THE TAPHONOMIC FIDELITY OF SEED SIZE IN FOSSIL ASSEMBLAGES: A LIVE-DEAD CASE STUDY

The fossil record provides key data for testing ecological hypotheses on geologic time scales but it is critical to assess the taphonomic filters before interpreting patterns. Seed size is related to reproductive strategy, dispersal, growth form and phylogeny. The seed size distribution of species within a flora is correlated with climate and community type. Several studies have suggested that within-flora seed size distributions have changed over geologic time but it is not clear how well fossil seed assemblages reflect the original vegetation. Taphonomic filters such as transport, predation, biodegradation and abundance may affect the fidelity of fossil seed assemblages. Seeds may be selectively destroyed or transported on the basis of size, thus causing the distribution of a fossil assemblage to differ from that of the source community. To assess the effects of these taphonomic filters, a live-dead study was performed in which the seed size distributions of a potential fossil assemblage and the living plant community that it represents were compared. 20 six-inch diameter soft-sediment cores were collected in two transects parallel to the shore in a tidal estuary in Edgewater, Maryland. The transects grade from pebbly clay to silt to organic-rich mud. The sediment cores were sieved and the seeds assigned to 44 taxa (potential fossil or death assemblage). 40 seed plant species were identified on the adjacent hillside (life assemblage). Seed mass was determined from herbarium specimens and literature. Although species of different growth forms and dispersal mechanisms had significantly different distributions, results indicate that the differential production and dispersal of seeds in these categories does not appear to have had significant effects on the death assemblage distribution. A composite taxon list was drawn from all cores and the resulting combined death assemblage was not significantly different from the life assemblage (P=0.1259). This case study indicates that fossil seed size distributions can be reliable indicators of an important paleoecological variable of the original source vegetation when a reasonable sample size is considered, although similar tests must be considered for other depositional settings.