TESTING THE EVOLUTIONARY RELATIONSHIPS AMONG THE STROPHIC SPIRE-BEARERS USING FOSSILIZED BIRTH-DEATH MODELS

Spiriferinida and Spiriferida are two charismatic and morphologically similar orders of extinct brachiopods that are believed to be closely related but have been distinguished from one another primarily on the basis of a single “key” trait: the presence or absence of punctae (endopunctae). Punctae are microscopic perforations in the valves of some fossil and living brachiopods. However, the function, taxonomic and evolutionary significance of punctae are unclear. Some subordinal higher taxa are diagnosed by the presence of punctae (in Athyridida, Orthida, Rhynchonellida) while others in the same order are impunctate. At what level(s) of homology are punctae informative phylogenetically? In this study we use multiple phylogenetic methods to test both the traditional hypothesis that Spiriferinida and Spiriferida are distinct clades as well as orders, and a competing hypothesis that the punctate “spiriferinides” are a polyphyletic grouping within Spiriferida. Our study utilizes parsimony and Bayesian Mkv and fossilized birth-death models which incorporate stratigraphic information and the additional parameter of diversification rate (birth) and extinction rate (death). We sampled 50 species in total, representing all 17 currently recognized superfamilies over the stratigraphic record of both orders (Ordovician-Jurassic). Past studies have analyzed genera in each order separately and typically examined one time period or superfamily, and could not address the broader phylogenetic distribution of punctae. Our analyses recover two, not one punctate clade, largely consistent with spiriferinide suborders under the current classification system (Carter et al., 1994). Our results further differ from previous studies in that some recovered clades are composed of both punctate and impunctate species, and several impunctate superfamilies are polyphyletic. The results of our study highlight the need to propose and test specific competing phylogenetic hypotheses with multiple phylogenetic methods. In order to evaluate brachiopod macroevolutionary hypotheses, it is necessary to demonstrate that not all named higher taxa can be assumed to be monophyletic. Moreover, it is now possible to analyze the varied morphology and phylogenetic distribution of punctae more fruitfully.