Early−Middle Devonian brachiopod provincialism and bioregionalization at high latitudes: A case study from southwestern Gondwana

Abstract

The paleobiogeography of Early−Middle Devonian (Pragian−Eifelian) brachiopods from West Gondwana was assessed to determine any potential controls (regional climatic differences or global eustasy) that may have driven bioregionalization. The Pragian−Eifelian interval of West Gondwana was examined because work by previous authors suggested that this was a period when regionally extensive areas of provincialism among marine invertebrates were present and most pronounced. Factors of particular interest in this study were the controls over brachiopod bioregionalization at high (60°−90°) southern latitudes, which the Malvinokaffric Realm is thought to have entirely occupied. A large presence-absence data matrix was compiled consisting of the occurrences of 206 genera from 17 localities across West Gondwana, and an array of multivariate methods (cluster analysis, nonmetric multidimensional scaling, and network analysis) was employed to assess regional bioregionalization trends. The results of our study suggest that regional climatic differences brought on by latitudinal effects were the determining driver for bioregionalism of brachiopods during the Pragian−Eifelian, and these trends were coincident with a global cooling period during the Early−Middle Devonian. Our study further suggests that of the three regional paleobiogeographic realms thought to be present in West Gondwana during the Early−Middle Devonian (Eastern Americas, Old World, and Malvinokaffric), only the Malvinokaffric Realm is valid as a single biogeographic area. Its area, however, is reduced; it is interpreted to have been a second-order biogeographic area and not a first-order area as suggested by previous authors. Given these factors, we suggest a new demonym for this area, the “Malvinoxhosan,” given the racially charged connotations of “Malvinokaffric.” We present a new biogeographic framework for West Gondwana that is free of preconceived biogeographic area and rank biases, with the understanding that a more globally expansive study should be undertaken to elucidate these areas and their rank within the correct hierarchy. Two first-order paleobiogeographic areas are recognized in West Gondwana and are named according to their latitudinal extent, namely, the high-latitude (60°S−90°S) and temperate-latitude (30°S−60°S) bioregions. The temperate-latitude bioregion consists of a single second-order paleobiogeographic area, the “Colombian−West African” bioregion. Two second-order bioregions are present in the high-latitude bioregion, namely, the Amazonian and Malvinoxhosan bioregions. Here, the Amazonian (∼50°S−70°S), compositionally, was an intermediate region between the Colombian−West African (∼30°S−50°S) and Malvinoxhosan (70°S−90°S) end members. Latitudinal effects may also have been responsible for dividing the Malvinoxhosan bioregion into two tentative third-order paleobiogeographic areas, namely, the Andeo−South African (∼70°S−80°S) and the Paraná (∼80°S−90°S) bioregions.