The Olenekian (also known as the Yongningzhenian) is a stage of the Early Triassic epoch. It spans the time between 249.7 ± 0.7 Ma and 245 ± 0.7 Ma (million years ago). The Olenekian is divided into the Smithian and the Spathian age. It is defined as starting near the lowest occurrence of Hedenstroemia or Meekoceras gracilitatis ammonites, and of the conodont Neospathodus waageni. It is defined as ending near the lowest occurrences of genera Japonites, Paradanubites, and Paracrochordiceras; and of the conodont Chiosella timorensis. Archosaurs -- a group encompassing crocodiles, pterosaurs, dinosaurs, and ultimately birds -- are diapsid reptiles that first evolved from Archosauriform ancestors during the Olenekian.