I strongly disagree with Mike on this, though. Not the luck part, which is a big factor, but on the lack of a pattern.
In the terrestrial pattern the size pattern is extraordinarily strong. Animals >5 kg simply did not survive. Even within turtles and crocodilians this is true: large crocs and large tortoise mimics (such as xinjiangchelyids) did not make it.
Within the marine realm planktonic taxa and animals that fed more directly on the plankton die off at rates higher than bottom-feeding benthic organisms. Large pelagic forms die off at rates higher than small demersal forms. And so on.
That said, once you are past these filters who lived and who died among the rest seems to be more stochastic. So why Aves and not Enantiornithes? Why some mammal clades and not others? These are less certain, and probably does have a lot to do about luck.