PHYLOGENETICALLY CLUSTERED ORIGINATION AND EXTINCTION FOLLOWING THE LATE ORDOVICIAN MASS EXTINCTION

Mass extinctions can have dramatic effects on the trajectory of life, but in some cases the effects can be relatively small even when extinction rates are high. For example, the Late Ordovician mass extinction is the second most severe extinction in terms of the number of genera eliminated, yet its effects on the dominance of major taxonomic groups was relatively minor. In contrast the end-Cretaceous mass extinction was less severe but brought about notable shifts to the evolutionary trajectories of major clades. Incorporating the phylogenetic relatedness of taxa into studies of mass extinctions can add insight into the factors driving diversification, many of which are thought to be phylogenetically conserved and should therefore promote diversification of closely related taxa. Here, we test whether there was phylogenetic selectivity in extinction and origination across the Late Ordovician mass extinction boundary using brachiopod genera from the Middle Ordovician through the Devonian. Using an index of taxonomic clustering (R_CL), we find that both extinctions and originations shift from phylogenetically random in the Ordovician to significantly clumped within families in the Silurian and Devonian. Of the eight stratigraphic intervals analyzed for the Ordovician, only 3 show taxonomically clumped extinctions (the Late Ordovician mass extinction is one of these, but is only moderately significant) and one shows clumped originations. However, 7 of the 10 stratigraphic intervals analyzed from the Early Silurian through the Late Devonian show significantly clumped extinctions and 9 show significantly clumped originations. Therefore, the Late Ordovician mass extinction marks a significant and permanent shift from phylogenetically random to clustered origination and extinction. This pattern is opposite to results for the end-Cretaceous, which produced a shift from clustered to random extinction patterns.