scienceWatch / Saul Scheinbach
What Really Happened to the Dinosaurs?*
“Now in New Mexico we have fossils of dinosaurs that were there right at the end and when we compare them with the only other fossils accurately dated from this time, from further north, we can see they are much different. There clearly were many types of dinosaurs thriving up until that moment the asteroid ended it all.” – Steve Brusatte
Dinosaurs arose about 230 million years ago (MYA) and dominated the planet for over 160 million years. But they vanished 66 MYA marking the end of the Cretaceous period (146-66 MYA). In the early 1900’s scientists identified a geological “boundary layer” found worldwide and later dated to 66 MYA. Below this layer many dinosaur fossils exist. Above it zero. The explanation for their demise was that intense volcanic activity occurring then caused global warming and acid rain which gradually killed off the dinosaurs because they couldn’t adapt. Even today we call someone a “dinosaur” to mean he can’t adapt to change.
That changed in 1980 when researchers assayed the boundary layer for iridium, a rare element on earth but more plentiful in extraterrestrial objects like dying stars and asteroids. “Cosmic dust” forms from these objects and falls to earth at a constant rate. So, they hoped to use iridium levels as a clock to see how long the extinction process took. To their surprise iridium levels were 200-fold more than expected and they concluded that rather than dinosaurs failing to adapt to climate change, they were wiped out by a large asteroid hitting the planet. Others showed that the boundary layer contained shocked quartz, tektites, glass spherules and charcoal, indicative of cataclysmic high heat, pressure and fire. Debate raged between proponents of the “impact theory” and the “volcano theory” until 1991 when geologists confirmed the existence of a huge crater in the Caribbean just off the coast of the Yucatan peninsula. This brought many paleontologists into the impact camp. But others maintained it was just the final blow to a group already in decline.
The impact was catastrophic. The 10-mile-wide asteroid—the size of Manhattan—hit the earth with the force of 10 billion Hiroshima atom bombs, heating the site to 10,000 degrees F, which instantly vaporized the asteroid and surrounding rock. Any life within 1,000 miles was incinerated. Initially earthquakes and 1,000-foot-high tsunami waves destroyed coastlines. Within hours the vaporized rock cooled as it rose, solidifying into trillions of spherules, which fell back to earth as shooting stars, heating the atmosphere and causing global forest fires. Worst of all, dense clouds of dust and ash enveloped the earth blotting out the sun for almost two years. Photosynthesis stopped, causing ecosystem collapse. Seventy-five percent of all species perished including all the dinosaurs.
Most paleontologists now agree that the dinosaurs didn’t slowly die out but were flourishing until the impact wiped them out. However, some still disagree, saying only one region located in the badlands of Montana and Wyoming contains thriving dinosaurs and has been dated to the end of the Cretaceous. What about other sites? Another area in New Mexico had a different flourishing dinosaur population but was never precisely dated. Writing in the October 23, 2025, issue of Science, a team led by Andrew Flynn, New Mexico State University, Las Crucis, NM, and Stephen Brusatte, University of Edinburgh, Scotland, UK, report they have dated rocks from that area using magnetism.
About every half million years the earth’s magnetic poles reverse, allowing scientists to precisely date sedimentary and volcanic rock strata based on the alignment of magnetic particles within the strata. Their orientation is “frozen” when the strata solidify, providing a “time stamp.” This bar code of a magnetically aligned “stripes” in the strata are compared with a global polarity time scale to determine their age.
Magnetic analysis of the New Mexican rocks revealed they were formed during the years leading up to the impact. “What we found is that these rocks were deposited in the last 380,000 years of the Cretaceous period. These are the very last dinosaurs alive in New Mexico before the asteroid impact,” said Dr. Flynn. Like the badlands site, the dinosaurs in New Mexico showed no drop in diversity near the end of the Cretaceous, indicating that dinosaurs across North America were not failing but thriving. “Dinosaurs were still going strong up to the moment the asteroid hit. There is no sign that they were gradually wasting away to extinction as many paleontologists once believed,” said Dr. Brusatte.
*“Impact: The End of the Age of the Dinosaurs” is now on view at The American Museum of Natural History.