Researchers have identified a fragment of fossilized skin that is at least 21 million years older than previously described skin fossils. The skin belonged to an early Paleozoic reptile species and had a grainy surface resembling crocodile skin.

It represents the oldest preserved epidermis, which is the outermost layer of skin in land reptiles, birds, and mammals. This skin fossil was a key adaptation for the transition to life on land. The fossil is described in the journal Current Biology along with other specimens collected from the Richards Spur limestone cave system in Oklahoma.

Occasionally we get a rare chance to peek back in time, says lead author Ethan Mooney, a paleontology graduate student at the University of Toronto who worked on the project as a student with paleontologist Robert Reisz. Discoveries like this can really enrich our understanding of these pioneering animals.

Soft tissues like skin rarely fossilize, but researchers believe this skin was preserved due to the unique conditions in the cave system. Fine clay sediments slowed decomposition, oil seepage occurred, and the cave provided an oxygen-free environment.

The animals would have fallen into this cave system in the early Permian period and gotten buried in very fine clay sediments, slowing decomposition, explains Mooney. Interestingly, oil was also seeping through the caves during the Permian, and interactions between oil and tar likely allowed the skin to preserve.

The tiny skin fossil, smaller than a fingernail, showed epidermal tissues under a microscope. This is a distinctive feature of amniote skin, the group of land vertebrates including reptiles, birds, and mammals that evolved from amphibious ancestors in the Carboniferous period.

We were very surprised by what we saw, as it didn’t resemble what we expected, says Mooney. Finding such an ancient skin fossil is a rare chance to glimpse the past and see what skin may have looked like on some of these early animals.

The skin shares traits with both ancient and modern reptiles, including a grainy surface like crocodile skin and articulated regions between epidermal scales resembling snake and lizard skin structures. However, without being associated with a skeleton or other remains, it’s impossible to identify what species or body region the skin came from.

The fact this ancient skin resembles modern reptile skin demonstrates how important these structures were for terrestrial survival. The epidermis was a critical adaptation for vertebrate survival on land, says Mooney. It provides a crucial barrier between the body’s internal processes and the harsh external environment.

Researchers suggest this skin represents the ancestral skin structure of land vertebrates in the early amniotes, allowing for the eventual evolution of feathers in birds and hair follicles in mammals.

The skin fossil and other specimens were collected by lifelong paleontology enthusiasts Bill and Julie May at Richards Spur, an active Oklahoma limestone cave system that has preserved many of the earliest land animal fossils due to its unique conditions. The specimens are housed at the Royal Ontario Museum.


Cell Press | Ethan D. Mooney, Tea Maho, et al., Paleozoic cave system preserves oldest-known evidence of amniote skin. Current Biology, January 11, 2024.

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