QUANTIFYING COLLECTION BIASES IN BULK, MUSEUM AND LITERATURE-BASED MOLLUSCAN SAMPLE DATA

Researchers are increasingly using previously collected information (e.g., monographic data, museum specimens and internet databases) to answer questions about the history of biodiversity and related macroevolutionary patterns. Such compilations provide an independent and cost-effective method to acquire enormous amounts of data that may not be feasible to collect otherwise. However, such faunal compilations may misrepresent the true nature of ancient faunal assemblages in terms of abundance structures, preservation and median taxon size. This research provides a quantitative comparison between museum-based collections and subsequent publications to those obtained by bulk field collection.

Museum collections assessed are from the Chesapeake Group housed at the Virginia Museum of Natural History in Martinsville, VA. To quantify potential differences between bulk and museum collections, bulk samples (5-10 kg) were field collected from the Calvert, Choptank, St. Marys, Eastover and Yorktown formations in the Chesapeake Bay area. Abundance counts for specimens larger than 1 mm were taken for each sample, and individual specimens were tabulated for various taphonomic characteristics. Field collected bulk samples were compared with museum collections, monographic literature, or faunal lists (field notes) obtained from the same localities. Extensive museum collections and associated literature often permitted a bed by bed comparison.

Preliminary data from the Eastover Formation (upper Miocene) suggest that sample diversity is higher in literature-based sources (monographs and unpublished field notes) (S=88) than either museum-based collections (S=32) or bulk samples (S=50). Bulk samples provide the closest approximation to true relative taxon abundances. Museum-based collections are better preserved and suggest that bulk-collected specimens of large taxa may consist predominantly of juvenile or intermediate-sized specimens (µ=10.3 cm museum; µ=4.6 cm bulk for a within-horizon comparison of average collected widths of the bivalve Chesapecten middlesexensis). Thus, bulk samples may not be adequate for various types of studies where large numbers of an individual taxon are needed, but significant taphonomic and collection biases exist in museum collections and faunal lists within literature derived from the same localities.