Evrim Yazgin
Cosmos science journalist
A fossil trackway in the Australian state of Victoria has caused palaeontologists to push back the emergence of the first land-based vertebrates – our direct ancestors – by about 40 million years.
The discovery is detailed in a paper published today in Nature.
All land-based vertebrates (and the ones, like whales, which returned to the seas) can trace their evolution back to the first tetrapods to make the transition out of Earth’s ancient oceans onto land. Humans, dinosaurs, dogs and birds are all descended from these creatures which evolved into the first reptiles.
Australia has some of the oldest evidence in the world for these pioneering tetrapods. Cosmos has discussed this in a magazine feature, “Victoria through prehistoric time.”
Now, more evidence in Victoria shows that the first tetrapods are even older than previously thought. The newly discovered trackway comes from the Mansfield district of northern Victoria and dates to the start of the Carboniferous period, about 350 million years ago (mya).
“Once we identified this, we realised this is the oldest evidence in the world of reptile-like animals walking on land – and it pushes their evolution back by 35 to 40 million years older than the previous records in the Northern Hemisphere,” says lead author John Long, a palaeontologist at South Australia’s Flinders University.
“The implications of this discovery for the early evolution of tetrapods are profound,” says Long.
Palaeontologists had previously thought that the ancestors of the first tetrapods evolved over a long period into modern terrestrial groups.
Fossils of the earliest amniotes – the group that includes mammals, birds and reptiles – are about 318 million years old. The earliest remains of tetrapod bodies date from 334 mya. The oldest trackways of early tetrapods are from 353 mya. All of this occurred within the Carboniferous period (359–299 mya) suggesting that this is the period of Earth’s history when tetrapods emerged.
The new discovery raises the possibility that the evolution into land-based animals occurred much quicker and earlier. The new tracks have clear indentations that must have been made by claws on a reptile – an amniote which, according to the earlier theory, wouldn’t have evolved for another 35–40 million years.
“We now present new trackway data from Australia that falsify this widely accepted timeline,” he says.
Long says the find shows that the first tetrapods must have emerged during the earlier Devonian period (419–359 mya). This suggests that there is more evidence of tetrapods in the Devonian to be found than was previously thought.
Locals found the trackways which palaeontologists had initially assumed were from an amphibian. But closer inspection revealed a hooked claw coming off the digits – something found in reptiles.
Cross-referencing fish in the surrounding rocks with known species in the fossil record helped the team date the track to the early part of the Tournaisian – the first stage of the Carboniferous occurring between 359 and 347 mya.
The trackway was scanned to create 3D models which were analysed in detail.
“We study rocks and fossils of the Carboniferous and Devonian age with specific interest to observe the very important fish-tetrapod transition,” says co-author Alice Clement, also at Flinders. “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.”
“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,” adds co-author and Flinders-based ancient trackway expert Aaron Camens.
“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,” says Long.
“The Mansfield area has produced many famous fossils, beginning with spectacular fossil fishes found 120 years ago, and ancient sharks,” he adds. “But the holy grail that we were always looking for was evidence of land animals, or tetrapods, like early amphibians. Many had searched for such trackways, but never found them – until this slab arrived in our laboratory to be studied.”
