A giant extinct species of carnivorous platypus about a metre long, the largest ever found, has been discovered in Australia, debunking the theory that the creature shrunk in size over time.
Unlike the living species, it had fully functional teeth that may have been used to kill and consume a wide range of animals that lived alongside it in ancient pools and lakes.
Until recently, the fossil record indicated that the platypus lineage was unique, with only one species inhabiting Earth at any one time.
The finding describes a new, giant species of extinct platypus that was a side-branch of the platypus family tree.
The new species, named Obdurodon tharalkooschild, is based on a single tooth from the famous Riversleigh World Heritage Area of northwest Queensland.
While many of Riversleigh's fossil deposits are now being radiometrically dated, the precise age of the particular deposit that produced this giant platypus is in doubt but is likely to be between 15 and 5 million years old.
"Monotremes (platypuses and echidnas) are the last remnant of an ancient radiation of mammals unique to the southern continents. A new platypus species, even one that is highly incomplete, is a very important aid in developing understanding about these fascinating mammals," said Rebecca Pian, lead author of the study.
Based on the size of tooth, it is estimated that this extinct species would have been nearly a metre long, twice the size of the modern platypus. The bumps and ridges on the teeth also provide clues about what this species likely ate.
"Like other platypuses, it was probably a mostly aquatic mammal, and would have lived in and around the freshwater pools in the forests that covered the Riversleigh area millions of years ago," said Dr Suzanne Hand, from the University of New South Wales.
"Obdurodon tharalkooschild was a very large platypus with well-developed teeth, and we think it probably fed not only on crayfish and other freshwater crustaceans, but also on small vertebrates including the lungfish, frogs, and small turtles that are preserved with it in the Two Tree Site fossil deposit," said Hand, a co-author of the study.
The oldest platypus fossils come from 61 million-year-old rocks in southern South America. Younger platypus fossils are known from Australia in what is now the Simpson Desert.
Before the discovery of Obdurodon tharalkooschild, these fossils suggested that platypuses became smaller and reduced the size of their teeth through time.
The modern platypus completely lacks teeth as an adult and instead bears horny pads in its mouth. The name Obdurodon comes from the Greek for "lasting (obdurate) tooth" and was coined to distinguish extinct toothed platypuses from the essentially toothless modern species.
The study was published in the Journal of Vertebrate Paleontology.
