The Greenhorn Limestone is a rock formation composed primarily of chalky limestone and shale that was deposited in the Late Cretaceous Western Interior Seaway of North America. Many dental and skeletal remains of fossil fishes were collected from the basal Greenhorn Limestone (early Turonian; ca. 93 Ma) at a locality along the western border (Plymouth County) of Iowa. The fish fauna consists of at least eight chondrichthyan and 12 osteichthyan taxa that are represented by diverse trophic forms, including durophagous forms, pelagic carnivores, and opportunistic feeders. Sedimentary particles of the basal Greenhorn Limestone at the locality include a relatively large quantity of quartz grams (e.g., compared to the basal Greenhorn Limestone in southeastern Colorado). This observation suggests that the deposition took place at a position closer to the shoreline than other Greenhorn localities.
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