Breakthrough fossil study reveals origins of living amphibians

Microscopic photograph of the lower jaw of Funcusvermis gilmorei shortly after it was discovered during microscopic sorting of sediments from the Thunder Ridge Fossil Site at the Petrified Forest National Park paleontology laboratory. (CREDIT: Ben Kligman for Virginia Tech)

The smallest fossil ever discovered could upend what paleontologists know about our history.

A team of paleontologists from Virginia Tech and the U.S. Petrified Forest National Park, among others, have discovered the first “unmistakably recognizable” fossil of a Triassic-era worm — the oldest known worm fossil — thus expanding the record of this small burrowing mammal by about 35 million years. The find also fills a gap of at least 87 million years in the known historical fossil record of the amphibious creature.

The fossil was first discovered by Ben Kligman, a PhD student in the Department of Earth Sciences, part of the Virginia Tech College of Sciences, in Arizona’s Petrified Forest National Park during an excavation in 2019. Kligman named the fossil Funcusvermis gilmorei. extends the history of the caecilians by 35 million years to the Triassic period, about 250 to 200 million years ago.

Prior to this new study, published today in the journal Nature, only 10 finds of fossil worms dating back to the early Jurassic period, about 183 million years ago, were known. However, previous DNA studies have estimated the evolutionary origin of the caecilians back to the Carboniferous or Permian era, roughly 370 million to 270 million years ago, according to Kligman, marking a gap of 87 million years. However, no such fossils have been found.

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“The discovery of the oldest caecilian fossils highlights the crucial nature of the new fossil evidence. Many of the biggest unanswered questions in paleontology and evolution cannot be solved without such fossils,” said Kligman, who previously discovered a 220-million-year-old species of cynodont, or stem mammal, a precursor to modern mammals. “Fossil caecilians are unusually rare and are found by chance when paleontologists are looking for fossils of other, more common animals. Our discovery of one of them was completely unexpected and changed the trajectory of my scientific interests.”

The fossil discovery was made in 2019 by Kligman and Petrified Forest National Park student intern Xavier Jenkins, now Ph.D. student at the University of Idaho, while the duo studied fossil deposits from the park, nicknamed Thunder Ridge, using a microscope. Funcusvermis was discovered in a layer of the Chinle Formation dating back about 220 million years ago, when Arizona was located near the equator in the central part of the Pangea supercontinent, Kligman said. This region at that time was subject to a hot and humid climate. It’s still hot in Arizona today, but the humidity is low.

“When I saw under the microscope the first jaw with the characteristic double row of teeth, goosebumps ran down my spine,” Kligman said. “We immediately knew it was a worm, the oldest worm fossil ever found, and a once-in-a-lifetime discovery.”

Ben Kligman at the Thunder Ridge Fossil Site in the National Petrified Forest in Arizona. (Credit: Adam Marsh)

Prior to this discovery, the 87-million-year gap in the fossil record obscured the early evolutionary history of the caecilians, leading to years of debate among scientists about the relationship of the caecilians to their amphibian relatives, frogs and salamanders.

“Funcusvermis expands on the wet equatorial distribution pattern seen in all known fossil and living caecilians, suggesting that the biogeographic history of the caecilians was driven by the limitation of these environmental conditions, probably due to moisture-related physiological constraints, and was held back by drifting continental plates during wet equatorial zone and out of it after the fragmentation of Pangea,” said Kligman.

Modern caecilians are legless amphibians with a cylindrical body and a compact, bullet-shaped skull that helps them dig underground. Currently, caecilians live exclusively in South and Central America, Africa and South Asia. They spend their lives burrowing through leaf litter or soil in search of prey such as worms and insects. This underground existence has made it difficult for scientists to study worms. Kligman ironically describes modern worms as “an eyeless sock puppet with a worm’s body”.

In the summer of 2018, paleontologists from Virginia Tech and Petrified Forest National Park are excavating at Thunder Ridge. (Source: Ben Kligman)

Funcusvermis actually shares skeletal features associated with early frog and salamander fossils, supporting evidence for a common origin and close evolutionary relationship between caecilians and the two groups. Funcusvermis also shares skeletal features with an ancient group of amphibians known to paleontologists as the dyssorophoid temnospondyls. Kligman adds: “Unlike modern caecilians, Funcusvermis lacks many of the adaptations associated with burrowing underground, indicating a slower acquisition of traits associated with underground life in the early stages of cauliflower evolution.”

Call this tune

And now for the fun part: The genus name Funcusvermis was inspired by the Ohio Players’ 1972 song “Funky Worm” from their Pleasure album, a songwriter’s favorite that was often played during the Thunder Ridge fossil digs. “Funcus” comes from the Latinized form of the English word “Funky” for an upbeat, rhythmic form of dance music, and “vermis” comes from the Latin word for “worm”. (That’s a great song, by the way. Instant earworm, so to speak.)

The specific name gilmorei honors Ned Gilmore, collections manager for the Academy of Natural Sciences at Drexel University in Philadelphia. (Kligman, originally from Philadelphia, volunteered to work with the Gilmore Herpetology Collection as an undergraduate student. “He was an important mentor who helped spark my interest in fossils and amphibians,” Kligman said.)

The co-authors of the study are Michelle Stoker, Associate Professor, and Sterling Nesbitt, Assistant Professor of Geosciences at Virginia Tech University and members of the Center for Global Change, which is part of the Fralin Life Sciences Institute. Other authors include Adam Marsh, a leading paleontologist; Matthew Smith, museum curator; and William Parker, Chief of Science and Resource Management, all in Petrified Forest National Park; and Brian Gee, Research Fellow at the University of Washington’s Burke Museum and Department of Biology.

“As the song of the same name says, this is the coolest worm in the world,” Marsh joked.

Stoker added: “What we collect really defines what we can say about what animals were present, how many there were, and what they looked like. Without the use of these methods to collect and analyze fossils, we would have missed the opportunity to learn so many important aspects of this Triassic ecosystem. Now that we have a search image of what bones to look for and how to look for them, it will be interesting to see what other fossil sites have preserved these early lissamphibians.”

Nesbitt said finds like this could reset the paleontology game board in the best possible way. “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.

What has happened since 2019

In Petrified Forest National Park, where the first discovery was made in 2019, the mandibles of at least 70 Funcusvermis have been recovered as of summer 2022, making the area “the most abundant fossil worm bone ever found.” Kligman said.

According to Kligman, only a few Funcusvermis bones were found, including the upper and lower jaws, a vertebra, and part of a hind limb. All found bones were dismembered, and not as whole skeletons. Without complete skeletons, Kligman and his fellow researchers cannot accurately determine the body length of Funcusvermis, but inference from individual features, such as a lower jaw length of less than a quarter of an inch, indicates that Funcusvermis was a tiny animal.

“Since its discovery in 2017, Thunder Ridge has developed a diverse community of more than 60 animals, from freshwater sharks to dinosaurs,” Kligman said. “Recently, several other new species have been described from this site. Many other new species from this site are currently being studied and will be published in the coming years.”

In other words, expect a bigger upheaval of what paleontologists know about fossil history.

For more science news, visit our New Discoveries section at The bright side of the news.

Note. Submissions provided above by Virginia Tech. Content can be edited for style and length.

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