Morphological Patterns in Late Ordovician and Early Silurian Graptolites

The Late Ordovician extinction event brought about an enormous change in pelagic graptolites. While several major clades were extinguished entirely, a single group of graptolites survived to diversify in the Early Silurian. A morphological analysis through the extinction event and into the following recovery (Katian through Rhuddanian) illuminates the morphological pressures placed on the graptolite clades by the extinction event. Data were obtained from a representative sampling of one-hundred and seventy-one species drawn from among the graptolite clades (defined by prior phylogenetic studies). Our data set consists of forty-five discrete characters that capture morphological details but also include four measurements of size. The diversity of morphological form was analyzed using measurements of disparity and a non-metric multidimensional scaling analysis (NMDS) to ordinate the data. Late Ordovician graptolites clustered in several areas of the morphospace, and our results suggest they underwent morphological selection in the end Ordovician that greatly restricted the range of realized morphologies eliminating taxa other than those bearing a particular conservative colony form. During the subsequent radiation, graptolites returned to those same areas of the morphospace from which mass extinction had removed them. This morphological convergence suggests that graptolite morphology was highly dependent on extrinsic ecological constraints.