A New Subgenus Of Neritid Gastropod From The Upper Cretaceous Of Baja California, Mexico
Numerous specimens of the neritid gastropod Nerita (Bajanerita) n. subgen. californiensis (White, 1885) are present in the Upper Cretaceous Rosario Formation at Punta Banda, Baja California, Mexico (Figure 1). Marincovich (1975) assigned these strata to a Late Campanian to Early Maastrichtian age. The strata contain extensive biostromal deposits of the caprinid rudistid bivalve Coralliochama orcutti White, 1885, that probably lived below mean wave base in a shallow-water, low-energy environment periodically affected by storm waves or currents (Marincovich, 1975). The nearby shoreline was apparently defined by steep wave-washed bedrock cliffs and local pocket beaches that formed along the margin of a forearc basin (Yeo, 1984). Scattered about in the sandstone matrix among the rudistid remains are the small-sized specimens of N. (B.) californiensis, which commonly weather out as resistant, complete shells on the surface of the rock. Saul (1970) concluded that the neritid and other shallow-water gastropods at Punta Banda accumulated in sediment-trapping depressions within the Coralliochama buildups. These gastropods had originally roamed over the algal pastures of these buildups. The color patterns preserved on many of the specimens of N. (B.) californiensis also provide evidence that the depth of water was shallow and within the photic zone. Furthermore, Sohl (1971) reported that Campanian and Maastrichtian gastropod assemblages of the Baja California region are mostly associated with rudist buildups, and warm-water rocky intertidal neritids are among the dominant faunal elements. Lowenstam and Epstein (1959), using oxygen-isotope studies of an ammonite, suggested that the Punta Banda rudistids lived at a marginally tropical temperature of about 19°C.