Organism interactions and their environmental significance, as exemplified by the Pliocene-Pleistocene fauna of the Kettleman Hills and Humboldt Basin, California

Abstract

Evidence of organism interactions (predatory and non-predatory) in recent samples from the Texas Gulf Coast and Pliocene - Pleistocene samples from the Humboldt Basin and Kettleman Hills of California were studied in order to determine their usefulness in interpreting the fossil record. The Recent samples were analyzed as potential fossil communities to determine the ecologic factors controlling the distribution of organism interactions. The Pliocene - Pleistocene samples of California were analyzed to determine latitudinal and temporal distribution of interaction types and interaction pairs. Seven types of interactions were identified from the Recent and fossil material. They included gastropod predation, algal/fungal borings, clionid sponge borings, polychaete borings, encrusting bryozoans, attached bivalves, and encrusting barnacles. The Texas Gulf Coast samples indicate that the distribution of interaction types was controlled by environmental stress at the sediment/water interface as it controls the availability of "host" shell materials (epifauna) and the viability of "guest" species. The intensity of interaction, with the exception of predation, was dependent on the presence of live epifauna and/or the presence of physical/biological processes which bring shell material to the surface and maintain it there. The Humboldt Basin samples represented a depositional gradient from basin to shoreface. Predation by naticids occurred at all stratigraphic levels independent of water depth, with intensity increasing upwards as a function of available, preservable prey species. The other types of interactions did not appear until stable, shelfal conditions were established. The number of incidents of interaction increased upward in the section with an abrupt decline in the shoreface environments. The Kettleman Hills samples were from relatively stable environments. The interactions did not show changes through time, and host-guest preferences remained invariant. The Kettleman Hills data indicated that interaction relationships did not change within the time frame represented (approximately 4 million years) and that the changes seen in the Humboldt Basin were ecologically, not evolutionarily, produced. Organism interactions provide useful tools for understanding shell accumulations and for making paleoecological interpretations. Detailed study of specific interaction pairs is needed to fully understand the importance of these pairs in the interpretation of the fossil record.