Ancient Cave System Preserves Oldest Known Fossilized Skin
A fragment of fossilized skin was found to be at least 21 million years older than previously described skin fossils.
Background
Much of what is known about animals that went extinct millions of years ago comes from their fossilized remains. The fossil record is rich and varied, but soft tissue such as skin, organs, and connective material are found less frequently since they break down more rapidly than bone. They often breakdown before stone has a chance to preserve its structure.
Fortunately, the infilled cave system of Richards Spur in Oklahoma is an environment with ideal conditions to preserve soft tissue. The sediment is unusually soft and fine, and the lack of oxygen would delay the decomposition of soft tissue. During the Permian Period (299 to 251 million years ago), the cave was an active oil seep. The hydrocarbons in the petroleum and tar permeating the sediment would have also helped to preserve tissues.
Very few skin samples from one of the most important evolutionary periods in vertebrate history have been found. 350 million years ago descendants of amphibian animals known as amniotes started evolving to live on land instead of water. This transition required the development of protective scales.
Summary
The fossilized skins of unknown amniotes are the oldest discovered, dating back 289 million years, which is 21 million years older than the previously oldest discovered skins. The skin samples all possess nonoverlapping scales, though the scale sizes, distribution, and abundance varies. It is estimated that the samples come from different places on the amniote body and possibly different species.
The skins show intricately patterned scales with resemblance to those of the modern crocodile. They also contain hinged regions between epidermal scales that resemble skin structures in snakes and worm lizards.
The epidermis is the outermost layer of skin and was a critical feature for survival on land. It acts as protection from the mechanical injury, retains water to keep the skin hydrated, creates new skin cells to replace old ones, and provides protection from the environment and ultraviolet light. Early amniotes rapidly diversified their scale-forming mechanisms to support a variety of drought-adapted bodies to survive on dry land with the rising temperatures. Through a process called epidermal cornification, the outermost skin layer develops and becomes watertight and mechanically hardened in land-dwelling animals.
Based on the observations of highly patterned mini scales in these fossilized skin samples, it is plausible that embryonic skin underwent rapid patterning into spot-like areas corresponding to sites where scales would form. On hatching, developing scales likely hardened rapidly by cell differentiation, which makes the scale cells distinct from the other surrounding tissue. Scale pattern formation on the epidermis is thought to give rise to this resilient yet flexible waterproof skin containing scales to protect the ancient amniote.
This skin may represent the ancestral skin structure for terrestrial vertebrates in early amniotes that allowed for the eventual evolution of bird feathers and hair follicles in mammals.
