We infer the body-size scaling slope of metabolic rate in a trilobite by applying a cell-size model that has been proposed to explain metabolic scaling in living organisms. This application is especially tractable in fossil arthropods with well-preserved compound eyes because the number and size of eye facets appear to be useful proxies for the relative number and size of cells in the body. As a case study, we examined the ontogenetic scaling of facet size and number in a ∼390-Myr-old local assemblage of the trilobite Eldredgeops rana, which has well-preserved compound eyes and a wide body-size range. Growth in total eye lens area resulted from increases in both facet area and number in relatively small (presumably young) specimens, but only from increases in facet area in large (presumably more mature) specimens. These results suggest that early growth in E. rana involved both cell multiplication and enlargement, whereas later growth involved only cell enlargement. If the cell-size model is correct, then metabolic rate scaled allometrically in E. rana, and the scaling slope of log metabolic rate versus log body mass decreased from ∼0.85 to 0.63 as these animals grew. This inferred age-specific change in metabolic scaling is consistent with similar changes frequently observed in living animals. Additional preliminary analyses of literature data on other trilobites also suggest that the metabolic scaling slope was <1 in benthic species, but ∼1 in pelagic species, as has also been observed in living invertebrates. The eye-facet size (EFS) method featured here opens up new possibilities for examining the bioenergetic allometry of extinct arthropods.

Metabolic rate is commonly thought to scale with body mass to the 3/4-power as a result of universal body-design constraints. However, recent comparative work has shown that the metabolic scaling slope may vary significantly among species and higher taxa, apparently in response to different lifestyles and ecological conditions, though the precise mechanisms involved are not well understood. To better understand these under-appreciated ecological effects and their causes, it is important to control for extraneous phylogenetic and environmental influences. We demonstrate how this may be done by comparing the ontogenetic scaling of resting metabolic rate among populations of the same species (the amphipod Gammarus minus) in mid-Appalachian freshwater springs with similar, relatively constant environmental conditions, except for the varying presence of the predatory fish Cottus cognatus. We found that populations of G. minus exhibit significantly lower metabolic scaling slopes (0.54 to 0.62) in three freshwater springs with C. cognatus than in two springs without these fish (0.76 to 0.77). We tested multiple hypothetical causes for these population differences. Our results best supported the hypothesis that metabolic scaling was influenced by the effects of size-selective predation on the ontogeny of growth, a metabolically expensive process. The body-size scaling of growth is significantly less steep in the populations inhabiting springs with versus without fish, thus paralleling the interpopulation differences in metabolic scaling. Prematurational growth of G. minus is as high or higher in the fish springs, whereas postmaturational growth is significantly lower, often approaching zero. Similarly the amphipods in the fish springs tend to have higher metabolic rates at small sizes, but lower metabolic rates at large sizes, compared to those in the fishless springs. Our results do not support other hypothetical causes of the interpopulation variation in metabolic scaling, including differential scaling of cell size or low-metabolism body components (fat and mineralized exoskeleton), or possible effects of other environmental factors associated with the presence of fish. However, fish-induced population differences in adult behavioral activity may influence metabolic scaling in G. minus, a possibility under current study. We conclude that ecological factors may significantly influence metabolic scaling, contrary to common belief.