Most of our knowledge of very ancient life comes from fossilized remains of hard tissues—bones, shells, and teeth. Now, the exquisitely preserved fossil of a tiny mammal from the time of the dinosaurs reveals a variety of soft tissues, including skin, fur, and spines; even remnants of its external ear were fossilized. The find pushes back the earliest record of mammalian internal organs and well-preserved fur by more than 60 million years, and shows that ancient fur and spines formed just as they do in today’s mammals.
“Finding complete fossils like this raises the bar for the rest of us,” says Richard Cifelli, a vertebrate paleontologist at the University of Oklahoma, Norman, who was not involved with the new study. “My breath is taken away.”
The fossil was unearthed from 125-million-year-old rocks in central Spain. In life, the creature likely measured about 24 centimeters (9.4 inches) in length and weighed between 50 and 70 grams—about the size and proportions of a juvenile rat, says Zhe-Xi Luo, a vertebrate paleontologist at the University of Chicago in Illinois. Luo and colleagues dubbed the new species Spinolestes xenarthosus. Spino refers to the creature’s hedgehoglike spines, and the Greek suffix lestes means "robber"—a term commonly used in the names of ancient mammals because of their presumed nocturnal habits. The species name xenarthrosus refers to the odd way the creature’s spinal vertebrae interlock and stiffen the back, similar to modern-day armadillos, anteaters, and other mammals in a group called xenarthrans.
Spinolestes belongs to a once widespread but now extinct group of rat-sized-or-smaller mammals called triconodonts, which got their name from the three conical projections on each of their molars. Whereas fossils of triconodonts have been found in many areas of North America and Eurasia, Spinolestes is the first from the group to be found in Spain, Luo says.
The finely layered limestones that entombed the fossil were deposited in a freshwater wetland. Rapid burial of the ancient carcass in sediment, as well as low concentrations of oxygen in the ancient marsh, likely contributed to its exceptional preservation, Luo says. The overall shape and arrangement of bones in the creature’s feet suggests that it was a ground-dweller and may have dug into the soil to forage for grubs or other prey.
This is the first time some of the features preserved in the fossil are seen in dinosaur-era mammals. For instance, the shafts of individual hairs have the same three-layered structure seen in modern mammals, Luo says. Some of the skin’s follicles have more than one hair growing from them, and in other cases the structures emanating from several different follicles have merged to form tiny spines like those seen in modern-day hedgehogs, the researchers report online today in Nature. In some places on the body, hairs appear to have broken off close to the skin and were discolored near the broken tips—signs that the creature may have had a fungal infection called dermatophytosis that’s common in living mammals.
Although much older mammalian fossils include remnants of hair, Luo notes, those structures are mere impressions and don’t preserve the detail seen in the newly described fossil.
The fossil also includes internal organs. Within the ribcage, there are patches of soft tissue that contain tubular structures in a branching pattern, which the team interprets as preserved lung tissue. Farther down in the abdomen is a large oval region of reddish brown material—likely the remnants of the creature’s liver, Luo says. The sharp boundary between the two suggests that Spinolestes had a strong muscular diaphragm, which in turn hints at the ability to rapidly breathe and fuel an active lifestyle.
“Diaphragm, liver, lungs; it’s amazing to see all that detail,” Cifelli says. Plus, he notes, Spinolestes has all the hair categories seen in modern mammals “and that’s not a small thing.” Only fossils unearthed in recent years provided evidence that ancient mammals had fur that looked like that seen in modern relatives.