Dinosaurs split into "lizard-hipped" and "bird-hipped" dinosaurs. Of these two groups, only the "lizard-hipped" (Saurischia) survived (these names are backwards).
Of Saurischia, only Theropoda survived. The diverse Sauropodomorpha clade died out fully. Of Theropoda, only Tetanurae survived. Of Tetanurae only Coelurosauria survived. And so on. By the time we got to Aves (birds) we are (according to wikipedia) 46 levels deep (if I counted correctly) with most levels unnamed. Aves is defined as the smallest clade that includes all living birds. In addition, we can go several levels outward to Avemetatarsalia and still have birds the only living group thereof (why we don't consider everything in Avemetatarsalia a "dinosaur" is unclear to me).
In contrast, the evolution of mammals does not follow such extreme pruning, starting from Synapsida (the sister clade to Reptila), there are 17 steps to take us to Mammalia according to wikipedia. Of course, there are probably many more branches along the lineage of birds and mammals than is known form the fossil record, but a 2.5:1 ratio in known branches is still intriguing.
- Is it true that the tree leading to birds had more "pruning" extinctions than most other clades such as mammals?
- If so, why would this be?