Spatial and temporal heterogeneity in late Cretaceous and early Tertiary shallow-shelf benthic marine assemblages

The Mesozoic - Cenozoic transition is a pivotal time in the evolution of benthic marine assemblages but the details of the timing and drivers of these changes are poorly known. The Atlantic and Gulf Coastal Plains of the United States contain assemblages preserved as original aragonitic and calcitic material in unconsolidated sediments. This makes Coastal Plain assemblages ideally suited to paleoecological analyses. Data derived from bulk samples of the Coffee (lower/middle Campanian: Mississippi) as well as published faunal lists from comparable samples of the Severn (Maastrichtian: Maryland), Providence (Maastrichtian: Georgia & Alabama), Stone City (Eocene: Texas) and Gosport (Eocene: Alabama) formations are used to assess changes in ecomorphological group (life habit and trophic group) composition through this time interval.; These analyses find significant decreases in rarefied sample species-richness from the Campanian through the Eocene. With the notable exception of the Stone City Fm., increases in carnivore (neogastropod) richness and abundance occurred before the Campanian. Epifaunal suspension-feeding species are a smaller proportion of the sample richness in the Eocene samples than in the Cretaceous samples. Decreased relative epifaunal suspension-feeder biomass but unchanged relative numbers of epifaunal suspension-feeder individuals suggests a relative decrease in epifaunal suspension-feeder size. The relative biomass of infaunal-deposit feeders decreased. Changes found in the structure of local shallow-shelf benthic assemblages from the Campanian through the Eocene are generally small relative to the variability between samples. Formation-level variation between assemblages is high relative to the magnitude of the temporal signal, emphasizing the need for investigators to include multiple formations per interval in tests of temporal trends.; Four bulk-samples from the Coffee Formation are used to assess inclusion criteria and relative abundance metrics as factors biasing paleoecological comparisons. Choice of relative abundance metric (number of specimens, number of individuals, and four different sources of measurements for calculating biomass) is found to have a greater effect than inclusion criterion (minimum specimen completeness required to be counted) on the relative abundance of ecomorphological groups. These results suggest that sample size is more important than inclusion criterion in obtaining reliable abundance data from fossil assemblages.