Short answer
- The non-avian dinosaurs went extinct 66 million years ago, at the end of the Cretaceous Period, in the event called the Cretaceous-Paleogene, or K/Pg, extinction.
- Dinosaurs as a group did not entirely die. Birds are living dinosaurs, the surviving branch of the theropod family tree. Every bird alive today is a dinosaur by descent and by classification.
- The main cause was an asteroid about 10 km (6 miles) wide that struck the Yucatán coast of Mexico, leaving the Chicxulub crater. The evidence chain runs from a worldwide layer of iridium-rich clay (1980) to the identification of the crater itself (1991).
- About 75 percent of all species on Earth died, including the pterosaurs, the great marine reptiles, and the ammonites. Small mammals, crocodilians, turtles, frogs, and birds were among the survivors.
- Huge volcanic eruptions in India (the Deccan Traps) were stressing the climate at the same time. Scientists debate how much they contributed, but the consensus is that the impact delivered the decisive blow.
Introduction
The dinosaurs went extinct 66 million years ago. That date, the end of the Cretaceous Period, marks one of the five great mass extinctions in Earth’s history, the event scientists call the K/Pg extinction. But the sentence needs its qualifier immediately, because it is not the whole truth: it was the non-avian dinosaurs that died out. One branch of the dinosaur family tree, the birds, came through the catastrophe, and on the current scientific consensus birds are not just descended from dinosaurs. They are dinosaurs, the one lineage that survived.
This article gives the date and how it is known, walks through the evidence chain that convicted an asteroid, presents the genuine scientific argument about the giant volcanic eruptions that were happening in India at the same time, and finishes with the part of the story that is just as interesting as the dying: what lived.
The Date: 66 Million Years Ago
The Cretaceous Period, the last chapter of the Mesozoic Era, ended 66 million years ago, and the Paleogene Period began. The boundary between them is written in rock worldwide as a thin layer of clay, and everything below that layer belongs to the world of the dinosaurs while everything above it belongs to the world that followed. Non-avian dinosaurs had dominated life on land for well over 160 million years by then.
Older books and some long-standing web pages give the date as 65 million years, and you will still find that figure in places, including older United States Geological Survey material. It is not a different event; it is an older measurement. Modern radiometric dating has tightened the number. As Professor Paul Barrett of the Natural History Museum in London puts it, recent redating has refined the date of the dinosaur extinction to 66.0 million years ago, and the boundary layers can now be dated very precisely. Published argon-argon dating of the boundary puts it at about 66.04 million years, with an uncertainty of only a few tens of thousands of years, a remarkable precision for an event so ancient.
The name of the event encodes its geology. K is the traditional abbreviation for the Cretaceous, from Kreide, the German word for chalk, and Pg stands for the Paleogene Period that followed. You may also see the older name K-T extinction, from when the following period was called the Tertiary.
The Asteroid Evidence Chain
The case against the asteroid was built in stages, and each stage answered the weakness of the one before.
The iridium layer (1980). In 1980, the physicist Luis Alvarez and his son, the geologist Walter Alvarez, reported something strange in the thin clay layer that marks the boundary near Gubbio, Italy: it was rich in iridium. Iridium is one of the rarest metals in Earth’s crust but is far more common in meteorites. The Alvarezes proposed that a large asteroid had struck Earth at exactly the moment of the extinction, scattering iridium-rich debris worldwide. The same iridium spike was then found in boundary layers all over the planet, always at precisely the extinction horizon.
The supporting traces. Around the Gulf of Mexico and the Caribbean, geologists found the debris such an impact should leave: tiny glass spherules, droplets of rock melted by the impact and scattered; grains of shocked quartz, quartz whose internal structure had been deformed by a shockwave far beyond anything volcanoes produce; and deposits consistent with enormous tsunamis around the ancient Gulf coastline.
