ASSESSING THE USEFULNESS OF LITERATURE-DERIVED ESTIMATES OF BODY SIZE

Trends in body size may be related to a range of evolutionary, paleoecologic and taphonomic questions. Efficient assessment of these trends is therefore important for our understanding of the history of life. Here, we evaluate the bias inherent in the use of size data provided by photographs in monographs.

The observed monographic bias can be assessed with respect to four different end-member outcomes. In the best case, monographs yield unbiased estimates of the central tendency of the sampled population. Alternatively, the monographs may yield biased but predictable (“inaccurate but precise”) estimates of size. This is acceptable as long as the bias does not vary across time, space, or taxa. Finally, monograph data would be deemed unacceptable for the reconstruction of size trends if they follow either an “accurate but imprecise” or “inaccurate and imprecise” model (i.e., high imprecision implies very low informative value of monograph estimates).

We targeted several species of Neogene bivalves (n=20) and Ordovician brachiopods (n=10) for which we could acquire, from the same locality, data on both monographic and bulk sample estimates for a given species. For each species, we then compared the central tendencies of the monographic measurements against the central tendencies of the bulk sample. The size bias of monographic specimens was also estimated by its percentile value, relative to the corresponding bulk sample estimates. These two approaches allow us to quantify the magnitude and consistency of the monographic bias and test our data with respect to the four end-member scenarios.

Results suggest that monographic specimens are consistently larger than the average sizes of those from bulk samples for the corresponding species. For brachiopods and bivalves, the majority of monographic specimens are located above the 60th percentile of the corresponding bulk samples. However, the central tendencies of monograph estimates for each species show significant correlations with estimates derived from the corresponding bulk samples. Thus, the data follow the “inaccurate but precise” scenario very well. Just as importantly, the bias appears to be consistent across higher taxa and through time. Thus, monographic data should yield meaningful estimates of body size trends through time.