The Biogeography and Macroevolutionary Trends of Late Paleozoic Cephalopods in the North American Midcontinent Sea: Understanding the response of pelagic organisms to changing climate during the Late Paleozoic Ice Age

Abstract

Geographic range is an important macroevolutionary parameter that is frequently considered in paleontological studies. Species distribution and geographic range size are determined by a variety of biotic and abiotic factors that are well known to affect the differential birth and death of species. Thus, considering how species distributions and geographic range sizes fluctuate over time can provide important insight into evolutionary dynamics across the geologic time scale. In this study, I examine how geographic range size and rates of speciation and extinction changed throughout the Pennsylvanian and Early Permian in the North American Midcontinent Sea in an important pelagic clade, the Cephalopoda, using Geographic Information Systems (GIS). This period is particularly interesting for biogeographic and evolutionary analysis because it is characterized by repetitive glacial cycles, a global transition from an icehouse to greenhouse climate during the Late Paleozoic Ice Age and sluggish macroevolutionary dynamics, i.e. low speciation and extinction rates, that have been repeatedly documented in studies of other marine invertebrate taxa from both Gondwana and Pangea. The analyses presented herein indicate that cephalopod species diversity fluctuated throughout the Pennsylvanian and Early Permian matching the findings of other studies that examined diversity components of the Late Paleozoic marine invertebrate fauna. However, contrary to studies that focused on benthic taxa, my analyses found that: mean geographic range size of cephalopod species did not change significantly through time, despite numerous climate oscillations throughout this period; and further, geographic range size did not correlate with macroevolutionary rates. This result suggests that pelagic organisms may have a different response to climate change than benthic organisms and thus additional consideration of this issue is needed. Finally, these analyses indicate that, in the case of cephalopods, macroevolutionary patterns during the Late Paleozoic were more ‘dynamic’ than previously characterized. Thus, the macroevolutionary patterns recovered may depend on the taxon analyzed.