An extraordinary discovery of fossilized skin provides new information about the appearance of early land vertebrates around 289 million years ago. The fossil discovered in a cave in the US state of Oklahoma is the oldest known evidence of animal epidermis. According to the analyses, the tiny, three-dimensionally preserved piece of skin shows an astonishing similarity to the skin of today’s crocodiles. Although the fossil cannot be assigned to a specific animal, the researchers assume that it comes from a prehistoric reptile, an ancestor of the dinosaurs.
When the first vertebrates walked on land around 385 million years ago, they not only had to adapt their locomotion to the new conditions. Life outside of the water also placed completely new demands on their skin. What was required was a stable and at the same time flexible shell that protected the inside of the body from drying out, heat, cold and other harmful influences such as pathogens. The first vertebrates to completely adapt to a dry life were the reptiles around 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.
Exceptionally well preserved
However, it is largely unclear what the skin of the earliest land vertebrates looked like and how it developed 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 find provides new insights into the skin of prehistoric reptiles: In a recent publication, a team led by Ethan Mooney from the University of Toronto Mississauga in Canada describes a three-dimensionally fossilized piece of skin that is probably between 286 and 289 million years old .
The fossil is smaller than a fingernail, but even tiny structural details can still be seen today. It owes this good state of preservation to the unique conditions where it was found: the Richards Spur limestone cave system in Oklahoma. “Animals probably fell into this cave system in the early Permian and were buried in very fine clay sediments, which delayed the decomposition process,” explains Mooney. “But the kicker is that this cave system was also an active source of oil during the Permian, and the interactions with the 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 top layer of skin in reptiles, birds and mammals. “The epidermis was a crucial feature that enabled the survival of vertebrates on land,” says Mooney. “It is a crucial barrier between the internal body processes and the harsh external environment.” Many features of the million-year-old skin can still be found in reptiles today. It has a scaled surface, similar to the skin of today’s crocodiles. The hinge areas between the individual scales are reminiscent of similar structures in snakes.
Because the skin fossil was found without an associated 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 half-aquatic. “This rare soft tissue fossil provides important initial evidence of the anatomical changes that mark the transition from aquatic and semi-aquatic lifestyles to a fully terrestrial lifestyle,” said the research team.
Source: Ethan Moony (University of Toronto Mississauga, Ontario, Canada) et al., Current Biology, doi: 10.1016/j.cub.2023.12.008