PHYLOGENETIC RELATIONSHIPS AMONG EXTANT RHYNCHONELLIDE BRACHIOPODS

The evolutionary relationships within rhynchonellide brachiopods have yet to be analyzed in a phylogenetic framework despite the fact that rhynchonellides occupy an important place in the evolution of all articulated brachiopods. Rhynchonellida is the oldest and most basal of the extant rhynchonelliform brachiopod orders and the first order to have true calcareous lophophore support structures, the crura. Fourteen rhynchonellide superfamilies, four of which are extant, have been defined based mostly on crural type. It remains unclear how these taxa are related to one another, if these groups imply anything about the evolution of the crura, or even if these groups are monophyletic. The purpose of this research is to analyze the phylogenetic relationships among extant rhynchonellide genera using morphological characters in order to investigate the monophyly of extant superfamilies and provide a phylogenetic hypothesis for use in analyses of character evolution. We completed the analysis using fifty-five characters of the interior and exterior of the shell that vary among the nineteen extant genera. Two early rhynchonellide genera and one pentameride genus were selected as outgroups to determine character polarity. The type species was chosen as a representative for each genus. We completed multiple analyses, experimenting with character removal and differential character weighting. We predicted the four extant superfamilies to be monophyletic since many past systematists have attempted to follow an evolutionary framework when erecting the rhynchonellide classification. Initial results suggest that two of the extant superfamilies, Hemithiridoidea and Dimerelloidea, are monophyletic, while the others appear to be paraphyletic, results that conflict with recent molecular phylogenetic hypotheses. This research also indicates that the raduliform crural type is the most basal type and the more derived Recent rhynchonellides are characterized by small size and greatly reduced shell ornamentation. This research suggests that characters currently used to unite the superfamilies may need revision. This phylogenetic hypothesis will be tested in future analyses by the inclusion of fossil genera and molecular data in order to further study the relationships and character evolution in this important order.