Paleolatitudinal morpho-gradient of the early Silurian brachiopod Pentameroides in Laurentia

Pentameroides is a large-shelled pentameride brachiopod, which was widespread throughout Laurentia in the early Silurian (Telychian). Evolving from Pentamerus in the early Telychian, it dispersed from its subtropical – high tropical origin to subequatorial intracratonic seas by the late Llandovery. In this study, large collections of reef-dwelling Pentameroides septentrionalis from the Attawapiskat Formation, Akimiski Island, Nunavut, and level-bottom-inhabiting Pentameroides subrectus from the Fossil Hill Formation, Manitoulin Island, Ontario, and the Jupiter Formation, Anticosti Island, Quebec, were biometrically analyzed for seven external morphological features. Bivariate and ordination analysis (principal components analysis) revealed that P. septentrionalis has a more globose, more biconvex shell with a larger ventral umbo than P. subrectus. These morphological differences, coupled with their excellent preservation in shallow-water reefal environments, suggest that P. septentrionalis was adapted to a relatively low-turbulence, hurricane-free, nutrient-stressed environment near the equator (<10°S) while P. subrectus lived in storm-dominated higher tropical latitudes (∼15°S–25°S). This interpretation is corroborated by the morphology of level-bottom-dwelling Harpidium and Sulcipentamerus from the paleoequatorial lower Silurian of North Greenland, which exhibit morphological features similar to those of P. septentrionalis, even though they did not inhabit a reefal environment. Principal components analysis reveals that P. septentrionalis resembles P. subrectus in early ontogeny, suggesting that P. septentrionalis evolved from P. subrectus as it moved northwards from high tropical to near-equatorial settings.