Multivariate analysis of 197 well-preserved specimens among 11 species of late Late Ordovician (Katian and Hirnantian) plaesiomyid brachiopods from North America revealed two trends of morphological changes: (1) increasing globosity and dorsal convexity from the early to late Katian, and (2) coarser, but fewer ribs in species from the paleoequatorial intracratonic seas compared with species from the mid- to high-tropical pericratonic shelves and platforms. Another notable change, albeit not quantified, was the enlargement of cardinal process in plaesiomyids from early to late Katian, characterized by a predominantly trilobed cardinal process in the intracratonic species versus a bilobed cardinal process in pericratonic species. Increases in dorsal convexity and shell globosity are interpreted to have allowed for development of a larger lophophore, improving the capacity and efficiency of filter feeding and gas exchange. Smaller but more frequently bifurcating ribs would have led to more numerous setae near the margin of the shell, resulting in increased sensitivity to environmental stimuli. Increased size and number of lobes of the cardinal process would have allowed for larger diductor muscles to open a larger, heavier shell. In this study, two new species, Plaesiomys periosa sp. nov. and Plaesiomys lenzi sp. nov., are proposed, and two existing species are transferred from Dinorthis to Plaesiomys.