Springtime. A rock about ten kilometres wide strikes the sea off what is now Yucatán, Mexico, some 3,000 km away. Within minutes the ground here shudders; within the hour, glass beads and hot debris rain from a glowing sky. Fish buried that day at a Hell Creek site called Tanis still have the beads in their gills, and the growth rings in their bones record the season. Then comes the dark and the cold. Three-quarters of all species die — every dinosaur that is not a bird among them. Ferns are the first plants to return. Small burrowing mammals like you are already there, waiting. Multituberculates carry on for another 30 million years, and long after the Mesodma line fades, the world belongs to mammals.