Permian Gondwana-Boreal antitropicality with special reference to brachiopod faunas

In a reconstructed global Permian marine biogeographic framework, antitropi cality is defined as a biogeographic phenomenon whereby a taxon occurs only in the Boreal and Gondwanan Realms and is lacking completely (or nearly so ) in the intervening Palaeoequatorial Realm. We recognise three types of an titropical distributions: bipolar (high-latitude, generally >50 degrees lat itude in each hemisphere), bitemperate (approximately between 30 degrees an d 50 degrees latitudes in each hemisphere) and a combination of both. Evide nce of Permian antitropicality has been reported from almost all major mari ne invertebrate groups, both pelagic or benthic forms, and is best known at the generic level although an increasing number of antitropical species ha ve also been recognised. Antitropicality appears to have occurred throughou t most of the Permian, but its strength varied from stage to stage, judging by the number of brachiopod genera shared between the two hemispheres. The phenomenon manifests itself most strongly from the Sakmarian to the Kazani an (?Wordian), suggesting a prolonged interval of enhanced biotic interchan ges between the Gondwanan and Boreal Realms. Possible migration pathways an d mechanisms are reviewed and discussed. Among a number of existing scenari os invoked to explain the origin of both modern and geological antitropical ity, we here present data particularly supporting (1) a south-to-north 'ste pping-stone' migratory mechanism via island terranes scattered in the easte rn Palaeotethys, (2) a vicariance model exemplified by the spatio-temporal distribution of Tomiopsis, and (3) a shelf, north-to-south migration along the western coast of the Palaeotethys, as suggested by Sowerbina. In additi on, we also consider dispersal along the western coast of Pangea, facilitat ed at times by upwelling systems, as an additional migration pathway for in tercontinental biotic exchanges during the Permian, although corroborating data are required at present. (C) 2000 Elsevier Science B.V. All rights res erved.