The New York Times “Trilobites” column has a story about the extinction of the non-avian dinosaurs some 66 million years ago. A long standing question about the extinction has been whether the age of dinosaurs came to a sudden end or were dinosaurs in decline when the asteroid struck the Yucatan Peninsula. The diversity of dinosaur species in northern North America at the extinction event has been shown to be less than it had been just few million years earlier. Dr. Andrew Flynn, a paleontologist at New Mexico State University and an author of a paper published Thursday in the journal Science looked at a rich dinosaur locality near Farmington, New Mexico, discovered in the early 1900s, and found much higher biodiversity. The dinosaurs in Texas and New Mexico were enjoying a much better time than their northern counterparts. Dr. Flynn found tyrannosaurs, ceratopsians, and even sauropods, which were long extinct from northern regions.

“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.