Local paleocommunities and paleocommunity types, but no paleocommunities: An example from the Yorktown Formation (Pliocene) of eastern Virginia.

G.M. Daley
Geological Society of America Abstracts with Program 31

The terminology of paleocommunity studies sometimes obscures the patterns that exist in fossil assemblages. To quantify the patterns seen at different temporal and spatial scales, it has been suggested that a multilevel terminology be used. The most spatially constrained level, the local paleocommunity, is derived from aggregates of statistically similar local living communities, i.e., the remains of organisms that lived in a certain place over a period of time. The next scale of resolution is the paleocommunity level, which has been defined as the aggregate of all local paleocommunities that are similar enough to each other that they can not be shown to be statistically different. The least spatially constrained level is the paleocommunity type, which is the aggregate of local paleocommunities that are similar but statistically distinguishable. The fossil assemblages of the well-exposed and grossly fossiliferous Pliocene Yorktown Formation were used to test the occurrence and recurrence of local paleocommunities, paleocommunities, and paleocommunity types.

Replicate samples of the silty sands of the Rushmere Member and the sandy silts of the Morgart's Beach Member were taken at 30 cm intervals at several localities in eastern Virginia. The 145 samples yielded nearly 30,000 specimens of mostly bivalves and gastropods which were assigned to 140 species. Each species was further assigned to one of 17 ecologic guilds based on its life habit. The data set was analyzed using principle components analysis, ANOVA, MANOVA, and discriminate canonical analysis.

At the outcrop level, samples taken from similar paleoenvironments within an outcrop were very similar, and frequently statistically indistinguishable. Therefore, local paleocommunities at each outcrop were well defined. At the largest spatial scale, local paleocommunities tended to be similar enough to plot together in PCA space, especially when the guilds were considered, but MANOVA of local paleocommunities at different localities showed they were statistically different. Therefore, while the local paleocommunity and paleocommunity types are both found, the paleocommunity is absent.