Detection of hooked claws on the trackways of early reptiles and amphibians at the Aboriginal area of Berrepit
The three sets of tracks in the study have clear footprints with indentations from claws, a feature of reptiles but not amphibians. “Having these hooked claws on the trackways indicates they’re definitely a reptile-like animal,” says Long
The larger structure that is the sandstone block has already been dated to the early Carboniferous on the basis of both radioactive and tectonic evidence. Fossilized tracks of various aquatic animals were also dated to this time period.
The claw tracks were found in a sandstone block on the bank of the Broken River at Barjarg in the state of Victoria, by two co-authors of the paper who are not professional scientists. The Indigenous people who own the land call this area of the river Berrepit.
The trackway record casts some additional light on this phenomenon. The oldest records of the amphibians, synapsids, sauropsids and limbed stemtetrapods are all from the 1930s and 1940s. It is well known from later parts of the vertebrate fossil record that trackway assemblages often capture taxa that are not seen in associated body-fossil assemblages48, and this also applies to the Devonian and Carboniferous record. The earliest known high-diversity tetrapod trackway assemblage, from the mid-Tournaisian of Blue Beach, Canada, contains taxa that are not represented among the associated body fossils (for example, temnospondyls)7. The Mansfield Group contains no known tetrapod body fossils14,15,16,17. The Givetian Valentia Slate Formation of Valentia Island, Ireland and the Eifelian Wojciechowice Formation of Zachemie, Poland both contain publishing tetrapods, but the former yields only fish. The trackway record is a good proxy for the incomplete body-fossil record, and it is also important that we flesh out the picture of early tetrapod diversity.
The last common progenitor between modern salamanders and amniotes must have existed at a later time in time, according to the co-author. The groups were in different parts of the world about 400 million years ago.
Early amniotes evolved to lay eggs on land, because they were encased in an amniotic membrane that stopped them drying out. Before this study, the earliest known amniote fossils had been found in Nova Scotia, Canada, and were dated to the mid-Carboniferous period, about 319 million years ago. The Carboniferous period was around 355 million years ago and the amniotes have been found there.
The earliest stem lungfish, Diabolepis, is nearly 500 million years old, and so the inferred age for the lungfish–tetrapods is a tad too young. If, as a thought experiment, the amniote crown-group node and the lungfish–tetrapod node are fixed, respectively, to the Devonian/Carboniferous boundary (358.9 million years) and the mid-Lochkovian (415 million years), and the aforementioned relative branch lengths are applied, they place the tetrapod crown-group node at a median age of 379.7 million years (early Frasnian). The approximate mid-point of a wide zone of possibility can be understood as this. However, a much younger age, at or close to the Devonian/Carboniferous boundary, can be rejected because the internode to the amniote crown-group node becomes implausibly short and incompatible with the substantial branch lengths consistently recovered by molecular phylogenies (Supplementary Information Part 1). Conversely, as neither the amniote crown-group node nor the lungfish–tetrapod node has a constrained maximum age, all three nodes could in fact be considerably older than indicated. The expanded number of genomes creates a more robust future analysis that can use the Snowy Plains Formation tracks as a calibration point to estimate the date of the crown group.