Long-limbed and sporting dramatic horns above its eyes, Guenther’s marsupial frog is a bizarre-looking amphibian from the misty cloud forests of the Andean slopes. Like other marsupial frogs this species, known as Gastrotheca guentheri, doesn’t produce free-swimming tadpoles, but rather rears its young under a flap of skin on its back.
But it has one even more puzzling trait: It possesses a full set of teeth.
Wait. Frogs have teeth? Actually, yes: Most frogs have a small number of them on their upper jaws. But virtually all 7,000 species of living frogs lack teeth along their lower jaws—except for G. guentheri.
Unfortunately, this species hasn’t been sighted since 1996—and even before then, was seldomly found or studied. There are precious few samples of the animals in museum collections—less than 30 specimens may exist in the entire world. As a result, no real images of the teeth themselves existed.
This lack of study has left many questions outstanding, including a very simple one: What does this weirdo frog’s jaw look like?
Daniel Paluh, a herpetologist pursuing a PhD at the University of Florida, wanted to fill in this gap in knowledge. Along with colleagues at the Florida Museum of Natural History, he used a micro-CT scanner to peer into the skulls of six G. guentheri specimens preserved for decades in alcohol.
The images and analysis, published November 10 in the journal Evolution, provide the first in-depth glimpse of the species’ jaws and teeth.
Teeth lost, teeth regained
The study also helps resolve other questions. Around 230 million years ago, the ancestors of modern frogs lost the teeth along their bottom jaws for good. So why does Guenther’s marsupial frog have teeth, and how did they arise?
To begin with, Paluh and colleagues have shown that these teeth are indeed genuine, and formed by bony tissue called dentin and encased in enamel, refuting suggestions that these structures were possibly “pseudo-teeth.” The researchers also found that these teeth, sourced from the University of Kansas Museum of Natural History, closely resemble teeth those along the upper jaws of other marsupial frogs—making them more likely to be the real deal.
These findings provide tantalizing evidence disproving a century-old evolutionary theory known as Dollo’s Law of Irreversibility. Coined by the paleontologist Louis Dollo, the theory posits that once a trait is lost in a group of organisms, it is gone for good. An organism cannot re-evolve something its ancestor lost, like how humans have not re-evolved tails, the thinking goes.
While the theory’s logic seems sound, evolutionary biologists have poked holes in Dollo’s Law with examples ranging from lizards redeveloping egg-laying to stick insects losing and then regaining wings.
But the re-evolution of teeth in G. guentheri may be the most unlikely case yet. In 2011, evolutionary biologist John Wiens reconstructed the evolutionary relationships between 170 different species of frogs to create a timeline between when frogs lost their lower teeth 230 million years ago and when G. guentheri regained its teeth. He found that the teeth were not regained until around 20 million years ago, an “unprecedented” length of time between the loss of a trait and its re-evolution.
Wiens, who currently works at the University of Arizona and was not involved with the recent study, believes that G. guentheri had one advantage when it came to re-evolving teeth—it still had a functional network of genes to create teeth along its upper jaw.
“It’s not like they had to re-evolve teeth from scratch,” Wiens says. “It’s just a question of putting them in a place that they haven’t been in 200 million years.”
That process would have probably been impossible in other hopping amphibians, such as toads, which are completely toothless. John Abramyan, a biologist at the University of Michigan-Dearborn who was also not involved in the study, recently investigated the genes coding for enamel in toads, which completely lost their teeth around 60 million years ago. He found the genes had essentially degenerated into pseudogenes over millions of years.
“These genes are essentially jobless,” and nonfunctional, Abramyan says. “[But] since most frogs still produce teeth in their upper jaws, they theoretically have all the tools to make a functional tooth, so it’s less of an evolutionary leap.”
An evolutionary puzzle
Nevertheless, this doesn’t tell us why or how this species regained their lower teeth, though diet certainly plays a role, Paluh says. As the primary tool animals use to bite and chew their food, teeth are often molded by what is on the menu. Paluh believes that most frogs’ penchant for small insects, and the use of sticky tongues to snag prey, made teeth less important to some species. However, G. guentheri possesses a healthy appetite that includes prey as large as lizards and other frogs. When going after big game, it may help to have lower teeth to secure squirming prey.
But if the teeth re-evolved to help Guenther’s marsupial frog engulf larger prey, why haven’t teeth re-evolved in other carnivorous frogs? Some frogs, like South America’s bulky “Pacman” frogs, sport jagged fangs along their lower jaws to secure prey. But these fangs are pseudo-teeth—bony extensions of the mandible, lacking both dentin and enamel.
Some answers may be hiding in the treefrog’s embryos, according to Alexa Sadier, an evolutionary biologist at the University of California-Los Angeles. While she primarily explores the evolution of bat teeth, she recently reviewed several cases where lost traits remained in the early stages of a creature’s development. She believes that comparing the development of G. guentheri with the embryos of other frog species may help yield insights into how and when genes turn tooth formation on or off.
She expects that if researchers do scan embryos, they’ll find more evidence of teeth that disappear during development—as well as the accompanying genetic wiring.
Paluh also hopes to do some developmental genetic work on the frog, but fresh embryos are not an option—a living G. guentheri specimen has not been spotted in the wild since 1996, not even in the damp, volcanic foothills of Ecuador’s Cotacachi Cayapas Ecological Reserve where they once thrived. While little is known about them, their numbers have dwindled as agriculture and logging devastate the cloud forests of Ecuador and Colombia. Some fear the species is already extinct.
However, the sudden rediscovery of a presumed extinct frog is not unprecedented. In 2018, for example, researchers found the horned marsupial frog (Gastrotheca cornuta) after failing to spot one for 13 years in the same Ecuadorian cloud forests where G. guentheri were once observed.
Paluh hopes that Guenther’s marsupial frog likewise reappears—not least because living samples of this amphibian will be crucial for learning more about their teeth, and solving this evolutionary enigma.
nationalgeographic.com, 11 November 2021