Tiny Ordovician brachiopods from the eastern United States: An assessment of their systematics, phylogenetic, and biogeographic trends

The evolution and biogeography of three genera of Late Ordovician anazygid brachiopods were investigated using a time-calibrated species-level Bayesian phylogeny and probabilistic biogeographic models. Specifically, the species-level phylogenetic relationships were estimated for the three genera within the family Anazygidae: Anazyga Davidson, 1882; Zygospira Hall, 1862, and Catazyga Hall and Clarke, 1894. Recovered evolutionary relationships indicate that the genera’s common ancestor origination was at 453 Mya. The phylogeny resolved into the two subfamilies: Anazyginae (Anazyga + Zygospira) and Catazyginae (Catazyga). Anazyga and Zygospira do not correspond to two distinct monophyletic clades as previously interpreted, but are instead combined herein into a single genus, Zygospira. Additionally, two new Zygospira species were described, Zygospira multicostata n. sp. from the Montoya Group, TX; and Zygospira idahoensis n. sp. from the Saturday Mountain Formation, ID. The recovered evolutionary relationships were used to develop an evolutionary biogeographic framework for the clade using the R package BioGeoBEARS. The reconstructed timing of cladogenetic events, geographic patterns, and the timing of dispersal and vicariance speciation events were used to correlate biotic changes with tectonic shifts and eustasy. Results indicate that dispersal was the main speciation mode in this clade. The geographic patterns observed within species’ dispersal pathways are supported by pulses of sea level rise which promoted connections between Laurentian midwestern, and southern basins influenced by the Sebree Trough. Estimated speciation and extinction rates indicate that the three genera experienced the highest diversification in the early Late Ordovician (Katian 3), explained by higher dispersal events, and became extinct during the mid-Late Ordovician (Katian 4).