Scales helped reptiles conquer the land—when did they first evolve?

Their feet left copious traces in muddy Permian floodplains, leaving tracks scattered across ancient sediments. But in one slab of such trackways, scientists uncovered something more: the trace of an animal’s tail as it dragged across the ground. Strikingly, these tail prints come complete with scale impressions—at 300 million years old, they’re 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 this past May describes this discovery in detail.

A rare find

The particular slab holding these traces was discovered in 2020 at the Piaskowiec Czerwony quarry in Poland. Mining had stopped to enable paleontologists to search the red sandstone rocks for fossils. Gabriela Calábková described climbing upon “a huge pile of rubble” only to discover a sizable slab of fossil tracks at the very top. There, among one set 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 ever encountered that kind of trace fossil before, but they “quickly understood that 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 by Polish, Czech, and German scientists studying the Permian in Poland—a geologic time period that stretches from 289.9 to 252 million years ago. The Piaskowiec Czerwony quarry is the world’s second-largest producer of one particularly recognizable type of Permian track known as Ichniotherium cottae. (If you were asked to draw hands, you might come up with something that looks like Ichniotherium cottae. The five bulbous imprints of the digits are almost cartoonish.)

Footprints and tail drags are examples of ichnofossils, or trace fossils. As their name suggests, these are the fossilized marks or traces made during the life of an animal. Matching the exact animal to its trace is almost impossible, especially when body fossils are absent, so traces themselves are often given scientific names.

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

Bromacker has also provided a helpful verification of the exceedingly rare tail drag found in the Piaskowiec Czerwony quarry. The only two other known tail drags associated with I. cottae trackways were found there. They, too, have similar corn cob-shaped scales.

Tipping the scales

Sebastian Voigt is the director of the Urweltmuseum GEOSKOP in Germany and the lead author of this paper. Evidence of segmentation within the toes of their footprints along with the tail drag scale impressions are “a further argument that diadectids had scales at least on the underside of the tail and feet,” he told Ars. “One cannot automatically assume that scales developed evenly and quickly all over the body. For example, scales could have developed first on the parts of the body that were most likely to be in contact with the ground in terrestrial vertebrates (feet, belly, underside of the tail).”

But either way, “the epidermal scales in diadectids and other terrestrial tetrapods prevented the evaporation of water from their bodies in a dry environment,” Calábková noted, which may have helped them survive “the desert climate that prevailed on [the supercontinent of] Pangea during the Permian.”

Adriana Cecilia Mancuso is a researcher from Argentina who specializes in the evolution of terrestrial ecosystems. She was not involved in the research, but says this discovery is “highly significant. Fossil skin impressions are rare in the deep geological record. Furthermore, the quality of the reported impressions provides strong evidence regarding the origin of epidermal scales in early tetrapods.”

“By studying these fossil traces,” she explained to Ars, “scientists can reconstruct ancient ecosystems, understand how organisms responded to environmental and climatic changes, and trace the evolution of key features in the history of life on Earth. This information not only helps us better understand our geological and biological past but also offers insights into how modern ecosystems may respond to current and future environmental challenges.”

Help on the ground

Access to the two Polish quarries where these fossils are found is a direct result of “support from the employees and management of the quarries,” said Izabela Ploch, co-author and stratigrapher and paleontologist with the Polish Geological Institute. She added that “in the case of very heavy sandstone slabs, they help us extract them from the quarry, often using heavy equipment.”

“We often talk about fossils to quarry workers,” she continued, “which really piques their interest. The result of this is, for example, the creation of [an exhibit space] in Radków, initiated by the management of a nearby quarry after they saw what beautiful trace fossils we found there.”

“We want local people to understand what they have in this area and what interesting stories the ichnofossils tell,” she said. And that includes plans to encourage geotourism to the area as well as exhibits displaying the trace fossils found in the quarries.

Ploch mused about how much we have yet to uncover. “There are so many mysteries still to be discovered,” she said. “Without knowing our past, we will not know our future and the processes that guide the evolution of life on Earth. By learning about even a modest fragment of the Earth's past, we add an element to a larger puzzle.”

Biology Letters, 2024.  DOI: 10.1098/rsbl.2024.0041