MEASURING SHELL SIZE WITHOUT MEASURING SHELL SIZE: A TIME-EFFICIENT APPROACH TO BODY SIZE ANALYSES

Body size estimates derived from skeletal remains are one of the most important quantifiable parameters accessible in the fossil record. Unfortunately, to study body size patterns, it is often necessary to estimate a sample-level proxy of central tendency (e.g., sample mean, sample median, etc.) via individual measurements of large numbers of specimens. However, this time-consuming process can potentially be simplified by exploiting an obvious relationship between size and weight. If there is a correlation between average body size of a known number of specimens and the total weight of that set of specimens, then the average size of complete specimens in a sample could be estimated by the expedient method of weighing the entire sample.

To test this approach, we acquired linear size and mass measurements for 363 complete valves of the infaunal bivalve Varicorbula gibba, representing 14 samples collected from the Upper Quaternary sequences of Po Plain, Italy. For each sample, all complete specimens were counted and bulk weighted using a high-precision balance. For each complete specimen, valve length and width were measured to the nearest 0.1mm and valve mass was estimated to +/- 0.0005g. Results show that valve dimensions correlate tightly with valve mass, regardless of taphonomic alterations. More importantly, sample weight and average valve length/width correlate strongly, indicating that average length can be predicted reliably by simply weighing and counting all complete specimens. Given the importance of sample-level estimates of body size, the approach proposed here may offer an attractive alternative to the tediousness of countless measurements. Replacing hundreds of measurements per sample, with a simple sample-weight-specimen-count protocol may make it possible to routinely incorporate body size data into large-scale quantitative studies.