BODY SIZE TRENDS IN BIVALVIA: AN ASSESSMENT OF BIAS IN THE USE OF DATA FROM MONOGRAPHS

Body size trends among taxa within major clades are potentially related to important evolutionary, paleoecologic and taphonomic transitions in the history of life. For this reason, interest has grown recently in how best to assess these trends. Arguably, the most direct way to determine size trends of any taxon is to directly measure specimens from a representative sample collected from each interval of interest, a feat that is impractical given the large number of taxa that must be assessed in any stratigraphic interval. One alternative, which is evaluated here, is to measure for each taxon a more limited number of individuals illustrated in primary monographs.

To evaluate bias in the collection of size data from monographs, specimens were measured from bulk-collected samples of 20 Neogene bivalve species from the Miocene and Pliocene of Virginia and Maryland. Linear measurements were taken from at least 50 specimens of each species to obtain representative mean and median values for each collection, which were then compared directly to a corresponding set of measurements made directly from a limited number of monographic illustrations of the same species.

Results indicate that specimens figured in monographs are consistently larger than the median sizes obtained from bulk collections, typically greater than the 80th percentile, indicating that monograph-based data reliably capture values near the maximum body size for these species. Additionally, regression of the monographic means onto the bulk sample means for the 20 species demonstrates a significant linear relationship with a slope near unity across a broad size spectrum, suggesting that size trends based on monographic data faithfully mirror those based on bulk collection data. Thus, to the extent that these results can be extrapolated more broadly to the Phanerozoic record, the use of illustrations in the taxonomic literature may dependably capture trends in body size that would have been evident through the direct measurement of much larger numbers of specimens.