The Few the Brave, the Marine: taxonomic structure of Neogene benthic associations

Mollusk-rich benthic associations dominate the Cenozoic marine fossil record and are a key source of data for studying secular trends in metazoan biodiversity and other marcoevolutionary and macroecological patterns. We evaluate the taxonomic structure of those associations using a geographically extensive, facies-controlled dataset of fossiliferous samples of the Late Cenozoic age (Miocene – Quaternary). A total of 489 bulk samples, including 8493 occurrences of 379 genera, were processed using a uniform sampling methodology and standardized taxonomy. These samples -- collected from Miocene, Pliocene, and Pleistocene sequences of the U.S. Coastal Plain (western Atlantic), North Sea Basin (eastern Atlantic), central Europe (Paratethys), and Italy (Mediterranean Sea) -- represent fully marine, siliciclastic depositional environments from mid latitudes and are overwhelmingly dominated by benthic gastropods and bivalves. The genus and family level analyses indicate that (1) the diversity-abundance structure of the pooled data fits closely a log-normal distribution; (2) the same several genera and families consistently dominate all analyzed samples, regardless of their stratigraphic position and geographic location; (3) the dominant genera represent small-sized forms from lower trophic levels (i.e., suspension feeders, grazers, deposit feeders), and all belong to groups often categorized as r-selective strategists; and (4) the persistent dominance of those few genera is unlikely to be the “garbage taxon” effect (the patterns persist at the family level and most genus occurrences are monospecific). Although samples encompass multiple continents and time intervals, the pooled data fit a classic diversity-abundance model as tightly as the best-behaving ecological datasets from single habitats: either the Neogene marine biosphere was extremely homogenous or the diversity models are impervious to our dataset, which samples primarily common taxa. Despite high diversity captured in our samples, the dominant taxa are not only ecologically uniform, but also limited, through time and space, to repeated occurrences of the same lower taxa. The abundance structure of the Neogene marine benthic associations appears to have been extremely homogeneous both ecologically and taxonomically