A site in the San Juan Basin of northwestern New Mexico is providing a rare glimpse into the last days of the dinosaurs.

Rocks and fossils at the Naashoibito Member site show an ecosystem that was filled with a diverse population of dinosaurs just before they disappeared from Earth.

Paleontologists have long debated if the dinosaurs suddenly went extinct when a 6.2-mile-wide (10-kilometer-wide) asteroid crashed into Mexico’s Yucatán Peninsula 66 million years ago at the end of the Cretaceous Period, or if they were in a gradual decline and living in weakened ecosystems ahead of the catastrophic event.

Answering that question requires finding fossils and dating the surrounding rock to come up with an accurate timeline of the site. But identifying fossils in an area accurately dated to just before the extinction event is rare.

A key site of interest for paleontologists has been the well-studied Hell Creek and Fort Union Formations, located in what is now Montana, Wyoming and the Dakotas.

Hell Creek has preserved evidence of multiple species of Triceratops and Edmontosaurus, as well as rocks, dated to the end of the Cretaceous Period, or just before the dinosaurs went extinct. But the dinosaur community there completely lacked any long-necked species, causing scientists to wonder if those had already disappeared.

Now, new research has dated rocks in the Naashoibito Member to the same time period as the Hell Creek Formation