14-05-2025
355-Million-Year-Old Footprints Just Rewrote Reptilian History
The origin of four-limbed animals known as tetrapods was thought to be fairly straightforward: Fish flopped onto land in the Devonian, evolved, and eventually diversified into the reptiles, birds, mammals, and other creatures that cover the Earth today. But now, a slab of sandstone small enough to be carried by a single person has thrown that tidy timeline into chaos.
The slab is from southeastern Australia and dates to about 355 million years ago, shortly after the end of the Devonian. Discovered by two amateur paleontologists (co-authors of a new study describing the find), the rock preserves a set of remarkable footprints: long-toed impressions with unmistakable claw marks. These trace fossils now represent the oldest clawed tetrapod tracks ever found.
'The key impact is that it pushes an important part of the tetrapod evolutionary tree—in essence, all the branches that go below the reptile-mammal split (also known as the amniote crown-group node)—back in time,' said Per Ahlberg, a researcher at Uppsala University and the lead author of the study, in an email to Gizmodo.
Ahlberg explained that although scientists agree tetrapods evolved in the Devonian, those early creatures were believed to be fish-like, only beginning to adapt to life on land. The researchers published a Nature study today describing the fossils and their significance for the tree of life.
'If we already have reptiles at the beginning of the Carboniferous, which is what our Australian trackways indicate, that can't be true,' Ahlberg said.
'When I saw this specimen for the first time, I was very surprised,' said co-author Grzegorz Niedźwiedzki, also a researcher at Uppsala University. 'After just a few seconds I noticed that there were clearly preserved claw marks.'
Claws are significant. They're a signature of early amniotes—the group that includes reptiles, birds, and mammals. Other tetrapods, like early amphibians or so-called 'fishapods' such as Tiktaalik, didn't have them. Which means the creature that made the fossilized tracks was likely a primitive reptile—and it was scurrying around much earlier than anyone expected.
'It collides head-on with the often-repeated claim that Tiktaalik and similar 'fishapods' were not only closely related to tetrapods (which they were), but actual tetrapod antecedents or even ancestors,' Ahlberg said. 'In fact, they lived long after the earliest tetrapods, and if our branch-length inferences are correct Tiktaalik was more or less contemporary with the tetrapod crown-group node!'
In other words, the appearance of reptiles—and by extension, the evolutionary branch that leads to humankind—gets pushed back by 35 million years. The team's conclusions also mean that the split between amniotes and amphibians had to happen even earlier.
By combining DNA-based family trees with fossil dates, the team found that the crown-group node likely lies way back in the Devonian—contemporary with Tiktaalik. That suggests advanced tetrapods were already on the scene while Tiktaalik was figuring out how to prop itself up.
Currently, the claw print fossil slab 'represents the entire fossil record of tetrapods from the earliest Carboniferous of Gondwana—a gigantic supercontinent comprising Africa, South America, Antarctica, Australia and India,' said Ahlberg in an Uppsala release. 'Who knows what else lived there?'
The key for the team now is more fieldwork—in Australia and elsewhere—that could yield more clues about the early amniotes that occupied Gondwana. More footprints would be good, Ahlberg said, but body fossils would be even better.
