Sixty-six million years ago, the reign of the dinosaurs, which had dominated Earth for an incredible 180 million years, came to a cataclysmic end. This abrupt extinction event, one of the most dramatic shifts in our planet’s history, is now widely attributed to a devastating asteroid impact. But where exactly did this world-altering meteor strike?
Professor Paul Barrett, a leading dinosaur researcher at the Natural History Museum, sheds light on the events of that fateful day and the location of the impact that changed life on Earth forever.
The Asteroid Theory: Evidence of a Cosmic Collision
The theory that an asteroid impact led to the dinosaur extinction gained prominence in 1980, thanks to the groundbreaking work of Nobel Prize-winning physicist Luis Walter Alvarez and his geologist son, Walter. They proposed that a unique layer of clay, rich in the rare element iridium, found globally at the geological boundary marking the end of the Cretaceous period, was evidence of a massive extraterrestrial impact. Iridium is scarce in Earth’s crust but more common in asteroids and meteorites.
This hypothesis suggested that the immediate destruction at the impact site and the subsequent global environmental fallout caused by the asteroid were responsible for the sudden disappearance of the non-avian dinosaurs and numerous other life forms. While initially met with skepticism, the asteroid impact theory has become the most accepted explanation for the mass extinction that marks the end of the Mesozoic Era.
Luis Walter Alvarez and his son Walter Alvarez, proponents of the asteroid impact theory.
Asteroids are essentially large space rocks, remnants from the early solar system, orbiting the Sun. These rocky bodies can vary in size from just a few meters to hundreds of kilometers in diameter. When an asteroid fragment survives its fiery descent through Earth’s atmosphere and strikes the ground, it is then known as a meteorite.
Unearthing the Crater: Pinpointing the Impact Location
‘The asteroid impact theory is strongly supported by compelling evidence, most notably the discovery of the impact crater itself,’ explains Professor Barrett. ‘This massive crater, now largely submerged beneath the seafloor off the coast of Mexico, dates back precisely to the time of the non-avian dinosaur extinction, a timeline consistently recorded in rock layers across the globe.’
This impact crater, named the Chicxulub crater, is centered on the Yucatán Peninsula in Mexico. Its discovery provided the crucial missing piece of the puzzle, confirming the location of the dinosaur-killing meteor’s landing site.
Scientists estimate that the asteroid itself was a colossal object, likely between 10 and 15 kilometers in width. However, the immense energy released upon impact, due to its high velocity, carved out a far larger crater, measuring approximately 150 kilometers in diameter. This makes the Chicxulub crater the second-largest confirmed impact crater on Earth, a scar left by an event of unimaginable magnitude.
Pieces of iridium, an element linked to extraterrestrial impacts and found in the geological layer coinciding with the dinosaur extinction.
The impact at Chicxulub unleashed catastrophic forces. Vast quantities of rock and debris were ejected into the atmosphere, while colossal tsunamis surged across the ancient oceans, inundating coastlines as far away as the American continents. Geological records from this period also reveal widespread evidence of massive wildfires that ignited across the planet in the immediate aftermath of the impact.
For many years, the dinosaur extinction was believed to have occurred 65 million years ago. However, advancements in radiometric dating techniques have refined this timeline.
Professor Barrett notes, ‘The dating of the iridium-rich clay layers found worldwide is remarkably precise, accurate to within a few thousand years. Recent redating efforts have further refined the extinction event’s timing, now placing the dinosaur extinction at 66.0 million years ago.’
The Mechanics of Extinction: How a Meteor Caused Global Devastation
Around 75% of all plant and animal species on Earth, including the non-avian dinosaurs, vanished in this relatively short geological timeframe. How could a meteor impact on the coast of present-day Central America trigger a global mass extinction?
Professor Barrett elucidates the chain of events: ‘The asteroid struck Earth with incredible velocity, essentially vaporizing upon impact. This created an enormous crater and unleashed immediate, localized devastation. A powerful blast wave and intense heatwave radiated outwards, and colossal amounts of pulverized rock and dust were hurled into the upper atmosphere.’
‘This ejecta included vast quantities of soot from global wildfires, which spread around the planet. While the sunlight wasn’t completely blocked, the amount of solar radiation reaching Earth’s surface was significantly reduced. This had a profound impact on plant photosynthesis and growth.’
Artist's depiction of the asteroid impact on Earth, illustrating the immediate shockwave and ejecta plume.
This reduction in plant life initiated a cascading collapse of the food chain. Herbivorous dinosaurs, deprived of their primary food source, struggled to survive. Consequently, carnivorous dinosaurs, dependent on herbivores for prey, also faced starvation.
Breeding seasons likely became shorter and environmental conditions more hostile. Every living organism, both on land and in the oceans, would have been affected in some way by the disruption.
