Abstract: Paired Foraminiferal Ecophenotypes in Gulf Coast Estuaries--Ecologic and Paleoecologic Implications

C. Wylie Poag

The shallow, brackish-water environment of San Antonio Bay, Texas, supports a benthic foraminiferal fauna whose major constituents are widespread around the margin of the Gulf of Mexico, the southern Atlantic Coast of the United States, the West Indies, and in low latitudes along the Atlantic and Pacific Coasts of South America. Several species of Ammotium, Ammonia, and Elphidium have been recognized by most authors as the dominant taxa in these estuaries. A scanning electron microscope (SEM) analysis of the exterior test morphology in five species from San Antonio Bay (Ammonia parkinsoniana, Elphidium gunteri, E. galvestonense, Palmerinella palmerae, and Ammotium salsum) reveals that two distinct phenotypes are present within each species. Each phenotype of a given pair s linked to the other by transitional phenotypes whose taxobases vary clinally. The distribution of each member of a phenotypic pair is correlated directly with the distribution of salinity and temperature in the bay. Thus, the paired phenotypes are ecophenotypes. Smaller, thinly calcified ecophenotypes having fewer chambers characterize environments that are near optimum for the respective calcareous species; the agglutinant species A. salsum is small, thin, and made of fine grains, in near-optimum environments. Larger, thickly calcified ecophenotypes having more numerous chambers are characteristic of environments that approach minimum tolerances of each calcareous species; A. salsum becomes larger, more inflated, and composed of larger grains in near-minimum environments. Field and la oratory evidence demonstrates that this paired ecophenotypy is caused by contrasting results of delayed reproductive maturation in minimum environments, versus accelerated maturation in optimum environments. Longer growth periods produce larger, thickly calcified tests; shorter growth periods produce smaller, thinly calcified tests. The phenomenon of paired ecophenotypy, although rarely mentioned, has persisted in low- latitude estuaries since at least the early Miocene, as demonstrated by a review of published records. Recognition of this characteristic among the cited and other species (living and fossil) will clarify much of the taxonomic and ecologic confusion that has arisen from close morphologic similarity among estuarine and nearshore marine phenotypes. It also will help provide ore accurate paleoecologic interpretation and correlation of marginal marine strata.

AAPG Search and Discovery Article #90965©1978 GCAGS and GC Section SEPM, New Orleans, Louisiana