Overview
- A peer-reviewed study in Science precisely dates the Naashoibito fossils to 66.4–66.0 million years ago, contemporaneous with the Hell Creek Formation.
- The assemblage features a diverse mix including Tyrannosaurus rex, Alamosaurus, Ojoceratops and Ojoraptorosaurus, signaling robust ecosystems at the end of the Cretaceous.
- Ecological and geographic analyses identify distinct northern and southern bioprovinces across western North America, driven largely by regional temperature differences.
- The results strengthen the case for an abrupt, impact-driven extinction rather than a long-term decline, with researchers noting the current picture is constrained to North America and requires more well-dated sites.
- The findings rest on a decade of fieldwork using detrital sanidine 40Ar/39Ar geochronology and magnetostratigraphy on U.S. Bureau of Land Management lands, with related patterns carrying into early Paleocene mammal communities.