 However, later on, both of these classification systems were found to be paraphyletic, or "artificial", clades.For example, troodontids are now known to be deinonychosaurs, or "raptors", closely related to dromaeosaurids and birds.The discovery of basal tyrannosauroids, such as "Guanlong", which lacked an arctometatarsus, also helped to disprove this theory.