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“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.

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.”