Neogastropods (Melongenidae, Fasciolariidae, Turridae) from the Cannonball Formation (Paleocene: Thanetian), North Dakota and South Dakota

The Cannonball Formation of Paleocene age is a shallow-marine, clastic sequence exposed primarily in southwest-central North Dakota and northwestern South Dakota. The formation is characterized by alternating, poorly consolidated, relatively thin sandstone and thick mudstone units. The Cannonball neogastropod families Melongenidae, Fasciolariidae, and Turridae contain 20 species assigned to 15 genera based on 334 individuals from 50 localities. Four genera, Alticollarum, Obtusicarina, Vitticoncha, and Ericksonia are new, whereas Serrifusus, Mesorhytis, Rhombopsis, and Deussenia are first reported occurrences from Tertiary strata. Marshallaria is newly reported from North America and likely from the Northern Hemisphere. Twelve species are additions to the Cannonball fauna; of these, Acamptogenotia varicosta, Serrifusus sohli, Deussenia minuta, and Ericksonia clivilinea are newly described. Mesorhytis dakotensis is the most abundant species; the other species are generally rare. No stratigraphic zonation of species was observed. The middle informal unit has the most diverse fauna with 16 species, followed by that of the lower with 15 and the upper with 11. The larger number of occurrences is in the sandstone lithology, whereas the greater number of individuals occurs in the mudstone. Cluster analyses indicate little or no correlation between substrate type and species occurrence and that these neogastropods have no paleoenvironmental significance. These neogastropods are inferred to have been semi-infaunal or shallow infaunal predators. Comparisons between the neogastropod faunas of the Cannonball and Late Cretaceous Fox Hills and Pierre Formations suggest that 60 percent of the Late Cretaceous genera and all of the species became extinct prior to Cannonball time. Five Cannonball neogastropods occur in the Agatdal Formation of West Greenland, two in the greensands of Copenhagen, Denmark, and one in the Aquia Formation of Maryland and Virginia. A northerly or northeasterly source for the Cannonball Sea is inferred as the simplest explanation for the large number of Cannonball species occurring in West Greenland. Furthermore, a connection with the North Atlantic most readily explains the mixture of cool and warmer water gastropod and bivalve genera in the Cannonball fauna, since most of these genera also occur in the West Greenland fauna. Absence of West Greenland tropical\u27\u27 forms in the Cannonball fauna is attributed to oceanic cooling caused by mixing of North Atlantic and Arctic waters. Occurrences of Cannonball neogastropods are restricted entirely to other late Paleocene faunas, and, therefore, a late Paleocene (Thanetian) age for the Cannonball Formation is tentatively suggested