SEPARATING THE CROWN FROM THE STEM: DEFINING BRACHIOPODA AND PAN-BRACHIOPODA DELINEATES STEM-BRACHIOPODS

Phylogenetic relationships among Recent and fossil members of the clade Lophotrochozoa, in particular Brachiopoda and Phoronida within Lophotrochozoa, remain unsettled. Recent molecular analyses place phoronids within Brachiopoda, not in the more traditional position as the brachiopod sister-group. These two alternatives affect how lophotrochozoan morphological character evolution is viewed, in particular the homology and polarity of characters relevant to the ability to mineralize a skeleton, the number of mineralized elements, their chemical composition and microstructure, and their relations to body plan and axis of symmetry. All these characters depend for their interpretation on the evidence supporting hypotheses of phylogenetic relationship among lophotrochozoan taxa.

Stem-brachiopod fossils can be clearly identified as such only by contrast with a defined crown clade Brachiopoda (the most recent common ancestor of all living brachiopods and all its descendants), and with the total clade Pan-Brachiopoda (Brachiopoda and all organisms that share more recent common ancestry with Brachiopoda than with any other mutually exclusive crown clade). The stem-group identity of several fossil taxa (e.g., Micrina, Tannuolina, Eccentrotheca, Acanthotretella, Mickwitzia, Lingulosacculus, Paterimitra, and other halkieriids, tommotiids, and other poorly understood early fossils) cannot be confirmed or denied until the boundary between the stem and the crown has been established. We recently defined the crown clade Brachiopoda and the total clade Pan-Brachiopoda (in the Companion Volume to the PhyloCode). These definitions clarify the boundaries of the stem, and allow us to evaluate, for the first time, the phylogenetic position of these putative stem-group fossils, in light of topological analyses of the structures preserved, taphonomic analysis of the fossils, and the interpretative model organisms chosen for comparison (Donoghue & Purnell, 2009). We further discuss the implications of these interpretations for hypotheses of morphological homology. We will show how different definitions of crown and stem lead to very different evolutionary interpretations of fossil morphology and evolution.