ankles, jaws and teeth. One of the major differences between placental and nonplacental eutherians is that placentals lack epipubic bones, which are present in all other fossil and living mammals (monotremes and marsupials).”

    As I understand, only if the researchers could reconstruct the pelvis sufficiently to ensure that no epipubic bones existed could they definitively determine that these animals had placentas. The Nature article abstract made no mention of epipubic or pelvic bones or evidence for a placenta – only that the discovery belongs to the “the eutherian–placental clade”. Please see:

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  7. 7. JDaniel 11:59 am 08/25/2011

    JamesDavis: Her thinking is just fine. Yes, humans are placental mammals. No, we did not evolve from modern monkeys. Modern monkeys and humans both evolved from a monkey-like (aka early primate) ancestor though. Yes, that early primate evolved from a rat-like ancestor. So yes, humans evolved from a rat-like ancestor as well, just not directly. Yes, the modern rat also evolved from an early rat-like mammal, just like us. The rat just happens to look a whole lot more like our shared ancestor than we do.

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  8. 8. JDaniel 12:18 pm 08/25/2011

    My biggest complaint with this is the assumption of the placenta. All modern eutherians are placental, fine, it is a defining character. However, there is no a priori reason to think that every single species from the beginning of the taxon had that trait. There are a whole bunch of extinct animals between Juramaia and any modern eutherian. There is no reason to believe that tooth characters are necessarily correlated with placentas and every reason to think they evolved at different times. The name Eutheria is an arbitrary designation. There is nothing that states that every Eutherian has to be a placental, only that the basal animals are more closely related to modern animals with placentas than they are to modern metatherians.In Witmer’s EPB, this is a level II inference, certainly plausible but not conclusive.

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  9. 9. JDaniel 12:30 pm 08/25/2011

    I would also like to point out that Luo et al. never specifically said anything about the origin of the placenta in their paper. They discussed inferences of diet, habitat, and mobility based on skeletal features and note that the split between eutherians and metatherians seems to relate to a “major ecomorphological diversification, notably scansorial adaptation, which made it possible for therians to exploit arboreal niches.” They do NOT however, mention reproductive strategies.

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