Overview
- Researchers dated the Naashoibito Member in northwestern New Mexico to roughly 66.4–66.0 million years ago using argon isotope analyses and magnetic polarity data.
- The site preserves a diverse latest‑Cretaceous fauna that includes the giant sauropod Alamosaurus, Tyrannosaurus, horned dinosaurs and duck‑billed species.
- Comparisons with the contemporaneous Hell Creek Formation show a clear north–south split in dinosaur communities across western North America, with sauropods absent in the north.
- The results, published in Science, strengthen evidence that non‑avian dinosaurs in western North America were not in long‑term decline and were abruptly wiped out by the impact.
- Independent experts welcome the precise dates but stress that conclusions come from a single region and that more well‑dated sites are needed to assess broader continental and global patterns.