Some of the world’s last dinosaurs were healthy and thriving until the day they were wiped out by asteroid Armageddon, a new study of fossils in New Mexico has found.
Scientists have debated for decades whether non-avian dinosaurs were in decline before a giant asteroid hit Mexico’s Yucatan Peninsula at the end of the Cretaceous period (145 million to 66 million years ago). The new study, published Thursday (October 23) in the journal Science, supports previous evidence that dinosaurs were attacked during their heyday and may still be roaming the Earth if it weren’t for those pesky asteroids.
Researchers have determined the age of rocks at a Naashoibit fossil site in New Mexico. It preserved a rich Cretaceous ecosystem that was home to a variety of dinosaurs, including a 70-foot (21-meter) long-necked sauropod known as Alamosaurus, a carnivorous tyrannosaurus, horned dinosaurs, and duck-billed dinosaurs. Dating reveals that this ecosystem existed just before the Chicxulub asteroid impact, suggesting New Mexico’s last dinosaurs were thriving before the giant space rock brought death from the sky.
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“They seem to be thriving, at least before mass extinction events,” study lead author Andrew Flynn, an assistant professor in New Mexico State University’s Department of Geological Sciences, told Live Science in an email. “Dinosaur populations appear to be healthy, as New Mexico’s Nashoibito Member is home to a diverse dinosaur fauna.”
This asteroid triggered a mass extinction event that wiped out approximately 75% of modern species, including all dinosaurs except for birds. Previous studies have suggested that environmental factors such as climate change made dinosaurs more vulnerable to catastrophe and reduced dinosaur diversity as part of the reconstruction of the Maastrichtian period of the Cretaceous period (72.1 million to 66 million years ago). However, other researchers argue that the asteroid interrupted an era of dinosaur prosperity.
Much of what scientists know about the Cretaceous-Paleogene (K-Pg) boundary – the rocks that mark the end of the Cretaceous Period – comes from geological formations such as Hell Creek and Fort Union in the northern Great Plains of the United States, meaning there is a lot of uncertainty about what was happening elsewhere.
Paleontologists knew that members of the Nashoibito featured the last known non-avian dinosaur in New Mexico, but the exact age of these fossils was controversial. In the new study, the researchers sought to resolve this uncertainty by combining two rock dating methods.
“We wanted to have two different and independent methods for determining the age of rocks,” Flynn said.
The first of these methods involved measurements of radioactive decay in argon isotopes. The second uses magnetic fields, where the Earth’s magnetic field flips between a “normal” state (as it is today), where magnetic north is north, and an inverted state, where magnetic north is south. Researchers know when these reversals occurred throughout Earth’s history, so by measuring the orientation of a rock’s magnetic poles, they can estimate their age.
Dating methods have determined that the Naashoibit member is between about 66.4 and 66 million years old. This means that the dinosaurs there lived within about 340,000 years of the asteroid impact. The researchers also found that New Mexico’s dinosaurs were unique, suggesting that there were areas of distinct dinosaur diversity in western North America.
“These revised estimates of dinosaur diversity during the Maastrichtian period still fall short of the clear peak of dinosaur diversification in the preceding Campanian period, or North America,” North Carolina State University paleontologist Lindsay Zanno, who was not involved in the study, said in an accompanying opinion piece published in the journal Science. “Nevertheless, current estimates of Maastrichtian biodiversity are still higher than most other Late Cretaceous estimates.”
A new study paints a picture of an asteroid impact that caused the sudden extinction of dinosaurs, followed shortly by the rapid rise of mammals. However, it is still unclear whether this was the case everywhere.
“This study really highlights the need to address new, previously understudied regions at this incredibly important time in Earth’s history,” Flynn said. “Just by adding one new locality of older dinosaurs in western North America, we can see this really interesting dinosaur picture.”
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