BIODIVERSITY UNHINGED: ASSESSING MEASURES OF BIVALVE DIVERSITY IN A PLEISTOCENE SHELL BED

Using fossil counts to determine biodiversity is critically important to quantifying the history of biodiversity over geologic time scales. Some fossil taxa, like brachiopods, contain easily recognizable, non-repeating morphological features that can be reliably counted as individual specimens. Other fossil taxa, e.g., many echinoderms, lack convenient non-repeating morphologic features for identifying individuals within bulk samples. How many individuals do 10 crinoid columnals or echinoid spines represent? However, even with fossil organisms with fewer moving parts, choosing what and how to count individual specimens can affect perceived biodiversity.

Each valve of a bivalve has only one umbo, which is frequently the most durable part of the valve in addition to being attached to hinge structures that are useful for identifying the bivalve’s taxonomic classification. It is therefore logical to count bivalve umbones when determining how many individual bivalve valves are present in a bulk sample. However, there are other non-repeating elements in the valves of individual taxa that could also be used (e.g., the adductor scar in oysters or the pallial sinus in some burrowing clams), as long as enough taxonomic information remained on the fossil to identify the bivalve. Limiting the sampling of countable specimens only to those with umbones does limit the recorded species richness as specimens that could be classified and added to the species list. Those without umbones are not counted. To address how this issue affects measures of biodiversity, all identifiable bivalve fragments for one bulk sample of the Ft. Thompson Formation (Pleistocene, Florida) have been classified and counted. In addition, a sample of a smaller size fraction (1-4 mm sediment size versus >4 mm) of the same bulk sample was analyzed to determine what, if any, size effects were present. The specific biases of using this technique for quantifying biodiversity will be addressed.