A typical feature of many reptiles is their calloused skin scales. Paleontologists in Thuringia have now discovered the oldest impressions of such scales. They come from an early representative of the reptiles who was resting on a muddy spot around 298 to 299 million years ago. As the fine mud fossilized, it preserved the scaly skin structures of this animal, which was only about the size of a lizard. They document for the first time the scaling pattern of the core group of today’s reptiles and show that the epidermal scales of these early lizards were already similar to those of modern reptiles. Also unusual is the impression of a slit-shaped opening at the base of the tail of this impression fossil, named Cabarzichnus pulchrus. This could be the oldest evidence of a cloaca.
Not only did prehistoric animals leave behind fossils in the form of fossilized bones, footprints and even impressions of their scales, feathers or other skin structures are sometimes preserved. They provide valuable insights into the appearance of these animals, but also into the evolution of the skin structures that are still typical of many animal groups today. This also applies to the calloused skin scales of many reptiles. Unlike the bone scales of fish and the earliest land vertebrates, which are formed from deeper layers of skin, reptile scales arise in the epidermis, the outer layer of skin. Typically they are angular and form regular rows and grid-like patterns. However, it is still unclear when the reptile scales were created. All that is known is that the first reptiles developed from early amniotes around 300 million years ago at the end of the Carboniferous Age. These common ancestors of reptiles and mammals were the first vertebrates able to reproduce on land thanks to their drought-resistant eggs.
Lying traces and scale impressions of a prehistoric reptile
But so far there has been a lack of fossils with sufficiently well-preserved skin and other soft tissue from the transition from the Carboniferous to the Permian. “Such soft tissue structures are extremely rare in the fossil record – and the further we go back in Earth’s history, the more extraordinary they become,” explains first author Lorenzo Marchetti from the Museum für Naturkunde Berlin. This makes a find that he and his colleagues have now made in two Thuringian quarries all the more spectacular. The so-called Goldlauter Formation comes to light there, a layer of rock that was deposited in the late Carboniferous. In this layer, the paleontologists discovered fossilized tracks and footprints of a prehistoric reptile. What can be seen is the approximately nine centimeter long body print of a lizard-like animal with a triangular head, tail and legs lying on its sides, which once rested in a muddy area of its habitat. Using volcanic ash deposits in the rock formation, Marchetti and his team were able to date the prints relatively precisely. They therefore date from 298 to 299 million years ago.
Not only footprints and body outlines were preserved in the primeval mud, but also details of the skin structure of this trace fossil, named Cabarzichnus pulchrus. The impressions of scales can be seen on the body, legs and underside of the tail, which lie close together in orderly rows. “The scales of the trunk are diamond-shaped and about twice as long as they are wide,” the team writes. “They show a similar morphology and arrangement to the epidermal scales of modern reptiles.” On the legs, the scales of Cabarzichnus are arranged in longitudinal rows and have a rather rectangular shape. At the rear end of the tail, the scales taper towards the back and are offset and partially overlapping, as the paleontologists report. According to them, the great age of these skin impressions and their characteristics indicate that these are early forms of the epidermal scales typical of reptiles.
Possible impression of the cloaca
“The epidermal scales of Cabarzichnus are the oldest definitive evidence of such skin structures in an early reptile,” state Marchetti and his colleagues. They discovered another peculiarity when they examined the fossil impressions of the base of the tail more closely. There was a narrow slit surrounded by several differently shaped scales. “The modified scales around this opening are consistent with the scale shapes around the cloacal opening of modern squamous reptiles,” report the paleontologists. Unlike life-bearing mammals, almost all other land vertebrates do not have separate openings for feces, urine and eggs. Instead, in reptiles and birds, for example, both the intestines and the sexual organs open into the same outlet, the cloaca. However, the fine structures of this opening are almost never preserved in fossils.
In this respect too, the new find is a real rarity. “It is the earliest fossil evidence of a cloacal opening in amniotes,” write Marchetti and his colleagues. The next oldest fossil is around 170 million years younger and comes from a Cretaceous reptile. The cloacal impression of Cabarzichnus also reveals that the opening of its cloaca was more horizontal. This means that it differs in shape and orientation from the cloaca of crocodiles and is more similar to the opening of turtles, lizards and snakes, as the paleontologists explain. This provides new insights into the evolution of this body opening in early land vertebrates. “Trace fossils are much more than just footprints,” emphasizes Marchetti. “They preserve details of anatomy that would otherwise be completely lost and make a crucial contribution to better understanding the evolution of early land vertebrates.”
Source: Lorenzo Marchetti Museum of Natural History – Leibniz Institute for Evolution and Biodiversity Research, Berlin) et al., Current Biology, doi: 10.1016/j.cub.2026.01.036