Footprints found in Australia change theories about the origin of reptiles
Distinctive clawed footprints found on a 356-million-year-old slab of rock from Australia suggest that reptile relatives emerged 35 million to 40 million years earlier than previously believed.
The footprints also push back the origin of amniotes, a group that includes reptiles , birds and mammals, and provide new evidence about how animals made the transition from living exclusively in the seas to living on land.
Amniotes represent a crucial part of the transition from aquatic to terrestrial life, as 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 dated back 318 million years in Canada. But the new findings, published May 14 in the journal Nature , challenge those long-held assumptions and indicate that the transformation from water-dwelling tetrapods to land-dwelling ones likely occurred much more quickly than scientists had imagined.
“I am astonished,” study co-author Per Erik Ahlberg, a professor of evolutionary and developmental biology at Uppsala University in Sweden, said in a statement. “A single slab of footprints that a person can carry calls into question everything we thought we knew about when modern tetrapods evolved.”
The location of the discovery suggests that Australia, once a central part of the ancient southern supercontinent Gondwana — which also included present-day Africa, South America, Arabia, Madagascar, Antarctica and India — could be an ideal place to look for more fossils of amniotes and reptiles — and where they originated, according to the study authors.
The rock slab, found by amateur paleontologists and study co-authors Craig Eury and John Eason in the Snowy Plains Formation in Victoria, Australia, appears to show two sets of footprints from the same animal that represent the oldest clawed tracks ever discovered.
The shape of the feet is similar to that of a modern water monitor, and while the exact size of the animal is unknown, it may have resembled a small goanna-like creature, about 32 inches (80 centimeters) long, according to the study's lead author, John Long, a strategic associate professor of paleontology at Flinders University. Asian water monitors are large lizards native to South and Southeast Asia, while goannas are large lizards common in Australia.
Curved claws, a key reptile-specific feature, may have allowed this early tetrapod to dig and climb trees.
The animal that made the footprints is the oldest known reptile and also the oldest known amniote, according to Ahlberg. And it is helping scientists decipher how tetrapods evolved.
“Our new discovery implies that the two major evolutionary lineages that led to modern tetrapods—one, the line of modern amphibians, and the other, the line leading to reptiles, mammals and birds—diverged from each other much earlier than previously thought, probably as early as the Devonian Period, about 380 million years ago,” Long says.
Prior to this discovery, the Devonian Period was believed to be a time of early fish-like tetrapods and “fishapods” such as Tiktaalik, which exhibited both fish and early tetrapod characteristics and began exploring the coastlines in a limited manner.
But the new study reveals a diversity of tetrapods large and small, some aquatic and others largely or entirely terrestrial, that likely lived at the same time.
“One implication of our research is that tetrapod diversity at this time was greater and included more advanced forms than previously thought,” Ahlberg wrote in an email.
It's critical to understand when life made the transition from being fully aquatic to terrestrial because it represents one of the biggest milestones in the evolution of life, Long said. This transition showed that animals were no longer dependent on living in or near water.
The transition occurred in part because amniotes evolved to reproduce with hard-shelled, rather than soft-shelled, eggs.
“The migration of vertebrates onto land was an important part, and within that a key step was the evolution of the amniotic egg in the immediate ancestors of reptiles and mammals,” says Ahlberg. “So these events form a key episode both in our own ancestry and in the history of the planet.”
The new study pushes the origin of amniotes much further back into the Carboniferous Period, to 299 million to 359 million years ago, which allows for a much longer period for the diversification of early reptiles, according to Stuart Sumida, president of the Society of Vertebrate Paleontology and a professor of biology at California State University, San Bernardino. Sumida, who wrote a companion paper to be published with the study, was not involved in the new research.
The search for the origin of amniotesLong has been studying ancient fish fossils from the Mansfield district, where the slab was found, since 1980.
“The Mansfield area has produced many famous fossils, starting with spectacular fossil fish found 120 years ago, and ancient sharks. But the ‘holy grail’ we were always looking for was evidence of land animals, or tetrapods, such as early amphibians. Many had looked for such tracks but had never found them – until this slab came to our lab for study,” he says.
Fossils from the Mansfield district have already helped shed light on how sex organs may have first evolved in ancient armored fish.
Now, researchers want to know what other beings lived in Gondwana alongside the ancient reptile they found.
The findings inspired researchers to expand the search for fossils of early amniotes and their close relatives to the continents of the Southern Hemisphere, Sumida said.
“Most of the early amniote skeletal fossil discoveries are known from continents that originated from the northern components of Pangaea,” Sumida said in an email. “These discoveries suggested that the origin of amniotes could be in these regions. It now seems clear to me that we need to expand our search to Early Carboniferous localities in Australia, South America and Africa.”
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