‘Funky worm’ shimmies into gap in fossil record

27 Jan 2023

Artistic reconstruction of Funcusvermis gilmorei. Artwork by Andrey Atuchin. Image: National Park Service and the Petrified Forest Museum Association

A caecilian fossil from the Triassic period, discovered in Arizona, helps Virginia Tech researchers understand the evolution of the species.

It’s 2019 and just like any other day for PhD student Ben Kligman and intern Xavier Jenkins. Together they are processing fossiliferous sediment from Thunderstorm Ridge in Petrified Forest National Park in Arizona. They probably have music on as they work. Looking through a microscope, they spot a jawbone that will make a huge mark on their discipline.

“Seeing the first jaw under the microscope, with its distinctive double row of teeth, sent chills down my back,” Kligman said. “We immediately knew it was a caecilian, the oldest caecilian fossil ever found, and a once-in-a-lifetime discovery.”

Modern caecilians are limbless amphibians with cylindrical bodies and a compact, bullet-shaped skull that helps them burrow underground, or as Kligman describes them, “eyeless sock puppets with the body of a worm”.

These mysterious, mostly underground creatures have a chequered past. Prior to Kligman et al’s study, published in Nature on 25 January, only 10 fossil caecilian occurrences were known, and these dated to the early Jurassic period, about 183m years ago. Yet, DNA studies estimated the evolutionary origins of caecilians as far back as the Carboniferous or Permian eras, some 370m to 270m years ago. This left an 87m-year gap in the fossil record of these wily worm-like amphibians.

‘The funkiest worm in the world’

The newly discovered fossil was named Funcusvermis gilmorei. The genus Funcusvermis was inspired by the Ohio Players’ 1973 hit, Funky Worm, a song often played by the researchers as they worked. The species name, gilmorei, honours Ned Gilmore, collections manager at the Academy of Natural Sciences in Drexel University, Philadelphia. Gilmore was a mentor to Kligman and inspired his interest in fossils and amphibians.

Funcusvermis was found in a layer of earth dated to about 220m years ago, placing it in the Triassic period.

At the time, the region was hot and humid. This shows that Triassic-era caecilians lived in similar humid equatorial regions as modern caecilians that now live in Central and South America, Africa and southern Asia.

Scientists have long debated the relationship between caecilians and their amphibian relatives, frogs and salamanders. This little Arizonan fossil unlocks some answers to the early evolutionary history of the species.

Funcusvermis has skeletal features that are similar to early frog and salamander fossils, strengthening evidence for a shared origin and close evolutionary relationship between these three groups.

Kligman said, “Unlike living caecilians, Funcusvermis lacks many adaptations associated with burrowing underground, indicating a slower acquisition of features associated with an underground lifestyle in the early stages of caecilian evolution.”

Ben Kligman, paleontologist, poses for a picture at a dig sit with a shovel in his hand, wearing a grey t-shirt and dark trousers. There are hills and a cloudy sky in the background.

Ben Kligman at the Thunderstorm Ridge fossil site. Image: Adam Marsh

A lucky find

Since the initial discovery, fossil fragments of at least 70 individual Funcusvermis have been found, making the area “the most abundant fossil caecilian-producing bonebed ever discovered,” according to Kligman.

Unfortunately, as of yet no full skeletons have been discovered, without which the researchers can only estimate the creature’s size, but it’s safe to assume it was tiny.

A co-author on the study, Dr Sterling Nesbitt, associate professor in the Department of Geosciences at Virginia Tech, said finds such as this are game-changing for palaeontology.

“This find clearly demonstrates that some fossils that you can barely see can greatly change our understanding of entire groups that you can see today,” he said.

And to think this remarkable little fossil was only found by chance. As Kligman explained:

“Fossil caecilians are extraordinarily rare, and they are found accidentally when paleontologists are searching for the fossils of other more common animals. Our discovery of one was totally unexpected, and it transformed the trajectory of my scientific interests.”

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Rebecca Graham is production editor at Silicon Republic