PHYLOGENETIC PATTERNS OF STROPHID BRACHIOPODS AND THEIR RELATIONSHIP TO THE ORDOVICIAN MASS EXTINCTION EVENT

The order Strophomenida was an ecologically abundant and taxonomically diverse group of Palaeozoic brachiopods that originated in the earliest Ordovician and went extinct in the Carboniferous. The group has classically been divided into two major superfamilies, the Strophomenoidea (strophids) and the Plectambonitoidea (plectams). Despite being treated as separate clades, it has often been assumed that the plectams form a paraphyletic grade into the strophids. During their long geologic range, the Strophonmenida survived two of the “Big Five” mass extinction events, the end Ordovician and the late Devonian, suggesting that they are potentially informative taxa for studying the evolutionary effects of these two distinct mass extinctions, each with drastically different forcing mechanisms. However, while there have been previous studies on species relationships within single genera of the Strophomenida, the higher level relationships of the group are still largely unknown. A detailed higher level parsimony based phylogenetic analysis of the Strophomenida is presented, consisting of 70 characters and 65 exemplar species sampled from every classically defined family. Several species of basal chonetids have also been included in this analysis, since they have traditionally been considered to be closely related to the Strophomenida and share several characters with both the plectams and strophids. The results of this study suggest that the plectams, as originally defined, are paraphyletic to the monophyletic strophids. Furthermore, the chonetids are reconstructed as a monophyletic group that is sister to the strophids, suggesting that their proper placement might be within the Strophomenida. The taxonomy is redescribed to ensure monophyletic taxonomic groups, and the overall topology is analyzed to determine if the end Ordovician and late Devonian mass extinction events had demonstrable effects on the overall evolution of the group; i.e. if major clades were driven to extinction or were merely pruned and did not completely go extinct.