Our planet also lost the ammonites and our mighty marine reptiles — mosasaurs, ichthyosaurs and plesiosaurs.
Never again would pterosaurs, flying reptiles, cruise our skies. Their departure gave rise to the age of mammals and the diversity we see today.
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.
In 2016, scientists drilled a rock core inside the underwater part of Chicxulub, pulling up a sample stretching deep beneath the seabed. This rare peek inside the guts of the crater showed that the impact would have been powerful enough to send deadly amounts of vaporized rock and gases into the atmosphere and that the effects would have persisted for years.
And in 2019, palaeontologists digging in North Dakota found a treasure trove of fossils extremely close to the K-Pg boundary, essentially capturing the remains of an entire ecosystem that existed shortly before the mass extinction. Tellingly, the fossil-bearing layers contain loads of tiny glass bits called tektites—likely blobs of melted rock kicked up by the impact that solidified in the atmosphere and then rained down over Earth.