Scales helped reptiles conquer the earth. When did they first evolve?

Enlarge / Top left: a reconstruction of Diadcetes. Below: false color images of his paw and tail prints. Right: The section of the tail that left the imprint.

Voigt et. al./Urweltmuseum GEOSKOP.

Their feet left abundant footprints in the muddy floodplains of the Permian, leaving footprints scattered across ancient sediments. But in one of those footprints, scientists discovered something else: the trail of an animal’s tail as it crawled along the ground. Amazingly, these tail prints come complete with scale impressions: at 300 million years old, they are among the earliest scale impressions we have.

This may seem small, but it shows us that some of the hardened skin structures necessary for our ancestors to survive on land had evolved much earlier than previously suspected. A paper published in Biology Letters last May describes this discovery in detail.

A rare find

The particular slab containing these footprints was discovered in 2020 at the Piaskowiec Czerwony quarry in Poland. Mining had stopped to allow paleontologists to search for fossils in the red sandstone rocks. Gabriela Calábková described climbing “a huge pile of rubble” only to discover a sizeable slab of fossil footprints at the top. There, between a pair of footprints, was something new.

She called to her colleagues to join her at the top of the pile. None of them, she said, had found that kind of trace fossil before, but “they quickly realized it must be a body impression,” she explained to Ars.

Calábková is a paleontologist at the Moravian Museum. She and her co-authors are part of a joint effort of Polish, Czech and German scientists studying the Permian in Poland, a geological period spanning from 289.9 to 252 million years ago. The Piaskowiec Czerwony quarry is the world’s second largest producer of a particularly recognizable type of Permian track, Ichniotherium cottae. (If you were asked to draw hands, you might come up with something that looks like Ichniotherium cottae. The five bulbous digit imprints are almost cartoonish.)

Footprints and tail drags are examples of ichnofossils or trace fossils. As the name suggests, these are the fossilized marks or footprints made during an animal’s lifetime. Linking the exact animal to its track is nearly impossible, especially when there are no body fossils, so the tracks themselves are often given scientific names.

In this case, however, we may have identified the source, which is why the world’s premier site for these same footprints is in nearby Germany, where the spectacular Bromacker site has not only yielded abundant Permian footprints but has also provided fossils of animals with feet that appear to match the prints. They belong to the diactymomorphs, four-legged vertebrates (tetrapods) that were a distant precursor to mammals.

Bromacker has also provided a useful verification of the extremely rare tail drag found in the Piaskowiec Czerwony quarry. The only other two known tail trawls associated with I. cottae tracks were found there. They also have similar corncob-shaped scales.

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