I've heard that approximately 60% of all species went extinct at the KT boundary. Hardest hit were ocean species(
1). Most any species that depended entirely on sunlight was wiped out. A T-Rex for instance fed on plant-eating animals who in turn fed on plants which relied on sunlight. Whatever event caused the KT boundary (probably an impact) seemed to have shrouded the earth in darkness wiping out any food chain that couldn't deal with extended darkness.
Luckily mammals were mostly burrowing creatures which ate insects, worms, and roots. A good wiki page:
Cretaceous?Tertiary extinction event - Wikipedia, the free encyclopedia
Also, dinosaurs didn't so much go extinct. We still have birds which are avian dinosaurs.
~modest
