OK, and I didn’t do a great job with the whole comment. What I’m trying to get at is, I picture the first air-breathing tetrapod and there’s all this LAND with all these various terrains and climates and such, and evolution can go off wildly in many directions. But once those niches are filled, I would think that the capacity for radical innovation would taper severely. Could that not be the limiting factor?

one that claimed the dinosaurs

and

this myth that dinosaurs went extinct at the end of the Cretaceous

That event certainly claimed dinosaurs. Lots of them went extinct. A vast majority, I would presume. Mr. Zimmer did not write “all” dinosaurs.

Some of the sexiest mass extinctions, like the one that claimed the dinosaurs 65 million years ago

Shame on you, Mr. Zimmer! As one of the best and a leading science journalists out there (and, might I add, one I respect a great deal), I find it surprising that you would perpetuate this long-standing myth about the Cretaceous-Paleogene extinction event! Surely you know that birds are extant dinosaurs (birds evolved from theropod dinosaurs and therefore are theropod dinosaurs, just as humans evolved from primates and therefore are primates), so the quoted sentence is patently false as stated. Is it true that the end-Cretaceous extinction claimed most of the dinosaurs? Certainly. Is it still a “sexy” extinction event (in the sense of being evocative for the non-specialist and attracting lots of scientific attention)? Of course. But we’ve known for over 20 years now that dinosaurs did not go extinct at the end of the Cretaceous because some of them survived and even thrived in the Cenozoic and supercede mammals in terms of diversity today! Please…let’s kill this myth that dinosaurs went extinct at the end of the Cretaceous!