Permian Gondwana-Boreal antitropicality with special reference to brachiopod faunas

In a reconstructed global Permian marine biogeographic framework, antitropicality 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 antitropical distributions: bipolar (high-latitude, generally > 50°latitude in each hemisphere), bitemperate (approximately between 30°and 50° latitudes in each hemisphere) and a combination of both. Evidence of Permian antitropicality has been reported from almost all major marine invertebrate groups, both pelagic or benthic forms, and is best known at the generic level although an increasing number of antitropical species have also been recognised. Antitropicality appears to have occurred throughout 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 Kazanian (?Wordian), suggesting a prolonged interval of enhanced biotic interchanges between the Gondwanan and Boreal Realms. Possible migration pathways and mechanisms are reviewed and discussed. Among a number of existing scenarios invoked to explain the origin of both modern and geological antitropicality, we here present data particularly supporting (1) a south-to-north 'stepping-stone' migratory mechanism via island terranes scattered in the eastern 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 addition, we also consider dispersal along the western coast of Pangea, facilitated at times by upwelling systems, as an additional migration pathway for intercontinental biotic exchanges during the Permian, although corroborating data are required at present.