The crater (1991). For a decade the theory had a predicted culprit but no crime scene. Then geologists identified the Chicxulub crater, centered on the Yucatán Peninsula in Mexico and now buried beneath later sediments, partly under the Gulf of Mexico. Its age matched the boundary. Britannica’s account gives the crater as more than 115 miles (about 180 km) across; some institutional sources put it nearer 150 km, a disagreement about where the disturbed rock ends rather than about whether the crater exists. The asteroid itself is estimated at roughly 10 km (6 miles) wide, with published estimates ranging up to about 15 km. Britannica, using an impact calculator developed by geophysicists, puts the energy of the strike at the equivalent of about 100 trillion tons of TNT.
That chain, a worldwide chemical fingerprint, physical impact debris concentrated around one gulf, and a crater of the right size in the right place at the right age, is why the Alvarez hypothesis, controversial in 1980, is now the most widely accepted explanation of the extinction.
What the Impact Did
The asteroid struck a shallow sea at the edge of the Yucatán, at an estimated 40,000 miles per hour. Near the impact, the destruction was total and immediate: rock vaporized, earthquakes beyond anything in human experience, and tsunamis that Britannica describes as reaching as high as 1,000 feet (305 m) in coastal regions.
The global killing, though, was done by what went up, not by what came down. The impact blasted vast quantities of debris, soot, and sulfur-rich vapor into the upper atmosphere, where it spread around the planet. Daylight faded to a constant twilight that lasted for months and possibly years. With sunlight throttled, photosynthesis collapsed, first on land and then in the surface waters of the oceans. Temperatures dropped. Food chains that began with living plants failed from the bottom up: herbivores starved, and then the predators that ate them starved too. Fires and acid rain added regional misery, but the core mechanism was darkness and cold in a world built on green plants.
This is also why the extinction was selective rather than total. The creatures that died fastest were the ones tied most directly to living vegetation and open sunlight. The creatures with a way to wait out the dark, by burrowing, by living in fresh water, or by eating dead matter and seeds, had a chance.
The Volcano Question: The Deccan Traps
No honest account can leave the asteroid alone on stage, because at almost exactly the same time, one of the largest volcanic episodes in Earth history was under way in what is now central India. The Deccan Traps are the stacked lava flows of eruptions that poured out around the end of the Cretaceous, burying a vast region under basalt. Eruptions on that scale do not just make lava. They pump carbon dioxide and sulfur gases into the atmosphere over long periods, warming and stressing the climate in pulses.
Some scientists argue that this volcanic stress did real damage before the asteroid arrived: that ecosystems were already under pressure from climate swings caused by the eruptions, and that some of the extinction belongs to the volcanoes. Others, dating the lava flows ever more precisely, have argued over whether the largest eruptions came mostly before the impact or continued after it. The Natural History Museum’s summary frames the relationship this way: the last dinosaurs were living at a time of genuine environmental change, some of it driven by the Deccan eruptions, and the asteroid was the final, killer blow.
A major review of the evidence by Brusatte and colleagues in Biological Reviews reached a similar balance: the abruptness of the dinosaur extinction in the fossil record points to a key role for the impact, while the coarseness of the record makes the volcanoes’ exact contribution hard to test. That is the fair state of the science. The impact as the decisive trigger is the consensus; the size of the Deccan contribution is a live, attributed debate, not a settled number.
What Died, and What Survived
When it was over, roughly three-quarters of life had perished. Britannica puts the toll at between 75 and 80 percent of life on Earth; the Natural History Museum uses about 75 percent of Earth’s animals. The victims included far more than dinosaurs.
Died out entirely: all non-avian dinosaurs; the pterosaurs, the great flying reptiles (close relatives of dinosaurs, but not dinosaurs themselves); the mosasaurs and plesiosaurs, the giant marine reptiles (again, not dinosaurs, despite the popular image); the ammonites, the coiled shellfish that had filled the oceans for hundreds of millions of years; and many kinds of plankton, whose loss tore the base out of ocean food chains. Every land animal heavier than about 25 kilograms (55 pounds) died, according to the Natural History Museum. Many individual bird lineages died too; survival was not guaranteed by feathers.
