Ontogenetic mechanisms of size change: Implications for the Lilliput effect and beyond

Abstract

Body size has a long history of study in paleobiology and underlies many important phenomena in macroevolution. Body-size patterns in the fossil record are often examined by utilizing size data alone, which hinders our ability to describe the biological meaning behind size change on macroevolutionary timescales. Without data reflecting the biological and geologic factors that drive size change, we cannot assess its mechanistic underpinnings. Existing frameworks for studying ontogeny and phylogeny can remedy this problem, particularly the classic age-size-shape space originally developed for studies of heterochrony. When evaluated based on metrics for age, size, and phenotype in populations, proposed mechanisms for size change can be outlined theoretically and tested empirically in the record. Using this framework, we can compare ontogenetic trajectories within and between species and determine how changes in size emerge. Here, we outline ontogenetic mechanisms for evolutionary size change, such as heterochrony, as well as how geologic factors can drive apparent, non-biological size change (e.g., taphonomic size sorting). To demonstrate the utility of this framework in actual paleobiological problems, we apply it to the Lilliput effect, a compelling and widely documented pattern of size decrease during extinction events. However, little is known about the mechanisms underlying this pattern. We provide a brief history of the Lilliput effect and refine its definition in a framework that can be mechanistically tested. Processes that likely produce Lilliput effects include allometric and sequence repatterning (including heterochrony) and evolutionary size-selective sorting. We describe these mechanisms and highlight relevant examples of the Lilliput effect for which feasible empirical tests are possible.