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.

Yet as exciting as the remains were, paleontologists weren’t sure how old they were, Dr. Flynn said. Some suggested that they hailed from a slightly earlier stage of the Cretaceous period, from which the dinosaur fossil record is significantly better known.

Beginning in 2013, Dr. Flynn started investigating the age of the New Mexican remains. He and his colleagues relied on an unusual ally: magnetism.

Throughout history, Dr. Flynn said, the polarity of Earth’s magnetic field has flipped about every half-million years. By measuring the magnetic pole direction in the rocks from the formation — and comparing them with the geochemical age of the crystals in the surrounding sandstones — the team was able to get a direct fix on the formation’s age.

“The rocks were deposited in the last 380,000 years of the Cretaceous period,” Dr. Flynn said, around the same time as the more famous dinosaur ecosystems of the North. “These are the very last dinosaurs alive in New Mexico before the asteroid impact.”

Dr. Brusatte noted that the dinosaurs of the Southwest and the Badlands in the North “are very different from each other.” While the New Mexican population shared animals like Tyrannosaurus and Triceratops, its most common herbivores were crested hadrosaurs and giant long-necked sauropods, both totally absent further north.

In these fossils, the researchers saw evidence of radically different northern and southern dinosaur communities. They therefore argued that North America still hosted a diverse population of dinosaurs.

The evidence, the team argues, suggests that the asteroid arrived as a brutal shock to a thriving array of species.

“Dinosaurs were still going strong up to the moment the asteroid hit,” Dr. Brusatte said. “There is no sign they were gradually wasting away to extinction as many paleontologists once believed. It really does seem like the asteroid fell out of the sky one day and struck down dinosaurs in their prime.”

Michael Benton, a paleontologist at the University of Bristol in England who did not work on the paper, said the diversity of the New Mexican dinosaurs shown in the new research does not mean declines weren’t happening elsewhere in North America or other parts of the world. Western North American dinosaurs seem to have dropped from 43 known species earlier in the Cretaceous to 30 during the last six million years of the period, Dr. Benton added, even if there were habitats rich in different faunas “where climates were favorable.”

But Philip D. Mannion, a paleontologist at University College London who was not involved in the study, called the analysis “robust.”

If it hadn’t been for a sudden astronomical accident, he said, “the Age of Dinosaurs would almost certainly have continued for much longer and might even still be the case today.”