THE IMPORTANCE OF ONTOGENY AND PHYLOGENY IN EVALUATING BODY SIZE CHANGE IN THE FOSSIL RECORD: A CASE STUDY OF LYSTROSAURUS (THERAPSIDA, ANOMODONTIA) IN THE KAROO BASIN, SOUTH AFRICA

Patterns of body size change, such as Cope’s Rule, Bergman’s Rule, Foster’s Rule, and the Lilliput Effect, are widely recognized in the fossil record. However, despite the large body of work on body size trends, there is little consensus on the mechanistic underpinnings of these patterns. Existing approaches, including heterochronic frameworks and phylogenetic comparative methods, provide ways to investigate the mechanisms of body size evolution. The age-size-shape space developed by Pere Alberch and colleagues is especially relevant and provides the means to quantify body size and other aspects of phenotype, and to compare ontogenetic trajectories between taxa. Furthermore, evolutionary patterns of body size change cannot be properly evaluated without a robust phylogeny because clade-level patterns of size change only make sense in a stemward-crownward context. Many studies of fossil body size change utilize stratigraphy alone to evaluate the polarity of changes in size, but apparent patterns of size change can look entirely different in the context of a phylogeny.

An excellent example of these issues is the therapsid genus Lystrosaurus, an abundant, widespread taxon that lived during the End Permian Mass Extinction (EPME). In the past decade, Lystrosaurus has been recognized as an example of the Lilliput Effect, a pattern of size decrease during mass extinctions. In addition to a stark decrease in average body size, Triassic individuals appear to be developmentally younger than equivalently-sized Permian counterparts based on bone histology. In a purely stratigraphic context, it appears that Lystrosaurus gets smaller during the EPME, but the current phylogeny implies that Triassic and Permian species diverged before the EPME from a small ancestor. Lystrosaurus species boundaries are notoriously contentious, however, so additional taxonomic and phylogenetic work is needed to ensure that patterns of size change are reconstructed accurately. Furthermore, patterns of size change need ontogenetic context. Although we have relevant data regarding size and age, further data are needed to appropriately relate measures of ontogenetic shape change to age and size. Without these data, the mechanisms driving size change, including ontogeny, species sorting, and taphonomic bias, cannot be identified.