The feared extinct species is the only frog with real teeth on its lower jaw – Florida Museum Science

In / A new study, biologists have ended a century-old debate by confirming that just one species of frog, out of more than 7,000 living today, has real teeth on its lower jaw. The culprit, a large marsupial frog named Gastrotheca guentheri, has puzzled scientists since its discovery in 1882 to possess what appeared to be a complete set of serrated dagger-shaped teeth on the top and bottom of its mouth.

Scientists have since wondered about the exact nature of these structures. Real teeth are made up of specific tissues, including dentin and enamel, which are notoriously difficult to observe in frog teeth due to their small size.

“They’re incredibly small, each the size of a grain of sand,” said lead author Daniel Paluh, a doctoral student in the Biology Department at the University of Florida. “There is no way to confirm the presence of dentin and enamel in frog teeth without using high resolution techniques. “

computed tomography of Gastrotheca guentheri The skulls revealed what appeared to be rows of identical teeth on the upper and lower jaws, which the researchers later confirmed by dissection.

Image from Florida Museum by Dan Paluh

Frogs have not had teeth on their lower jaws since their first appearance in the fossil record over 200 million years ago. A single living species with a complete dentition therefore seemed unlikely at best, and contradicted a long-held biological theory called Dollo’s Law, which states that once a complex trait is lost in an organism, it never returns.

But frogs are notorious for flouting the rules when it comes to teeth. Although the shape and anatomy of their basic body has remained largely unchanged since the Jurassic Period, Paluh and his colleagues recently determined that frogs have lost their teeth more than 20 times and may have them. found six more times in their evolutionary history.

Some species, such as those that feed on ants and termites, are entirely toothless and instead rely on their sticky projectile tongues to coil their food. Frogs that prey on larger chunks are often equipped with a row of teeth on their upper jaw and a serrated, serrated palate on top of their mouth, which helps keep wiggling prey from s ‘escape.

In rare cases, some species have developed large bony fangs that protrude from their lower jaw and superficially resemble teeth, but lack the telltale tissues of dentin and enamel. And instead of the conveyor belt tooth replacement system in other frogs, the fangs only grow once and cannot be replaced.

For decades no one was sure that the structures on G. guentheri the lower jaw were bones masquerading as teeth or the genuine object. Finding a specimen to settle the matter was not easy either. Native to the cloud forests of Colombia and Ecuador, the last recorded sighting of G. guentheri was made in 1996, raising concerns that the species has since succumbed to extinction.

A handful of G. guentheri are kept as museum specimens, but given their rarity, biologists are reluctant to subject them to the kind of destructive analyzes that would be required to study their teeth. But Paluh has been able to capitalize on a particularity of Gastro-librarybiology of using a preserved embryo as a sample rather than a full-sized adult.

Instead of laying their eggs in ponds or streams, female marsupial frogs carry them in a pocket on their back. “In the case of G. guentheri, these eggs will skip the tadpole stage and hatch straight into miniature adult versions called frogs, ”Paluh said.

Image of a marsupial frog of the genus Gastrotheca
Frogs of the genus Gastro-library have a pocket on the back, which they use to transport and protect the fertilized eggs.

Photo by José Ulisses Araújo de Souza, CC-BY-NC

The researchers performed CT scans of the embryo’s jaws and carefully stained razor-thin sections of individual teeth with dyes that bind to enamel and dentin, with unequivocal results. G. guentheriIts teeth were virtually identical to those of other frog species in general shape, development, and the tissues that make them up.

“It was surprising given the extremely long time since they were lost and recovered,” Paluh said. “We expected that if they got any teeth back they would be somehow different, but that’s not what we saw at all.”

As to how structures that had been absent for millions of years reappeared in an otherwise unpretentious frog, Paluh suspects the answer lies in the complex pathway of tooth development preserved in the majority of living amphibians. While the location of teeth can vary from species to species, the same basic genetics likely underlie their development, and producing them on the lower jaw can be as simple as flipping a switch.

Paluh plans to take advantage of several genetic tools in the near future to map the contours of tooth development and evolution in frogs, but for G. guentheri and a host of other species at risk in the American tropics, finding the exact answer may no longer be possible. DNA degrades over time in plants and animals kept in museum collections, and the limited number of aging G. guentheri specimens are unlikely to be viable resources for genetic study.

“We have seen many species in this group become endangered due to climate change, habitat degradation and disease from the chytrid fungus,” Paluh said. “We will not be able to answer these questions in any other group because these traits do not exist anywhere else in the Frog Tree of Life.”

Researchers published their findings in Evolution.

David Blackburn and Edward Stanley of the Florida Museum and Gareth Fraser and Wesley Dillard of the Department of Biology at the University of Florida co-authored the study.

Research funding was provided by the National Science Foundation.

Sources: Daniel Paluh, [email protected]

Editor: Jerald Pinson, [email protected], 352-294-0452

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