What Happened To The Other Mesozoic Mammals?

In 2003, a fossil belonging to a mammaliaform was discovered in an ancient lakebed in what's now China. It was an almost complete skeleton the size of a platypus, a find that complicated the history of mammaliaforms. It painted a picture of their explosive diversification, their mysterious disappearance, and how our own ancestors might have survived thanks to a leg up from some leafy allies…