Mammals recover after the asteroid (Corral Bluffs)

Corral Bluffs is a line of eroded sandstone cliffs east of Colorado Springs that holds one of the best records of the world after the asteroid impact. A team from the Denver Museum of Nature and Science, led by Tyler Lyson and Ian Miller, collected thousands of fossil leaves and many mammal skulls and jaws from rock laid down in the first million years of the Paleocene. They dated the layers against the impact boundary and published the results in Science in 2019.¹²

Before the impact the Denver Basin supported dinosaur-era forests. The asteroid wiped out roughly three quarters of species, and in the cliffs the first rocks above the boundary show a fern spike, the signature of ground that had to be recolonized. Mammals had lived in the shadow of dinosaurs for about 160 million years, as small insect, seed and fruit eaters, and a few of them survived the crisis. With the great reptiles gone, the survivors faced almost no larger competitors or predators.¹²

The Corral Bluffs fossils show a rapid mammal comeback. Within roughly a hundred thousand years mammal diversity had about doubled, and by about 300,000 years the largest mammals were around three times heavier, while the plants lagged behind. Only about 700,000 years after the impact does a flora rich in legumes appear, plants that fix nitrogen and feed the soil. The team argues that this shift in food supply let mammals grow still larger and diversify further, so plants and mammals recovered together. The result is a real-time picture of how a post-catastrophe ecosystem is rebuilt, and of the world in which primates, hoofed mammals and early carnivores began their rise.¹²³