THE PHYLOGENY OF ATRYPIDA, IMPLICATIONS FOR SPIRALIA EVOLUTION

Atrypide brachiopods have great potential for understanding the role that changes in the geometry and orientation of the lophophore play in brachiopod evolution but the ability to test such questions is limited by uncertainty in their phylogenetic relationships. Atrypides are differentiated from other spire bearers by their medially to dorsally oriented spiralia, which support the lophophore. While the lophophore is not preserved in most brachiopods, the spiralia, which show its general structure often are. Because of this, atrypide brachiopods provide an opportunity to study how changes in lophophore geometry affect atrypide evolution by looking at morphological changes in spiralia. In this study we conduct parsimony and multiple Bayesian analyses to determine how the different assumptions underlying these methods affect hypotheses of atrypide evolution and provide a foundation for better understanding atrypide character evolution.

A character-taxon matrix consisting of 55 atrypide genera and 5 outgroup genera were described by 94 contingent characters. We focused on taxa that were well known with published serial sections so that internal and external characters could be coded consistently between taxa. A parsimony analysis, an uncalibrated (non-clock) Bayesian analysis, and three Bayesian analyses utilizing fossilized birth death models (FBD) were performed. The three FBD models employed three alternative evolutionary clock models to incorporate stratigraphic data: strict, rates of character evolution are constant across the tree; independent gamma rates, rates of character evolution among branches evolve at independent rates; and autocorrelated rates, rates of closely related taxa are more similar to each other. The fit of three different fossilized birth death employing different clock models was accessed using Bayes Factor.

Of the three different FBD clock models employed, a strict clock best fit the data. This study found that overall, spiralia change in orientation from medially oriented to dorsally oriented but there are examples where more taxa with medially oriented spiralia (such as medio-dorsal or dorso-medial) evolved from an ancestor with more dorsally oriented spiralia. Clades with large body sizes tended to have more dorsally oriented spiralia suggesting a relationship between body size and spiralial orientation.