Why did crocodiles, birds and tiny mammals survive when three quarters of species died?

The extinction was selective, and the survivors tended to be animals whose food did not depend on living plants. The K–Pg extinction was global, rapid and selective. Species that depended on photosynthesis declined or became extinct as atmospheric particles blocked sunlight. Surviving mammals and birds fed on insects, worms and snails, which in turn fed on detritus, dead plant and animal matter. Omnivores, insectivores and carrion-eaters survived, perhaps because their food became more available.

Streams and lakes show the pattern. Few animal groups became extinct there, including crocodile relatives, because those ecosystems rely less on living plants and more on detritus washed in from land. Modern crocodilians can live as scavengers and survive for months without food, and their young are small, grow slowly and feed largely on invertebrates and dead organisms in their first few years.

Size and shelter mattered too. The extinction killed all non-avian dinosaurs and most other tetrapods (four-limbed vertebrates) weighing more than 25 kilograms, with exceptions such as sea turtles and crocodilians. There is no evidence that late non-avian dinosaurs could burrow, swim or dive. Birds, on the other hand, may have survived by diving, swimming or sheltering in water and marshlands, and many can build burrows or nest in tree holes or termite nests. The mammals that survived were generally small, comparable in size to rats.

Then came the opportunity. Many groups underwent adaptive radiation, a sudden and prolific divergence into new forms in emptied niches. After about 700,000 years some mammals had reached 50 kilograms, a 100-fold increase over the weight of the survivors. Most paleontologists regard birds as the only surviving dinosaurs.