Species dominance in benthonic foraminifera, represented by percent of the assemblage composed of the single most abundant species, shows little change in observed range of values from shallow into deep-marine waters in 1,005 samples from the Gulf of Mexico, Atlantic, and Arctic margins of North America. This finding contrasts with the model that species dominance is highest in shallow-marine environments and decreases offshore into deeper marine waters. In warm waters off the southeastern U.S., the dominant species composes 10-50% of the benthonic assemblage in inner neritic environments; this range of dominance percentages remains unchanged down to depths of 5,000 m. Analysis of a large sample subset from the northeastern Gulf of Mexico, which was placed into three depth intervals of 0-200, 201-1,000, and greater than 1,000 m, indicates differences, however, between the means of dominance values in the three intervals. The mean of dominance is statistically higher in the deepest depth interval of greater than 1,000 m (mean of 24.0%) than in the shallowest interval of 0-200 m (mean of 20.0%). The lowest mean of dominance (15.3%) occurs in the 201-1,000 m interval. In cooler waters off the northeastern U.S. Atlantic coast, the most dominant species composes from 15-82% of the assemblage in waters 0-200 m deep, and also has a similar range of 9-90% in waters 201-5,000 m deep. Here, the highest mean of dominance, 42.5%, is found in the 0-200 m depth interval; this mean is significantly higher than the mean of 32.4% found in the deepest interval of greater than 1,000 m. The lowest mean of dominance, 27.0%, again is found in the continental slope interval of 201-1,000 m. A significantly higher mean of dominance occurs in the northern Atlantic data subset compared to that found in the northeastern Gulf of Mexico subset. Thus, use of species dominance in paleobathymetric interpretation of open-marine strata should be applied with caution. Marginal-marine assemblages consistently have high dominance (30-95%) and low diversity (2-11 species); a combination of these two characteristics makes these environments more recognizable in the fosil record. Equitability (E), the relation of all species abundances within an assemblage, also shows little change between the values found in shallow-marine assemblages and those found in assemblages from deeper water environments. Considerable variation in E values exists within all depth intervals, and it is of no discernible use for paleoenvironmental interpretation. Surprisingly high scatter exists among the relationships of equitability (E), species diversity (S), and dominance values of the assemblages. Equitability and dominance values found in 421 assemblages from Paleocene, Eocene, Miocene, and Pleistocene strata of the Atlantic and eastern Gulf of Mexico coastal plains are similar to the modern values.
