Newly Described Fossil From Wyoming Sheds Light on When Frogs and Toads Lost Their Teeth

More than 110 million years ago, a now-arid stretch of Bighorn Basin, Wyoming, was covered by a balmy bayou. These primeval wetlands were home to turtles, crocodiles and several species of dinosaurs. Scuttling among these giant reptiles was a palm-sized forebearer of modern frogs and toads. Unlike other ancient amphibians in the swamp, this critter’s upper jaw lacked teeth, making it one of the oldest-known cases of a toothless species in the frog and toad family tree.

Matthew Oreska, a paleontologist in the Department of Paleobiology at the National Museum of Natural History, first came across the toothless jawbone fragment while sifting through a trove of tiny fossils from Wyoming’s Cloverly Formation, a geological layer dating back to the Early Cretaceous. Over the past few decades, Smithsonian scientists have collected tens of thousands of specimens from this deposit. Oreska originally placed the jawbone fragment into a box labeled “unknown frog” and turned his attention to other samples.

The fossil resided in anonymity until Dave DeMar, a former postdoc and now paleobiology research associate at the museum, took a second look at Oreska’s fossil pile.

“Dave pointed out the absence of maxillary teeth and said that was unusual for a frog that old,” Oreska said. “It is a trait exclusive to this specimen and not present in the four other frog species we’ve identified from the Cloverly Formation.”

In September, Oreska, DeMar, NMNH curator of Dinosauria Matthew Carrano and the Royal Tyrrell Museum’s Jim Gardner, described the jawbone in the Journal of Vertebrate Paleontology. They named the new species Ostrombatrachos nodos. The genus name combines “Ostrom,” an homage to pioneering paleontologist John Harold Ostrom, who devoted years to investigating fossils from the Cloverly Formation, and “batrachos,” the Greek word for frog. The species name, “nodos”, is Greek for toothless.

According to the researchers, O. nodos represents the oldest Northern Hemisphere case of edentulism, the trait of toothlessness, in the order Salientia, which includes frogs, toads and their extinct relatives. Edentulism has independently evolved at least 22 times among modern members of the taxonomic order. O. nodos’s toothless jaw represents an early example of this phenomenon.

“The ancient ancestors of frogs would have had teeth on both their upper and lower jaws,” Oreska said. “The lower jaw teeth were lost along the line, but many modern toads and frogs have teeth on their upper jaw, despite the popular imagery of Kermit the Frog.”

Oreska speculates that the gene responsible for tooth development in salientians is influenced by changes in feeding behavior over time. Perhaps, the presence or absence of teeth depends on how a species captures its primary prey, ranging from tiny insects to small mammals.

“There is research noting a possible connection between the loss of teeth in some fossil salientian groups and the emergence and diversification of ants and termites during the Early Cretaceous,” Oreska said. With a buffet of small invertebrates newly available, perhaps ancient amphibians could chow down without needing as many teeth.

This “on/off” expression of teeth is highly unusual among animal groups, and scientists are still trying to understand why it has repeatedly occurred within the frog family tree.

The bottom edge of O. nodos’s jawbone appears to have a cutting surface. However, due to the fossil’s fragmentary nature, Oreska and his team can only hypothesize about this critter’s behavior and feeding ecology. If it was terrestrial, it could have been snacking on termites. If it was primarily aquatic, it may have been snapping up larvae and other small creatures.

Because Oreska and the team have only found a single O. nodos specimen among the thousands of Cloverly fossils in the museum’s collection, it is likely that this ancient creature was a relatively rare member of the Cretaceous wetland community. By contrast, the team has uncovered multiple specimens belonging to an ancient salamander-like amphibian that Carrano and Oreska named Albanerpeton ektopisitkon, as well as bits and pieces from other ancestral amphibians.

While the jawbone’s isolation from other anatomical features presents challenges, it has the benefit of being fully exposed. More complete frog remains are often found entombed under layers of finely-grained silt. The weight and compression of the stacked earth can flatten the skull, essentially “pancaking” the organism and damaging or concealing critical surfaces associated with the head, including the upper jaw.

The new fossil also preserves some intriguing anatomical features that mark this species as unique among ancient salientians. Researchers identified new bumps on the upper jaw's surface that served as cartilage connections, and the bottom of an eye socket that suggests O. nodos’s eyes were perhaps smaller or placed higher on its head compared to  other frogs and toads.

The fossil’s age, estimated at 111 million years old, is notable, as it provides further context of when frogs and toads first began to lose their teeth. “Before O. nodos, there was a 90 million year gap between the oldest known toothless salientian, Notobatrachus reigi from Argentina, and the next oldest example,” Oreska said. “Our newly described species helps us begin to investigate how and why edentulism evolved so many times throughout Salientia history.”

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Chihiro Kai | READ MORE

Chihiro Kai is a Science Writing Intern at the Smithsonian National Museum of Natural History. She covers natural history research for the museum's online magazine, Smithsonian Voices. Chihiro is a double major in Journalism and Evolutionary Biology, Ecology and Organismal Biology at the University of Kansas. She is particularly fond of dinosaurs and was a research volunteer for two field seasons at the KU Vertebrate Paleontology Lab. Previous to the Smithsonian, Chihiro was a freelance reporter covering the Tokyo 2020 Olympic and Paralympic Games, specializing in the COVID-19 pandemic and Japanese domestic and international policy. One of her favorite pastimes is watching old "Bill Nye the Science Guy" episodes.