New evidence reveals dinosaurs were thriving right up to the moment the asteroid hit
Newly dated fossils from New Mexico challenge the idea that dinosaurs were in decline—and suggest instead they had formed flourishing communities.
Around 66 million years ago, during the Northern Hemisphere’s spring, a six-mile-wide asteroid struck what is now the Yucatán Peninsula. The resulting disaster drove 75 percent of Earth’s species to extinction, including nearly every dinosaur lineage except for beaked birds.
The impact was devastating. Its aftermath, apocalyptic. But what was happening right before the cataclysmic event?
Paleontologists have long debated whether the dinosaurs were already in decline when the asteroid hit or if they were flourishing and giving rise to new species. Fossils from New Mexico that date to within about 340,000 years before the asteroid struck paint a vivid picture: the dinosaurs were thriving right until the moment of impact, a new study suggests.
(Would dinosaurs have died without an asteroid strike? Here's the science.)
Among the dinosaurs that dominated the landscape in prehistoric New Mexico during this period were true giants, like the immense long-necked herbivore Alamosaurus.
“I can imagine the scene, one minute a jet plane-sized dinosaur was shaking the ground as it walked. The next minute the whole Earth was shaking with the energy unleashed by the asteroid,” says National Geographic Explorer Stephen Brusatte, a paleontologist from University of Edinburgh in Scotland and an author of the study.
The finding, published Thursday in Science, supports the idea that North America’s dinosaurs were not fading away before they went out with a bang. The study also provides insight into the diversity of dinosaur species that lived in the southwestern region of North America at the end of the Cretaceous period.
New dates for New Mexico’s fossils
A great deal of what paleontologists know about the fate of the non-avian dinosaurs comes from fossils discovered in western North America, particularly at the Hell Creek and Fort Union Formations. These rock outcrops in Montana, the Dakotas, and Wyoming preserved snapshots of land-dwelling species before and after the collision. They show that dinosaurs such as Tyrannosaurus, Triceratops, Edmontosaurus, and Ankylosaurus inhabited the ancient floodplains preserved around the Rocky Mountains.
But further to the south, in Cretaceous-era New Mexico, a different assemblage of dinosaurs wandered the lowlands. The researchers looked to the southwestern dinosaurs—which included the shovel-beaked Kritosaurus, the three-horned Torosaurus, and the armored Glyptodontopelta —to investigate how they were faring just prior to the asteroid strike.
(Rare fossils reveal a stunning scene from the final days of the dinosaurs)
Piecing together the timeline for these dinosaurs has taken years of research and fieldwork looking for clues in the rocks and sediments.
“This project was over a decade in the making, with some of the first geochronology samples collected before I started graduate school,” says Andrew Flynn, a paleobotanist from New Mexico State University, and the first author on the paper.
Flynn and his team studied rock layers found within New Mexico’s San Juan Basin, an area known to geologists as the Naashoibito Member. Up until now, the exact age of these rocks has been uncertain. Previous research estimated they were about 70-million-years-old—millions of years older than the asteroid impact.
But the new research from Flynn and his team has revised the dates to 66.4 to 66-million-years-old, meaning the dinosaurs found within these New Mexico rocks lived within the last half-a-million years before the asteroid strike.
That suggests these dinosaurs were living around the same time as the ones found in the Hell Creek and Fort Union Formations of western North America. In fact, it shows that dinosaurs like Alamosaurus in prehistoric New Mexico were even closer to the impact site in Chicxulub Mexico, than the likes of Triceratops in ancient Montana.
(A second asteroid may have struck during the dinosaurs' demise)
To properly place the dinosaurs in time, Flynn and colleagues took multiple geologic samples from New Mexico’s Naashoibito Member and ran them through a special dating technique that analyzed tiny crystals inside the rocks to determine how long ago they were deposited. While previous studies proposed that the Naashoibito dinosaurs lived millions of years before the impact, the new study refined the dates to about 340,000 years before the end of the Cretaceous.
It suggests that North America was home to multiple dinosaur communities, evolving alongside each other in different basins, just before disaster struck.
“I think the San Juan Basin deposits being dated to the latest Cretaceous is a significant and important contribution to our knowledge of the end of the period,” says Manabu Sakamoto, a palebiologist from the University of Reading in England, who was not involved in the study.
Diverse and thriving dinosaurs
Not only were dinosaurs thriving right until the extinction, Flynn and his colleagues also found that North America’s last dinosaurs were divided into different communities of species depending on where they lived.
Experts know this as “provinciality,” where new species evolve in geographic pockets and are often separated from each other by differences in vegetation, temperature, and other natural phenomena.
(These fossils may capture the day the dinosaurs died.)
Some of the last Cretaceous dinosaurs roaming New Mexico were very similar to those found further to the north. Tyrannosaurus, best known from skeletons found in places like Saskatchewan, Canada and Montana, also stalked New Mexico right before the asteroid strike.
But others were startlingly different, like Alamosaurus.
The largest dinosaur to wander Cretaceous New Mexico, Alamosaurus, could exceed 80 feet in length and weigh more than 30 tons. It marked the return of large sauropod dinosaurs to western North America from titanosaur ancestors that lived further to the south.
“Nothing illustrates how dinosaurs were thriving up to the very end than the fact that Alamosaurus—one of the biggest dinosaurs ever—was there to witness the asteroid,” Brusatte says.
With the new dates for the rock layers established, Flynn and his colleagues compared dinosaur species that lived across the span of western North America between 75 and 66 million years ago. That comparison would reveal whether the continent was home to just a handful of dinosaur species before the extinction event or to many kinds of dinosaurs.
The team's findings add to a growing body of evidence that ancient life formed regional communities of different species rather than a single dinosaur community spanning the whole continent, Sakamoto says.
The main feature that divided the northern and southern dinosaur communities, Flynn and colleagues propose, was temperature.
(Different dinosaur species may have really traveled together like in the movies)
The giant herbivore Alamosaurus lived in warmer southern regions at the end of the Cretaceous but were absent from cooler habitats to the north that hosted many more duckbilled and horned dinosaurs. Given that these dinosaurs were large animals capable of moving long distances, and some species like Tyrannosaurus lived in both regions, it appears that temperature was more important in creating these different dinosaur communities than a geographic barrier like a mountain range or river system, the researchers say.
Such sensitivity to temperature and natural conditions on the continent hint that even more unique dinosaur communities await to be discovered, not only in North America but elsewhere around the world.
“I think our new work shows that we need more research done from new areas on the K/Pg mass extinction,” Flynn says, especially from the Southern Hemisphere.
(Scientists find a new titanosaur dinosaur species in Patagonia)
Research underway in South America is beginning to piece together a picture of the continent’s last dinosaurs and how they compared to those found elsewhere on the planet. Identifying the key species and when they lived is an essential part of piecing together a truly global picture of the extinction event.
Even though the fate of the great dinosaurs is well-known, paleontologists are only just beginning to find the dinosaurs that witnessed the close of the Cretaceous.
