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US Great Plains (SD, ND, MT, CO, KS) · Late Cretaceous, 81–69 million years
Marine shale deposited in the Western Interior Seaway that split North America in two. A deep, muddy sea floor that preserved ammonites, marine reptiles, and fish.
Baculites, Placenticeras (often iridescent ammolite), mosasaurs, plesiosaurs, and fish.
Source matters: private ranch land is legal; federal land is not. Iridescent ammonite (ammolite) and mosasaur material is popular. Confirm land type and landowner release. Mosasaur jaws are frequently composited from multiple specimens.