Evolution, palaeoecology, and palaeobiogeography of the Late Ordovician–Early Silurian brachiopod Epitomyonia

Epitomyonia is characterized by various types of dorsal ridges, which may be transverse, longitudinal, or highly convoluted and probably served as skeletal supports for lophophores of various complexity. Multivariate analyses suggest that the Epitomyonia-bearing brachiopod associations lived in relatively shallow-water environment in the Late Ordovician, and inhabited mainly deep-water environments in the early Wenlock. The temporal and spatial change in the faunal distribution may be explained by three alternative scenarios: (1) Epitomyonia followed the broad evolutionary trend of the Palaeozoic Evolutionary Fauna to shift from shallow- to deeper-water settings over time; (2) the dicoelosiid communities could not compete with the large-shelled pentameride communities in continental shelf settings during the Early Silurian; or (3) only the shallow-water Epitomyonia died out in the Late Ordovician mass extinction event, whereas some poorly known deep-water Late Ordovician forms survived into the Early Silurian. Epitomyonia paucitropida n. sp. from the lower Whittaker Formation (late Katian) of the Mackenzie Mountains, northwestern Canada, is reported as the first known Ordovician species of Epitomyonia from the palaeocontinent of Laurentia, characterized by a small shell with weak, transverse dorsal ridges that are most primitive for the genus.