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= Why did the dinosaurs go extinct? =

Learn about the mass extinction event 66 million years ago and the evidence for what ended the age of the dinosaurs.
Abundant fossil bones, teeth, trackways, and other hard evidence have revealed that Earth was the domain of the dinosaurs for at least 230 million years. But so far, not a single trace of dinosaur remains has been found in rocks younger than about 66 million years. At that point, as the Cretaceous period yielded to the Paleogene, it seems that all nonavian dinosaurs suddenly ceased to exist.

Along with them went fearsome marine reptiles such as the mosasaurs, ichthyosaurs, and plesiosaurs, as well as all the flying reptiles known as pterosaurs. Ancient forests seem to have flamed out across much of the planet. And while some mammals, birds, small reptiles, fish, and amphibians survived, diversity among the remaining life-forms dropped precipitously. In total, this mass extinction event claimed three quarters of life on Earth.

Over a thousand dinosaur species once roamed the Earth. Learn which ones were the largest and the smallest, what dinosaurs ate and how they behaved, as well as surprising facts about their extinction. Piecing together what happened has been a massive effort for paleontologists, and theories for what killed the dinosaurs and the rest of the planet’s Cretaceous inhabitants have ranged from the plausible to the downright zany. For now, two leading ideas are battling it out within the scientific community: Were dinosaurs victims of interplanetary violence, or more Earthly woes?

Death from above One of the most well-known theories for the death of the dinosaurs is the Alvarez hypothesis, named after the father-and-son duo Luis and Walter Alvarez. In 1980, these two scientists proposed the notion that a meteor the size of a mountain slammed into Earth 66 million years ago, filling the atmosphere with gas, dust, and debris that drastically altered the climate.

Their key piece of evidence is an oddly high amount of the metal iridium in what’s known as the Cretaceous-Paleogene, or K-Pg, layer—the geologic boundary zone that seems to cap any known rock layers containing dinosaur fossils. Iridium is relatively rare in Earth's crust but is more abundant in stony meteorites, which led the Alvarezs to conclude that the mass extinction was caused by an extraterrestrial object. The theory gained even more steam when scientists were able to link the extinction event to a huge impact crater along the coast of Mexico’s Yucatán Peninsula. At about 93 miles wide, the Chicxulub crater seems to be the right size and age to account for the dino die-off.

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