Fossil tip-dating reveals novelties on evolutionary and diversification trends in three Late Ordovician brachiopod genera (Atrypida, Anazygidae)

Abstract During the Ordovician, the brachiopod order Atrypida originated and diversified. However, speciation patterns and evolutionary drivers within three atrypide Laurentian genera, Anazyga , Catazyga , and Zygospira , remain poorly known. Herein, we propose a Bayesian phylogenetic framework for these clades. Morphological character data, including 34 internal and external characters, were collected for 20 species, including seven species assigned to Zygospira , six species previously assigned to Anazyga , and seven species previously assigned to Catazyga. Morphological data were analyzed in BEAST2.5 via an Mk Model of morphological character evolution implementing the fossilized birth–death (FBD) model. In addition, FBD-based rates of extinction, origination, and fossil sampling were estimated across four Late Ordovician time intervals. Primary results include: (1) each genus was polyphyletic as traditionally established; (2) proposed transfer of all Anazyga and two Catazyga species to Zygospira based on reconstructed evolutionary relationships; (3) recognition of one new species, Zygospira idahoensis Vilela-Andrade n. sp., and the elevation of Z. multicostata Howe, 1965, originally a subspecies, to species status; and (4) updated understanding of clade origination and speciation. The origination age for the clade is calibrated to be 453 Ma (Sandbian 2). Recovered biodiversification rates indicate that the highest speciation and extinction rates occurred in the Katian 3 stage slice, when the number of species in the clade peaked. Subsequently, a decline in biodiversification led to an extinction episode in the Katian 4 for all lineages. Catazyga and Zygospira are interpreted as early Late Ordovician atrypids that experienced both rapid radiation and extinction during the Katian. UUID: http://zoobank.org/442febc2-f7bb-4c04-b454-d43180237ebc