Frogs have lost teeth over 20 times during the course of their evolution, which is more than any other vertebrate group, according to a new study from the Florida Museum of Natural History. The experts report that some species have even re-evolved teeth millions of years after losing them.
Out of more than 7,000 species of frogs, only one species has teeth on both the upper and lower jaws. Some have tiny teeth on their upper jaws or on the roof of their mouth, while others have no teeth at all.
The researchers identified a link between the absence of teeth in frogs and a specialized diet on small insects, such as ants and termites. The investigation is the most comprehensive of its kind to study tooth evolution in frogs, involving CT scans of nearly every living amphibian genus.
“Through this study, we have really been able to show that tooth loss in vertebrates is largely a story about frogs, with over 20 independent losses,” said study lead author Daniel Paluh. “Only eight other groups of living vertebrates, including seahorses, turtles, birds and a few mammals, have also evolved toothlessness.”
Teeth first evolved more than 400 million years ago, supporting the diversification of sharks, fish, and ultimately terrestrial vertebrates. Not many studies have focused on frog teeth because they are incredibly small.
“If you open a frog’s mouth, chances are you will not see teeth even if they have them, because they’re usually less than a millimeter long,” or smaller than the tip of a pencil, said Paluh.
In the past, it was very challenging to determine which frogs had teeth, as it would have required tedious work that destroyed portions of preserved specimens. Furthermore, frogs are so diverse that it would have been a time-consuming task.
However, the Florida Museum leads a massive multi-institutional effort to CT scan 20,000 vertebrate specimens, giving researchers the ability to study animals in ways that were not previously possible.
The project, called oVert, allows the public to access 3D models that are derived from the CT scans, depicting features such as bones, internal organs, muscle tissue, and teeth.
The study provides a powerful example of the research that can be accomplished with open-access data, said study senior author David Blackburn.”We effectively crowdsourced the data collection across our lab, including people that were not in the U.S. at that time.”
The results showed that frogs have undergone “rampant tooth loss,” said Paluh, with toothlessness popping up in groups as distantly related as toads and poison dart frogs.
The experts discovered a strong correlation between the presence or absence of teeth in frogs and their eating habits. For example, frogs without teeth were found to eat tiny insects.
“Having those teeth on the jaw to capture and hold on to prey becomes less important because they’re eating really small invertebrates that they can just bring into their mouth with their highly modified tongue,” said Paluh. “That seems to relax the selective pressures that are maintaining teeth.”
Blackburn noted that innovative techniques, such as those used in the oVert project, are beginning to underscore knowledge gaps and limitations like these, but they also open up the field to new discoveries.
“We now have lots of new questions in my lab inspired by the surprising things turning up from 3D imaging from the oVert project, and those will lead us both back into museum collections and to the field to see what these animals are doing in the wild.”
The study is published in the journal eLife.