Survived: a few lineages of birds, who carried the dinosaur line through; the mammals, mostly small at the time, many of them burrowers able to shelter and to eat almost anything; crocodilians; turtles; lizards and snakes; frogs, which came through remarkably well; and much of freshwater life generally, which Britannica notes typically fared better than life on land. Plants were hit hard as forests collapsed, but seeds and pollen could wait out the dark in the soil in a way no large animal could, and flowering plants rebounded to dominate the recovered world.
The pattern in that list is the lesson of the event. Survival had little to do with being impressive. It favored the small, the sheltered, the undemanding eaters, and the creatures of rivers and wetlands over the giants of the open plains.
The Last Dinosaurs
The final non-avian dinosaurs were not sickly remnants. In the rocks of the Hell Creek Formation in Montana and the Dakotas, laid down in the last stretch of the Cretaceous, the classic final cast is all present: Tyrannosaurus, Triceratops (which the Natural History Museum calls one of the last non-bird dinosaurs), horned and duck-billed herbivores, armored ankylosaurs. Recent research reported by Smithsonian Magazine, based on precisely dated fossils from New Mexico, argues that North American dinosaurs were diverse and thriving right up to the moment of the impact, with no sign of a long decline in that record.
That finding is part of the wider argument rather than a contradiction of it. Whether dinosaur diversity was gently falling worldwide, holding steady, or varying by continent in the last ten million years is still debated, and the very last chapter is documented in detail mainly in western North America. What is not debated is the ending: below the boundary layer, non-avian dinosaur fossils are abundant; above it, across the entire world, there are none. No verified non-avian dinosaur fossil has ever been found in rock younger than the boundary.
How Scientists Know the Date
The 66 million year figure is not read off the fossils themselves. It is measured in the rocks around them, mainly by radiometric dating, methods that use the steady decay of radioactive elements as a clock. For the boundary, the workhorse is argon-argon dating: volcanic ash layers and impact-melted rock near the boundary contain minerals whose argon isotope ratios record how long ago they crystallized or were reset by the impact’s heat. Dating laboratories in different countries, working on samples from different continents, converge on the same figure of about 66 million years, which is why the date is trusted.
The boundary layer helps in a second way. Because the iridium-rich clay was deposited worldwide in what was, geologically, an instant, it works as a global marker horizon. A dinosaur fossil found below it anywhere on Earth is older than the impact; the sterile layers above it define what came after. It is the combination of that marker and the radiometric clock that lets scientists say not only when the reign ended, but that it ended everywhere at once.
Frequently Asked Questions
Did all the dinosaurs go extinct?
No. All the non-avian dinosaurs died out, but birds survived, and birds are dinosaurs: they descend from small feathered theropod dinosaurs, and their anatomy and fossil history place them inside the dinosaur family tree. The more than 10,000 species of birds alive today are the surviving dinosaurs. Crocodiles, by contrast, are close dinosaur relatives but are not themselves dinosaurs.
Why do some sources say dinosaurs died out 65 million years ago?
Because that was the best measurement available for many years, and older pages and books, including some long-standing United States Geological Survey material, have not all been updated. Modern radiometric dating has refined the boundary to 66.0 million years ago. It is the same extinction, measured more precisely.
What does K/Pg mean?
It is shorthand for the boundary between the Cretaceous Period and the Paleogene Period that followed. K abbreviates the Cretaceous from Kreide, German for chalk, and Pg stands for Paleogene. The event is also called the end-Cretaceous extinction, and in older writing the K-T event, with T for the Tertiary.
How big was the asteroid that killed the dinosaurs?
