Palaeolatitudinal distribution patterns of higher rhynchonelliform brachiopods in the early Ordovician

During the Arenig-early Llanvirn, rhyrichonelliform brachiopod assemblages were mostly dominated by orthidines, syntrophidines, plectambonitoids and clitambonitidines. A survey of about 150 genera from 24 localities throughout the world, including new data from Argentina, shows that inferred palaeolatitude and composition of assemblages at ordinal/subordinal level are significantly correlated, especially in orthidines and syntrophidines. These non-random, latitudinally-related, global distribution patterns were strongly controlled by temperature-related factors, such as the nature of the substrate and nutrient availability. Brachiopod assemblages inhabiting carbonate and clastic environments are different in life strategy and ecospace milisation, and these differences are reflected in high-level taxonomic composition. The early orthidines, which possessed a generalised and conservative morphology, were less constrained by palaeoenvironmental and palaeoclimatic controls. They were dominant in cold-water shelves occupying the 'attached-low, suspension feeder' guild. In contrast, articulated brachiopods inhabiting tropical carbonate shelves experienced a relatively rapid morphological radiation through successive stages in the occupation of offshore, soft substrates. Those brachiopods displaced to deeper and colder waters on the equatorial shelves were physiologically and morphologically best adapted to occupy the empty habitat space in cold-water communities. A corollary is that the relative abundance of higher brachiopod taxa, expressed as the 'high brachiopod taxa ratio' may serve as a tool in locating palaeolatitudinally unconstrained terranes.