Extinction Theories We’re going to take a brief break from exhibit spotlights, then cover our Argentine dinosaurs during the month of September. This week we cover one of the most frequently asked questions about dinosaurs: how did they all die? Science does not yet have a definitive answer, but plenty of theories have been suggested—some much more plausible than others.

• Asteroid Impact: Most people seem familiar with the basics of this one. As a hypothesis it predicted evidence of a colossal meteoroid hitting the earth roughly 66 million years ago. Several lines of evidence do indeed support the idea that a large asteroid did just that. First, geologists discovered an impact crater of the right age on the edge of the Yucatan Peninsula in the late 1970s. Since its center lay near the town of Chicxulub (chick-shoe-lube), the crater and the 6-mile wide asteroid that caused it are also called Chicxulub. Second, shocked quartz deposits many miles from the crater give evidence of an impact so tremendous that the biggest known volcanic eruption delivered less than one quarter of one percent of the energy generated by the Chicxulub impact. Third, deposits of the same age as the impact contain a metal common to asteroids but rare on Earth: iridium. Layers of clay found at the boundary between Cretaceous rocks and Tertiary rocks exhibit as much as 1000 times the amount of iridium as the surrounding layers.

As bad as all that sounds, it gets worse: Chicxulub may have travelled with friends. Other impact craters from similar strata show that a number of cataclysmically large collisions may have followed Chicxulub like aftershocks, exacerbating an already dire ecological situation. Most scientists now agree that a catastrophic asteroid event must have played a significant role in the extinctions that doomed the rule of reptiles and paved the way for the age of mammals. It may not have doomed the dinosaurs completely, though: fossil evidence suggests the possibility (not proof) that some populations of dinosaurs may have survived as many as 2 million years after the impact.

• Extreme Volcanic Activity: a little over 66 million years ago, volcanoes in India started erupting like crazy, forming one of the largest geologic formations of volcanic material in the world. Called the Deccan Traps, the eruptions continued for roughly 30,000 years. Remember how the eruption in Iceland a few years ago disrupted much of Europe, or how ash from Mt. St. Helens darkened skies across the west? Now picture that sort of thing more or less constantly for a period longer than the history of humanity for an illustration of the effect that might have had on the ecology of the world at the time.
• Fluctuating Ocean Levels: the Cretaceous extinction features the deaths of a bizarre array of animal groups that survived plenty of prior mass extinction events on both land and in the ocean. A clear drop in ocean levels during the last few million years of the Cretaceous could explain extinctions on both land and sea during that period. The great plains, for example, formed the bed of a shallow seaway that spread across North America throughout the Cretaceous. Eastern Europe formed an archipelago. All those shallow seas teemed with life, and while they had always fluctuated levels somewhat, they finally dried out for good towards the end of the Mesozoic. This deprived many of the classic seagoing reptiles and other animals dependent on those seas of vital habitat, but it also would have drastically altered climate and the ecology of several continents, killing off terrestrial biomes as well.

Dozens of other theories have been put forward over the years—too many to fit in here. These represent the strongest currently accepted candidates for the smoking gun at the end of the Cretaceous, though in reality they all probably worked in concert to spell the dinosaurs’ doom, as well as that of roughly three-quarters of all species living at that time. Horrific as that sounds, it was only the third worst mass extinction on record: the worst came at the end of the Permian and killed off an estimated 83% of all species. That event has proven even more mysterious than the more famous K-T extinction we’ve just discussed. —Jeff Bond