World's Oldest Mammalian Fossil Uncovered In Spain

A 125-million-year-old hedgehog fossil was discovered in Spain, and this new mammalian fossil pushes back the earliest record of preserved mammalian hair structures and inner organs by more than 60 million years, according to scientists.

The fossil specimen, called Spinolestes xenarthrosus had several features intact, such as guard hairs, underfur, tiny hedgehog-like spines, along with evidence of a fungal hair infection. The fossil also had an external earlobe, soft liver tissues, and one of its lungs and diaphragm were intact, scientists from the Autonomous University of Madrid, University of Bonn and the University of Chicago revealed in a news release.

The presence of microscopic structures of hair and spines in Spinolestes are now classified as the earliest-known examples in mammalian evolutionary history.

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

The researchers revealed that the specimen was about 24 cenitmeters in length and weighed between 50 to 70 grams, which is about the size of a modern-day juvenile rat.

Spinolestes' teeth and skeletal features showed that the creature was a ground-dweller, which ate insects, according to the researchers. Its features were similar to a modern hedgehogs and African spiny mice.

"Hairs and hair-related integumentary structures are fundamental to the livelihood of mammals, and this fossil shows that an ancestral, long-extinct lineage had grown these structures in exactly the same way that modern mammals do," Luo said. "Spinolestes gives us a spectacular revelation about this central aspect of mammalian biology."

Spinolestes existed during the Cretaceous period, and they belonged to an extinct species of early mammals known as triconodonts. The structures of the Spinolestes's lung, along with the iron-rich residues represented the earliest-known record of mammalian organ systems.

"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.

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