A fossil of an extinct species of platypus, twice the size of its modern-day counterpart, has been found in Australia, paleontologists say. While scientists had believed the platypus lineage was unique, with only one species of the odd duck-billed egg-laying creature inhabiting Earth at any one time, the new giant species of extinct platypus discovered in northwest Queensland has been determined to be a side-branch of the platypus family tree, they said. The new platypus species, Obdurodon tharalkooschild, is based on a single fossil tooth thought to be between 15 million and 5 million years old, the Society of Vertebrate Paleontology reported Tuesday. "Monotremes [platypuses and echidnas] are the last remnant of an ancient radiation of mammals unique to the southern continents," lead study author Rebecca Pian, a doctoral candidate at Columbia University, said. "A new platypus species, even one that is highly incomplete, is a very important aid in developing understanding about these fascinating mammals." Based on the fossil evidence, the ancient platypus may have grown to more than 3 feet long, about twice the size of modern individuals. Likely a mostly aquatic animal like the modern platypus, the discovered well-developed tooth suggests its diet could have consisted of crayfish and other freshwater crustacean along with the occasional frog or even small turtle, the researchers said. The chosen name Obdurodon comes from the Greek for "lasting [obdurate] tooth" and was coined to distinguish extinct toothed platypuses from the essentially toothless modern species.