The Middle-Late Ordovician brachiopod Plectorthis from North America and its paleobiogeographic significance

Plectorthis is a commonly reported Late Ordovician brachiopod genus from Laurentia and several other tectonic plates. Examination of shells from the Cincinnati Arch type area shows that species previously assigned to the genus have a considerable range of variability, such as ribbing style, shell outline, and musculature. The type species, Orthis plicatella Hall, 1847 is characterized by generally simple costae, with double-rowed aditicules on each rib, and a cordate ventral muscle field. Some species previously attributed to Plectorthis have a subtriangular ventral muscle field and hence a closer affinity to the plaesiomyid genus Austinella. Orthis fissicosta is here demoted to a subspecies of Plectorthis plicatella based on its similar ribbing style to some specimens of the type species. In Laurentia, Plectorthis first appeared in the Darriwilian-Sandbian boundary interval, achieved a relatively high species diversity and wide paleogeographic distribution in pericratonic seas during the early Katian; the subequatorial species attained a notably larger shell than those in the higher paleotropics. The genus did not invade the intracratonic basins of Laurentia and became extinct by the late Katian.