The discovery of a furry, prehistoric animal could shed new light on the way early mammals evolved, according to a study published today in the journal Nature. A fossilized skeleton of the extinct mammal, named Spinolestes xenarthrosus, was discovered in remarkably good condition, with its fur, skin, ears, and some soft tissue still intact. The specimen dates back 125 million years to the Mesozoic Era (252 to 66 million years ago), and is the oldest record of a mammal with spines and hair that can be examined on the cellular level.
Spinolestes xenarthrosus, which is comparable in size to a small opossum, was discovered in 2012 in the wetlands of Las Hoyas, Spain, a paleontological site known for its well-preserved fossils. The study's authors say the mammal had features similar to hedgehogs, with sharp spines on its back, and strong forelimbs that it likely used to forage for insects. It also shares characteristics with modern anteaters, armadillos, and sloths.
Previous fossil discoveries have shown that early mammals had hair, but most traces were in fossilized impressions, which gave little indication of the hairs' microstructure. Spinolestes xenarthrosus is the latest in a series of recent discoveries showing that early mammal species were far more diverse and complex than previously believed.
"another little stone in the mosaic."
"The general textbook knowledge, until the last 20 years, was that Mesozoic mammals were primitive, and not very specialized," says Thomas Martin, a paleontologist at the University of Bonn in Germany, and lead author of the study. "But now we know that they are highly diversified, and our Spinolestes xenarthrosus is another little stone in the mosaic."
Upon discovering the specimen, Martin and his colleagues analyzed its bones under an electron microscope to learn more about how it behaved. They saw that it had strong forelimbs and that its vertebrae structure was similar to modern anteaters. Its large ears also suggested that it was nocturnal, while the arrangement of its lungs and liver showed that it may have had a diaphragm, which would suggest that it was warm-blooded. They believe that the animal was terrestrial and that it used the sharp spines on its lower back to defend against predators.
Experts not involved in the study say it's highly unusual to find a complete skeleton of a Mesozoic animal with its hairs fully preserved. Steven Sweetman, a paleobiologist at the University of Portsmouth, describes Spinolestes xenarthrosusas a "truly remarkable specimen," adding that the animal's preserved soft tissue and skeleton allow for "unparalleled insights into [its] lifestyle and ecological adaptations." Alistair Evans, a paleontologist at Monash University, adds that it's "very rare" to find an early mammal in such "exquisite" condition, saying that most previous discoveries were in China.
"We always need new specimens."
"For a lot of these guys, we only found a few teeth, sometimes found a few jaws," Evans says. "And that tells us a little bit but nowhere near as much as this type of stuff does."
The study's authors say they'll next examine the chemical conditions at Las Hoyas to determine how such an old specimen was preserved. Yet despite the exquisite preservation of its remains, there are aspects of Spinolestes xenarthrosus that remain unknown. It's not clear, for example, whether the animal's spiny fur was widespread among other mammals at the time, or a unique adaptation. At one point, Martin says he and his colleagues thought that the animal may have carried eggs in an area below its stomach, though the evidence was not clear.
"The question of whether it laid eggs or gave birth to live young — that's a very interesting question, but we don't have conclusive information from this specimen," Martin says. "We would need a new specimen... We always need new specimens."