Reptiles originated 35 million years earlier than we thought

Newly discovered fossilised footprints with long toes and claws, found in a small rock slab in Victoria, are believed to have been made by a small goanna-like creature about 355 million years ago.

This is the oldest evidence in the world of reptile-like animals walking around on land — and it suggests they evolved 35 to 40 million years earlier than we thought, thus overthrowing the established evolutionary timeline of backboned land animals known as tetrapods. Published in the journal Nature, the study was led by Uppsala University and also involved Australian researchers.

Tetrapods evolved from a group of fish that left the sea about 390 million years ago, during the Devonian period. They became the ancestors of all modern backboned land animals, ie, amphibians and amniotes — the latter of which includes mammals, reptiles and birds.

The oldest amniote fossils prior to this latest discovery were found in the Northern Hemisphere and come from the later part of the Carboniferous period, making them about 320 million years old. But the Australian rock slab, discovered in the Mansfield district of northern Victoria, shows that reptiles must have already existed 35 million years earlier, at the beginning of the Carboniferous, originating in the ancient southern supercontinent of Gondwana.

“Locals Craig Eury and John Eason (co-authors on the paper) found this slab covered in trackways and, at first, we thought they were early amphibian trackways, but one in the middle has a hooked claw coming off the digits, like a reptile — an amniote, in fact,” said Professor John Long, from Flinders University.

“It was amazing how crystal clear the trackways are on the rock slab. It immediately excited us, and we sensed we were onto something big — even though we had no idea just how big it is.”

The Flinders palaeontology team working on the project included Dr Alice Clement, who scanned the fossil footprints to create digital models that were then analysed in detail, working closely with Uppsala University researchers led by Professor Per Erik Ahlberg.

“We study rocks and fossils of the Carboniferous and Devonian age with specific interest to observe the very important fish–tetrapod transition,” Clement said.

“We’re trying to tease apart the details of how the bodies and lifestyles of these animals changed, as they moved from being fish that lived in water, to becoming tetrapods that moved about on land.”

Study co-author Dr Aaron Camens, who studies animal trackways from around Australia, produced heatmaps that explain details of the fossil footprints much more clearly. He noted, “A skeleton can tell us only so much about what an animal could do, but a trackway actually records its behaviour and tells us how this animal was moving.”

An impression of what the amniote would look like from 350 million years ago. Image credit: Martin Ambrozik.

Because Long had been studying ancient fish fossils of this area since 1980, he had a clear idea that the rock deposits in the Mansfield district dated from the Carboniferous period, which started about 359 million years ago.

“This new fossilised trackway that we examined came from the early Carboniferous period, and it was significant for us to accurately identify its age — so we did this by comparing the different fish faunas that appear in these rocks with the same species and similar forms that occur in well-dated rocks from around the world, and that gave us a time constraint of about 10 million years,” he said.

The researchers received further support for the interpretation that reptiles emerged around this time from newly discovered fossil reptile tracks from Poland. They are not as old as the Australian rock slab, but substantially older than the previous oldest known examples.

Moving the origin of reptiles back in time changes the whole timeline of tetrapod evolution. Previously, scientists believed that the last common ancestor of amphibians and amniotes lived around 355 million years ago. But as the ancestor must be older than the oldest reptiles, that is now being called into question.

“All stem-tetrapod and stem-amniote lineages must have originated during the Devonian period — but tetrapod evolution proceeded much faster, and the Devonian tetrapod record is much less complete than we have believed,” Long said.

By combining the dating of fossils with the DNA of living descendants, the researchers are now trying to estimate when the last common ancestor of amphibians and amniotes might have lived. Their analysis shows that it was probably at the beginning of the latter part of the Devonian period, which was previously thought to be populated exclusively by primitive fish-like tetrapods and transitional forms such as Tiktaalik.

“This means that advanced tetrapods were already living at a time when it was previously thought that only primitive ‘four-legged fish’ were dragging themselves along the shores and just beginning to explore the land,” Ahlberg said.

Ahlberg added that the Australian rock slab is currently the only tetrapod fossil of earliest Carboniferous age we have from the whole of Gondwana, which comprised Africa, South America, Antarctica, Australia and India. “Who knows what other animals may have lived there?” he pondered.

Top image: Professor John Long compares the trackways with a modern iguana foot. Image credit: Traci Klarenbeek.