Peter M. Sheehan, David E. Fastovsky, Claudia Barreto, Raymond G. Hoffmann; Dinosaur abundance was not declining in a “3 m gap” at the top of the Hell Creek Formation, Montana and North Dakota. Geology ; 28 (6): 523–526. doi: https://doi.org/10.1130/0091-7613(2000)28<523:DAWNDI>2.0.CO;2
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An ∼3 m stratigraphic interval that entirely lacks dinosaur fossils or has very few fossils has been reported at the top of the Hell Creek Formation in the upper Great Plains of North America. The presence of the “3 m gap” in fossil distribution has been cited as evidence that dinosaurs had either become extinct or were on the verge of extinction prior to the bolide impact at the end of the Cretaceous Period. A survey in two areas of North Dakota and Montana reveals that dinosaur fossils in the upper 3 m of the Hell Creek Formation occur in numbers that are comparable to their abundance in other levels of the formation. Evidence for a gradual extinction is absent, and data are consistent with an abrupt extinction associated with an impact.