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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.

  1. Is it true that the tree leading to birds had more "pruning" extinctions than most other clades such as mammals?
  2. If so, why would this be?
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    $\begingroup$ Note that "steps" or "levels" in the phylogeny have no biological reality. They will vary depending on the level of details humans get access to and care to describe. Comparing the shapes of different branches of the tree of life is a legitimate question, but relying on such steps or levels might be a problem. That said, I don't have a better proposal. $\endgroup$
    – bli
    Dec 3 '21 at 9:23
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    $\begingroup$ @bli: That is what my first question gets at, is the difference in depth "real" or just because humans happen to have more details about the evolution of birds than of mammals? $\endgroup$ Dec 3 '21 at 17:22
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    $\begingroup$ keep in mind dinosaurs were successful for a LOT longer than mammals so it is not surprising there was a lot more diversification. T-rex is closer to the present day than it is to the first dinosaur by a large margin. we also have a LOT more dinosaur fossils than early mammals. also think about how severe the KT extinction was, 75% of all species, that alone is responsible for most of the pruning. $\endgroup$
    – John
    Dec 5 '21 at 1:02

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