Living populations of the oysters Crassostrea virginica and Crassostrea gigas occupying several environmental situations are used to revise the taxonomy of the Cretaceous oysters Crassostrea subtrigonalis and Crassostrea glabra; and to interpret the paleogeographic settings of various Late Cretaceous oyster beds exposed in south-central Alberta, Canada.
Valves of the modem oysters Crassostrea virginica and Crassostrea gigas are described herein: the former from North Inlet, South Carolina, and from Copano and Aransas Bays, Texas; the latter from the Pacific coastline of North America. Varied and abundant valves of Crassostrea subtrigonalis are described from the Horseshoe Canyon and St Mary River formations (both Upper Cretaceous) of south-central Alberta. Analyses and comparisons of the measurements of the right valve of modem and Cretaceous oysters indicate that a single species - C subtrigonalis - occurs in the Cretaceous study region; it is characterized by a resilifer length/hingeline length ratio of 2.0-2.4 (mean 2.19). Crassostrea glabra is judged to be a junior synonym of C. subtrigonalis. Variations in the size, shape, and convexity of the left valve of the modem and Cretaceous oysters facilitate recognition of ecophenotypes of C. subtrigonalis indicative of subaerial exposure, salinity, rates of sedimentation, and food supply. Accordingly, the six C. subtrigonalis ecophenotypes recognized can be characterized as follows:
Ecophenotype T: third-order tidal creeks dissecting the high marsh;
Ecophenotype S: second-order tidal creeks dissecting the low marsh;
Ecophenotype F: first-order tidal creeks flowing through low marsh from nonmarine to marine environments;
Ecophenotype uE: upper estuary; Ecophenotype IE: lower estuary; and
Ecophenotype L: lagoons with strong marine influence.
The detailed stratigraphy and faunal composition of the oyster beds in the following seven study areas of south-central Alberta are described: Willow Creek, Drumheller, Queenstown, Travers Reservoir, Monarch Fault Zone, Magrath, and Del Bonita. The invertebrate fauna associated with Crassostrea subtrigonalis is usually very limited. The following taxa have been identified and discussed herein: bivalves, Mya (A.) simulatrix, Brachidontes (B.) dichotoma, Modiolus (M.) galpinianus, Corbicula (C.) occidentalis, C (C.) ventricosa, C. (C.) obliquata, Anomia (A.) perstrigosa, Corbula (C.) perangulata, and C. (C.) subtrigonalis; gastropods, Euspira obliquata, Pachymelania wyomingensis, and Viviparus mokowanesis; bryozoans, Eokotosokum bicystosum and Villicharixa lintonensis; ostracods, Cypridea sp. and Pontocypris sp.; and ichnofossils, Entobia cretacea and Oichnus simplex.
Within each study area, distinctive associations of the Crassostrea subtrigonalis ecophenotypes with particular species enable definition of recurring bioassemblages. The latter are considered to characterize specific environments within the study areas. Two sedimentary models proposed for the Willow Creek and Drumheller areas reflect depositional and ecological conditions interpreted from the lateral and vertical distribution of the bioassemblages. The Willow Creek Model represents a system of three orders of tidal creeks dissecting a salt marsh. The Drumheller Marine Tongue Model incorporates a series of oyster accumulations of ecophenotypes uE, IE, and L that are distributed distally within an estuary. These models are applied to interpret the remaining five study areas. The Queenstown, Monarch Fault Zone, and Del Bonita study areas are similar to the Drumheller Marine Tongue Model; while the Travers Reservoir and Magrath study areas are consonant with the Willow Creek Model.
The significance of this study lies in (a) specifying criteria enabling unequivocal recognition of Crassostrea subtrigonalis; (b) defining and interpreting the significance of ecophenotypes; and (c) constmcting the Willow Creek and the Dmmheller sedimentaryecological models that may be used for the interpretation of quantitatively varied occurrences of species of Crassostrea.