USING GIS TO STUDY THE PALEOBIOGEOGRAPHY AND MACROEVOLUTION OF SOFT-BODIED CAMBRIAN ARTHROPODS

Soft-bodied Cambrian arthropods from Burgess Shale-type deposits are some of the most famous and extensively studied fossil invertebrates, but global species-level assessments of their biogeographic patterns have not previously been explored quantitatively. We developed a new occurrence database of Cambrian soft-bodied arthropods from literature sources and museum collections and used this database to explore the paleobiogeographic context of the Cambrian radiation of arthropods. This was done using Geographic Information Systems (GIS) analysis and PaleoGIS, which allowed fossil locality positions (n=51) to be mapped onto Cambrian paleogeography. We compared spatio-temporal distributional patterns of soft-bodied arthropods with those of co-occurring trilobite taxa. Distributional patterns of individual species were assessed using both distance and area based methods, which give correlated results. We found that soft-bodied Cambrian arthropods show both statistically larger geographic ranges and greater stratigraphic longevities than co-occurring trilobite species. Further, we found a significant correlation between geographic range and survivorship in the non-trilobite arthropods. Non-trilobite arthropods appear biogeographically buffered from extinction, relative to trilobites, and also probably experienced reduced rates of speciation. Many of the most widely distributed soft-bodied arthropods were likely members of the Cambrian nekton; a notable exception to this pattern is provided by the most widely distributed species, Naraoia compacta, which almost certainly had a benthic habit. Our analysis also uncovered a possible example of biological invasion across widely separated regions: Anomalocaris saron may have invaded western Laurentia from China sometime between the early and Middle Cambrian. Our results suggest that soft-bodied Cambrian arthropods, including many stem-group taxa, were exceptional dispersers; they may be ecological analogues to certain extant marine arthropods. Further, many of the biological and environmental factors that today govern the distribution of organisms were also important early in the history of animal evolution and also influenced macroevolutionary patterns in a manner similar to other time periods in the Phanerozoic.