One rainy day, millions of years ago, a large reptile-like creature scuttled across a patch of mud and then scuttled back again. Its fossilised footprints, revealed in this week’s Nature, are now forcing experts to reconsider when life hauled itself out of the sea and onto land.
The emergence onto land of four-limbed creatures called tetrapods was a key moment in the evolution of land-living animals. Until now, the story was that tetrapods evolved during the Devonian period, then diversified into the ancestors of amphibians and amniotes (the group that includes reptiles, birds and mammals).
Before this study, the earliest known reptile fossil was around 318 million years old. This suggested that the earliest amniotes must have appeared somewhere between 320 and 330 million years ago, and that the full transition from water to land probably took around 90 million years.
The fossil footprints change all that.
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The footprints were discovered by amateur palaeontologists in a slab of sandstone from the bank of the Broken River in Barjarg, Victoria, Australia. The rock, which is pitted with raindrop prints, bears the footprints of a four-footed, five-digited creature that had claws.
“The combination of the claw scratches and the shape of the feet suggests that the track maker was a primitive reptile,” says Per Ahlberg from Uppsala University, Sweden, who studied the specimen.
The rock bears no marks of the animal dragging its tail or belly, so it’s thought the reptile held itself aloft. At an estimated 80cm long, the authors think it might have resembled the modern-day Asian water monitor.
Dated at 356 million years old, the fossil has now become the earliest known reptile. This pushes back the origin of reptiles by around 35 million years, which in turn, impacts the whole timeline of tetrapod evolution.
“I’m stunned,” says Ahlberg. “A single track-bearing slab, which one person can lift, calls into question everything we thought we knew about when modern tetrapods evolved.”
A second fossil, also described in the paper, comes from Poland. It too contains the impression of clawed reptilian feet but is dated at 330 million years old.
The common ancestor of modern amniotes must have been alive before both of these specimens, and if this is the case, then the transition from water to land happened more quickly than was previously estimated. All that action, in a ‘mere’ 50 million years!
Find out more about the study: Earliest amniote tracks recalibrate the timeline of tetrapod evolution
Main image: the image shows a reconstruction of the reptile. Illustration: Marcin Ambrozik
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