GRAPTOLITE BIOGEOGRAPHY: USING PALEO-GIS TO EXAMINE THE EVOLUTIONARY DYNAMICS OF EARLY PALEOZOIC ZOOPLANKTON

The relationships between geographic range and evolutionary dynamics, particularly taxonomic duration, have been analyzed for a number of fossil groups, mostly with benthic lifestyles. The exceptionally well controlled species longevities documented for planktonic graptolite species (in addition to their abundant distribution and well sampled fossil record) make them ideal fossils for testing hypotheses regarding the relationship of geographic range to evolutionary dynamics. Using the Mitchell et al. (2007) Diplograptoidea phylogeny as the basis for our study, we coded 87 graptolite species for presence-absence at 31 different Middle and Late Ordovician localities around the world, as well as for their sub-clade membership and biotope association. For each species we obtained a stratigraphic duration in millions of years from a CONOP9-generated graptolite range chart and timescale (Sadler et al., 2010), and calculated a geographic range in km using Paleo-GIS software (Scotese, 2004, 2006). Sub-clade associations included undulograptid, normalograptid, eoglyptograptid, dicranograptid, retiograptid, climacograptid, and diplograptid. Biotope assignments were to cosmopolitan epipelagic (CE), cosmopolitan mesopelagic (CM), or endemic cratonic (CrE). We then tested for correlations between stratigraphic duration and geographic range among all the Diplograptoidea, as well as among groups defined by sub-clade membership and biotope association. Using Pearson’s correlation, stratigraphic duration and geographic range were significantly correlated (r = 0.392; p = 0.004) for all Diplograptoidea. Within biotopes, only among members of the CE biotope exhibited a significant correlation between range and duration (r = 0.707, p = 0.000). Among the Diplograptoidea sub-clades, diplograptids and dicranograptids had significant range/duration correlations using Pearson’s correlation (r = 0.669, p = 0.002; and r = 0.574, p = 0.007, respectively). Using non parametric tests, diplograptids, dicranograptids, and climacograptids all had significant range/duration correlations. Future studies will examine the effects of sampling and fossil recovery biases on the likelihood of a taxon actually occurring at a particular locality and time but being coded as absent.