In the Carboniferous, a geological period from 359 to 298 million years ago, some tetrapods came out of the water. Now they have discovered skin from 289 million years ago of an amniote, a fully terrestrial animal. The integument preserves several of its layers and the patterns it presents, with scales and protuberances, everything indicates that they had an epidermis similar to that of lizards. This innovation was key for amniotes, ancestors of reptiles, birds and mammals, to conquer a whole new territory, the continents.
The skin, like the rest of the tissues and organs, gets along poorly over time. The bacteria inside and all the microorganisms outside, the action of oxygen and the chemical reactions with the minerals with which it comes into contact, leave any vertebrate in the bones. It is the teeth and bones that are best preserved, such as that of a whale from 40 million years ago, which is considered the heaviest animal that has ever inhabited the Earth.
But, sometimes, the soft parts fossilize as they mineralize, leaving them as a relief spot on the stone. Even rarer is the discovery of skin that preserves its three-dimensional structure, like what Canadian paleontologists have just discovered. And this case is even more exceptional because the epidermis in question is from one of the first land animals.
The skin, found at the site of Richard's Spur, Oklahoma (USA), is fragmented and all the pieces together are no larger than a fingernail, but its study under a microscope revealed epidermal tissues, a hallmark of amniote skin. . As the researchers detail in the scientific journal Current Biologythe integument preserves convex tubercles, like the protuberances on crocodile skin, and the outermost part is made up of scales ending in horny tissue, made up of hardened keratin.
“Fully terrestrial”
For paleontologist Robert Reisz from the University of Toronto (Canada), senior author of this research, “the presence of this type of skin indicates that we are dealing with a fully terrestrial animal.” It is believed that in the long transition to the new way of life, there must have existed groups of animals that, like amphibians, had semi-aquatic characteristics. “Geological and paleoclimatological work indicates that this animal lived in a tropical environment, with a monsoon-type climate,” adds Reisz.
An epidermis like the one described could not be an amphibian. The integument of these animals has glands that keep it moist, something that was not needed in a dry environment. In addition, their bare skin helps them breathe oxygen, which they could not do with keratinized scales. “The first terrestrial vertebrates also included amphibians, but here we are talking about amniotes. This means that the first amniotes, the common ancestor of living reptiles, birds, mammals and their extinct relatives, had this type of skin,” concludes Professor Reisz.
Amniotes are four-legged vertebrate animals. One of the greatest innovations that allowed life on the terrestrial portion of the planet was the development of up to four layers that protect the embryo, housing food and moisture. With this, oviparous reproduction in a dry environment became possible. But these newcomers did so with other innovations, such as exclusively lung breathing or dry skin. Thanks to the accumulation of keratin in the epidermis, the scales formed a barrier that, while preventing dehydration, protects from external aggressions, such as rain or sunlight.
Along with this skin, traces of fossilized epidermis were also found attached to the vertebrae of a small animal, the Captorhinus agouti, an amniote of just 30 centimeters. This leads Ethan Mooney, first author of the study and also from the University of Toronto, to suggest that the skin they have discovered is from this same animal: “It is reasonable to assume that these isolated skin fragments are actually from a C. agouti because they have horny components and we also see horny structures arranged in bands on the skeleton of one of these animals.”
Furthermore, hundreds of thousands of bones have been found at the same site and the majority are of this species. “This animal was the most abundant at this site and is also one of the first reptiles, so we consider that there is evidence that it has at least similar skin structures. So it's likely that they belong to him, but we can't say for sure,” Mooney adds.
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