Paleontologists Uncover Fossil Impressions of Giant, Alligator-Like Amphibians

More than 255 million years ago, in what’s now South Africa, enormous amphibians floated and swam through the ancient shallows. Paleontologists know this not from petrified bones but from marks left in the sediment by the living creatures. The Dave Green paleosurface, in the eastern portion of South Africa’s Karoo Basin, is covered with the tracks and body impressions of amphibious creatures that grew to more than five feet in length. “The more you look, the more you find,” says paleontologist David Groenewald of the University of the Witwatersrand in Johannesburg.

Groenewald and colleagues described the fossils, attributed to animals called rhinesuchid temnospondyls, Wednesday in PLOS One. Formed near the end of the Permian Period, prior to the world’s worst mass extinction, the expanse of olive-green rock was laid down by rivers that threaded across the prehistoric landscape. Those waterways, it would seem, were home to large amphibious vertebrates that paleontologists know as temnospondyls—roughly salamander-like animals that could grow to impressive sizes, effectively the alligators of their time. While temnospondyl bones have given paleontologists a general idea of what these animals looked like, the trace fossils are effectively the prehistoric behavior of these animals locked in stone.

Determining what sort of organism made a trace fossil, including those on the Dave Green paleosurface, can be a challenging task. Body fossils, such as bones, are rarely found with footprints and body impressions. Museums of Western Colorado paleontologist Julia McHugh, who wasn’t involved with the new study, notes that sometimes paleontologists find tracks or other trace fossils before they find bones. Experts have to work backward from those footprints and other traces through the list of creatures that could have possibly left such imprints. In this case, based on the size of the trace fossils, their shape and the wealth of previous research on animals that lived in the area at the end of the Permian, a rhinesuchid temnospondyl is the best fit, McHugh says.

Most of what paleontologists have come to understand about temnospondyls has come from their bones, including broad, flat-topped skulls that gave their noggins a passing resemblance to a toothy toilet seat. But the trace fossils—scrapes and scours in the rock—reveal the shape and behavior of animals that lived millions of years before the dinosaurs. “I feel like each time I go, I find something new or something that I hadn’t seen before,” Groenewald says. Tracks left by the temnospondyls as they slowly moved around the rivers dot the stone, as well as whole-body impressions created when the amphibious creatures rested on the silty bottom.

Altogether, Groenewald and colleagues report, the fossil surface preserves 15 distinct trackways that may have been made by a single temnospondyl. The spindle-shaped body impressions are similar in size, seem to follow each other over the surface and are sometimes even connected by furrows created as the animal swam along the river bottom. If Groenewald and co-authors are correct, the trace fossils could essentially be a day in the life of the temnospondyl that created them.

The ancient vertebrates weren’t just plumped-up versions of today’s hellbenders. Some of them had tough coverings of keratin on their skin, for example, or bony armor to make them a harder bite for other predators of the time.

Previously, paleontologists based their ideas of how temnospondyls moved on comparisons of fossil skeletons with living creatures. Experts weren’t quite sure whether the animals used their tails, their arms and legs, or some combination of the two as they moved through rivers, lakes and swamps. The new traces suggest that these were primarily tail-propelled amphibians.

“These tracks are certainly consistent with temnospondyl skeletal morphology,” McHugh says, “but what I find interesting is the depth of the prints.” The tail impressions, especially, seem to be very deep and indicate that the animals were resting the full weight of their tails on the bottom. This might mean the amphibians had thick, powerful tails that they relied upon for getting through the water more than their limbs. When crocodiles slide into the water, Groenewald and colleagues note, they move by undulating their long tails and their bodies, often keeping their limbs tucked against their sides. The impressions on the Dave Green paleosurface show similar behavior: the temnospondyls essentially wiggling through the water and keeping their limbs close to their bodies as they slid into a good resting spot.

But footprints exist on the fossil surface, too. The impressions show the temnospondyls spread their arms and legs out and sometimes even walked along the bottom of the river, leaving footprints alongside the smooth body drags. The overall pattern is similar to some living salamanders, smaller amphibians that rely more on their tails than their arms and legs to move through the water.

Trace fossils often act as a check on what paleontologists determine from skeletons. Biomechanical models can often set limits on what it was possible for an extinct animal to do, and previous research was unclear on whether temnospondyls used their tails, limbs or some combination to move through the water. But trace fossils are actual records of how prehistoric species behaved. “The discovery of this impressive ichnological site is wonderful,” McHugh says, “and provides empirical evidence to support the conclusions of years of biomechanical research.”