Newly Discovered Fossil Tracks May Rewrite Early History of Reptiles
Fossil claw prints found in Australia were probably made by the earliest known members of the group that includes reptiles, birds and mammals, according to a study published in Nature today. The findings suggest that this group — the amniotes — originated at least 35 million years earlier than previously thought.
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 latest findings suggest that amniotes also existed in the early Carboniferous period, around 355 million years ago.
'This discovery is exciting, and if the tracks have been interpreted the right way, the findings have important implications for our understanding of tetrapod evolution,' says Steven Salisbury, a palaeontologist at the University of Queensland in Brisbane, Australia.
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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. This area of the river is known as Berrepit to the Indigenous Taungurung people who own the land.
The sandstone block is part of a larger structure that had already been dated to the early Carboniferous on the basis of radiometric and tectonic evidence. Fossilized tracks of aquatic invertebrates and fish found in the same layer were also dated to this time period.
The three sets of tracks in the study have clear footprints with indentations from claws, a feature of reptiles but not of amphibians. 'Having these hooked claws on the trackways indicates they're definitely a reptile-like animal,' says John Long, a palaeontologist at Flinders University in Adelaide, Australia.
There are no marks of dragging bellies or tails, and the authors suggest that the amniotes that left the tracks were able to keep their bodies and tails off the ground while they walked on land. But Salisbury questions that interpretation, because it would mean the animals had developed sophisticated structures for complex locomotion, which would be surprising given how early they are. 'It seems more likely that the tracks were made by an animal that was 'punting' in shallow water, rather than walking on land,' he says.
Until now, evidence suggested that the last common ancestor of modern amphibians and amniotes lived around 352 million years ago. But if the ancestors of reptiles existed during the early Carboniferous, their split from amphibians must have occurred even earlier, says Long. Dating by the team suggests that the groups diverged in the Devonian period, about 380 million years ago.
To estimate the probable time of divergence, Long and his colleagues used several dating methods. One included geological evidence from radioactive decay in volcanic rock layers above and below the fossil tracks. They also used molecular phylogenetics, which compares similarities and differences in the DNA of living species to estimate their evolutionary relationships and how recently their last common ancestor lived.
The discovery could also shift the origin of amniotes to the Gondwana landmass. This formed the southern portion of the Pangaea supercontinent and gave rise to multiple current landmasses, including Africa and Australia. Previously, the earliest known amniotes were found in North America, leading palaeontologists to think that the group originated in the Northern Hemisphere. But more evidence from Australian fossils is needed before definitively shifting their origin site, says Long. 'Australia is a vast area with fewer palaeontologists on the ground,' Long says. 'We've got a lot more unexplored fossil sites where new things like this keep turning up.'
This article is reproduced with permission and was first published on May 14, 2025.
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