Ancient mammal fossil reveals skin, hair evolution

Scientists have discovered a new 125-million-year-old fossil mammal in Spain that has pushed back the earliest record of preserved mammalian hair structures and inner organs by more than 60 million years.

The specimen, named Spinolestes xenarthrosus, was fossilised with remarkably intact guard hairs, underfur, tiny hedgehog-like spines and even evidence of a fungal hair infection.

The unusually well-preserved fossil also contains an external ear lobe, soft tissues of the liver, lung and diaphragm.

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

"Spinolestes is a spectacular find. It is stunning to see almost perfectly preserved skin and hair structures fossilised in microscopic detail in such an old fossil," said study co-author Zhe-Xi Luo, professor of organismal biology and anatomy at University of Chicago.

This Cretaceous furball displays the entire structural diversity of modern mammalian skin and hairs, he added.

Spinolestes xenarthrosus lived in the Cretaceous period and belonged to an extinct lineage of early mammals known as triconodonts.

The specimen measured roughly 24 cm in length and is estimated to have weighed around 50 to 70 grams, about the size of a modern-day juvenile rat.

Its teeth and skeletal features indicate it was a ground-dweller that ate insects.
Spinolestes had remarkably modern mammalian hair and skin structures, such as compound follicles in which multiple hairs emerge from the same pore.

It had small spines around a tenth of a millimeter in diameter on its back, similar to modern hedgehogs and African spiny mice which appeared to be formed by the fusion of filaments at follicles during development.

The team even found abnormally truncated hairs that are evidence of a fungal skin infection known as dermatophytosis, which is widely seen among living mammals.

Spinolestes is also the first example of a Mesozoic mammal in which soft tissues in the thoracic and abdominal cavities are fossilised.

"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 concluded.
The findings were described in a study published in the journal Nature.