Why is the traditional taxon class Reptilia considered paraphyletic?

Question

Quizplus · Accepted Answer

The traditional taxon class Reptilia is considered paraphyletic because it includes an ancestor and some, but not all, of its descendants. Specifically, it includes the common ancestor of turtles, lizards, snakes, crocodilians, and birds, but it excludes all of the descendants that evolved into birds. Birds are now recognized as a separate class in the phylogenetic classification system as they evolved from within the reptile lineage.

Why Is the Traditional Taxon Class Reptilia Considered Paraphyletic