Fish remains are extremely abundant in several Cretaceous formations of the Rocky Mountains and Great Plains, but except in the Niobrara formation of Kansas, a fish skeleton well enough preserved for description or identification is the greatest rarity. The fishes are represented by separate scales, in some places associated with a few vertebrae and other fragmentary bones or by isolated teeth. In the original descriptions of both the Mowry and the Aspen shales of Wyoming the presence of fish scales is mentioned as a characteristic feature. Fossils of other classes are usually very rare in beds containing many fish scales. Many of the scales are beautifully preserved and show varied forms and more or less complex structure.
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USGS Numbered Series
Some American Cretaceous fish scales, with notes on the classification and distribution of Cretaceous fishes