POPULATION DYNAMICS OF VENERICARDIA HATCHEPLATA

The early Eocene Hatchetigbee Formation at Hatchetigbee Bluff, southwestern Alabama, is characterized by the abundance of the bivalve Venericardia hatcheplata. The majority of individuals are found in discrete horizons only a single shell thick, and most are articulated and in life position. Shells are generally in excellent condition, with little to no post-mortem damage to valves. Size-frequency distributions of articulated specimens indicate that within a bed, there are modal size classes with relatively little variation around them, suggesting that individuals come from one or several contemporaneous cohorts. These observations lead us to conclude that the Hatchetigbee Bluff venericard horizons represent census populations rather than time averaged assemblages characteristic of the majority of the fossil record.

Such snapshots in time allow us to explore spatial and temporal variation in real fossil populations, rather than inferring these variables from time-averaged data. Because expansive bedding plane exposures were unavailable, a 40-meter transect along the outcrop trace of one bed recorded the positions of articulated clams with respect to each other as a proxy for spatial distribution in two dimensions. Comparison of spatial data with modeled Poisson distributions indicates that the distribution of individuals along the transect is random, rather than clumped or overdispersed. This is consistent with observations of other shallow-burrowing marine bivalves, and likely reflects the vagaries of larval spatfall on an effectively homogeneous sediment surface.

Morphometric analysis of clams provides quantitative measurements of mean morphology and variability in morphology within each fossil population. Comparison of analyses from successive bedding planes allows the examination of small-scale evolutionary changes within a species on a time scale generally unavailable to both paleontologists and ecologists, and hence can provide some important information about the evolutionary process.