Distinct clawed footprints found on a slab of 356 million-year-old rock from Australia suggest that reptile relatives appeared between 35 million and 40 million years earlier than previously believed.
The tracks also push back the origin of amniotes, a group that includes reptiles, birds and mammals, and provide new evidence about how animals transitioned from existing solely in the seas to living on land.
Amniotes represent a crucial part of the transition from aquatic to terrestrial life because they were the only tetrapods, or four-limbed creatures, that evolved to reproduce on land.
Previously, the oldest body fossils and footprints associated with amniotes were dated to 318 million years ago in Canada.
But the new findings, published on May 14 in the journal Nature, challenge such long-held assumptions and signal that the transformation of tetrapods living in water to living on land likely occurred much more rapidly than scientists thought.
“I’m stunned,” said study coauthor Per Erik Ahlberg, professor of evolution and developmental biology at Uppsala University in Sweden, in a statement. “A single track-bearing slab, which one person can lift, calls into question everything we thought we knew about when modern tetrapods evolved.”
The location of the discovery indicates that Australia, once a central part of the ancient southern supercontinent of Gondwana that also included present-day Africa, South America, Arabia, Madagascar, Antarctica and India, may be the ideal place to look for more amniote and reptile fossils — and where they originated, according to the study authors.
Rewriting evolutionary history
The rock slab, found by amateur paleontologists and study coauthors Craig Eury and John Eason in the Snowy Plains Formation in Victoria, Australia, appears to show two sets of tracks from the same animal that represent the earliest clawed footprints ever discovered.
The shape of the feet is similar to a modern water monitor’s, and though the animal’s exact size is unknown, it may have resembled a small goanna-like creature about 80 centimeters (31 inches) in length, said lead study author John Long, strategic professor in paleontology at Flinders University. Asian water monitors are large lizards native to South and Southeast Asia, while goannas are large lizards commonly found in Australia.
Hooked claws, a key feature specific to reptiles, might have enabled the primitive tetrapod to dig and climb trees.
The animal that made the footprints is the oldest known reptile and oldest known amniote, Ahlberg said. And it’s helping scientists crack the code on how tetrapods evolved.
“Our new find implies that the two main evolutionary lines leading to modern tetrapods — one, the line to modern amphibians, and two, the line leading to reptiles, mammals and birds — diverged from each other much earlier in time than previously thought, likely back in the Devonian Period about 380 million years ago,” Long said.
Prior to this finding, the Devonian Period was believed to be a time of primitive fishlike tetrapods and “fishapods” like Tiktaalik, which exhibited traits of fish and early tetrapods and began to explore shorelines in limited ways.
But the new study reveals a diversity of large and small tetrapods, some aquatic and others largely or entirely terrestrial, likely lived at the same time.
“One of the implications of our research is that tetrapod diversity at this time was higher, and included more advanced forms, than had been thought,” Ahlberg wrote in an email.
It’s crucial to understand when life shifted from being entirely aquatic to terrestrial because it is one of the biggest steps in the evolution of life, Long said. This transition showed that animals were no longer dependent on living in or near water.