Skin is hard to find in the fossil record. The outer flesh of ancient organisms is much more prone to being eaten by scavengers or rotted away than sturdy bones are. But sometimes paleontologists get a lucky break. Within the roughly 288-million-year-old recesses of a cave in Richards Spur, Oklahoma, paleontologists have found truly ancient fossil skin impressions that were preserved in a very unusual way.
Described Thursday in Current Biology, these tatters of fossil skin and various skin impressions come from some of the earliest amniotes. The lizard-like animals were some of the first creatures to live their entire lives on dry land, away from the water’s edge, and their bodies were covered in pebbly, scaly skin. The newly uncovered fossils are the oldest record of preserved skin yet found, about 130 million years older than the previous record holder, containing not only the exterior texture of the skin but also the internal structure of the epidermis.
Paleontologists have been going back to Richards Spur for years. “It’s an exceptional locality,” says study author and University of Toronto paleontologist Ethan Mooney. Hundreds of millions of years ago, fine-grained sediment filled in fissures of an ancient cave system—burying the remains of early amniotes and other animals within. What makes Richards Spur special, however, is a unique interaction between the clay-rich cave sediment, a lack of oxygen, and oil that seeped from the cave rocks.
Most living things decay and completely break down after death, leaving no trace in the fossil record. Preservation is rare to begin with, and to find skin is even more unusual. In this case, the ancient animal remains buried within the cave were likely left in a relatively dry area with low oxygen. The tough skin, with scales made out of the same material as your fingernails, was able to desiccate and stand a better chance of being buried with the animal’s bones.
When the remains of prehistoric animals became buried in the Richards Spur cave, hydrocarbons seeped out of the cave’s stone to combine with sediment to create what Mooney calls “exquisite fossils.” That oil came from the transformed remains of even older fossil organisms that lived in the seas about 330 million years ago. The organic remains essentially turned into petroleum, which then interacted with the skin and bones of the animals buried in the cave and preserved the skin in three-dimensional detail. Those fossils helped to create new fossils that have survived for hundreds of millions of years.
Mooney and colleagues have identified multiple samples of preserved skin and skin impressions among the Richards Spur sample. All the samples came from creatures that lived at about the same time, though most are isolated shreds of skin detached from the animals they once covered. Some show scale patterns reminiscent of modern crocodiles. Most impressive of all, however, is the preserved skin on a fossil of a creature called Captorhinus aguti. The fossil of the early reptile includes part of its skin arranged in 24 bands behind its skull. The remains are the closest paleontologists can get to seeing what Captorhinus would have looked like in the flesh.
The new finds not only are rare but also help close a knowledge gap about what the body coverings of early amniotes were like, says Altmühltal Dinosaur Museum paleontologist Frederik Spindler, who was not involved in the new study. “I’ve seen a lot of Richards Spur fossils, but this is among the most meaningful,” he says.
The findings of the study are more than skin deep. When the researchers used CT scans to look at the details inside the fossil skin, Mooney says, they found that the internal structure was like that of modern crocodiles and was also suited to the side-to-side, lizard-like walking motions that researchers expect of early amniotes.
From the outside, the skin of Captorhinus looks scaly. The find speaks to some of the changes early amniotes were undergoing as they adapted to a more terrestrial life than their amphibious ancestors. “There was a transition from amphibian skin types with bones beneath to fully terrestrial skin with tough outer scales,” Spindler says. Not only were early amniotes able to lay eggs on land, away from the water, but a scaly outer layer of skin also helped keep these creatures from drying out. “The major role of thickened skin is to not only act as a protective barrier,” Mooney says, “but also to prevent critical water loss.” Scaly skin, just like eggs contained by a shell, were ways early amniotes kept moisture inside as they ventured further and further into the terrestrial realm.
While the skin from Richards Spur is now the oldest of its kind yet known, the evolution of scaly skin must have happened earlier. “Other fossils will tell us about the timing of this evolutionary step,” Spindler says, “but the new study is fundamentally important for the diversification of skin types.” The fossils are a kind of milepost for when amniotes were settling in on land and establishing the foundational traits of later creatures like fuzzy mammals and feathery dinosaurs. Of all the evolutionary innovations related to living on land, tough skin is high on the scale.