Earliest reptile footprints could be the first known signs of true terrestrial life

A series of fossilised footprints could push back the origins of true life on land by as much as 40 million years.

The Devonian, which lasted from around 420 to 359 million years ago, was a landmark period for life on Earth as vertebrates emerged onto land for the first time. Gradually, these pioneering fish gave way to a group of four-limbed animals known as the tetrapods.

While the tetrapods initially still needed water to reproduce, the evolution of the egg and internal fertilisation led to the first truly terrestrial vertebrates. Some of these animals, known as the amniotes, became the earliest ancestors of mammals, reptiles and birds.

Until recently, a combination of fossil and molecular evidence suggested that these ancestors didn’t evolve until around 319 million years ago. As such, the discovery of 356-million-year-old footprints in Barjang, southern Australia, has surprised researchers studying amniote evolution.

Professor Per Ahlbergopens in a new window is a co-author of a new paper describing the footprints, which are believed to belong to an early reptile. He says that while it’s “very unusual” to see such a shift in the age of a major group, it’s not entirely unexpected.

“It is important to realise just how incomplete the early record of tetrapods is,” Per explains. “As an extreme example, the track-bearing slab we’ve described is currently the only earliest Carboniferous tetrapod fossil from the whole of Gondwana – the supercontinent which included South America, Africa, Antarctica and many other landmasses.”

“Globally, the Devonian record is only marginally better, and is strongly biased in favour of environments where fossils are readily preserved. Only new discoveries in the field will give us truly novel data that can advance our understanding of these animals.”

The findings of the study were published in the journal Natureopens in a new window.

Walking back in time

The footprints first came to the attention of the researchers after they were highlighted by two community scientists. They had found a slab in the Broken River, and identified what looked like trackways on its surface.

The first challenge was to figure out how old this slab might be, because it was found loose in the river. As it has a similar structure to nearby rocks, and doesn’t show any signs of washing in from elsewhere, the researchers think it is probably about the same age as its immediate surroundings – which has been dated to between 359 and 354 million years old.

The fossil captures the aftermath of a rain shower from all this time ago. While the ground was still moist, an animal around the size of a monitor lizard – estimated at 80 centimetres long – left its footprints. After the ground had hardened slightly, another set of footprints were left by the same kind of animal.

Once the tracks had dried out completely, they would subsequently have been buried and started to fossilise, ready to be found more than 350 million years later.

While the team can’t be completely certain which animal made the footprints, there are a few clues that suggest it was made by an early reptile.

“The footprints show definite claw impressions, and claws are a characteristic trait of the amniotes,” Per says. “While there are some examples of clawed amphibians, such as the ‘clawed frog’ Xenopus, they’re very rare.”

“The overall shape of the footprints also closely matches that of known reptile footprints from the Late Carboniferous. The relative size and arrangement of the toes, as well as the curious way the claws tend to lie down sideways, are very different from the footprints left by early members of the mammal lineage.”

How did amniotes evolve?

If the team’s findings are confirmed, the fossil changes widely accepted timings of how the tetrapods and amniotes evolved.

Tetrapods are believed to have split from their close relatives, the lungfish, around 420 million years ago. Fossils of the earliest confirmed lungfish, Diabolepis, have been found from just five million years later.

The first signs of limbed tetrapods, tracks found in Polandopens in a new window and Ireland, don’t appear until around 390 million years ago. It’s unclear whether the track-makers are among the ancestors of modern tetrapods, or relatives that split off beforehand, but the new timeline suggests that these animals must have already been more diverse than expected.

At some point before the Australian tracks were left 356 million years ago, the ancestors of all modern amniotes also must have evolved. The team speculate that the rise of the amniotes might have been driven by a mass extinction at the end of the Devonian, which led to about three quarters of all species becoming extinct.

More fossils, particularly body fossils of early tetrapods, will be crucial to firm up the dates of these key transitions. Dr Marc Jonesopens in a new window, our fossil reptile curator, adds that the paper’s “really exciting” findings show just how much there is still to discover.

“This research highlights just how little we know about the earliest history of the amniotes,” Marc says. “However, it also shows the importance of fieldwork to find new data, particularly from southern continents such as Australia.”