[Animals] The mystery of the frog that, millions of years later, recovered its teeth

[Noticias ALEJANDRO]

 

The toothed marsupial frog is a strange-looking amphibian from the cloud forests of the Andean slopes in South America. Like other marsupial frogs, this species, known as Gastrotheca guentheri, does not produce free-swimming tadpoles, but instead rear its young under a flap of skin on its back.

But it has an even more puzzling trait: it has a full set of teeth.

Waiting. Do frogs have teeth? Well yes: most frogs have a small number of teeth in their upper jaws. But virtually all 7000 species of living frogs lack teeth in the lower jaw, except for the G. guentheri frog.

Unfortunately, this species has not been sighted since 1996, and even earlier, it has rarely been found or could be studied. There are very few specimens of these animals in museum collections: there may be fewer than 30 specimens in the world. Therefore, there are no actual images of teeth.

This absence of study has left us in the inkwell many pending questions, including a very simple one: What does the jaw of this strange frog look like?

(Related: Frogs Have Teeth Again)

 

 

Faced with this question, Daniel Paluh, a herpetologist who is pursuing a doctorate at the University of Florida (United States), set out to do something about it. Together with colleagues at the Florida Museum of Natural History, Paluh used a microtechnology scanner to examine the skulls of six specimens of G. guentheri preserved for decades in alcohol. His discoveries have not gone unnoticed.

The images and analysis, published Nov. 10 in the journal Evolution, provide the first in-depth look at the jaws and teeth of the species.
Lost teeth ... and recovered

The study also helps to solve other questions. About 230 million years ago, the ancestors of modern frogs permanently lost their lower jaw teeth. That said, an obvious question arises: why does the toothed marsupial frog have teeth, and how did they emerge?

To begin with, Paluh and his colleagues have shown that these teeth are authentic, and that they are made up of a bony tissue called dentin (coated with enamel), thus refuting suggestions that these structures were possibly "pseudo teeth." The researchers also found that these teeth, from the Museum of Natural History at the University of Kansas (USA), closely resemble the teeth of the upper jaws of other marsupial frogs, so they are more likely to be authentic.

These findings provide proof (yet to be confirmed) that would disprove a centuries-old evolutionary theory known as Dollo's Law of Irreversibility (or Dollo's Law). Coined by the Belgian paleontologist Louis Dollo, the theory argues that once a trait is lost in a group of organisms, it disappears forever; an organism cannot re-evolve something that its ancestor lost, in the same way that humans have not re-evolved the tail.

Although the logic of the theory seems sound, evolutionary biologists have hitched at Dollo's Law with examples ranging from lizards that return to lay eggs to stick insects that lose and then regain their wings.

 

 

But the reevolution of the teeth in G. guentheri could be the most unique case seen so far. 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 your teeth. He discovered that teeth did not recover until about 20 million years ago, an "unprecedented" period of time between the loss of a trait and its re-evolution.

Wiens, who currently works at the University of Arizona (USA) and was not involved in the recent study, believes that G. guentheri had an advantage when it came to re-evolving its teeth: it still had a functional network of genes to create a denture along your upper jaw.

(Reading: Dart frogs have evolved to be immune to their own venom.)

"Not that they had to re-evolve the teeth from scratch," says Wiens. "It was just about putting them in a place they hadn't been in 200 million years."

That process would probably have been impossible in other jumping 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 that code for enamel in toads, which completely lost their teeth about 60 million years ago. He discovered that genes had essentially degenerated into pseudogenes over millions of years.

"In essence, these genes do not have any work" and are not functional, says Abramyan. "[But] since most frogs still produce teeth in their upper jaws, theoretically they have all the tools to make a functional tooth, so it's a minor evolutionary leap."
An evolutionary puzzle

However, this does not give us clues about how or why this species recovered its lower teeth, although diet certainly plays a role, says Paluh. As the main tool animals use to bite and chew their food, teeth are often shaped by what is on the menu. Paluh believes that most frogs' predilection for small insects (and the use of sticky tongues to hook their prey) made teeth less important to some species. However, G. guentheri has a healthy appetite, and its regular menu includes prey as large as lizards and other frogs. When hunting big game, it can be helpful to have lower teeth to secure resisting prey.

But if teeth have evolved to help the toothed marsupial frog gobble up larger prey, why haven't the teeth of other carnivorous frogs undergone a similar transformation? Some frogs, such as the bulky "Pacman" frogs of South America, have serrated fangs along their lower jaws to secure their prey. But these fangs are pseudo teeth, bony extensions of the jaw that lack dentin and enamel.

According to Alexa Sadier, evolutionary biologist at the University of California-Los Angeles (United States), perhaps we can find some answers in the study of embryos of the tree frog. Although Sadier primarily explores the evolution of bat teeth, she has recently been reviewing several cases in which lost features remained in the early stages of a creature's development. She believes that comparing the development of G. guentheri with embryos from other frog species could help understand how and when genes turn teeth on or off.

Sadier believes that if the researchers scan the embryos, they will find more evidence of teeth that disappear during development, as well as the genetic wiring that goes with them.

Paluh is also hoping to be able to do some genetic study on the development of the frog, but unfortunately the use of fresh embryos is not a realistic option: no live specimens of G. guentheri have been seen in the wild since 1996, not even in the humid volcanic foothills of Ecuador's Cotacachi Cayapas Ecological Reserve, where they once thrived. Although little is known about them, their numbers have declined as agriculture and logging have devastated 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 not seeing one for 13 years in the same Ecuadorian cloud forests where G. guenthe was once observed.

 

https://www.nationalgeographic.es/animales/2021/11/el-misterio-de-la-rana-que-millones-de-anos-despues-recupero-sus-dientes