STATISTICAL DISCRIMINATION OF CORBICULA SPECIES (MOLLUSCA: BIVALVIA) FROM A MIXED FRESHWATER-MARINE MOLLUSCAN ASSEMBLAGE IN THE FOX HILLS FORMATION (LATE CRETACEOUS: MAASTRICHTIAN) OF NORTH DAKOTA

In a sandstone tidal channel facies of the Late Maastrichtian Fox Hills Formation in south central North Dakota, multiple morphotypes of the freshwater bivalve Corbicula occur in a rich, mixed assemblage with brackish and fully marine mollusks.  At one site specimen are numerous with many showing obvious abrasion indicative of transport.  These Corbicula had been assigned to at least four Linnaean species (Meek and Hayden, 1856), although it seems contrary to ecological parsimony to have four sympatric species.  If the number of species present were accurately determined, one might then begin to assess habitat separation and preferences thereby producing a useful tool for paleoecological analysis within the complicated deltaic-estuarine marginal deposits of the Fox Hills Seaway.

Major revisions of the Fox Hills bivalve fauna were made by Feldmann in 1967 and Speden in 1970.  Feldmann recognized two species C. cytheriformis (Meek and Hayden)  1860 and C. moreauensis (M&H) 1856, whereas Speden was unsuccessful in defining species because he felt that greater than 100 specimens were required for significant separation.  He included morphologies assignable to C. subelliptica (M&H) 1856, C. nebrascensis (M&H) 1856, and C. occidentalis (M&H) 1856, in his Corbicula sp. A.  The new collection of over 300 specimens is used here to clarify their species concepts.  Meek (1876) described Corbicula species primarily by characteristics of shell length, height, and width rather than by dentition or prosopon which vary little.  In our study, we are able to analyze shell morphologies using H/L and W/L ratios to assess the species separation statistically within this assemblage.

Cluster analysis suggests validity of C. subelliptica, C. nebrascensis and C. occidentalis in the Fox Hills Formation. C. cytheriformis is a junior synonym of C. nebrascensis, and C. moreauensis, as used by Feldmann (1972), is a junior synonym of C. subelliptica.  Species are most clearly defined by their H/L ratios.  C. subelliptica mean height is 83 percent of its length with a σ=0.05. C. nebrascensis has a 1:1 ratio of height to length with a σ=0.03, while C. occidentalis has a height of 109 percent of its length with a σ=0.03.  Discrimination of these species will allow progress forward recognition of habitat separation among Fox Hills Corbicula spp.