Rewriting the history of frogs, toads and salamanders: 200 million-year-old fossil expands amphibious timeline by 15 MILLION years
A new analysis of a pair of tiny fossils found in the 1990s has helped scientists to uncover the backstory of the most mysterious amphibian alive.
Researchers used 3D X-rays to map out the remains of a now-extinct species that walked the Earth 200 million years ago.
They found that the species is a long-missing amphibious link that expands the known history of frogs, toads and salamanders by at least 15 million years.
The fossils, which scientists have dubbed Chinlestegophis jenkinsi, are the oldest relative of a mysterious group of amphibians known as caecilians that have baffled scientists for decades.
Today, these limbless, worm-like carnivores, with a backbone and two rows of sharp teeth, live underground and range in size from six inches (15 cm) to five feet (1.5 m).
The Chinlestegophis jenkinsi fossil was found to be a common ancestor of both the the mysterious caecilians and modern amphibians like the frog and salamander.
The discovery fills in a large gap in the evolutionary tree of amphibians and means that scientists have finally pinpointed the origin of the bizarre caecilians.
‘Our textbook-changing discovery will require palaeontologists to re-evaluate the timing of the origin of modern amphibian groups and how they evolved,’ said Dr Adam Huttenlocker, coauthor of the study and researcher at the University of Southern California.
The team’s finding closes a major gap in early caecilian evolution by connecting them to stereospondyls, animals with toilet-seat heads that were the most diverse amphibian group during the Triassic era more than 200 million years ago.
Scientists previously believed the story of the stereospondyl order was a dead-end because, although widespread during the Triassic Period, the animals seemed to be unrelated to anything alive today.
The two recently discovered fossils finally fill this gap and suggest that the amphibian lineage of today evolved from a common ancestor 315 million years ago.
‘Caecilians are hard to find in the fossil record because most are so small,’ Dr Huttenlocker said.
‘Chinlestegophis jenkinsi still preserves a lot of the primitive morphology that is shared with other Triassic amphibians, namely their four legs.’
Before C. jenkinsi, scientists had found only two other caecilian fossils from the Age of Dinosaurs.
Unlike the two used in the recent study, those fossils came later and had reduced limbs, more closely resembling their modern living relatives.
‘It’s possible that the things that frog and salamander tissue can do when it comes to scar-less healing are also present in human DNA but may be turned off,’ said Mr Jason Pardo, lead author of the study and a doctoral candidate in the Faculty of Veterinary Medicine at the University of Calgary in Alberta, Canada.
‘Because humans are also vertebrates, we enhance our understanding of our own evolutionary history and genetic heritage when we gain understanding of the amphibian lineage.’
There are currently fewer than 200 species of caecilians, which live in the wet, tropical regions of South America, Africa and Southeast Asia.
But the two ancient fossil amphibians found in the late 1990s were preserved in the fossilised burrows of Eagle County, Colorado.
The paleontologists used 3D X-rays to reassemble the fossil remains of two C. jenkinsi specimens.
Parts of a skull, spinal column, ribs, shoulder and legs survived in the fossils of the first specimen. Only the skull was distinguishable in the second specimen.
‘Twenty to 30 years ago, we weren’t even sure of the origins of birds,’ Mr Pardo said.
‘Now we are solving some of the final remaining mysteries when it comes to what sorts of animals the major vertebrate groups evolved from.
‘Caecilians, turtles and some fish are the only major vertebrate groups that paleontologists still have questions about.’
The burrows the fossils were preserved in were almost 2 inches (5 cm) wide, meaning they could not have been very big.
Their bullet-shaped skulls were just under 1 inch (2.5 cm) long, so the ancient caecilian was probably about the size of a small salamander, Dr Huttenlocker said.
The length of the animal is unknown because researchers do not have the full fossil remains of the animal, but Mr Pardo estimates that the ancient caecilian was between 6 inches (15 cm) to a foot (30 cm) long.
As a small carnivore, it probably ate insects.
Its eyes would have been functional but tiny – some of today’s caecilians do not have eyes or they are hidden under moist skin.
During the summer, this central Colorado area would have seen high temperatures, which is likely why these subterranean animals thrived.
Big dinosaurs like early relatives of the Tyrannosaurus rex and Triceratops could not have existed in such conditions, Dr Huttenlocker said.
‘The ancient caecilians lived in these burrows deep in the soil down to about the level of the water table so that they could keep wet and avoid the extreme aridity from the dry season,’ Dr Huttenlocker said.