Are Dinosaurs Extinct? Modern Birds Connect us to Our Ancient Past

[Paraleptopecten (Timothy Campbell)]

There are challenges with a strictly modern-cladistic definition of taxa, for instance:

@Paraleptopecten

Hoo boy. Here we get into the “Grades are useful” vs. “Clades are the only usable group” mess among taxonomists. Cladistically birds fall within Dinosauria, because some non-avian dinosaurs are closer to modern birds (e.g. Troodon ) than others (e.g. Ornithimimus ). However, if one defines “dinosaur” more like “something with diagnostic characters of Dinosauria, but not one capable of powered flight, or descended from something capable of powered flight”, then modern birds are not dinosaurs, and “dinosaur” refers to a grade. Grades may be quite useful, especially for paleontology, when in some cases the group wasn’t yet a grade.

More broadly, asking “Are paraphyletic groups useful in some cases?” is better than the all-too-common “only monophyletic groups are acceptable”. Especially where a common name is involved, (e.g. “fish”) paraphyly is probably fine.