The scaly fossil is the oldest-known piece of skin

Dry, scaly skin can be one of the funniest parts of winter. But in the broader scheme of things, the hard, waterproof hide helped the ancestors of modern reptiles, birds and mammals move inland while their thin-skinned amphibian relatives stayed close to water.

In a published study Thursday in the journal Current Biology, scientists announced the discovery of the oldest piece of fossilized skin. The pebble scrap, no bigger than a human fingernail, often belongs to ancient reptiles and offers a rare glimpse into the evolution of skin.

The skin fragment is one of countless traces of prehistoric life preserved in the Richards Spur limestone cave system near an oil spill in southwestern Oklahoma. When animals fell into caves 289 million years ago, conditions were ideal for preservation: fine clay deposits buried the bodies quickly, low levels of oxygen in groundwater slowed the decomposition process, and hydrocarbons from oil penetrated the tissues and degraded them. Inhospitable to bacteria. The tar seeped into the fossils and stained them.

In 2018, retired forensic scientist Bill May shared with University of Toronto Mississauga paleontologist Robert Reiss some small flakes from an unidentifiable Richards Spurr.

“We were very excited by what we saw under the microscope,” said Dr Rees, an author of the paper.

“The structure of the skin is very unique and interesting. It really stands out from other fossil materials. It's obviously not bone,” said Ethan Mooney. If anything, the fossil tissue bore a striking resemblance to the scaly skin of an alligator.

A Ph.D. student and another author of the paper, Dee Maho, used a diamond-tipped knife to slice a small section of skin into hair-thin layers. The outer layers consisted of tough structures made of keratin, a protein found in mammalian hair and nails. These hardened structures, or cornifications, are a hallmark of the skin of amniotes—land-dwelling, vertebrate animals including reptiles, birds, and mammals. The ancestors of amniotes could live and reproduce out of water, unlike their amphibian relatives.

The tough, impenetrable skin was a key evolutionary adaptation for ammonites to conquer land, “because in order to survive in a terrestrial environment, you want to not dry out,” says Mr. Mooney said.

Fossilized skin was found on its own, not attached to bone. However, Richards Spur has yielded numerous fossils of a small lizard-like reptile called Captorhinus aguti. Scientists c. Although Agudi did not find the fossil with the skin attached to it, they did identify one with the remains of carnifications. Dr. Reiss said the skin came from the same animal.

Hans Suess, a paleontologist at the National Museum of Natural History who was not involved in the study, said he was “delighted” by the paper, acknowledging that it was “a really early fossil example” of skin.

“We have skin impressions, but here they can see the detailed structure under the microscope as if they were harvesting skin from a living animal,” Dr. Seuss said. “This is a very important finding.”

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