How Size, Diet and Burrowing Helped Some Animals Survive the Dinosaur-Killing Asteroid

About 66 million years ago, an asteroid struck Earth, and within hours the planet was transformed. Superheated rock thrown up by the impact formed a mushroom cloud that pushed temperatures in the upper atmosphere to 439 degrees Fahrenheit (226 degrees Celsius). Tsunamis a mile high swept the Gulf of Mexico and disturbed ocean basins half a world away. Fires burned plants and animals alike, shock waves flattened what lay in their path, and sulfur and other particles rose high enough to block the sun before falling back as acid rain.

Seventy-five percent of Earth's species did not survive, the nonavian dinosaurs among them. Some birds, turtles and mammals did. A feature from Live Science, drawing on two paleontologists, looks at what separated the two groups, and the first answer is size.

Kenneth Lacovara, founding executive director of the Edelman Fossil Park and Museum at Rowan University in New Jersey, says the largest animals were doomed from the moment of impact. That meant Tyrannosaurus rex and Triceratops on land, and plesiosaurs and the huge Mosasaurus in the sea. Their bulk made them likelier to be hurt in the initial blast, left them unable to hide through the aftermath, and demanded enormous amounts of food when little was available. On land, Lacovara says, the pattern is clear: to get through the event, you had to be small and you had to burrow. Some lizards and mammals, none larger than a badger, could find shelter. Small turtles and fish that needed less food were protected in water. The bird lineage that led to modern birds likely survived because its members were small, strong fliers with powerful chest muscles, and because their chicks grew fast enough to fend for themselves without overtaxing their parents.

The result was a shift in the typical size of animals. On land, most of the largest survivors were about the size of house cats. In the water, Lacovara says, the largest were about the size of a run-of-the-mill shark.

Diet mattered too, says Roger Benson, a curator of dinosaur paleobiology at the American Museum of Natural History in New York. With the sun obscured for up to a decade, plant eaters and the animals that preyed on them were hit hardest. Even small size was not enough for some lizards and turtles that depended too heavily on photosynthesizing plants. In the water, the die-off of plankton collapsed food webs and starved large marine animals. Creatures that fed on dead organic detritus fared better, including sea sponges, sharks of the genus Carcharias, and mollusks such as the lineage leading to today's chambered nautilus. Seed eaters, including birds, and foraging insect eaters such as the tree-dwelling primate Purgatorius janisae also had an edge, since seeds and insects were not wiped out by the temperature swings and darkness.

Generalists did well. Purgatorius coracis, a small mammal, ate insects, fruits and seeds. The same logic is expected to hold for modern animals facing climate change, with crows and raccoons likely to cope. Luck played a part as well. Aquatic turtles such as Hutchemys rememdium ate shelled animals that lived on detritus, a habit called durophagy that a 2026 study linked to relatively high survival. Benson adds that greater reproductive capacity or behavioral flexibility may have helped.

Some cases do not fit. Many modern bivalves feed on microscopic organisms that depend on sunlight, yet among bivalves that survived, sun reliance did not strongly predict survival. Night lizards that lived through the event near the impact site have small litters, which cuts against the idea that fast reproduction helps after extinctions, though their slow metabolisms probably helped. A 660-pound (300 kilogram) land crocodile, Tewkensuchus salamanquensis, survived in what is now Argentina, raising the possibility that the impact's effects were milder in the Southern Hemisphere. Diverse plant fossils there suggest vegetation regrew faster at southern latitudes.

Lacovara notes that nobody knows why mammals emerged as the dominant larger animals. One idea is that mammals resist fungal infection better than reptiles, which would have helped through the "mini age of fungus" that followed. What came after, the age of mammals that includes humans, is where the story leads.