Overview
- Argon-isotope measurements and paleomagnetic stratigraphy place the Naashoibito Member at roughly 66.4–66.0 million years ago, within about 340,000 years of the Chicxulub impact.
- The assemblage includes Alamosaurus, Tyrannosaurus, Torosaurus, duckbilled and armored dinosaurs, indicating diverse, functioning ecosystems immediately before the extinction.
- Comparisons with the contemporaneous Hell Creek Formation reveal clear north–south provinciality, with sauropods like Alamosaurus in the south and none in the north, and different hadrosaur types between regions.
- Authors interpret the data as evidence against a long-term decline in North American dinosaurs and note rapid early Paleocene mammal diversification and persistence of bioprovinces after the boundary.
- Independent experts welcome the precise dating but emphasize that conclusions derive from a single well-dated locality, urging more sites to test regional versus global patterns; the study appears in Science.