The latest issue of Nature magazine features a paper co-authored by Grzegorz Niedźwiedzki (PhD), an employee of the Polish Geological Institute – National Research Institute. The most important achievement resulting from the research is the discovery of fossilised amniote footprints, i.e. a group of vertebrates capable of embryonic development on land (reptiles, birds and mammals) in Australia.

These tracks were found on a sandstone slab from the Snowy Plains Formation in Victoria, dated to the Early Turonian, the earliest part of the Carboniferous period, probably between 358.9 and 354 million years ago. The key feature of these tracks, allowing their creators to be identified as amniotes, is the presence of distinct claw impressions, which is a characteristic feature of amniotes and is not found in known stem forms of this group.

The tracks from the Snowy Plains formation resemble the ichnomorphs Notacalacerta, Varanopus and Dromopus, which are considered to be sauropsida tracks, indicating that they were left by a primitive sauropsida or an animal close to the crown group of amniotes.

This discovery is of fundamental importance for understanding the early evolution of tetrapods, as it pushes back the probable time of origin of the amniotes by at least 35–40 million years earlier than previously accepted. This means that the crown group of amniotes must have originated shortly after the Devonian–Carboniferous boundary.

Consequently, the crown group of tetrapods, which unites the amphibian and amniote lineages, must be placed much deeper in the Devonian. Based on molecular analyses, the new estimate for the age of this lineage is approximately 379.7 million years (early Fran), although this is only an approximate midpoint of a wide range of possibilities.

In addition, new Notacalacerta-like tracks discovered in Poland, dated to the Middle Serpukhovian to Early Bashkirian, also push back the earliest confirmed traces of amniotes in Euramerica to around eight million years earlier than previously thought. These results suggest that the evolution and diversification of tetrapods in the Devonian occurred much more rapidly than previously thought, and also imply that the ancestral lineages of tetrapods and amniotes must have originated in the Devonian.

This discovery also affects the interpretation of the so-called Romer's gap, suggesting that the mass extinction at the end of the Devonian period was not the only factor driving the emergence of more advanced lineages, as some of them were already present before this event.

Traces of amniotes

Access to the paper: Earliest amniote tracks recalibrate the timeline of tetrapod evolution | Nature