Revision of the Gastropoda of the Fox Hills Formation, Upper Cretaceous (Maestrichtian) of North Dakota
Nearly one hundred years have passed since F. B. Meek completed his monumental studies of invertebrate fossils in the Upper Cretaceous rocks of the Nebraska and Dakota Territories. The desire to examine molluscan distributions of the Western Interior in new ways has prompted a need for revision of the molluscan taxonomy and faunal lists of the "upper Missouri country." The Fox Hills Formation (Maestrichtian) of North Dakota and South Dakota, a complex of nearshore and littoral, delta platform and lagoonal sandstones, siltstones, and clays, contains a moderately large molluscan faunal record of the closing marine event of the Mesozoic Era in that region. By the end of his career Meek had described approximately 25 gastropod species from the formation, only four of which were definitely taken from localities in North Dakota. The Bivalvia having been revised earlier by Feldman, this revision updates the taxonomy and distribution of the gastropods of the Fox Hill Formation in North Dakota. As described herein the fauna contains 37 species. Those represent 24 genera in 18 families distributed among four orders and two subclasses of the Gastropoda. Five species, Neritina loganensis, n. sp.; Piesochilus feldmanni, n. sp.; Hercorhyncus (Haplovoluta) hollandi, n. sp.; Remera? cvancarai, n. sp.; and Cancellaria siouxensis, n. sp. are described. Eleven species in a like number of genera are newly reported from the Fox Hills Formation. Goniocylichna bisculpturata Wade and Euspira rectilabrum (Conrad) occur in common with the fauna of the Ripley Formation of the Mississippi Embayment, with which the Fox Hills Formation has its closest compositional affinity. Paleogeographic distributions of Fox Hills genera clearly indicate that North Dakota was a region of overlap of Tethyan migrants (for discussion of Tethys see particularly Sylvester-Bradley in Adams and Ager, 1967) from the Gulf Coast region and a less prominent "boreal" or northern Pacific stock emigrating from the north and northwest. Southern genera show a reduction of ornamentation reflecting cooler conditions than those in the Tethyan Realm. True endemism, formerly considered predominant in the Fox Hills fauna, is actually not that great, but was, and is, a manifestation of our still incomplete knowledge of distributions and stratigraphic occurrences of Gastropoda in the Cretaceous of the Western Interior. Observations regarding stratigraphy and sedimentation of the formation indicate that major storms swept the midcontinental seas during the late Cretaceous and were responsible for several major sediment accumulations within the formation. In addition an as yet unnamed member was recognized high in the section. As a blanket sandstone it seems to owe its depositional origin to subsidence of the deltaic platform and resulting re-transgression of the generally regressive Fox Hills sea.