RECOVERY AND RADIATION FOLLOWING THE END-ORDOVICIAN MASS EXTINCTION: TESTING FOR EXPANSION AND CONVERGENCE IN GRAPTOLITE MORPHOLOGICAL GUILDS

The Upper Ordovician extinction event brought about an enormous change in pelagic graptolite taxa. While several clades were extinguished entirely, a single group of graptolites survived to diversify in the Lower Silurian. After the extinction event, this new fauna developed a range of novel colony shapes as well as forms that closely resemble those of the extinct clades, suggesting convergent evolution. Specifically, we hypothesize that graptolites surviving the extinction re-radiated and produced a range of morphologies comparable to that existing pre-extinction, in order to conform to similar planktonic guilds as their extinct predecessors. Measures of disparity (the diversity of morphological form) may be examined by using a set of discrete characters and a dissimilarity metric. Our data set consists of 52 coded characters that capture morphological details such as overall rhabdosome shape and thecal aspect, but also include binned measurements of size. Data were obtained from a representative sampling of species of each of the major Ordovician and Silurian graptolite clades (defined by prior phylogenetic studies) that have been placed within a global chronostratigraphic framework constructed via CONOP9. Based on this data, we compare the disparity of Ordovician and Silurian graptolites (Katian to Llandovery) and test our hypothesis of convergence in form.