EVOLUTIONARY TRENDS IN LATE CRETACEOUS SPECIES OF HOPLOSCAPHITES FROM THE U.S. WESTERN INTERIOR

The Upper Cretaceous of the U.S. Western Interior (Baculites eliasi-B. grandis Zones) contains many species of Hoploscaphites. An examination of their shell shape and ornamentation through time reveals changes in response to changes in the environment of deposition. Six species of Hoploscaphites were examined: H. plenus, H. crassus, and H. peterseni, which occur in the B. eliasi Zone and lower part of the B. baculus Zone, and H. macer, H. criptonodosus, and H. sargklofak, which occur in the upper part of the B. baculus Zone and B. grandis Zone. We calculated the ratio of whorl width to whorl height (W/H) at midshaft in macroconchs (females) of each species to obtain a measure of the degree of compression or depression of the shell (values <1 denote compressed whorl sections; values >1 denote depressed whorl sections). We also counted the number of lateral tubercles on the exposed shell in each species. The results reveal that the values of W/H decrease in geologically younger species, implying that these species are more compressed. In addition, the number of lateral tubercles increases in geologically younger species. Both these trends are interpreted as an adaptation to a more nearshore, higher energy environment during the early Maastrichtian, which favored more hydrodynamically efficient forms with coarser ornamentation to protect them against increased levels of predation.