DEVELOPING A PHYLOGENETIC FRAMEWORK FOR TINY LATE ORDOVICIAN BRACHIOPODS FROM THE EASTERN UNITED STATES (ATRYPIDA: ANAZYGINAE AND CATAZYGINAE)

Speciation events are the main drivers of diversification; thus, generating well-constrained phylogenetic hypotheses to investigate speciation processes and facilitators can provide key data on links between biogeography, speciation, and diversification. The Ordovician epicontinental seas in eastern Laurentia provided an excellent environment to examine speciation, with sea-level fluctuations, shifting climatic conditions, and active tectonic settings. During the Ordovician, the brachiopod order Atrypida originated and diversified extensively, evolving novelties such as helical and skeletal-supported lophophores. However, speciation patterns and evolutionary drivers within two widely-distributed atrypid subfamilies, Anazyginae (Anazyga + Zygospira) and Catazyginae (Catazyga) remain poorly known. Herein, we develop a robust phylogenetic framework for these clades and use that framework to evaluate speciation processes and facilitators during the Late Ordovician.

Morphological character data, including 38 internal and external characters, were collected from literature and museum collections for a focal group of 18 species, six recently reviewed species of Zygospira, four species of Anazyga, and seven species of Catazyga. These data were analyzed in RevBayes, via three Bayesian models of character evolution: (1) morphological model (Mk Model), in which character states can change equally within branches, (2) clock model, which represents the rates of evolutionary changes through time slices, and the (3) fossilized-birth-death model (FBD), where rates of extinction, origination, and differential sampling are considered independently for each branch. Speciation mode and biogeographic patterns were then analyzed using BioGeoBears.

Primary results include (1) consistency in key morphological traits within genera, such as the number of shell ribs or spiral whorls (2) resolved phylogenetic structure between and within the clades, and (3) speciation and extinction rates associated with major geological events. Combining phylogenetic and biogeographic analyses of the Atrypida provides essential information for understanding the impacts of geological and biotic changes influencing the evolution and ecology of the shelly benthos across the Middle and Upper Ordovician.