‘The precise mechanisms of extinction and the duration of this extinction period are still subjects of ongoing scientific debate, with many aspects remaining unknown. However, it is clear that this was a truly massive event that impacted all life on Earth, from microscopic organisms to the mighty dinosaurs,’ Professor Barrett concludes.
The list of casualties from this extinction event is extensive. Ammonites, various types of plankton, and large marine reptiles all perished. However, this devastating loss paved the way for the rise of the modern world and the diversification of surviving lineages.
The Role of Climate Change and Volcanic Activity
While the asteroid impact is the primary driver of the dinosaur extinction, it’s important to acknowledge that Earth was already experiencing significant environmental changes prior to the impact. This pre-existing instability likely exacerbated the effects of the asteroid.
During the Late Cretaceous period, massive volcanic eruptions were occurring in what is now central India, forming a geological feature known as the Deccan Traps. This volcanic activity, unrelated to the asteroid impact, released enormous quantities of greenhouse gases into the atmosphere, disrupting global climate patterns.
Professor Barrett explains, ‘For approximately two million years leading up to the extinction, there was intense volcanic activity, spewing gases into the atmosphere and significantly impacting global climate.
‘Longer-term geological processes were also at play. The continents were in motion, drifting apart and altering ocean basins, which in turn influenced global ocean currents and atmospheric circulation. These large-scale changes had substantial effects on climate and vegetation patterns.’
A selection of ammonite fossils from the Natural History Museum collection, representing a group that went extinct during the K-Pg event.
The last non-avian dinosaurs lived during a period of considerable environmental upheaval, some of which began millions of years before the asteroid impact. The Chicxulub impact served as the final, decisive blow that pushed already stressed ecosystems over the brink.
Survivors and the Dawn of a New Era
During the Cretaceous-Paleogene (K-Pg) extinction event, plant life proved more resilient than animal life. Plant seeds and pollen possess greater longevity and can endure harsh environmental conditions for extended periods. Following the dinosaur extinction, flowering plants rapidly diversified and became the dominant plant group on Earth, continuing a trend that had begun in the Cretaceous. However, among land animals, any species weighing over 25 kilograms perished.
‘The survivors essentially represented the foundation for the modern fauna we see today. Many of the major animal groups present today were already in existence before the asteroid impact, and while they experienced varying degrees of extinction, the lineages that ultimately led to modern animals managed to persist,’ says Professor Barrett.
‘All non-avian dinosaurs became extinct, but dinosaurs did survive in the form of birds. While some bird lineages also went extinct, the ancestors of modern birds persevered.’
Initially, the surviving animal groups were small in size. Birds were among the first to evolve towards larger body sizes after the extinction event.
Although some lineages of gigantic, flightless birds emerged – both predatory and herbivorous forms – these did not persist for long and eventually also went extinct.
Fossilized skull of Diatryma, an extinct giant flightless bird from the Eocene Epoch, showcasing the evolution of large birds after the dinosaur extinction.
‘It wasn’t until approximately 15 million years after the non-avian dinosaurs disappeared, during the Oligocene Epoch, that truly large mammals began to reappear. This period marks the re-emergence of rhino-sized animals and other megafauna. In the interim, the world was largely populated by smaller animals, a stark contrast to the dinosaur-dominated world that preceded it. It took considerable time for body size to catch up in the aftermath of the extinction.’
Dinosaurs remain the largest land animals to have ever lived. Only whales, in the marine realm, have surpassed their size.
What If? Could Dinosaurs Have Survived?
Intriguingly, recent research suggests that the location of the asteroid impact played a crucial role in the severity of the extinction event. Had the asteroid struck a different location on Earth, the outcome for life on our planet might have been significantly different. If the impact had occurred just minutes later in its trajectory, it would have landed in deeper ocean waters. This scenario could have resulted in less vaporized rock being ejected into the atmosphere, potentially reducing the extent of sunlight blockage and diminishing the severity of the mass extinction.
Considering this, Professor Barrett speculates on the fate of dinosaurs if the asteroid had missed Earth entirely.
A Triceratops skeleton, representing one of the last non-avian dinosaurs and prompting contemplation about dinosaur survival without the asteroid impact.
‘I suspect that some non-avian dinosaur lineages might still be around today. Our knowledge of the final 10 million years of their reign is limited, primarily based on fossil records from western North America. This region provides excellent fossil evidence of iconic last non-avian dinosaurs like Tyrannosaurus and Triceratops.
‘In this part of the world, dinosaur populations appeared to be thriving in terms of numbers, although the diversity of dinosaur species may have been declining. Whether this pattern held true globally remains a significant mystery.’
Without the asteroid impact, dinosaurs might have persisted for longer. However, with the rise of modern birds, mammals, and reptiles already underway, their dominance might have gradually waned over time, even without the catastrophic cosmic collision at Chicxulub.
