SYSTEMATIC REVISION OF THE MIDDLE AND LATE DEVONIAN BRACHIOPODS Schizophoria (Schizophoria) AND 'Schuchertella' FROM NORTH AMERICA

摘要

Schizophoria and 'Schuchertetla', two common genera of Devonian brachiopods from North America, are systematically revised based on dadistic analysis. Several Early Devonian schizo-phoriid species are placed within S. (Eoschuchertella), while all other North American Devonian and Mississippian species studied are placed within S. (Schizophoria). Schizophoria meeki Fenton & Fenton is synonymised with S. macfarlanii Meek. Ancestral forms of Schizophoria (Schizophoria) were present in Europe and the Appalachian basin and inhabited outer platform environments through the Frasnian. Several independent lineages of S. (Schizophoria) persisted through the Late Devonian biodiversity crisis and subsequently colonised nearshore environments. Most cladogenetic events within North American species of S. (Schizophoria) occurred via dispersal mechanisms, while vicariant speciation was rare. Cladistic analysis of North American species assigned to 'Schuchertella' revealed that all species through the Frasnian belong to Floweria, an impunctate genus, while Famennian and later species belong to Schuchertella. The generic placement of the Devonian 'schuchertellids' is revised accordingly. Schuchertella and Floweria are superficially similar and exhibit an analagous (albeit temporally offset) evolutionary transition from free-resting ancestors to descendants with attached pedicle valves. Floweria also exhibits reduced rates of vicariant speciation compared to speciation by dispersal. The relative reduction in vicariant speciation has been documented for other Devonian taxa (bivalves, phyllocarids and trilobites) and may. be an important control on faunal dynamics in North America during the Late Devonian biodiversity crisis.