"I'm stunned." says Per Ahlberg of Uppsala University, who coordinated the study; "A single track-bearing slab, which one person can lift, calls into question everything we thought we knew about when modern tetrapods evolved."
The story of the origin of tetrapods began with fishes leaving the water, and ended with the descendants of these first colonists on land diversifying into the ancestors of the modern amphibians and amniotes (the group that includes reptiles, birds and mammals). The timeline of these events has seemed clear-cut: the first tetrapods evolved during the Devonian period and the earliest members of the modern groups appeared during the following Carboniferous period. The earliest amniote fossils are from the late Carboniferous, about 320 million years old. This led researchers to conclude that the beginning of the evolutionary radiation of the modern groups, the point on the evolutionary tree where the ancestors of amphibians and amniotes separated (also known as the 'tetrapod crown-group node'), lay in the earliest Carboniferous around 355 million years ago. The Devonian period was seen as inhabited by more primitive fish-like tetrapods and transitional 'fishapods' such as Tiktaalik.
A sandstone slab from the earliest Carboniferous of Australia, approximately 355 million years old, discovered by two amateur palaeontologists who co-authored the study, changes all this. It carries well-preserved footprints of long-toed feet with distinct claw impressions at the tips. These are by far the earliest clawed footprints ever discovered.
"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." says Grzegorz Niedźwiedzki of Uppsala University, co-author of the study.
"Claws are present in all early amniotes, but almost never in other groups of tetrapods." adds Per Ahlberg; "The combination of the claw scratches and the shape of the feet suggests that the track maker was a primitive reptile."
If this interpretation is correct, it pushes the origin of reptiles, and thus amniotes as a whole, back by 35 million years to the earliest Carboniferous. Further support comes from new fossil reptile footprints from Poland, also presented in the study; not as old as those from Australia, but also substantially older than previous records. This recalibration of the origin of reptiles impacts the whole timeline of tetrapod evolution. The tetrapod crown-group node must be older than the earliest amniotes, because it is a deeper branching point in the tree, but how old exactly? The authors address this problem by combining data from fossils and modern DNA.
"It's all about the relative length of different branches in the tree" explains Per Ahlberg; "In a family tree based on DNA data from living animals, branches will have different lengths reflecting the number of genetic changes along each branch segment. This does not depend on fossils, so it's really helpful for studying phases of evolution with a poor fossil record."
Their analysis, overlaying branch lengths from DNA analyses onto the known fossil dates, indicates that the tetrapod crown group node lies far back in the Devonian, roughly contemporary with Tiktaalik. This means that a diversity of advanced tetrapods existed at a time when, it has been thought, only transitional 'fishapods' were dragging themselves around muddy shorelines and starting in a small way to explore the land. But perhaps that should not surprise us.
"The Australian footprint slab is about 50 cm across", says Per Ahlberg, "and at present it represents the entire fossil record of tetrapods from the earliest Carboniferous of Gondwana - a gigantic supercontinent comprising Africa, South America, Antarctica, Australia and India. Who knows what else lived there?"
"The most interesting discoveries are yet to come and that there is still much to be found in the field. These footprints from Australia are just one example of this." says Grzegorz Niedźwiedzki.
