The oldest crocodile-like skin fossil found today

The extraordinary discovery of fossilized skin provides new information about the emergence of early land vertebrates about 289 million years ago. The fossil discovered in a cave in the US state of Oklahoma is the oldest known evidence of animal skin. According to analyses, the small piece of skin preserved in three-dimensional form shows a striking resemblance to today's crocodile skin. Although the fossil cannot be assigned to a specific animal, researchers hypothesize that it comes from a prehistoric reptile, an ancestor of dinosaurs.

When the first vertebrates walked the Earth about 385 million years ago, they not only had to adapt their movement to new conditions. Life outside the water also placed entirely new demands on their skin. What was needed was a stable and at the same time flexible shell that would protect the interior of the body from dryness, heat, cold and other harmful influences such as pathogens. The first vertebrates to fully adapt to dry life were reptiles about 300 million years ago. Unlike amphibians, which spend parts of their lives in water, their skin contains the protein keratin, which is also found in the skin, hair and claws of mammals and in the feathers of birds.

Very well preserved

However, it is largely unclear what the skin of the first terrestrial vertebrates looked like and how it evolved over the following millions of years. Because skin is usually one of the first structures to rot after an animal dies, there is little fossil evidence. But now an extraordinary discovery is providing new insights into the skin of prehistoric reptiles: In a recent publication, a team led by Ethan Mooney of the University of Toronto Mississauga in Canada describes a three-dimensional fossilized piece of skin that may be between 286 and 289 million years old.

The fossil is smaller than a fingernail, but even small skeletal details can still be seen today. This good state of preservation is due to the unique conditions in which it was found: the Richards Spur limestone cave system in Oklahoma. “The animals likely fell into this cave system in the early Permian and were buried in very fine clay deposits, delaying the decomposition process,” Mooney explains. “But what is interesting is that this cave system was also an active source of oil during the Permian, and interactions with hydrocarbons from petroleum and tar likely led to the preservation of this skin.”

Similarity to today's reptile skin

Microscopic examinations revealed that it was epidermal tissue, the upper layer of skin in reptiles, birds and mammals. “Edermis was a crucial feature that enabled vertebrates to survive on land,” Mooney says. “It is a critical barrier between the body's internal processes and the harsh external environment.” Many features of million-year-old skin can still be found in reptiles today. It has a scaly surface, similar to today's crocodile skin. The hinge zones between individual scales are reminiscent of similar structures in snakes.

Because the skin fossil was found without a skeleton or other remains, it is unclear which animal it came from. However, the structure of the skin suggests that it must have been an early reptile that evolved from amphibians that were still semi-aquatic. “This rare soft tissue fossil provides important first evidence of anatomical changes that indicate a transition from aquatic and semi-aquatic to fully terrestrial lifestyles,” the research team said.

Source: Ethan Mooney (University of Toronto Mississauga, Ontario, Canada) et al., Current Biology, doi: 10.1016/j.cub.2023.12.008

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