GrrlScientist has written an excellent overview of a much talked-about article in Nature, which accompanies this lovely schematic of the Mammalian supertree.  The upshot of the article is that the common view that the mass extinction of non-avian dinosaurs 65 million years ago (the K/T boundary) cleared a path for the diversification of mammals is mistaken.  They were already well on the way to establishing a broad diversity of orders.  Says GrrlScientist:

However, contrary to this hypothesis, the new mammal "supertree"
shows that placental mammals had already diverged into several separate
orders by 93 million years ago — long before the bolide impact and at
a time when dinosaurs still ruled the planet. This supertree was
constructed from both fossil and genetic data (se supertree below). It
shows most of today’s mammalian lineages appeared between 100 and 85
million years ago. It also rebeals [sic] that the rate of mammalian evolution
barely changed after the dinosaurs had gone extinct. Despite this, it
was not until 10-35 million years after the K/T boundary when these
mammalian lineages began to thrive and further radiate into more modern
sub-groups… However, because most researchers now recognize that birds descended
from dinosaurs, at least one lineage of dinosaurs is still with us (the
birds) and there are more than 40 taxonomic orders of birds living
today…."There was a period of several million years at the end of this
period which witnessed several extinctions of non-avian dinosaurs,"
explained Jones. "So the old textbook idea that at the K/T boundary
dinosaurs disappeared and mammals appeared is a bit of a straw man."

Being the pedant I am, what is nagging me about this piece is the difference, in my mind, between the diversification of mammals, as documented above, and the "rise of mammals", the latter construct being more subjective. The long lag between the K/T extinction and the diversification of mammalian families seems to indicate that the extinction did not play a role in the rate of mammalian diversity. Is diversification synonymous with the "rise"? Sure, mammalian diversification occurred during the reign of the saurians, but it seems obvious to me that they would have remained the furtive, furry little critters hopping from hole to hole looking for a niche to squat.  It’s hard to imagine the lion, tiger, mastodon, or any other charismatic fauna characterizing the rise of mammals (let alone humans) in the absence of the K/T extinctions.