In the ancient limestone of Yunnan Province, a 244-million-year-old fossil named Austronaga has quietly overturned long-held assumptions about when reptiles first committed themselves fully to the sea. This archosauromorph — kin to the ancestors of crocodiles, dinosaurs, and birds — bore flippers where limbs once were and carried fish scales in its preserved gut, evidence of a life lived entirely in water far earlier than science had imagined. Its existence, in the shadow of the greatest extinction Earth has known, suggests that catastrophe and opportunity are often the same event, and that li