The oldest known fossilized skin shows how life adapted to land

At nearly 290 million years old, it’s one of the only fossilized skins found from the Paleozoic Era

A photo of a set of dark skin fossils

A dagger-shaped fossil cast (the two pieces left of the bottom right piece) is the oldest mummified skin specimen ever found. It and seven skin impressions (also pictured) belong to an early four-legged vertebrate called an amniote. The dark color comes from staining by petroleum and tar that seeped through sediments.

E.D. Mooney et al/Current Biology 2024

A dark sliver of rock pulled from an Oklahoma limestone quarry is the world’s oldest fossil cast of skin ever found.

The fossil is nearly 290 million years old, and once dressed an early member of the amniotes, a clade of four-legged vertebrates that evolved from amphibians and includes all reptiles, birds and mammals, researchers report January 11 in Current Biology. It’s 21 million years older than the only other reported fossilized skin from the Paleozoic Era, spanning 541 million to 252 million years ago, during which animals moved onshore and diversified.

“This is definitively the oldest [known] piece of mummified skin,” says paleontologist Ethan Mooney of the University of Toronto Mississauga. It fits “into a broader story of how the first animals left the water and went onto land.”

Fossil collectors Bill and Julie May found the cast, along with exquisitely preserved skin impressions, at an Oklahoma quarry in an ancient limestone cave system known as Richards Spur.

There, a special concoction of cave conditions contributed to the fossils’ superb preservation. Corpses were buried in fine sediments, which excluded oxygen and slowed decay, and were exposed to groundwater rich in iron, an element that helps preserve tissues. Also, the site was an ancient oil seep. Petroleum and tar permeated the remains, sealing them off from decaying conditions while also staining them black.

An illustration of a four-legged vertebrate walking on the ground.
Captorinhus, an early amniote (illustrated), walked on solid ground during the Permian period, which stretched from around 299 million to 252 million years ago. Michael deBraga

The skin samples all possess nonoverlapping scales, though the scale sizes, distributions and abundances vary. The specimens probably come from different places on the amniote body, Mooney’s team suggests, and possibly different animals too.

Cast cross sections revealed a thickened outer skin layer, or epidermis. The development of a robust epidermis would have protected the early amniotes from the elements while also helping it retain water.

That surficial innovation eventually led to bird feathers and mammalian hair follicles. Tough and bumpy amniote skin, Mooney says, was “the first stage.”

Nikk Ogasa is a staff writer who focuses on the physical sciences for Science News. He has a master's degree in geology from McGill University, and a master's degree in science communication from the University of California, Santa Cruz.

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