Fossils Shed Light on Evolution of Amphibians

Chinlestegophis jenkinsi was a tiny subterranean carnivore and is an ancient relative of frogs and salamanders. Illustration by Jorge Gonzalez.

(CN) – Recently discovered fossils of an extinct species from the Triassic period represent the missing link between amphibians and their colorful, worm-like relative, according to a new study.

Reporting in the scientific journal Proceedings of the National Academy of Sciences, scientists present findings that close a major gap in the early history of caecilians – limbless serpentines that have backbones and live underground.

The two new fossils connect caecilians to stereospondyls, the most diverse amphibian group of the Triassic period, which ended more than 200 million years ago, according to the team.

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

The research expands the known history of frogs, toads and salamanders by 15 million years or more, according to the study.

Studying the ancestry of stereospondyls was previously considered a dead-end as researchers thought they were unrelated to any current species, despite their widespread presence during the Triassic period. However, the new fossils debunk that theory and suggest that the modern amphibian lineage evolved about 315 million years ago from a common ancestor known as Chinlestegophis jenkinsi.

“Caecilians are hard to find in the fossil record because most are so small,” 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 caecilian fossils from the age of dinosaurs. Unlike the new samples, the previously discovered fossils were of younger caecilians with reduced limbs, more closely resembling their relatives living today.

“It’s possible that the things that frog and salamander tissue can do when it comes to scarless healing are also present in human DNA but may be turned off,” said Jason Pardo, lead author of the study and a doctoral candidate 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.”

The two amphibian fossils were found in the late 1990s by study co-author Bryan Small, a research assistant at Texas Tech University, and preserved in Eagle County, Colo.

The team used 3-D X-rays to reassemble the fossil remains of two C. jenkinsi specimens, which helped them analyze parts of a skull, shoulder, spinal column, ribs and legs. Only the skull could be examined in the second specimen.

“Twenty to 30 years ago, we weren’t even sure of the origins of birds,” 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.”

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