Science|Fossils of Some of the Last Dinosaurs in North America Have a Story to Tell

Scientists have found that a trove of fossils from New Mexico represents a group of some of the last dinosaurs that lived in North America. The discovery has the potential to settle a long-running paleontological argument about exactly how the dinosaurs went extinct.

When a six-mile-wide asteroid slammed into the Yucatán Peninsula 66 million years ago, the age of the dinosaurs came to an apocalyptic end. For decades, paleontologists have debated whether that extinction arrived as a bolt from the blue, wiping out a flourishing dynasty; or as a final stroke, clearing the decks at the late stages of the animals’ long decline.

For much of the 20th century, people studying the end of the period when the asteroid struck, the Cretaceous, have focused on the fossil-rich badlands around Montana and Wyoming. The area produced species like Tyrannosaurus rex, Triceratops and the duck-billed Edmontosaurus.

The presence in those regions of just one massive predator and only a few species of big herbivores represents a less biodiverse landscape than those in previous eras, said Andrew Flynn, a paleontologist at New Mexico State University and an author of a paper published Thursday in the journal Science.

But in New Mexico and Texas, paleontologists in the early 1900s uncovered a number of dinosaurs unlike those farther north, Dr. Flynn said.

A particularly notable set of fossils was found in the Naashoibito Member in the De-Na-Zin Wilderness Area, south of Farmington, N.M. It included the 100-foot-long, 80-ton Alamosaurus, which was “among the largest sauropods that ever lived, anywhere, at any time,” said Steve Brusatte, a paleontologist at the University of Edinburgh and an author of the paper.