A fragment of fossilized skin collected from the Richards Spur limestone cave system in Oklahoma, the United States, is at least 21 million years older than previously described skin fossils.
The fossilized skin of Captorhinus aguti. Image credit: Mooney et al., doi: 10.1016/j.cub.2023.12.008.
The newly-described fossilized skin belonged to Captorhinus aguti, a species of early reptile that lived during the Permian period, around 289 million years ago.
The specimen and an associated skeleton of the reptile were collected by lifelong paleontology enthusiasts Bill and Julie May at Richards Spur, a limestone cave system in Oklahoma that is an active quarry.
The skin fragment, which is smaller than a fingernail, has a pebbled surface and most closely resembles crocodile skin.
It’s the oldest example of preserved epidermis, the outermost layer of skin in terrestrial reptiles, birds, and mammals, which was an important evolutionary adaptation in the transition to life on land.
“Every now and then we get an exceptional opportunity to glimpse back into deep time,” said Ethan Mooney, a graduate student at the University of Toronto.
“These types of discoveries can really enrich our understanding and perception of these pioneering animals.”
Skin and other soft tissues are rarely fossilized, but Mooney and his colleagues think that skin preservation was possible in this case because of the Richards Spur cave system’s unique features, which included fine clay sediments that slowed decomposition, oil seepage, and a cave environment that was likely an oxygenless environment.
“Animals would have fallen into this cave system during the Early Permian and been buried in very fine clay sediments that delayed the decay process,” Mooney said.
“But the kicker is that this cave system was also an active oil seepage site during the Permian, and interactions between hydrocarbons in petroleum and tar are likely what allowed this skin to be preserved.”
The analysis of the specimen revealed epidermal tissues, a hallmark of the skin of amniotes, the terrestrial vertebrate group that includes reptiles, birds, and mammals and which evolved from amphibian ancestors during the Carboniferous period.
“We were totally shocked by what we saw because it’s completely unlike anything we would have expected,” Mooney said.
“Finding such an old skin fossil is an exceptional opportunity to peer into the past and see what the skin of some of these earliest animals may have looked like.”
The skin shares features with ancient and extant reptiles, including a pebbled surface similar to crocodile skin, and hinged regions between epidermal scales that resemble skin structures in snakes and worm lizards.
However, because the skin fossil is not associated with a skeleton or any other remains, it is not possible to identify what species of animal or body region the skin belonged to.
The fact that this ancient skin resembles the skin of reptiles alive today shows how important these structures are for survival in terrestrial environments.
“The epidermis was a critical feature for vertebrate survival on land. It’s a crucial barrier between the internal body processes and the harsh outer environment,” Mooney said.
“This skin may represent the ancestral skin structure for terrestrial vertebrates in early amniotes that allowed for the eventual evolution of bird feathers and mammalian hair follicles.”
The findings appear in the journal Current Biology.
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Ethan D. Mooney et al. Paleozoic cave system preserves oldest-known evidence of amniote skin. Current Biology, published online January 11, 2024; doi: 10.1016/j.cub.2023.12.008