PALEOECOLOGICAL IMPLICATIONS OF THE FOX HILLS FORMATION (MAASTRICHTIAN) REPTILIAN AND AMPHIBIAN FAUNA FROM SOUTH-CENTRAL NORTH DAKOTA

Fossil vertebrates of the Fox Hills Formation in the Missouri River valley of North Dakota play a singular role for interpretations of both paleoecology and paleogeography at the margin of the Western Interior Seaway (WIS). They link interpretations of terrestrial, estuarine, and marine ecosystems in a manner that invertebrates alone generally cannot. The Fox Hills Formation, which underlies and interfingers with the Hell Creek Formation, and associated marine tongues in the Hell Creek Formation indicate that the WIS persisted into the latest Cretaceous in North Dakota. Invertebrate fossils are common in the nearshore Fox Hills deposits consisting primarily of sandstone and siltstone. Deposition occurred during the Late Maastrichtian Jeletzkytes nebrascensis Western Interior ammonite Zone.

Remains of reptiles in the Fox Hills Formation in North Dakota are not common and consist mostly of teeth and other isolated skeletal parts. Only partial skeletons of any reptiles have been found in the Fox Hills Formation. The meager reptilian fauna, recovered from nearshore sandstone facies, consists of marine and terrestrial taxa. The marine taxa present include the mosasaurs Mosasaurus dekayi and Plioplatecarpus sp. Terrestrial taxa represented are dinosaurs, Tyrannosauridae indet., and the theropods Paronychodon sp. and Theropoda indet.; turtles, the trionychid Aspideretoides sp. and the nanhsuingchelyid Basilemys sp.; the crocodile Leidyosuchus? sp.; the Choristodera Champsosaurus sp. and the salamanders Opisthotriton kayi and an undetermined species.

This mixed fauna indicates that marine mosasaurs frequented shallow water areas of the WIS. The occurrence of Champsosaurus with the horseshoe crab, Casterolimulus kletti, exemplifies faunal mixing in an estuarine habitat. Dinosaurs, crocodiles, turtles, and salamanders inhabited Late Maastrichtian shoreline areas.