BODY SIZE TRENDS AND ENERGETIC CONSTRAINTS PREDICT DECLINING ABUNDANCE OF TRILOBITES IN THE ORDOVICIAN OF NORTH AMERICA

Energy use is the most fundamental metric of ecological importance, and energetic data are essential to a mechanistic understanding of ecosystem structure through time. Estimating the energy use of fossil taxa is inhibited by taphonomic loss of metabolic information, but among members of a single clade with stereotyped metabolic biochemistry the primary determinants of energy use are body size and abundance -both of which may be estimated in well-preserved fossil assemblages. We integrate body size and abundance data to examine a controversial ecological transition in the fossil record: the declining relative abundance of trilobites through the Ordovician Period in North America, which is apparently decoupled from trilobite diversity trends. Using a database of bed-scale paleocommunity samples and trilobite body sizes compiled from our own fieldwork and published sources we show that declining relative richness and abundance is accompanied by increased mean body size, so that total sampled biovolume is approximately invariant through this interval.

Because individual metabolic rates increase with mass, body size imposes a strong energetic constraint on maximum population density: numerous studies have shown that maximum abundance scales approximately as body size to the -3/4 power across ecologically and phylogenetically disparate taxa. This allometric relationship provides a null prediction of abundance change given any trend in body size distribution. To test the predictive power of body size trends, we constructed a simple model which generates simulated relative abundance distributions for all trilobite taxa in our dataset using –3/4 power scaling and compared the results to the actual relative abundance data. As would be expected given numerous taphonomic biases, the correlation is weak but highly nonrandom when the relative abundance of individual taxa within collections is considered. However, the model predicts the average abundance of trilobites within any time interval with high accuracy. These results suggest that energetic tradeoffs between body size and population density may be an important component of the Ordovician decrease in relative abundance of trilobites, and highlight the potential for extending macroecological insights into the fossil record.