In evolutionary terms, dinosaurs are best described as a region of the family tree which lies somewhere between: a) the last common ancestor of all birds, and

In evolutionary terms, dinosaurs are best described as a region of the family tree which lies somewhere between: a) the last common ancestor of all birds, and b) the last common ancestor of all birds and reptiles As such, whether they're classified as reptiles, birds, or in their own category is really just a matter of taste.