"Jurassic Mother" Is Our Earliest-Known Mammal Ancestor

By Joseph Castro | August 26, 2011 4:05 pm

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What’s the News: Researchers have now found a well-preserved fossil of the earliest known member of the animal group that encompasses today’s placental mammals, which includes humans. The shrew-like creature, named Juramaia sinensis, or “Jurassic mother from China,” dates back to 160 million years ago, 35 million years earlier than the oldest mammal fossil previously discovered. The Nature study gives some tangible support to genetic evidence suggesting that the two main types of mammals split well before the previous oldest mammal fossils.

How the Heck:

  • Zhe-Xi Luo, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh, and his colleagues analyzed the Juramaia fossil, which was unearthed in China’s northeast Liaoning Province, a hotspot for paleontology. The fossil consisted of an incomplete skull, a partial skeleton, a full set of teeth, and impressions of residual soft tissues like hair.
  • After comparing the fossil’s features with those of other ancient mammals, the researchers placed Juramaia among the eutherians, a group that includes placental mammals and their progenitors, rather than the marsupial group, the metatherians. “Specifically, eutherians have three molars, and five premolars. This is in contrast to metatherians characterized by four molars and three premolars,” Luo told BBC News.
  • Dating techniques revealed that Juramaia is at least 160 million years old and lived during the Upper/Late or Middle Jurassic epochs, when dinosaurs still roamed the Earth.

What’s the Context:

  • Previously, the oldest eutheria found was Eomaia, which dates back to 125 million years ago. The oldest fossil in the metatherian record so far belongs to another 125 million-year-old creature, Sinodelphys.
  • Even though the earliest fossils of the two groups are only 125 million years old, scientists believed that they split into their distinct lineages much earlier. DNA tests—the molecular clock technique—suggests that the two groups had a common ancestor over 160 million years ago. Juramaia now aligns the fossil record closer with the molecular evidence.
  • Looking even further back, proto-mammals (reptiles with mammalian features) existed over 200 million years ago.

Reference: Zhe-Xi Luo, Chong-Xi Yuan, Qing-Jin Meng, Qiang Ji. A Jurassic eutherian mammal and divergence of marsupials and placentals. Nature, 2011; 476 (7361): 442 DOI: 10.1038/nature10291

Image courtesy of Mark A. Klinger / Carnegie Museum of Natural History

CATEGORIZED UNDER: Living World
  • t cheney

    Where do these age dates come from? You know that dinosaur bones are routinely discovered with pliable bone marrow and red blood cells intact.
    It is time to rethink the ages of things we dig up. All age dating techniques are based on assumptions. These are not factual ages. Lets use the observational evidence in front of us and not the belief system of some scientists.

  • t bush

    i believe you, t cheney.

  • John Kwok

    @ t cheney -

    There are no “belief system of some scientists” involved in obtaining dates. As a former paleobiologist, I know this. Dates in geological time can be relative or absolute.

    They are relative on the basis of centuries worth of data in biostratigrapy, correlating geological formations from one region to another on the basis of key characteristic fossils which are known as index fossils (Contrary to creationist claims to the contrary, this is not a tautology since biostratigraphers have recognized certain species that are geographically widespread for them to be used successfully in assigning relative ages of, for example, Cretaceous marine rocks in Texas with those in Delaware and New Jersey. They can assign ages to other fossils found in these rocks based on the occurrence of the index fossils associated with these marine sedimentary rocks – what would be termed more accurately as sedimentary rock sequences or strata – in these locations.).

    Absolute ages are based on radiometric dating (which depends on the decay rate of radioactive elements, and these are absolute in the sense that they are accurate at a 95% or better probability.). In the case of the Liaoning fauna, the dates were derived by dating igneous rocks above and below the strata (sedimentary rock sequences) containing these mammalian fossils. That is why these Chinese researchers – whom I might add have been collaborating with colleagues at New York City’s American Museum of Natural History and elsewhere for years – can be reasonably confident about the approximate age of these fossils.

    You can find more information about the Liaoning fauna here:

    http://www.amnh.org/exhibitions/dinosaurs/

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