By Ella Davies
Earth News reporter
Frogs re-evolved "lost" bottom teeth after more than 200 million years, according to new research.
Tree-dwelling Gastrotheca guentheri are the only frogs with teeth on both their upper and lower jaw.
The reappearance of these lower teeth after such a long time fuels debate about whether complex traits are lost in evolution or if they can resurface.
Scientists suggest this new evidence identifies a "loophole" in previous theories.
The Gastrotheca genus of frogs carry eggs on their backs
Commonly known as "marsupial frogs", the Gastrotheca genus carry their eggs in pouches.
Unlike marsupial mammals such as kangaroos however, the frogs' pouches are on their backs.
The species Gastrotheca guentheri is even more unusual, being the only known frog to have teeth on its lower jaw.
Dr John Wiens led a team of scientists from Stony Brook University, New York to investigate this exceptional feature.
Their findings are reported in the journal Evolution.
"I combined data from fossils and DNA sequences with new statistical methods and showed that frogs lost their teeth on the lower jaw more than 230 million years ago, but that they re-appeared in G. guentheri within the past 20 million years," explains Dr Wiens.
In the past, scientists have argued that traits "lost" in evolution cannot return, an assertion known as Dollo's law.
The return of lower jaw (mandible) teeth in G. guentheri after more than 200 million years could make evolutionary biologists reconsider this law.
"The loss of mandibular teeth in the ancestor of modern frogs and their re-appearance in G. guentheri provides very strong evidence for the controversial idea that complex anatomical traits that are evolutionarily lost can re-evolve, even after being absent for hundreds of millions of years," Dr Wiens says.
Dr Wiens believes that this re-evolution can be considered a "'loophole' in Dollo's law".
He suggests that because the frogs have always had teeth on their upper jaw, the "mechanisms for developing teeth" have always been present.
"What G. guentheri did was to put teeth back on the lower jaw, rather than having to re-evolve all the mechanisms for making teeth 'from scratch'," says Dr Wiens.
"This "loophole" may apply to many other cases when traits appear to re-evolve, such as in the re-evolution of lost fingers and toes in lizards," Dr Wiens tells the BBC.
According to Dr Wiens, this theory could be applied to other recent studies that have suggested the re-evolution of lost traits.
In the last decade, scientists have identified and debated several attributes that have apparently "re-evolved" over time including stick insect's wings, coiling in limpet shells, larval stages of salamanders and lost digits in lizards.
G. guentheri live on the forested slopes of the Andes in Colombia and Ecuador.
The IUCN lists the species as vulnerable due to their "extremely fragmented" habitat.