Isolated sandstone and siltstone outcrops of the Late Cretaceous (Campanian-Maastrichtian) Fox Hills Formation occur at the Poison Springs Locality in northeastern Colorado. These nearshore deposits document the western margin of the Fox Hills Sea and have yielded elasmobranch and osteichthyan remains. Seventeen elasmobranch species from eleven families including Squatina hassei, Plicatoscyllium derameei, Cretorectolobus olsoni, Carcharias samhammeri, Odontaspis aculeatus, Cretalamna cf. C. feldmanni, Palaeogaleus navarroensis, Galeorhinus girardoti, Archaeotriakis rochelleae, Pseudomyledaphus sp., Pseudohypolophus mcnultyi, Ischyrhiza avonicola, Ptychotrygon winni, Ptychotrygon greybullensis, Ptychotrygon sp., Dasyatis sp., and Rhombodus levis and five osteichthlyan species from four families including Melvius sp., Paralbula casei, Enchodus dirus, Enchodus sp., and Hadrodus sp. were identified. This paleofauna is essentially the same as the Fox Hills Formation paleofauna recovered from North Dakota and extends the geographic range of the Late Cretaceous Fox Hills fish paleocommunity to far western shores of the Fox Hills Sea.
The Poison Springs fish paleofauna consists of a mix of freshwater, brackish water/estuarine and shallow marine, and open marine taxa reflecting the dynamic Fox Hills Sea coastal margin. This fish paleocommunity included predaceous, durophagous bottom feeders, shallow marine piscivores, and open water ambush predators. Some taxa are restricted to the northern Western Interior Seaway, but most are more cosmopolitan and range to the Texas-Gulf Coast, Mississippi Embayment, and Atlantic Coastal Plain. A few have also been found in Europe and North Africa. This Late Cretaceous Fox Hills Formation fish paleofauna provides additional evidence for extinctions of marine fish at the end of the Cretaceous.