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