125-million-year-old mammal fossil discovered in Spain

Scientists have found fossilised remains of a 125-million-year-old rat-sized mammal in Spain, pushing back the earliest record of preserved mammalian hair structures and inner organs by more than 60 million years.
The microscopic structures of hair and spines in the specimen, named Spinolestes xenarthrosus, 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 co-author Zhe-Xi Luo, professor at the University of Chicago.
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 grammes, about the size of a modern-day juvenile rat, researchers said.
Its teeth and skeletal features indicate it was a ground-dweller that ate insects. Its soft tissues, with discernable microscopic structures, were preserved through a rare process known as phosphatic fossilisation.

Individual hair follicles and bulbs, as well as the composition of individual hair shafts, could be identified using an electron scanning microscope.
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 millimetre 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.
"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 is also the first example of a Mesozoic mammal in which soft tissues in the thoracic and abdominal cavities are fossilised.

The team noted microscopic bronchiole structures of the lung, as well as iron-rich residues associated with the liver.
These areas were separated by a curved boundary that is thought to be a muscular diaphragm for respiration.
The fossil of Spinolestes contains a large external ear, the earliest-known example in the mammalian fossil record, as well as dermal scutes - plate-like structures made of skin keratin.
"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.

The study was published in the journal Nature.