Around 66 million years ago, a gigantic asteroid smashed into Earth and wreaked chaos globally.

Superheated rock from the impact spewed into the air, creating a mushroom cloud that heated Earth's upper atmosphere to a scorching 439 degrees Fahrenheit (226 degrees Celsius). Mile-high tsunami waves rushed through the Gulf of Mexico and disturbed ocean basins half a world away. Fires raged, burning animals and plants to a crisp. Shock waves propagated, blasting everything in their path. And particles from the collision, including sulfur, shot upward, blocking the sun and falling down as acid rain.

In all, 75% of Earth's species went extinct, including the nonavian dinosaurs. So how did some animals ‪—‬ including species of birds, turtles and mammals ‪—‬ survive the catastrophic asteroid collision and its aftermath?

Size, it turns out, played a crucial role. Earth's largest apex predators and herbivores — dinosaurs like Tyrannosaurus rex and Triceratops, and marine reptiles like plesiosaurs and the behemoth Mosasaurus — were doomed from the moment of impact, Kenneth Lacovara, founding executive director of the Edelman Fossil Park and Museum of Rowan University in New Jersey, told Live Science. That's because their enormity meant they were more likely to be harmed during the initial blast, were unable to hide in a safe place during the apocalyptic aftermath, and needed massive amounts of sustenance to survive in a time when food was scarce.

It's not too surprising that size would be linked to survival, Lacovara said. On land, "it's pretty clear in terms of correlation that you have to be small, and you have to be a burrower to get through this event," said Lacovara, who is also a professor of paleontology and geology at Rowan University. Small animals, such as some lizards and mammals, which were no larger than badgers at that time, were more likely to find shelter from the asteroid's immediate and long-term aftereffects. Other small animals that required less food, such as some turtles and fish, were sheltered in the water.

The avian group that led to modern birds survived likely because they were small and had powerful wings and chest muscles that allowed them to fly well and escape dire situations or find new opportunities. Their chicks also grew quickly, meaning they could soon fend for themselves and not overtax their parents.

This led to an overall shift in the average size of Earth's animals. On land, most of the largest surviving animals were about the size of house cats; in the water, the largest survivors were about the size of a "run-of-the-mill shark," Lacovara said.

What the survivors ate

Diet was likely another important factor, said Roger Benson, a curator of dinosaur paleobiology at the American Museum of Natural History in New York City. The plant eaters and those that ate them were especially hard-hit, as the sun was obstructed for up to a decade. Even with size on their side, some smaller creatures, such as certain lizard and turtle species, went extinct because their diets were too reliant on photosynthesizing plants, Benson said.

Aquatic ecosystems were a bit more shielded from the asteroid's initial impact, especially in deeper oceans and freshwater ecosystems. But as photosynthesizing plankton died off due to a lack of sunlight, food systems collapsed and large marine animals starved. Those that consumed dead, organic detritus had a better chance of survival. Some of those resilient marine creatures included the sea sponges, sharks of the Carcharias genus, and mollusks, including the lineage that led to today's chambered nautilus (Nautilus pompilius).

Seed eaters, including birds, and foraging insectivores, such as the tree-dwelling primate Purgatorius janisae, had a better shot at surviving because their food sources of seeds and insects weren't destroyed by extreme temperature changes and a lack of sunlight.

A common theory is that species with more generalist diets had a better chance of surviving the drastic environmental change. For example, Purgatorius coracis, a small mammal, survived the mass extinction thanks to their wide-ranging diet of insects, fruits and seeds. (This trend holds true for modern animals facing climate change: Those with generalist diets, such as crows and raccoons, are expected to do well because they have a wide range of foods to fall back on in case one food source disappears.)

Some species got lucky if their prey were also survivors. A few aquatic turtle species, like Hutchemys rememdium, had an adaptive feeding habit of eating shelled creatures which were living off detritus in aquatic ecosystems. A 2026 study found this adaptation, called durophagy, was linked to comparatively high survival rates during this mass extinction.

Certain behaviors, such as a higher capacity for reproduction or behavioral flexibility in a changing environment, may have enabled survival Benson said.

Mysterious exceptions

Despite current evidence, there are still unknowns surrounding mass extinction survivorship. Recent research indicates we don't know why certain adaptations helped some species but not others.

For example, many present-day species of bivalves feed on microscopic aquatic organisms that rely on the sun to survive. However, among the aquatic bivalves that survived the asteroid and its impacts, reliance on the sun didn't strongly determine survivorship. In another case, night lizards that survived near the impact site are known to have small litters, which goes against the hypothesis that fecundity is advantageous following extinctions. However, these night lizards' slow metabolisms were likely helpful for their survival, and they persist today, the study noted.

And the discovery that a large, terrestrial croc (Tewkensuchus salamanquensis) survived the asteroid strike in what is now Argentina raises questions about how this 660-pound (300 kilogram) species survived. This also raises questions whether the asteroid's impacts weren't as strong in the Southern Hemisphere as they were in the Northern Hemisphere. For example, the wide diversity of plant fossils uncovered in present-day Argentina suggests plants were able to regrow faster at southernmost latitudes.

"We also don't know why mammals emerged from the extinction as the dominant macrofauna," Lacovara said. One theory is that mammals are more resistant to fungal infections than reptiles are, giving mammals a better shot at surviving the "mini age of fungus" that followed the mass extinction.

After all, how evolution proceeded following the mass extinction is a part of humanity's history. The animals that survived ultimately ushered in the age of mammals, including humans, that continues to this day.

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