Gastropoda of the Fox Hills Formation (Maestrichtian) of North Dakota

The Fox Hills Formation (Maestrichtian, Upper Cretaceous) in North Dakota contains a gastropod fauna of thirty-seven species. Five species, loganensis n. sp., Piestochilus feldmanni n. sp., Hercorhyncus (Hercorhyncus) hollandi n. sp., cvancarai n. sp., and Cancellaria siouxensis n. sp. , and genus, Dakotia, are newly described. Twenty-four genera in eighteen families representing four orders and two subclasses of Gastropoda are present. Eleven species in a like number of genera are newly reported from the Fox Hills For mation. Goniocylichna bisculnturata and rectilabrum occur in common with the gastropod fauna of the Ripley Formation of the Mississippi Embayment, which has the closest compositional affinity with the Fox Hills fauna of North Dakota. In North Dakota, examination of the geographic distribution of Fox Hills genera indicates a blending of faunal elements from two sources. Genera such as Hercorhyncus, Goniocylichna, Amuletum, Remera, Cancellaria Semitriton, Pyropsis, and Neritina are prevalent in Gulf Coast and Mississippi Embayment faunas and represent a strong migrational influence from the southern midcontinent. A less prominent, but well established, portion of the fauna including and Serrifusus has close relatives among Pacific Coast and western Canadian faunas. The ranges of these groups overlap in the northern Western Interior where their zoogeographic distribution was probably at least partially controlled by their respective limits of tolerance of water temperatures. Four members and five major lithofacies resulted from deposition in a nearshore, deltaic and lagoonal complex. The gastropod fauna is compatible with data on the bivalve fauna compiled by other workers which indicate a range of water depth of zero to eighty fathoms for Fox Hills sedimentation. If modern analogues of glauconite formation and wave activity are valid, depths must have been at least sixteen fathoms over much of the area. Water temperatures of fifteen degrees Celsius occurred, and the Fox Hills sea is thought to have become periodically thermaily stratified over large, restricted areas. Interpretation of sedimentary regimes indicates that massive storms of hurricane intensity occasionally destroyed the thermoclines, scoured large portions of the nearshore sedimen tary complex, and redeposited some of the material farther inshore where it was subsequently reworked into barrier bars and spits. These storm deposits contain the most fossiliferous units of the Fox Hills Formation. An unnamed stratigraphic unit, probably of member rank, representing a minor marine transgression, or a major storm, late in the history of Fox Hills sedimentation deposited a blanket sandstone that frequently contains a re-worked, estuarine, Crassostrea-Dakotia faunal complex. This sandstone or correlative units, covered a wide area which probably included some terrestrial environments represented by sediments presently found in the Hell Creek Formation