US researchers have unearthed a fossil arm bone from a coquí frog, also known as rain frog, dating back to 29 million years. According to a study, which was published in the Biology Letters journal on Wednesday, the ancient coquí makes it the world’s oldest, surpassing the amber frog fossil discovered in the Dominican Republic in 1987. David Blackburn, Florida Museum curator of herpetology and the studys lead author, said: "Its a national treasure. Not only is this the oldest evidence for a frog in the Caribbean, it also happens to be one of the frogs that are the pride of Puerto Rico and related to the large family Eleutherodactylidae, which includes Floridas invasive greenhouse frogs." Senior Aauthor Jorge Velez-Juarbe found the fossil on a river outcrop in the municipality of San Sebastian in northwestern Puerto Rico. Velez-Juarbe and his collaborators previous collecting efforts at the site uncovered fossil seeds, sea cows, side-necked turtles and rodents in the Caribbean, dating to the early Oligocene Epoch, about 29 million years ago. He combed the deposits for half a day without much luck when a small bone, partially exposed in the sediment, caught his eye. "At the moment, I couldnt wrap my mind as to what it was. Then once I got back home, cleaned around it with a needle to see it better and checked some references, I knew I had found the oldest frog in the Caribbean," Velez-Juarbe said.