251 Million B.C.T. - The Olenekian Age Began

Around 251 million years ago, the Olenekian Age began.

In the geologic timescale, the Olenekian is an age in the Early Triassic epoch or a stage in the Lower Triassic series. It spans the time between 251.2 and 247.2 million years ago. The Olenekian follows the Induan and is followed by the Anisian. 

Archosaurs - a group encompassing crocodiles, pterosaurs, dinosaurs, and ultimately birds - are diapsid reptiles that first evolved from Archosauriform ancestors during the Olenekian.