What Caused the Dinosaur Extinction?
66 million years ago, a large asteroid crashed into Earth — causing the fifth mass extinction event and giving rise to the Age of Mammals.
Earth has experienced five events when at least 75% of all species died out over a short amount of geological time. The most recent of those ‘Big Five’ mass extinctions occurred 66 million years ago (MYA). While famous for killing the dinosaurs, the fifth mass extinction also shaped the wildlife we see today.
While there are several controversial explanations, the leading theory for the actual ‘kill mechanism’ is that a 10km-wide asteroid smashed into Earth, leaving a crater 180km across at Chicxulub in Mexico. A thermal pulse from heated debris spread across the globe, while the force of collision — equivalent to millions of atomic bombs — created shockwaves that triggered earthquakes, then tsunamis and wildfires.
The impact launched a vast dust cloud into the atmosphere, which coincided with gas released from volcanic activity and blocked out the sun for years. That led to nuclear winter and limited photosynthesis, which ultimately powers most life, and that in turn caused a collapse of our planet’s ecosystems.
So, you ask your self did all dinosaurs go extinct after all this? Not quite — because birds are living dinosaurs! To distinguish birds — the class Aves — from prehistoric reptiles, extinct groups are known as ‘non-avian dinosaurs. They were the dominant terrestrial vertebrates of the Mesozoic era, throughout the Triassic, Jurassic, and Cretaceous periods.
Because the fifth extinction occurred at the boundary between the Cretaceous (K) and subsequent Paleogene period, it’s also called the end-Cretaceous or K-Pg event. Other reptiles commonly mistaken for dinosaurs went extinct during the event, including the winged pterosaurs and aquatic plesiosaurs, and mosasaurs.
Mass extinctions aren’t random, they’re selective, and there are clues that help explain why some species were survivors while others were victims. One contributing factor is characteristics associated with feeding and diet. For example, after the fifth extinction, natural selection was biased against large bony fishes with fast-closing jaws.
An ability to use available food sources could also be the reason why some birds survived when all other dinosaurs died out: the ancestors of modern birds (‘neornithines’) had solid beaks instead of tiny teeth, which probably enabled them to break-open seeds.
Life has always bounced back from mass extinctions eventually, as the surviving groups colonize the ecological niches left vacant by their extinct competitors or predators. Although all classes of vertebrates recovered in the aftermath of the fifth extinction, mammals achieved the greatest success at taking advantage of the dinosaurs’ demise by evolutionary radiation — a rapid rise in diversity that gave the ongoing Cenozoic era its nickname, the Age of Mammals.
Scientists believe mammals became successful because they’re generally smarter than other animals but, for 10 million years following the fifth event, body size ballooned from small shrew-like forms. So at least at first, it was brawn before brains.
