A skin fossil the size of a fingernail is causing quite a stir because of the potential insights it holds into the evolution of vertebrates. The sample was found in the extensive limestone cave system known as Richards Spur in Oklahoma. Although there are many fossils in these caves, fossilised skin is extremely uncommon. Furthermore, compared to previously found skin fossils, this fossil, which is described in Current Biology, is at least 21 million years older.
This is the first skin-cast fossil found in the Paleozoic Era, and its early amniote ancestry is suggested by its crocodile-skin-like pebbled surface. Being the oldest preserved epidermis ever discovered, it offers a vital piece of information that advances our knowledge of evolution.
Because they were the first animals to successfully live on dry land, early amniotes are significant. These lizard-like organisms would have required this outermost layer of skin to retain moisture and shield themselves from pathogens before branching out to become mammals, reptiles, and birds.
"What we saw completely shocked us because it's not anything we would have expected," says first author Ethan Mooney, a graduate student studying palaeontology at the University of Toronto, who collaborated with palaeontologist Robert Reisz on the project while an undergraduate.
"This is a unique opportunity to look into the past and see what some of these earliest animals' skin may have looked like—finding such an old skin fossil."
Fortunately, Richards Spur offers the ideal habitat for this kind of fossil. The cave system has features like oil seepage, fine clay sediments that probably slowed down decomposition, and an oxygen-free cave environment.
"Animals would have been buried in very fine clay sediments that delayed the decay process when they fell into this cave system in the early Permian," says Mooney. The worst part is that during the Permian, this cave system was also a site of active oil seepage; interactions between the hydrocarbons in tar and petroleum are probably what preserved this skin.
The skin structure of early amniotes may have been modelled after this one, according to researchers, allowing for the eventual evolution of mammalian hair follicles and bird feathers.