About 10 km (6 miles) wide, with published estimates ranging up to roughly 15 km. It struck off the Yucatán Peninsula in Mexico and left the Chicxulub crater, which is on the order of 115 miles (about 180 km) across and now lies buried beneath sediment and the Gulf of Mexico.
Were pterosaurs and the giant sea reptiles dinosaurs?
No. Pterosaurs were flying reptiles closely related to dinosaurs, and mosasaurs and plesiosaurs were marine reptiles of a different lineage. All of them died in the same extinction, which is one reason they are so often bundled into the story, but the extinction was much bigger than the dinosaurs: it removed about three-quarters of all species.
Could a non-avian dinosaur have survived somewhere after the impact?
There is no evidence that any did. No verified non-avian dinosaur fossil has been found in rock laid down after the boundary layer, anywhere in the world, despite two centuries of searching on every continent. A handful of claimed later specimens have all been explained as older fossils eroded out and redeposited in younger rock.
Conclusion
The dinosaurs went extinct 66 million years ago, give or take a sliver of geological time, when an asteroid struck the Yucatán and, in the darkness that followed, about three-quarters of Earth’s species died with them. The date is among the most precisely measured in all of deep-time geology, pinned by radiometric clocks and a worldwide layer of iridium-rich clay. The argument that continues, over how much the Deccan volcanoes had already weakened the world the asteroid hit, is a debate about emphasis, not about the ending itself. And the ending has a footnote that never stops being astonishing: it was not quite the end. The dinosaurs’ smallest feathered branch survived the dark, and it is singing outside your window.
Sources & references
- Encyclopaedia Britannica (britannica.com), "What Happened to the Dinosaurs?": the strike 66 million years ago by an asteroid with the force of about 100 trillion tons of TNT; the crater more than 115 miles across on the Yucatán Peninsula; the asteroid theory first suggested in 1980 and the crater's identification more than a decade later; tsunamis as high as 1,000 feet (305 m); darkness lasting months to possibly years; between 75 and 80 percent of life perishing; small burrowing mammals and freshwater species faring better than most.
- Natural History Museum, London (nhm.ac.uk), "How an asteroid ended the age of the dinosaurs" (Professor Paul Barrett): the Alvarez hypothesis of 1980 and the iridium evidence; the Chicxulub crater centered on the Yucatán Peninsula, exactly the age of the extinction of the non-bird dinosaurs; the asteroid between 10 and 15 km wide and the crater about 150 km in diameter on that account; the redating of the extinction to 66.0 million years ago; about 75 percent of Earth's animals dying; all land animals over 25 kg dying out; birds as surviving dinosaurs; the Deccan Traps' climate effects and the asteroid as the final, killer blow; Tyrannosaurus and Triceratops as classic last non-bird dinosaurs.
- Smithsonian Institution (si.edu), "The Last American Dinosaurs: Discovering a Lost World" exhibition: sixty-six million years ago a massive asteroid slammed into the sea near present-day Mexico, ending the world of the dinosaurs, with the Hell Creek Formation preserving the evidence.
- Smithsonian Magazine (smithsonianmag.com): recent precisely dated research (Naashoibito Member, New Mexico) arguing North American dinosaurs were diverse and thriving right up to the impact, reported with the study authors' own assessment.
- Brusatte, S. L., et al. (2015), "The extinction of the dinosaurs," Biological Reviews (via PubMed, PMID 25065505): non-avian dinosaurs went extinct 66 million years ago, geologically coincident with a large impact during an interval of massive volcanic eruptions; little support for a global long-term decline across non-avian dinosaur diversity; the abruptness of the extinction suggests a key role for the impact; testing the effects of Deccan volcanism is difficult given the fossil record.
- U.S. Geological Survey (usgs.gov), "When did dinosaurs become extinct?": the extinction at the end of the Cretaceous Period after dinosaurs had lived on Earth for about 165 million years; noted in this article as carrying the older 65 million year figure that modern dating has superseded.
