NEW, RARE ECHINOID RECORDS FROM THE PIERRE—FOX HILLS TRANSITION (LATE CRETACEOUS: CAMPANIAN—MAASTRICHTIAN) IN THE WILLISTON BASIN

Echinoids are notably rare in the Late Campanian through Maastrichtian Pierre Shale –Fox Hills regressive facies of the Western Interior Seaway of the Williston Basin (WIS). Previously, in the Fox Hills Fm. of North Dakota only the holotype of the cassiduloid Hardouinia waagei Holland and Feldmann (1967) was known. In the top of the Pierre Shale (“transition beds”) in SE Montana Meek and Hayden (1857) described Hemiaster humphreysanus, a spatangoid, from one good and one poor specimen. Holland and Feldmann (1963) reported 7 additional specimens from the same position in SW ND.

We are reporting 2 specimens of the regular echinoid, Eurysalenia minima, from the Gregory Member of the Pierre Shale in Griggs Co., central ND (Hoganson et al., 1999). We are newly reporting 2 specimens of H. humphreysanus from the Pierre-Fox Hills transition in western ND, and eastern MT, 1 specimen, apparently a new species, of the holasteroid Pseudholaster from the base of the Fox Hills Fm. in Emmons County, ND, and 3 additional specimens Hardouinia waagei from sandstone facies of the Timber Lake Member in the Missouri Valley of central ND. Those collected from the Timber Lake Member have been found in association with the trace fossil Ophiomorpha major suggesting a depositional environment on/near the boundary of the littoral and sublittoral zones.

The Pierre-Fox Hills transition in eastern MT and western ND is Late Campanian in age as is the Gregory Member of the Pierre Shale in central ND (Baculites gregoryensis Zone). Eurysalenia minima was originally known only from 222 ft below the top of the lower unnamed member in the Red Bird Section of the Pierre Shale in eastern Wyoming (Kier, 1967). We believe E. minima and H. humphreysanus to have been Late Campanian in age. The new species of Pseudholaster from a concretion in the Trail City Member of the Fox Hills is from the Maastrichtian Jeletzkytes nebrascensis ammonite zone which is believed to extend upward through the Tancredia—Ophiomorpha biofacies of the Timber Lake Member in the Missouri Valley of ND. This is the first Maastrichtian record of Pseudholaster and the first record of the genus in North America. The stratigraphic distribution of these taxa suggests an offshore to onshore facies gradient and thus deeper to shallower environmental preferences for this suit of echinoid taxa in the WIS.