Obdurodon (Greek for "tough tooth"); pronounced ob-DOOR-oh-don
Swamps of Australia and South America
Miocene (23-5 million years ago)
Size and Weight:
About one foot long and a few pounds
Insects and crustaceans
Broad, flat bill studded with teeth
The prehistoric platypus Obdurodon used to count as one of the exceptions to the rule that every modern creature had a plus-sized ancestor lurking millions of years back in its family tree: this monotreme (mammalian egg layer) was about the same size as its modern playtpus relatives, but its bill was comparably broad and flat and (here's the main difference) studded with teeth, which adult platypuses lack. Judging by this dental equipment, paleontologists believe Obdurodon made its living by digging with its bill into the soft silt near lakes and rivers and eating whatever crawly things lay exposed (such as insects, crustaceans and the occasional small fish). As ancient as it was, Obdurodon wasn't the first platypus ancestor to appear on the prehistoric scene; there were also the early Cretaceous Teinolophos and Steropodon.
We say "used to" in the paragraph above because a new discovery has placed Obdurodon squarely in the "megafauna mammal" category: a three-foot-long species (diagnosed on the basis of a single tooth) that was recently discovered Down Under, in sediments dating from 15 million years ago. Besides its size, Obdurodon tharalkooschild was distinguished by its highly developed teeth, which it used to crush crawfish, crustaceans, small vertebrates including birds and lizards, and possibly even the occasional turtle!