Amateur fossil hunters find evolution-altering discovery

By Stephen Beech

The discovery of the earliest reptile footprints - dating back 355 million years - could rewrite the timeline of evolution, say scientists.

The sandstone slab was found by two amateur paleontologists in Australia.

It carries well-preserved footprints of long-toed feet with "distinct" claw impressions at the tips.

They are by far the earliest clawed footprints ever discovered, according to a study published in the journal Nature.

Study coordinator Professor Per Ahlberg, of Uppsala University in Sweden, said: "I'm stunned.

"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 fish leaving the water, and ended with the descendants of those 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 those events had seemed clear-cut, with the first tetrapods evolving during the Devonian period and the earliest members of the modern groups appearing during the following Carboniferous period.

The earliest amniote fossils are from the late Carboniferous, about 320 million years old.

That led scientists 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.

But the discovery of the sandstone slab from the earliest Carboniferous of Australia has changed everything, say scientists.

Co-author Dr Grzegorz Niedźwiedzki, also of Uppsala University, said: “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.”

Ahlberg said: "Claws are present in all early amniotes, but almost never in other groups of tetrapods.

"The combination of the claw scratches and the shape of the feet suggests that the track maker was a primitive reptile."

If that interpretation is correct, it pushes the origin of reptiles, and so 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.

The recalibration of the origin of reptiles impacts the whole timeline of tetrapod evolution, according to the research team.

They say the tetrapod crown-group node must be older than the earliest amniotes, because it is a deeper branching point in the tree.

The research team combined data from fossils and modern DNA to try and work out how much older.

Ahlberg said: "It's all about the relative length of different branches in the tree.

"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.

That means that a diversity of advanced tetrapods existed at a time when it had been thought, only transitional 'fishapods' were dragging themselves around muddy shorelines and starting in a small way to explore the land.

Ahlberg said: "The Australian footprint slab is about 50 centimetres across, 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?"

Dr. Niedźwiedzki added, “The most interesting discoveries are yet to come and there is still much to be found in the field.

"These footprints from Australia are just one example of this.”