New fossils provide missing link to mysterious amphibians

Fossils of an extinct species from the Triassic Period are the missing link that connects frogs, toads, and other amphibians to wormlike creatures with a backbone and two rows of sharp teeth, according to a new study.

The fossil, named Chinlestegophis jenkinsi, was from the oldest relative of the most mysterious group of amphibians: caecilians. They are limbless, colorful, serpentine carnivores that today live underground and range in size from 6 inches to 5 feet.

Paleontologists used 3-D X-rays to reassemble the fossil remains of two C. jenkinsi specimens discovered in Colorado. 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.

The study, published in Proceedings of the National Academy of Sciences, expands the known history of frogs, toads, and salamanders by at least 15 million years. It also 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.

“Our textbook-changing discovery will require paleontologists to re-evaluate the timing of the origin of modern amphibian groups and how they evolved,” said Adam Huttenlocker, senior author of the study and an assistant professor at the University of Southern California Keck School of Medicine.

The story of the stereospondyl order was considered a dead-end because, although widespread during the Triassic Period, the animals were believed to be unrelated to anything alive today, the new study said. The two recently discovered fossils dispel that theory and suggest that the amphibian lineage of today evolved from a common ancestor some 315 million years ago.

Before C. jenkinsi, scientists had found only two other caecilian fossils from the Age of Dinosaurs and — unlike the two recently unearthed — those came later and had reduced limbs, more closely resembling their contemporary living relatives, the study said.

“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,” said 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.

The burrows the fossils were preserved in were almost two inches wide, meaning they could not have been very big, the study reported.

Their bullet-shaped skulls were just under an inch long, so the ancient caecilian was probably about the size of a small salamander, Huttenlocker added.

During the summer, this central Colorado area would have been scorching, which is probably why these subterranean animals thrived. Big dinosaurs like early relatives of the Tyrannosaurus rex and Triceratops could not have existed in such conditions, 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,” Huttenlocker said. “I’m going back to Colorado this summer and hope to find more animals with more complete skeletons. We’ll find one. This is just the initial report.”

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By: David Beasley, Earth.com Staff Writer

Source: University of Southern California

Image Credit: Jorge Gonzalez