Footprints that redefine prehistory

A GROUND-BREAKING fossil discovery in the Mansfield Shire is set to shake-up the timeline of evolution, with the earliest known ‘reptile’ footprints found on the banks of the Broken River. Reported last week in Nature, the world's leading multidisciplinary science journal publishing peer-reviewed research, the find is of international significance. It places the evolution of four limbed-creatures (or tetrapods as they are scientifically known) from aquatic creatures to those living fully on land as happening faster than thought, predating previously discovered footprints and body fossils by tens of millions of years. The discovery also indicates that such animals originated in the ancient southern continent of Gondwana, of which Australia was a central part. The five-fingered fossilised tracks are thought to belong to an amniote – an early ancestor of the reptiles. Found on a slab of rock recovered from the Snowy Plains Formation, the tracks date back 356 million years and it is telltale claw marks that denote the tracks as those of most likely an amniote. “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 John Long, Strategic Professor in Palaeontology at Flinders University. “The implications of this discovery for the early evolution of tetrapods are profound.” The footprints are similar in shape to a modern water monitor and using this animal’s gait as a reference, researchers have estimated this ancient amniote may have been about 80 centimetres long. Professor Long speculates the animal resembled a small, stumpy goanna-like creature, however emphasises the exact proportions of the animals are unknown. An important step in the evolution of land-dwelling animals was the emergence of tetrapods out of the sea and onto the land. The only group of tetrapods that evolved to reproduce on land were the amniotes, a group that today includes reptiles, birds and mammals. The earliest known body fossil and trackways were previously dated to around 320 million years ago, suggesting that amniotes may have taken up to 90 million years to evolve after tetrapods first ventured out of the water. The local discovery has challenged this view, recalibrating the whole timeline of tetrapod evolution. “My involvement with this amazing fossil find goes back some 45 years, when I did my PhD thesis on fossils of the Mansfield district,” said Prof. Long. After decades studying ancient fish fossils of the area, he already had a clear idea of the age of rock deposits in the shire. “This new fossilised trackway that we examined came from the early Carboniferous period, and it became significant for us to accurately identify its age,” Prof Long said. “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.” Prof Long said 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, or tetrapods, like early amphibians,” he said. Many had searched for such trackways locally but never found them. Then the mudstone slab arrived at the Flinders University laboratory to be studied. “Two locals – Craig Eury and John Eason who co-authored the paper in Nature – found this slab covered in trackways and at first thought they were early amphibian trackways,” Prof Long said. That was until, under further investigation, a hooked claw was recognised coming off the digits like a reptile, or more correctly an amniote. “It is amazing how crystal clear the trackways are on the rock,” said Prof. Long. Prof Long admits it immediately excited both the original finders and later researchers at Flinders University. “We sensed we were onto something big – even though we had no idea just how big it was,” he said. John Eason said the rocks around Mansfield are already renowned for the many species of fossil fish they contain. “The fossil-bearing rocks of the South Blue Range and the Broken River are 400 to 350 million years old,” Mr Eason said. “These periods in Earth’s history – the late Devonian and early Carboniferous to give them their geological names – were when the first forests began to cover the land. “Purplish-brown mudstone from the Broken River is everywhere in Mansfield. “This rock was formed during the first part of the Carboniferous period on the east coast of the supercontinent of Pangaea (of which Gondwana was a part), in a strip of low-lying forested land crossed by sluggish meandering rivers. “Today’s rock surfaces are covered with innumerable ‘trace fossils’ – ripples, worm burrows, foraging marks, dried mud cracks and plant debris. “There is a good chance you will find some if you have a rock wall or paving-stones in your garden,” Mr Eason added. “This was the time when lobe-finned fish in shallow estuarine environments and rivers evolved into the first four-legged animals (tetrapods). “Somewhere among these abundant local trace fossils you would have thought you would find their footprints, but none had been found. “Up until recently,” he added. It was 2021, and for some years a group of interested locals had been scouring the countryside looking for fish fossils. “It was Craig who first spotted it – seven clear, five-digit footprints made by what we initially thought was a clawless primitive amphibian.” The slab was originally sent to Prof Long at Flinders University to scan and study, who then sent it from Adelaide to Sweden to Professor Per Ahlberg a vertebrate evolution expert at Uppsala University. Further scans would reveal a second set of tracks. “Three faint traces of an animal with claws,” said Mr Eason. With two sets of tracks identified, researchers believe they belonged to the same animal. Additional high-resolution scans would turn up even more faint footprints. These also showed claws. “Amphibians don’t have claws,” said Mr Eason. “Only amniotes – reptiles, mammals and birds in whom the fetus is surrounded by an amniotic sac – have claws.” Mr Eason said until recently the earliest known reptile fossil is Hylonomus, whose fossil bones were discovered in 318-million-year-old rocks in Nova Scotia. In an adjacent Canadian province, fossilised footprints ascribed to Hylonomus were also found in rock estimated to be 315 million years old. The footprints found in Mansfield Shire are similar to these Canadian footprints, suggesting reptiles evolved millions of years earlier than previously thought. “We have found the footprints,” said Mr Eason of the significant discovery. “There have got to be fossil remains out there somewhere.” It's now just a matter of finding them.

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