Why are reptiles considered a paraphyletic group in some classification systems?

Reptiles are considered a paraphyletic group in modern biological classification because the traditional definition of the class Reptilia does not include all the descendants of their most recent common ancestor.12

In phylogenetic systematics, a monophyletic group (or clade) must consist of an ancestor and all of its descendants. However, scientific evidence—including both fossil and molecular data—demonstrates that birds (class Aves) evolved from within the Archosauria, a lineage of reptiles. Because traditional classification systems treat birds as a separate class while grouping them apart from other reptiles, the group "Reptilia" excludes a significant branch of its own evolutionary history.1234

Consequently, when birds are excluded, the remaining group is paraphyletic—a "grade" rather than a true clade. While some conservative classification systems retain this grouping for convenience, many modern systematists argue that because birds are modified dinosaurs, they should be classified within the reptile lineage to accurately reflect evolutionary history.23