"Hi, I'm superorder Allotheria!
You may remember me from such roles as Dead Uncle in TMBG's song Mammal, or Unknown Fossil #4 in the Museum of Natural History..."
The Allotheria were one of the most successful groups of Mammals ever, but if you're not a They Might Be Giants fan, a natural history buff, or a Jeopardy! five-time champion, you've probably never heard of them. Their fossil record extends back into the Jurassic, over 160 million years ago--which is not to say they weren't around sooner--and they survived the K-T extinction. Their defining features were their teeth. The most recognizable was the greatly enlarged, serrated premolar, which was long and flat like the blade of a steak knife. But it is because of their molars with multiple cusps that some allotheria are defined separately from all other mammals as multituberculates. Other distinguishing features include a parafibula bone, a trait shared with monotremes.
After the K-T extinction, Allotheria (primarily the mulittuberculata) exploded onto the scene, growing to the size of beavers. They were the first group of mammals to specialize by living in trees (think sloths and squirrels), and the only Mesozoic mammals to survive the K-T extinction. Sadly, their own extinction is linked to the origin of rodents, a closely related member of the mammal family. Those dirty rats...