New acrotretid brachiopods from North and South America and an examination of their functional morphology

Acrotretid brachiopods are described for the first time from the Ordovician of northern Argentina. Four Ordovician (Arenig and Llanvirn) localities in the Argentine Precordillera yielded eight new species as well as one species previously described from Sweden. New species are also described from the Silurian of Oklahoma and the Canadian arctic, doubling the North American species diversity of the acrotretids for this time period. Three new species are from the Lower Silurian (Wenlock) of Cornwallis Island and one is from the Wenlock of the Northwest Territories. Two previously described species are identified from the Silurian of Oklahoma.;Based solely on the morphological features of both the larval and adult shell, the habitat of highly conical acrotretids is interpreted to have been planktonic. An extensive examination of the larval pits is provided and three general pitting arrangements are suggested. Two morphological groups are proposed: the 'benthic' group, is characterized by the densely built, compact shell of Artiotreta Ireland; and the 'epiplanktonic' group, which is characterized by the highly conical lightly built shell of Opsiconidion Ludvigsen.