Animal evolution theory turned upside down by new fossil discovery

The origin of reptiles on Earth has been pushed back by an astonishing 40 million years. Fossilized tracks unearthed in Australia provide compelling evidence that reptile-like animals existed far earlier than previously thought.

This finding challenges long-held assumptions about the evolution of tetrapods, the group that includes amphibians, reptiles, birds, and mammals.

A team led by John Long, a professor at Flinders University, discovered fossilized tracks of an amniote with clawed feet, dating to approximately 350 million years ago.

The tracks, found in the Mansfield district of northern Victoria, indicate that amniotes – a group that includes reptiles – were present in the Carboniferous period, much earlier than established records in the Northern Hemisphere.

“Once we identified this, we realised this is the oldest evidence in the world of reptile-like animals walking around on land – and it pushes their evolution back by 35-to-40 million years older than the previous records in the Northern Hemisphere,” said Professor Long.

First clawed reptile tracks in Gondwana

The track-bearing slab, discovered in Taungurung Country, belonged to the Snowy Plains Formation.

The animal that left these tracks likely resembled a small, stumpy Goanna-like creature. With its clawed feet, this primitive sauropsid walked across what was once part of the ancient southern supercontinent, Gondwana.

“The implications of this discovery for the early evolution of tetrapods are profound,” noted Professor Long. The finding suggests that stem-tetrapod and stem-amniote lineages began evolving in the Devonian period, much earlier than previously thought.

The rapid evolution of tetrapods during this period, combined with an incomplete fossil record, has obscured the true timeline of their emergence.

Reptile evolution timeline shifts 

The fossil record of crown-group amniotes, which includes modern reptiles, birds, and mammals, was previously thought to begin in the Late Carboniferous, around 318 million years ago.

Earlier estimates placed the emergence of crown-group tetrapods at 334 million years ago, with the oldest trackways dating back 353 million years.

However, the newly discovered Australian tracks, securely dated to the early Tournaisian, push the origin of crown-group amniotes back by 35 to 40 million years. This revised timeline suggests that tetrapod evolution proceeded faster and earlier than previously thought.

“We now present new trackway data from Australia that falsify this widely accepted timeline,” said Professor Long. The implications extend beyond the Australian site, challenging assumptions about the broader evolutionary history of tetrapods.

Fossil slab holds oldest reptile tracks

The sandstone slab, measuring about 50 cm across, carries the oldest known clawed footprints. The slab was discovered by two amateur palaeontologists, Craig Eury and John Eason, who initially thought the tracks belonged to early amphibians.

“When I saw this specimen for the first time, I was very surprised. After just a few seconds, I noticed that there were clearly preserved claw marks,” said Grzegorz Niedźwiedzki of Uppsala University, co-author of the study.

According to Niedźwiedzki, claws are a distinct feature of early amniotes, almost never seen in other tetrapod groups. This characteristic suggests the animal that left these tracks was a primitive reptile, marking the earliest known evidence of clawed feet among tetrapods.

Reptile tracks in Australia and Poland

In addition to the Australian tracks, similar trackways have been identified in Silesia, Poland. These tracks, dated to the mid-Serpukhovian to early Bashkirian, are slightly younger than the Australian finds but also display claw impressions.

The Polish tracks extend the known range of Notalacerta into the early Carboniferous. This suggests that amniotes were present across both Gondwana and Euramerica much earlier than previously thought.

The Snowy Plains Formation likely dates to the early Tournaisian, approximately 358.9 to 354 million years ago. This pushes the origin of crown-group amniotes back significantly, indicating that tetrapods evolved more rapidly than previously estimated.

Fossils reveal faster reptile evolution

To refine the evolutionary timeline, researchers combined fossil evidence with DNA analysis. Molecular data provide valuable insights into the relative timing of evolutionary events, particularly when the fossil record is incomplete.

“It’s all about the relative length of different branches in the tree,” said Per Ahlberg of Uppsala University. “This method helps us pinpoint evolutionary phases with a poor fossil record.”

By overlaying DNA branch lengths onto known fossil dates, the team estimated the age of the tetrapod crown-group node to be in the Devonian, contemporaneous with transitional ‘fishapods’ like Tiktaalik. This finding suggests that tetrapods diversified earlier and faster than previously thought.

Challenging previous theories

The Snowy Plains Formation trackway slab is more than just a piece of stone. It represents a pivotal moment in the history of life on Earth, a moment when reptiles first set clawed feet on land.

“A single track-bearing slab, which one person can lift, calls into question everything we thought we knew about when modern tetrapods evolved,” said Per Ahlberg. The slab, found in Taungurung Country, is now housed in Museums Victoria.

The tracks show a consistent foot morphology, with five slender digits splayed out in a fan shape. Claws are present on four digits, with the fifth digit carrying a shorter claw.

The trackmaker’s hip-to-shoulder distance suggests a body length of approximately 80 cm, similar to that of a modern water monitor.

Search for evidence in Gondwana

Mansfield, Victoria, has long been a hotspot for fossil discoveries. Since 1980, researchers have uncovered fossilized fish and sharks in the area, but finding terrestrial vertebrate tracks had remained elusive.

“The Mansfield area has produced many famous fossils, beginning with spectacular fossil fishes found 120 years ago, and ancient sharks. But the holy grail that we were always looking for was evidence of land animals,” explained Professor Long.

The Snowy Plains Formation trackways provide crucial evidence that reptiles and other amniotes were already present in Gondwana during the early Carboniferous. This challenges previous theories that placed the origin of amniotes solely in the Northern Hemisphere.

Reptile evolution: A broader timeline

The discovery of clawed tracks from the early Carboniferous has profound implications for the timeline of tetrapod evolution. It suggests that the tetrapod crown-group node must be located deep within the Devonian, not the Carboniferous as previously thought.

The findings also reveal that tetrapod evolution was a more rapid and complex process than previously recognized. The appearance of clawed amniotes in both Gondwana and Euramerica indicates that these groups may have evolved in parallel across different regions.

“This discovery rewrites this part of evolutionary history,” said Dr. Jillian Garvey of La Trobe University. “It indicates there is so much that has happened in Australia and Gondwana that we are still yet to uncover.”

The future of fossil research

While the Australian tracks provide valuable insights, they also raise new questions about the evolutionary history of reptiles. The fossil record in Gondwana remains incomplete, and future discoveries may further alter our understanding of when and where reptiles first emerged.

“The most interesting discoveries are yet to come and that there is still much to be found in the field,” said Grzegorz Niedźwiedzki. “These footprints from Australia are just one example of this.”

The Snowy Plains Formation slab stands as a powerful reminder that even a single piece of rock can reshape our understanding of the past.

As researchers continue to search the Devonian and Carboniferous formations of Gondwana, the story of reptile evolution may undergo even more dramatic revisions.

The study is published in the journal Nature.

Image Credit: Marcin Ambrozik.

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