The Long Toes of Evolution

A rock slab from Australia with newly discovered fossilized footprints containing long toes and claws is now suggesting that reptile-like creatures originated 35 million years earlier than previously thought. 

In fact, researchers say the discovery has turned the accepted evolutionary timeline of backboned land animals, or ‘tetrapods,’ on its head.  

“They are the oldest tracks of clawed feet in the world,” said study author Per Ahlberg in a statement. “It’s astounding that a single slab of rock, so small that one person can lift it, calls into question everything we thought we knew about the emergence of modern tetrapods.” 

Tetrapods originated from a group of fish that left the sea about 390 million years ago, in the Devonian period, and became the ancestors of all modern backboned land animals, including amphibians and amniotes, a group that encompasses mammals, reptiles, and birds. 

Before this study, the oldest known amniote fossils dated to the late Carboniferous period, around 320 million years ago. The Australian sandstone slab recently discovered, however, is around 355 million years old and shows that reptiles were already present at the beginning of the Carboniferous period, 35 million years earlier. 

“When I first saw the rock slab, I was very surprised,” said study author Grzegorz Niedźwiedzki. “After just a few seconds, I noticed that it bore clearly preserved claw marks. Claws are present in all early amniotes, but almost never in other groups of tetrapods. The combination of claw marks and the shape of the feet suggests that the tracks were made by a primitive reptile.” 

Further support for this timeline came from newly discovered fossil reptile tracks in Poland. Even if these are not as old as those recovered from Australia, researchers said, they are still much older than any previously known examples. 

Researchers tried to estimate when the last common ancestor of amphibians and amniotes lived by combining fossil dating with the DNA of living descendants. The results show that it must have been at the beginning of the latter part of the Devonian period, which was previously thought to be inhabited only by primitive fish-like tetrapods.