CONTRASTING BIVALVE LATITUDINAL DIVERSITY GRADIENTS SHAPED BY CENOZOIC ORIGINATION PATTERNS

The latitudinal diversity gradient is the most ubiquitous macroecological pattern on Earth, with a wide range of taxa declining in diversity with increasing latitude. Although pervasive, the structure of latitudinal gradients can vary considerably, even within major taxonomic groups. Here, we analyze this variability in marine bivalves using a taxonomically standardized database that currently includes species data from most families within 5 of the 7 major bivalve clades, Anomalodesmata, Arcoida, Mytiloida, Heteroconcha, and Pteriomorpha (N = 695 genera\subgenera, 4395 species, 16,481 occurrences 187 localities). Latitudinal diversity gradients were determined by binning taxa into (1) global 10 degree bins and (2) tropical, temperate, and polar realms. We characterized the diversity history of the major clades using (1) the first appearance of the clade in the geologic record, (2) the median age of all genera within that clade, and (3) the median age of living genera within that clade.

Preliminary results show strong correlations between the gradients of individual clades and the overall bivalve diversity gradient, with the striking exception of the Anomalodesmata. We find no significant correlations between the slope of a diversity gradient and a clade's age. Rather, gradient structures vary with the proportion of Cenozoic originations that have occurred in the tropics. All clades whose genera have a median age in the Cenozoic have 70% or more of their Cenozoic originations in the tropics and a strong, “normal” latitudinal gradient. In contrast, the Anomalodesmatans have no tropical Cenozoic originations and a median generic age in the Mesozoic, and, presumably as a consequence, show an anomalous diversity gradient with peak diversity in the temperate regions. Although we do not yet have a standardized taxonomic framework for the Protobranchs, this group also has a median age in the Mesozoic, low tropical origination rates in the Cenozoic, and a shallow diversity gradient. These results therefore suggest that variability in latitudinal diversity patterns between bivalve clades results from spatial variability in origination within the last 65 Myr, as opposed to extinction patterns, or the age or basal position of a clade.