During the Late Ordovician, a transgression flooded much of Laurentia. The fauna of these intracratonic basins became differentiated from the fauna of the pericratonic shelves and platforms, typically displaying gigantism and coarser shell ornamentation. In this study, 509 specimens from 11 species of the Plaesiomyidae brachiopod family from the Katian and Hirnantian were measured, of which 198 included in principal component analysis to quantify morphological changes over this interval. Three trends were revealed: 1) increasing globosity and dorsal convexity from the early to late Katian, 2) coarser, but fewer ribs on species from the paleoequatorial intracratonic seas compared to species from the mid- to high-tropical pericratonic shelves and platforms, and 3) enlargement of the cardinal process from early to late Katian and predominance of a trilobed cardinal process in the intracratonic species versus bilobed cardinal process in pericratonic species. Two new species were assigned, and two existing species were reassigned
