TEMPORAL CHANGES IN BODY SIZE: AN EXAMINATION OF SYNOPTIC AND SPECIMEN DATA, BIOGEOGRAPHIC, ENVIRONMENTAL, AND TAPHONOMIC CONTROLS

Body size is influenced by a range of physiological, ecological, and evolutionary processes, and as a result, analyses of temporal size trends may reveal the consequences of global environmental change and evolving ecosystem structure. For instance, increases in nutrient supply and primary productivity are often proposed to explain increases in animal size in the fossil record. Despite recent interest in patterns of body size among marine invertebrates across evolutionary transitions and biodiversity crises, few studies have rigorously quantified temporal changes in body size within a model group, where aspects of biogeography, environment, preservation, and data biases can be constrained.

Two complimentary datasets were constructed for molluscs from the Cenozoic of western North America. The synoptic dataset is composed of maximum reported body size data for unique species as derived from the literature. These measurements are used here as a proxy for the body size of given species across their geographic and temporal distribution. The specimen dataset comprises body size measurements of specimens belonging to a restricted number of species-rich families that have an extensive Cenozoic fossil record. Most data were generated directly from specimens captured through the Cenozoic of the Eastern Pacific (EPICC) digitization project at the Natural History Museum of Los Angeles County (LACMIP).

These data are used to investigate general trends in body size through the Cenozoic, their relationship to global environmental change, and potential biases. The synoptic dataset reveals a gradual decrease in the mean of maximum reported body sizes between the Paleocene and Eocene, and a sharp increase through the late Paleogene and Neogene, culminating in maximum sizes during the late Miocene. A sharp drop in mean maximum size to the present day is coincident with an increase in the number of small species described from the Recent that are reported in the late Neogene fossil record. This may relate to increased availability of unlithified sediments, the broad range of depositional environments preserved in the Quaternary fossil record, or taxonomic biases. Whether any of these changes reflect primarily ecological or environmental selective pressures on body size remains unclear.