UTILIZING PHYLOGENETIC PALEOECOLOGY WITH PALEOZOIC ARTHROPODS TO EXPLORE MACROEVOLUTIONARY AND MACROECOLOGICAL TRENDS

The importance of both intrinsic biological and external environmental factors in defining macroevolutionary patterns has been recognized since its popularization by G. G. Simpson. However, there is still much discussion about how the genealogical and ecological hierarchies interact, and the general repeatability of the evolutionary outcome of any given situation. Phylogenetic paleoecology, through combining tree-based frameworks of relationships with geologic paleoenvironmental data, is one way to explore how clades respond to broad-scale changes in environment. Arthropods are an excellent group on which to conduct such studies due to their character-rich hard external exoskeleton. Combining studies of phylogeny, empirical morphospace, and environmental occupation, I compare variations in diversity and disparity across the evolutionary history of two Paleozoic arthropod groups, the Eurypterida and Xiphosurida. Both groups undergo a change in evolutionary regime during the Late Devonian, linked to an ecological transition from marine to freshwater environments. In eurypterids the change manifests as a shift from a ‘boom and bust’ regime to ‘sluggish’ macroevolution resulting in the lineage exhibiting low morphological diversity alongside a decrease in endemism. Xiphosurids, however, show the opposite trend, with the invasion of non-marine environments resulting in a shift away from bradytely to ‘boom and bust’ conditions and a proliferation of new species that occupy novel regions of morphospace. Interestingly, other xiphosurid groups that later invade non-marine environments during the Mesozoic show a similar ecological pattern of differentiation, speciation and subsequent extinction. Shifts in ecology therefore have long-term impacts on a lineage’s macroevolutionary trajectory, with eurypterids experiencing a permanent shift in morphospace in response to the regime change in the Devonian. These results demonstrate that disparate lineages independently shifted environmental occupation in response to the Devonian biotic crisis, and that different clades may exhibit distinct evolutionary responses to same ecological changes, but that such responses may also be phylogenetically conserved within clades.