The important discovery was made by a research team as part of a National Science Centre project led by the Polish Geological Institute-NRI. During work in an active red sandstone quarry near Nowa Ruda, foot and tail prints of an amphibian from 300 million years ago were found. The results were published in the journal Biology Letters.

The tail marks discovered on the sandstone shoal are historically the first evidence of horn-like scales in amphibians closely related to oviparous amphibians (organisms in which fetal membranes appear during embryonic development). The discovery suggests that the origins of vertebrates' complete independence from water may be much earlier than previously thought.

Water-impermeable skin is one of the key features that paved the way for the first insectivores to fully colonise land, enabling the future success of reptiles, birds and mammals. This property is provided by a continuous stratum corneum, made up of densely cross-linked proteins. The evolutionary development and timing of the emergence of this feature is very difficult to trace, as skin impressions are extremely rare fossils.

The skin imprint described by the scientists comes from an amphibian of the diadectid group. These large amphibians roamed the areas of today's Sudetes during the Permian, leaving numerous tracks. Within the path trodden by the animal, the researchers came across the imprint of a tail fragment with which the individual had inadvertently touched the mud. The skin fragment is imprinted in the same layer as the tracks, next to the imprint of the hind limb according to the animal's march direction. The scientists had no doubt that they were dealing with a unique find. The skin imprints indicate adaptive features of the body cover to periodically dry palaeoenvironments. Terrestrial vertebrates have had to adapt to extreme weather conditions, such as possible night frosts.

The diadectid's tail was not naked as in today's amphibians, but covered with rows of scales - the kind seen in many reptiles. It turns out that the animals, which still resemble amphibians in terms of their bones, were probably well adapted to function permanently out of water as adults. This complicates the idea of amphibians as terrestrial animals and opens up a wide field for new interpretations of their ecological role in Earth's history. In addition to the discovery in question, scientists have also come across fish fossils in the sediments of periodic lakes.

The international research team that made the discovery consisted of scientists from: Polish Geological Institute-NRI, the Institute of Evolutionary Biology at the University of Warsaw, the Institute of Geological Sciences at the University of Wrocław, the Moravian Museum and Masaryk University in the Czech Republic, the GEOSKOP Museum of the Prehistoric World and the Natural History Museum in Germany.

As part of the project, Lower Permian lacustrine and flowing water sediments (sandstone, limestone, clay sediments, conglomerates and volcanic rocks) dating back 290 million years were studied in several quarries in the mid-Sudetic basin.

See more: Ars Technica.

Fossil samples showing traces of three diadectid passage routes and a partial imprint of a scaly body. The paw print is similar in size to a human hand. Reconstruction: Friderik Spindler

This is what a wandering diadectid might have looked like - the outline of the tracks left behind is superimposed on the drawing with a new reconstruction of the quadruped already having scales on the underside of its body

Fish fossils discovered

Text: Iza Ploch, Wojciech Pawlak (UW), Artur Baranowski