A sandstone slab imprinted with clawed footprints could mean that four-legged animals (tetrapods) transitioned from sea to land 35 million years earlier than previously thought. That finding of the earliest known clawed footprints — in Australia, by amateur paleontologists — may have major evolutionary implications, according to a paper in the journal Nature.

“I’m stunned,” Per Ahlberg, a researcher from Uppsala University and an author of the paper, said in a press release. “A single track-bearing slab, which one person can lift, calls into question everything we thought we knew about when modern tetrapods evolved.”

Challenging Tetrapod Evolution

The prevailing notion is that tetrapods split into two groups about 320 million years ago: ancestors to amphibians and a group named amniotes, which includes birds, reptiles, and mammals. The clawed footprints may require a revision to the evolutionary timeline.

“Claws are present in all early amniotes, but almost never in other groups of tetrapods,” Ahlberg said in the release. “The combination of the claw scratches and the shape of the feet suggests that the track maker was a primitive reptile.”

Comparing DNA to Fossil Record

If this interpretation is correct, reptiles emerged 35 million years earlier than previously thought, during the Carboniferous (about 359 million years ago to 299 million years ago), rather than the earlier Devonian Period (about 419 million years ago to 359 million years ago). To verify this hypothesis, the researchers turned to two other lines of evidence.

They first compared the Australian fossil to similar reptile footprints from Poland. The Polish prints fit somewhere in between the timeline of the older Australian prints and the previously known oldest reptile prints.

Next, they turned to DNA. If the tetrapods are, indeed older than the earliest known amniotes, the differences should be reflected in genetic changes over time. To follow that line of logic, the researchers paired DNA data with the fossil record.

“It’s all about the relative length of different branches in the tree,” Ahlberg said in the release. “In a family tree based on DNA data from living animals, branches will have different lengths reflecting the number of genetic changes along each branch segment. This does not depend on fossils, so it’s really helpful for studying phases of evolution with a poor fossil record.”

Read More: The Devonian Extinction: A Slow Doom That Swept Our Planet

A Key Moment for Evolution

After overlaying evolutionary branch lengths of DNA onto known fossil records, the researcher confirmed that tetrapods did, indeed split into two broad evolutionary groups during the Devonian period.

Their analysis, overlaying branch lengths of DNA onto known fossil dates, indicates that the tetrapod crown group node lies far back in the Devonian. This places early tetrapods alongside — not after — fishlike creatures (sometimes nicknamed fishapods) who may have used strong fins to explore muddy shorelines. The conventional wisdom has tetrapods emerging after the fishapods, not living alongside them.

Our understanding of this key moment in evolutionary time may continue to shift, as more fossils are discovered. Ahlberg noted that the newly reported Australian footprints represents a minuscule amount of life that existed on a massive supercontinent that included what is now Africa, South America, Antarctica, Australia, and India.

“Who knows what else lived there?” Ahlberg said in the release.

Read More: Fewer Skull Bones May Have Restricted The Evolution Of Land Animals

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Before joining Discover Magazine, Paul Smaglik spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.