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If this thing looks like a rat, it very nearly is.
It, and its relatives in the multituberculates thrived about a hundred million years ago, back when mammals were still making up their minds between being placental or marsupial. Some were as large as beavers, some as small as mice. They lived in the ground, and up in the trees. If you were a stegosaurus, you could hardly move without stepping on a multituberculate, which is convenient for modern fossil hunters.
The multituberculates occupied virtually every niche as modern rodents: there were squirrel-like multituberculates, hamster-like multituberculates, for all I know, even cute little bunny-rabbit-multituberculates. They were so much like rodents their own mamas must've thought they were; they had rodent-like bone structure, even long rodent-like incisors.
But they were not rodents; they were no more closely related to rodents than we are.
The rodents appeared about 66 million years ago, after xanclomys and the other multituberculates had already gotten a 60-million-year head start. But the rodents had one crucial advantage. The posterior of their incisors has no enamel, meaning it will grow the entire lifetime of the animal. When the animal gnaws, the incisor gets a razor-like edge. Instead of wearing away, like poor old xanclomys, the rodent is equipped with a set of ever-lasting, self-sharpening teeth.
The multituberculates had a good run of it; they lived longer than any other lineage of mammals, but they were no match for the rodents. Now there's not a single multituberculate left, not one, and our friend xanclomys is known to us by only one remaining fossil.
The moral of the story is, if you make your living acting like a rat, you run the risk of being outdone when a real rat comes along.
