METABOLIC RATE SCALING OF THE TRILOBITE ELDREDGEOPS RANA

Basal metabolic rate has long been presumed to scale with body mass (M) raised to the ¾-power. Subsequent efforts to document this relationship have found that, although power-law scaling of metabolism is ubiquitous in nature, the exact scaling exponent varies widely generally about 2/3 to 1. One explanation of power-law scaling, termed the cell-size model, predicts that in organisms that grow exclusively by cell proliferation, metabolic rate should scale as M¹, whereas in organisms that grow exclusively by cell enlargement, metabolic rate should scale as M^2/3. Intermediate scaling exponents are produced by combining both modes of growth. In living arthropods, there is evidence that the number and size of facets in the compound eye are a proxy for the total number and size of cells in the body, thus potentially allowing metabolic scaling to be estimated for trilobites, which often have well-preserved eyes. We measured changes in eye facet size and number during the ontogeny of Eldredgeops rana in order to estimate metabolic scaling for this extinct presumably benthic species. Specimens were obtained from museum collections (Paleontological Research Institute and the Smithsonian Institution) and digitally imaged. During the early stages of development, the schizochroal eyes of E. rana appeared to increase in size by a roughly equal contribution of facet (cell) proliferation and enlargement. Thereafter, the number of facets remained roughly constant and eye growth was almost exclusively due to facet (cell) enlargement. If the cell-size model is correct, these results suggest that E. rana exhibited allometric metabolic scaling, as documented for many modern arthropods. Species-specific developmental or functional (adaptive) visual mechanisms may also importantly influence the growth patterns of trilobite eyes. We are currently collecting additional data for other trilobite species with holochroal eyes and varying ecological lifestyles (e.g., putatively benthic versus pelagic) to test the robustness of our findings.