Most animals died, but the survivors were small, adaptable, and able to survive months without food. The asteroid impact 66 million years ago (the K-Pg extinction event) killed roughly 75% of all species on Earth in a matter of weeks to months. But some lineages made it through.
Birds — This is the big one. Modern birds ARE dinosaurs — they descended from theropod dinosaurs and survived the impact. Smaller species with lower metabolic needs and the ability to fly to less-affected regions had the best odds. Flying gave them access to resources and escape routes that ground-bound animals could not reach.
Mammals — Small, nocturnal, burrowing mammals survived. They could hide underground, required less food, and were already adapted to darkness (useful when an impact blocks out the sun for months). Every mammal alive today — including humans — descends from these survivors.
Crocodiles and Alligators — Semi-aquatic reptiles with slow metabolisms that can survive long periods without food. They hunted in murky water where they were shielded from the worst effects of the impact winter.
Turtles and Snakes — Many reptile lineages survived, particularly those that could burrow or were aquatic. Sea turtles survived in the oceans.
Fish and Sharks — Aquatic vertebrates, especially in deep water and ocean regions, fared better than terrestrial animals. Ocean acidification and disrupted food chains were problems, but less immediately lethal than the atmospheric collapse on land.
Amphibians — Frogs and salamanders survived, especially those in aquatic or burrowing niches. Their small size, low metabolic rate, and ability to survive in water or underground gave them advantages.
Insects — Insects were devastated but not eliminated. Beetles, flies, and other insects that could feed on decaying plant matter or survive dormant in soil and wood made it through.
The impact itself vaporized rock and created a massive fireball that ignited wildfires globally. Then came impact winter — dust and soot in the atmosphere blocked sunlight for months, dropping temperatures below freezing even at the equator. Photosynthesis stopped. Plants died. Herbivores starved. Carnivores starved chasing herbivores. Large animals with high metabolic needs and long generation times (like non-avian dinosaurs) could not survive the bottleneck.
Pro tip: The survival of birds is why we still have 10,000+ bird species today — they are living dinosaurs. The next time you see a chicken, a pigeon, or an eagle, remember: that animal is the direct descendant of theropods that outlasted an extinction event that killed 75% of all life on Earth.