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. 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. 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. 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. 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. 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 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. Learn which ones were the largest and the smallest, what dinosaurs ate and how they behaved, as well as surprising facts about their extinction. Over a thousand dinosaur species once roamed the Earth.
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