Hell Creek records the last dinosaurs
The final two million years of the Mesozoic, preserved in Montana right up to the boundary clay.
- When
- —
- Duration
- 2 million years
- Known to
- ± 1 million years
- Era
- Cretaceous, Mesozoic
The Hell Creek Formation of Montana and the Dakotas captures a floodplain ecosystem in the last stretch before the extinction, and is capped directly by the iridium-bearing boundary layer. That continuity is what makes it invaluable: it lets the question of whether dinosaurs were already declining be tested against a single well-sampled section rather than assembled from scattered sites. Its fauna includes Tyrannosaurus, Triceratops, Edmontosaurus, Ankylosaurus and a diverse assemblage of mammals, and the debate over what the section shows is still live.