The Cretaceous-Tertiary extinction was caused by a massive asteroid impact on the Yucatan Peninsula, killing off all non-avian dinosaurs and devastating life on land and sea. Birds and mammals thrived in the aftermath.
The Cretaceous-Tertiary extinction event was the second largest mass extinction of all time, but substantially closer in magnitude to the third, the Ordovician extinction, than the first, the Permian-Triassic extinction. Occurring about 65.5 million years ago, the Cretaceous-Tertiary extinction event is thought to have been caused by a massive asteroid impact on the Yucatan Peninsula in present-day Mexico.
The asteroid thought to have caused the Cretaceous-Tertiary extinction event was about 10 km (6 mi) in diameter and left a crater at least 180 km (110 mi) wide. The asteroid, traveling at about 17 km/s, arrived at a sharp angle as it traveled northwest. The resulting impact would have ignited every tree within approximately 800 miles of the point of impact and ejected more than 80 cubic miles of molten rock in the direction of the North American continent.
Within an hour or so, this would have rained molten fire over most of what is now the United States, directly killing many organisms. Tiny dust particles would rise into the stratosphere and stay there, blocking out the sun for up to a decade. This severely disrupted photosynthesis and caused the death of many plants.
The Cretaceous-Tertiary extinction devastated life on land and sea. In the oceans, half of all diatoms have been washed away, along with numerous benthic foraminifera (important marine microbes). Animals that depended on primary production (plants and algae) died out preferentially, while omnivores, detrivores and fungi survived. Numerous groups of cephalopods went extinct, including all ammonites and belemnoids, iconic marine animals of the Mesozoic. Rudists, the reef-building clams, went extinct, as did about 20% of the shark family and 30% of the echidnoderms (starfish and relatives). Mosasaurs and plesiosaurs, two gigantic varieties of reptilian marine predators, also went extinct during the Cretaceous-Tertiary extinction event.
Terrestrial animals suffered the greatest losses. All non-avian dinosaurs went extinct, including the theropods (flesh eaters like T. Rex), sauropods (huge quadrupeds like Brachiosaurus), and ornithischians (the rest, including herbivores like Triceratops). The reason for their extinction is probably threefold: their large size made them dependent on the availability of plenty of food, they lacked the ability to dig, swim or dive, meaning they had nowhere to hide from the worst environmental stresses, and were dependent on largely on primary production (plants), mostly extinct due to sun blockage.
Meanwhile, in the wake of the extinction of the dinosaurs, two groups began to diversify and thrive: birds, which descended from dinosaurs, and mammals, which had already existed for tens of millions of years as small insectivores and omnivores that roamed the shadows of dinosaurs.
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