Are Birds Reptiles?

According to phylogenetic classification, a "reptile" is any organism descended from the original group of reptiles, which includes birds. Thus, birds are reptiles by this method of classification, much like everything that descended from the first vertebrate, which is a vertebrate. When we appropriately calibrate our microscope, we find that birds are more closely related to dinosaurs, branching off from this group. The first reptile group split 300 million years ago. About 40 million years later, a group called therapsids branched off, eventually evolving into what we now recognize as modern mammals. Approximately 120 million years later, several branches diverged, one of which gave rise to dinosaurs—a highly successful species distantly related to snakes, lizards, and turtles, which had split off at different times.