Recently Found 125-Million-Year-Old Fossil Reveals Oldest Mammal Hair Structures And More

The specimen found in Spain was preserved by a rare fossilization process that preserved the hair structures and inner organs.

depiction of Spinolestes xenarthrosus
Illustration by Oscar Sanisidro via UChicago/Twitter 
Spinolestes xenarthrosus was about the size of a present-day juvenile rat.

When you picture the animals alive during the Cretaceous Period (that’s 145 to 66 million years ago), you’ll now have to add a furry Spinolestes xenarthrosus to the group. It’s a small animal about the size of a juvenile rat that had fur and tiny, hedgehog-like spines, according to a news release from Science Daily. Scientists from the Autonomous University of Madrid, the University of Bonn and the University of Chicago recently found it at the Las Hoyas quarry near the city of Cuenca east of Madrid. Las Hoyas is an area known to have exceptional fossil preservation.

Spinolestes is a spectacular find. It is stunning to see almost perfectly preserved skin and hair structures fossilized in microscopic detail in such an old fossil,” study co-author Zhe-Xi Luo, PhD, professor of organismal biology and anatomy at the University of Chicago said in the release. “This Cretaceous furball displays the entire structural diversity of modern mammalian skin and hairs.”

We’re getting to see this fossil and some of its preserved soft tissue thanks to a rare fossilization process called phosphatic fossilization, which results in extreme preservation. 

Besides giving us the oldest known mammal hairs, which the scientists were amazed to find similar to modern mammalian hair and skin structures, the specimen had some intact soft tissue from its thoracic and abdominal cavities, and it revealed other anatomic characteristics.

“With the complex structural features and variation identified in this fossil, we now have conclusive evidence that many fundamental mammalian characteristics were already well-established some 125 million years, in the age of dinosaurs,” Luo said in the release.

Spinolestes xenarthrosus was a triconodont, which is an extinct order of mammals. Although it’s not a direct ancestor of present-day rats and hedgehogs, it was a close relative of their ancestors.

The full article about what the discovery of Spinolestes xenarthrosus means was published by the scientists in the journal Nature.

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