A PHYLOGENETIC ANALYSIS OF THE BRACHIOPOD GENUS LEPTAENA

Leptaena is a taxonomically diverse and long-ranging genus that extends from the Middle Ordovician through the Lower Devonian that survived two of the ‘Big Five’ mass extinctions - the end-Ordovician and late-Devonian events. Given this genus’s perseverance, Leptaena is a potential source of information regarding survivorship in mass extinctions and can aid in predicting the responses of organisms to the current biodiversity crisis. This study aims to establish the phylogenetic relationships between species of Leptaena and compile occurrences of this genus in the PaleoDB Database. We scored Leptaena species for 36 characters involving internal and external shell features. Then, we conducted a parsimony and maximum likelihood analyses, the latter of which incorporated the stratigraphic data of each taxon in order to derive a complete tree that benefits from the information provided by the fossil record, as well as disparity analyses. Our results show high morphological disparity in the Ordovician with a significant decrease after the end-Ordovician mass extinction which suggests a common evolutionary behavior seen before in other studies where high rates of change occur in the early history of a genus. Assessing the end-Ordovician extinction proved to be more complex due to the possible convergence of younger Leptaena species towards older forms lost during extinction: although parsimony trees suggest many survivors, disparity analyses indicate a major drop of disparity after the Ordovician followed by slow re-expansion recapturing pre-extinction morphospace. Future work will involve the addition of more taxa in order to obtain a more complete tree and properly assess the behavior of evolutionary relationships during the first two Paleozoic mass extinction events. Further goals include investigating the ecologies and phylogenetic relationships between species that survived the end-Ordovician mass extinction, as this may help us to predict survivorship among biomineralizing benthic clades to ongoing global change.