Around 125 million years ago, an early mammal was scurrying around a wetland in what is now central Spain. Unfortunately for this little furry creature, it met its end in the watery depths and was quickly buried by fine sediment. In 2011, a team of scientists found the animal preserved in astonishing detail in the rocks of the Las Hoyas Quarry.
“Well the fossil itself is a spectacular find,” University of Chicago’s Zhe-Xi Luo, who co-authored the study describing the fossil published in Nature, told IFLScience. “When I first saw the original fossil in Madrid, it was absolutely astounding. The big revelations started to trickle in when scanning electron microscope results started coming in. We realized first that this mammal has compound hairs.” 
This was the first of many incredible details preserved in the fossil. The researchers were able to see individual pores and even follicles – the tiny skin organs that produce hair. A compound hair follicle, where one pore produces a single primary hair and other smaller secondary hairs, is a basic structure of mammalian hair. And it seems that this feature evolved early in the mammal lineage.
“The second feature that is amazing is that the spines are also clustered,” says Luo. We know from modern mammals that it takes multiple hair follicles to fuse together during the early stages of embryonic development in order to produce larger spines. These exquisitely preserved microstructures now show, according to Luo, “that this 125-million-year fossil actually has the same pattern in embryogenesis of this hair like structure.”
But it doesn’t end there, as the skin is not the only bit of soft tissue from the animal to have been preserved for all this time. The fossil also contains the earliest known record of mammalian organ systems, with the lung and liver also being preserved. So well preserved, in fact, that the liver appears red from iron-rich residues. But what is of more interest is that between these two organs, the researchers see a curved line that they think is a diaphragm. As modern mammals use this muscle to help breathe while running, its presence shows how the early mammal was probably actively hunting down the insects it fed on.
The little critter, called Spinolestes xenarthrosus, would have lived scurrying around the feet of the dinosaurs that dominated the watery landscape at the time, while early birds took to the skies. It is a member of a now extinct group of mammals called “eutriconodonts,” which on the evolutionary tree of mammals falls between the monotremes (egg-laying mammals like platypuses and echidna), and the therians (marsupials and all other mammals).
With such high levels of preservation, researchers can now conclusively say that complex structural features such as compound hair follicles and spines, many of which are fundamental mammalian characteristics, were already well established during the age of the dinosaurs, some 125 million years ago.
Top image in text: A reconstruction of what the mammal might have looked like in real life. Oscar Sanisidro
Bottom image in text: The actual fossil of S. xenarthrosus. Georg Oleschinski
