The story of the end-Cretaceous mass extinction is, at its core, the story of a single day — arguably the worst day in the history of complex life on Earth. On a morning 66 million years ago, a rock approximately 10–15 kilometres across, travelling at roughly 20 kilometres per second, entered Earth's atmosphere over what is now the Yucatán Peninsula of Mexico.
The Impact
The Chicxulub impactor — named after the town near the impact site — released energy estimated at 100 teratonnes of TNT, roughly ten billion times the energy of the atomic bomb dropped on Hiroshima. The impact excavated a crater approximately 180 kilometres in diameter and 20 kilometres deep. The rock that hit the seabed vaporised instantly; the resulting explosion expelled billions of tonnes of material into the stratosphere in seconds.
The immediate effects were catastrophic and essentially instantaneous across a wide radius. Mega-tsunamis hundreds of metres tall swept across coastlines. Wildfires ignited across North America from the thermal pulse — essentially a global oven effect as superheated ejecta re-entered the atmosphere. Earthquakes of magnitude 11 or greater shook every continent.
The Chicxulub crater was only definitively identified in 1991 by geophysicists Alan Hildebrand and William Boynton, following earlier work by Glen Penfield. For years, it had been overlooked because it was buried beneath a kilometre of sediment and partly submerged in the Gulf of Mexico.
The Long Night
The immediate violence, however catastrophic, was not what killed the majority of species. What followed was arguably worse: the impact winter. Billions of tonnes of sulphur dioxide (from the sulphate-rich Yucatán limestone), soot from global wildfires, and pulverised rock dust were injected into the stratosphere, where they spread globally within weeks.
The resulting aerosol layer reflected incoming sunlight with extreme efficiency. Global temperatures dropped by an estimated 10–15°C within months. Photosynthesis collapsed across much of the planet as sunlight was blocked. The food web unravelled from the bottom up: phytoplankton died, zooplankton followed, and the collapse cascaded upward through every ecosystem on Earth.
What Survived — and Why
Approximately 75% of all species on Earth went extinct at the K-Pg boundary. The pattern of survival is instructive. Animals that survived tended to share certain characteristics: small body size, ability to burrow or shelter, dietary flexibility (able to eat detritus, seeds, or insects rather than specific plants), and the ability to enter dormancy.
Crocodilians survived. Turtles survived. Small mammals survived. Birds — the avian dinosaurs — survived, specifically the lineages that were ground-nesting seed-eaters. Freshwater ecosystems, buffered by water's thermal stability and the availability of organic material from dead terrestrial vegetation, retained higher diversity than terrestrial communities.
The large, specialised, warm-blooded non-avian dinosaurs — dependent on specific food sources, unable to burrow or enter dormancy, requiring enormous caloric intake — had no realistic survival pathway. The last non-avian dinosaurs were almost certainly dead within months of impact, not millennia.
Was the Asteroid Alone?
Modern research suggests the Chicxulub impact delivered the fatal blow, but it may not have been striking a perfectly healthy patient. The Deccan Traps — a massive volcanic province in what is now India — had been erupting for around 500,000 years before the impact, releasing CO₂ and sulphur dioxide that were already stressing global ecosystems. The most recent research suggests that while the Deccan Traps contributed to background stress, the mass extinction itself correlates precisely and specifically with the impact — the Deccan Traps alone would likely not have caused it.
What we know with certainty is this: the Chicxulub impactor ended 165 million years of dinosaur dominance within a geological instant, and opened the ecological space that allowed our own mammalian ancestors to diversify and, eventually, to ask questions about what killed the dinosaurs.