A dark shred of fossilized material, just a few millimetres across, “shocked” palaeontologists when they realized what it was. It arrived in a tiny glass vial, sent by fossil collectors in Oklahoma. “We got very excited when we actually saw the texture,” says palaeontologist Robert Reisz at the University of Toronto Mississauga in Canada. The material in the vial was fossilized skin — skin from millions of years before the rise of the dinosaurs.
The skin is the oldest ever found for a group of animals collectively known as amniotes, which includes reptiles, birds and mammals — all terrestrial vertebrates except amphibians. It is around 289 million years old, making it 21 million years older than the current recordholder. It probably belonged to a small, lizard-like animal known as Captorhinus aguti, according to a paper published today in Current Biology.
Primeval skin in 3D
What makes the find so special is that it isn’t just an impression of skin: it is an actual 3D layer of fossilized skin. The researchers were able to cut through it and examine a cross-section, in which distinct layers of epidermis and dermis were visible. The epidermis includes corneous tissue, the topmost layers of epidermal cells that act as a tough barrier between the inside and outside worlds. Skincare aficionados know the human equivalent of this layer as the stratum corneum or skin barrier.
The skin was found by Bill and Julie May, who, for many years, have sifted through material from a limestone quarry in Richards Spur, a cave system in Oklahoma known for its fossils of C. aguti. The Mays have a long-standing collaborative relationship with Reisz, who investigated the skin with students Ethan Mooney and Tea Maho, also at the University of Toronto Mississauga.
The specimen is dark — almost black — because oil was actively seeping into the cave as the skin was fossilizing, causing, in Mooney’s words, “a total permeation and encapsulation” of the organic materials with hydrocarbons. Pickled in petroleum, with an instantly recognizable reptilian-esque texture, the fragments offer data on early adaptations to life on land.
The skin we’re in
“Obviously, we’re terrestrial organisms. So we don’t think about how harsh the terrestrial environment is, in comparison to life in the water,” says Stephanie Drumheller-Horton, a taphonomist at the University of Tennessee, Knoxville.
Tough, watertight skin was one of the key evolutionary adaptations that allowed animals to adopt completely terrestrial lives. It allows amniotes to carry their organs around in a kind of inner sea, protected from harsh temperatures, ultraviolet-light radiation and the constant threat of dehydration. Alas, because these hides are not very permeable, we amniotes had to forego skin breathing.
If skin hasn’t received the same palaeontological attention that skeletal features have, it’s because there’s just not that much available to study.
“Any time we’re asking questions in palaeontology, we’re at the mercy of what has ended up in the fossil record,” says Drumheller-Horton, who was not involved in the study. “That usually means some type of mineralized tissue: so bones, shells, things like that.” Getting something as soft and prone to decomposition as skin is a “fantastic window into the evolution of those groups.”
This article is reproduced with permission and was first published on January 11, 2024.