If an asteroid killed all the dinosuars… How did the crocidiles, sharks, snakes, and all the other animals survive?
The Cretaceous-Tertiary extinction event killed off more than just dinosaurs.
Some 15% of all marine families died out, including most major marine predators at the time (e.g., plesiosaurs, mosasaurs, ichthyosaurs). And these were major families, as well, as they included somewhere close to half of all marine genera, and 80%-90% of all marine species. Of the survivors, the Foraminifera, Echinoderms, and Molluscs took major hits as well.
On land, around 25% of all terrestrial animal families died out, including around 56% of all such genera. Little, if anything, larger than about 25kg survived. Of course, this includes all of the 900+ species of dinosaurs (with the exception of birds, of course) and pterosaurs. Many land plants took significant hits as well.
As for the survivors, it’s hard to say why any one species would survive over another in such an event. In general, the more wide-spread the species, the greater the chances of survival. Likewise, the more non-tropical the species, the greater the survivability. Any sort of specialization also hurts one’s chances of survival.
Creatures like sharks tend to be very diverse (being one of the oldest marine forms), thus it is not surprising that as a group, they survived. Small, cold-blooded reptiles (or even small mammals) could go into hibernation for a time, thereby increasing their survivability.
Ultimately, however, it is not, entirely clear why animals such as birds, turtles, and crocodiles did not die off. While the K-T asteroid (or comet) seems the most likely killer for many groups, the various scenarios resulting from such an impact have yet to explain fully the concomitant extinction (and survival) patterns.
Or, the short answer is: we don’t know.