Paleontologist: Dinosaurs belonged to two major taxonomic groups, the Saurischia and the Orlnithischia. Scientists have long known that the branch of the Saurischia from which birds are descended had primitive feathers. However, a recently discovered fossil of a previously unknown Orinithischia species also has feather-like structures. Thus, dinosaurs must have evolved feathers early in their history, before the Saurischia and Orinithischia diverged.

Which of the following is an assumption the paleontologist's argument requires?