Canadian researchers say they've found evidence that the ancient ancestors of modern-day frogs were once keen predators with thousands of teeth to help devour their prey.
The team from the University of Toronto examined fossils of animals believed to have evolved into the amphibians people are familiar with today.
The fossils, believed to be 289 million years old, show that frogs, salamanders and other amphibians have evolved significantly over time.
While modern frogs have several small teeth lining the edges of their mouths, their predecessors' jaws were much more menacing.
The ancient ancestors, known as dissorophoids, boasted thousands of tiny hooked teeth throughout the roof of their mouths, as well as large fangs meant to sink into their prey.
Senior researcher Robert Reisz says the findings raise intriguing questions about the way the species has evolved over the millennia.
"It's an interesting mystery," Reisz said in an interview. "It takes a lot of energy to make these teeth, and it may have been that they were not needed in the changeover from these ancient terrestrial predators to frogs and salamanders."
Reisz said the perfectly preserved fossils were discovered in caves located in Oklahoma alongside thousands of other bones.
He said the caves acted as natural traps for animals of the period and helped maintain their remains in excellent condition over the centuries.
Reisz said the preserved skull of a dissorophoid gave researchers a detailed look at the inside of the mouth.
The teeth that prevail in present-day frogs were still in evidence, but Reisz said the rest of the mouth bore little resemblance to today's amphibians.
They found thousands of tiny teeth not only embedded in the bone on the roof of the mouth, but also in the soft tissue that covers the palate.
"They're very cool and very interesting because they all point backwards," he said. "They're hooked ... and they would have probably just stuck out of the skin of the roof of the mouth, so they would be like tiny, tiny little grappling hooks that would allow for the food to go down the gullet, but would prevent it from moving out of the mouth."
Researchers initially speculated that the hooked objects were denticles, which are tooth-like projections that don't have characteristics of real teeth.
But Reisz and his team analyzed the projections and found that they matched the definition of actual teeth. They all featured pulp cavities, an enamel covering, and the hard, calcium-heavy material known as dentine that comprises the main part of proper teeth.
Reisz said the animals would have been replacing these teeth every few months.
Modern-day amphibians are carnivores that primarily survive on insects and other animals, but Reisz said the findings suggest their ancestors were "pretty effective little predators" in a different class from the creatures that succeeded them. The power of the small teeth would have been enhanced by larger fangs ideal for sinking into unsuspecting prey, he added.
Reisz said the next step of the research is to investigate how the process of replacing the teeth took place in the ancient dissorophoids, as well as to probe reasons why the teeth in the roof of the mouth are nowhere to be found in today's amphibians.