What's cooler than a venomous, duck-billed mammal that lays eggs? A giant one—and that's just what researchers have found.

A newly discovered species of three-foot-long (one-meter-long) platypus, dubbed Obdurodon tharalkooschild, swam through freshwater pools in Australian forests about 5 to 15 million years ago, according to a new study. That's a much bigger critter than a modern-day platypus, which at 15 inches (38 centimeters) long is about the size of a small domestic cat.

Scientists fleshed out the animal based on a single tooth found several years ago in limestone collected from the fossil-rich Riversleigh World Heritage Area of northwest Queensland (map).

The limestone fossils were stowed in a cupboard and forgotten until study leader Rebecca Pian, a Ph.D. student at Columbia University in New York City, pulled them out in 2012 while studying at Australia's University of New South Wales.

One tooth struck her as odd: It was bigger than any known platypus tooth. After closer study, "I said, 'Wait a second, not only is it quite big, it's quite different as well,'" Pian remembers. When she showed it to study co-author Mike Archer, he immediately agreed it was new.

For instance, the tooth clearly had the unique shape known to belong only to platypus teeth. But it also had bumps and ridges never before seen in the group. To estimate the size of the animal the tooth came from, Pian and colleagues compared the tooth with other platypus teeth and made a rough extrapolation of the size of the new species.

It was bigger than any platypus known before. The team had just shaken up platypus evolution.

Filling in the Gaps

The ancient platypus belongs to a tiny group of egg-laying mammals called monotremes, of which only three modern species remain: the platypus and two species of echidna, all of which are found in Australia and New Guinea. (See "Platypus Genome Reveals Secrets of Mammal Evolution.")

Only four extinct platypus species have been discovered, each in different periods of time, leading scientists to believe that either there are huge gaps in the fossil record or the platypus family tree is simply not very diverse. Part of the problem is that most of the time, only the teeth with their hardy enamel survive the wear and tear of time.

Now, with the discovery of O. tharalkooschild, researchers know that "the evolution of the platypus is potentially more complicated than we thought," said Pian, whose study was published in the November issue of the Journal of Vertebrate Paleontology.

That's because its larger size and possibly more carnivorous teeth suggest it had a different diet from other platypuses—which mostly eat soft invertebrates—possibly taking on bigger prey such as frogs, Pian said. Such a hearty diet may have also been why the newfound platypus was so big, she added.

The fact that the ancient species had such a big tooth was surprising, since older platypus fossils have suggested they evolved smaller and fewer teeth over time. Today's platypus, for example, only has teeth as a youngster. Later in life, an adult chews its soft prey using horny pads in its mouth. (Watch a video about platypus evolution.)

It's even possible the new fossil platypus was part of a now-extinct side branch of the main platypus lineage.

Solid Research

"This seems like a solid piece of research—if I'd found it, I'd have given it a new name as well," noted Timothy Rowe, director of the Vertebrate Paleontology Laboratory at the University of Texas, Austin.

Platypus teeth are so "exceedingly unique" that it's clear the new tooth is from a platypus, added Rowe, who wasn't involved in the new study.

The paleontologist also said the finding reinforces that "we don't really know" a lot about the evolution of platypuses and echidna.

But, he said, "we're starting to fill in some of the gaps, and that's always a happy thing."