A 125-million-year-old fossil of a mammal unearthed in Spain with preserved hair has pushed back the earliest record of mammalian hair structures by more than 60 million years, paleontologists say.

The fossil of Spinolestes xenarthrosus, about the size of a young rat, is also the oldest specimen with preserved, fossilized internal organs, they say.

"Normally, in paleontology, you only see mineralized hard parts, like the bones," says researcher Thomas Martin of the University of Bonn. "We know, from other mammals, that there was hair on mammals at this time — but you usually only see impressions of the fur in the fossil. Here the hair is preserved down to the cellular level."

Similar to fur on mammals today, on S. xenarthrosus, it grew from compound follicles, with multiple hairs growing from the same skin pore, the researchers report in the journal Nature.

It also has spikes formed from fused hairs on its back like those seen on hedgehogs.

"This Cretaceous furball displays the entire structural diversity of modern mammalian skin and hairs," says Zhe-Xi Luo of the University of Chicago.

There are even preserved but abnormally truncated hairs that are evidence the animal suffered from a fungal skin infection known as dermatophytosis, still often seen in living mammals.

Skeletal features and the creature's teeth suggest it was a ground-dwelling animal, probably nocturnal, with a taste for insects.

The fossil was discovered in Spain in the Las Hoyas Quarry, a site primarily known for well-preserved fossils of birds and reptiles until the discovery of S. xenarthrosus in 2011.

In addition to the hair, paleontologists found preserved structures of the creature's lungs, a muscular diaphragm for respiration and evidence of the liver, the earliest-known record of mammalian organ systems.

There is also a well-preserved, large external ear, the earliest example in the fossil record of mammals, they say.

All of the findings suggest a number of fundamental mammalian evolutionary characteristics were already well established in the age of the dinosaurs 125 million years ago, the researchers say.

"The fossil thus joins the ranks of an entire range of newer findings," Martin says. "We have to revise our thinking. Mammals were indeed very small during the time of the dinosaurs. But they were certainly not primitive."

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