Until the late 20th century, all groups of dinosaurs were believed to be extinct; however, the fossil record indicates that birds are the modern descendants of feathered dinosaurs, having evolved from theropod ancestors during the Jurassic Period,[2] and are now termed "avian dinosaurs". As such, birds were the only dinosaur lineage to survive the mass extinction event.[3] Throughout the remainder of this article, the term "dinosaur" is sometimes used generically to refer to both the avian and non-avian dinosaurs combined, while at other times it is used to refer to the non-avian dinosaurs specifically, and the avian dinosaurs are sometimes simply referred to as "birds". This article deals primarily with non-avian dinosaurs.