Dinosauriformes is a
clade of
archosaurian reptiles that include the
dinosaurs and their most immediate relatives. All dinosauriformes are distinguished by several features, such as shortened forelimbs and a partially to fully perforated acetabulum, the hole in the hip socket traditionally used to define dinosaurs. The oldest known member is
Asilisaurus, dating to about 245 million years ago in the
Anisian age of the middle
Triassic period.