Gondwana-Eurasian Permian biogeographic links: implications for Permian global marine biogeography, inter-continental correlations and climate change

SHI GUANG R.; TABOADA ARTURO C; KLETS ALEXANDR

China

Congreso; 13 Gondwana International Congress; 2008

Chinese Academy of Sciences, Institute of Geology and Geophysics

Biogeographic bipolarity (s.l.), also known as antitropicality, is defined as a pattern of biotic distribution in which forms of life “shun the Tropics” (Hubbs, 1952). This large-scale biogeographical phenomenon is featured very strongly in Earth’s present-day ecosystems, both in animals and plants, as it was during some intervals of the geological past (Crame, 1986; Shi and Grunt 2000). Two main schools of view have long been proposed to explain the origin and historic development of this feature: dispersal vs. vicariance. The ‘dispersalist’ view, typified by Darwin’s classic ‘centre of origin’ doctrine, assumes that all biotic disjunctions have been formed by long-distance dispersal from their centre of origin. The ‘vicarianist’ view, best represented by The´el’s ‘Relict Theory’ (The´el, 1911) assumes that modern biotic bipolarity may have originated as a result of an originally contiguous global biota, later halved into distinct hemispheres by tropical extinction due to either excessive low-latitude warming and/or competition. Despite considerable arguments between the two schools of view, at least one element is common to both, namely they both relate the origin of modern bipolarity to past global environmental changes. However, the exact mechanisms and cause-effect relationships between bipolarity and environmental processes and especially the significance of the fossil record in this debate are poorly known. The latter question is particularly important as the fossil record can provide unique historical data to track the evolutionary process of biotic distributions in space and time and therefore provides the only feasible approach to constrain and distinguish competing theories regarding the “deep-time” origin of modern bipolarity. Bipolarity has been documented in the fossil record especially for the Permian and Jurassic periods when it appears to have manifested itself most prominently, at a level comparable with or exceeding that of the modern biota. However, just as there has been an extensive debate surrounding the origin of the modern bipolarity, palaeobiogeographers are also deeply divided with regard to the initiation, dynamic development and underlying control mechanisms of this phenomenon in the fossil record. In a recent pilot study based on Permian brachiopod faunas from eastern Australia and northern Russia, we proposed several possible biogeographical models to explain the origin of the Permian bipolarity (Shi and Grunt, 2000). These models emphasise the role of tectonic vicariance events, surface ocean currents, palaeogeography and suitable climatic conditions in the long-distance dispersal of bipolar taxa. These ideas are however fundamentally different to those of several other recent studies who instead advocate the role of tectonic displacement or global circulation of cold ocean-bottom currents. Further, several latest studies have revealed that the Late Palaeozoic bipolarity extends well beyond the traditional cases of Australia and north Russia¾it has now also been confirmed from southern Tibet, northeast China, Japan, western USA, Argentina and East Asia. Therefore, it is certain that the stratigraphical and geographical distribution of the Late Palaeozoic bipolarity is much more widespread than previously recognized although the details of these distributions remain poorly constrained. This also means that existing models of Late Palaeozoic bipolarity require re-assessment in light of the new discoveries. In addition, there are two other important aspects of the Late Palaeozoic bipolarity that have not been addressed in previous studies: namely the biostratigraphical and palaeoclimatic/palaeoceanographical significance of the Late Palaeozoic bipolar faunas; and the implications of the Late Palaeozoic bipolarity for understanding the origin and evolution of the modern bipolarity. The present study is a continuation and extension from the work of Shi and Grunt (2000), but based on extensive recent discoveries from Siberia, Mongolia, Russian Far East, Northeast China, Argentina and Australia. A critical review of currently available Permian marine fossil records from these regions show that Permian bipolarity was a ubiquitous and conspicuous feature among many Permian marine invertebrate groups, manifested at all taxonomic levels (from species upwards). In particular, recent discoveries of Permian bipolar taxa from western and southern Argentina, with varying links to Antarctica, Australia and Siberia, suggests that it is possible to biostratigraphically align some Carboniferous and Permian marine faunal assemblages between these continents by means of these bipolarly distributed taxa and faunal assemblages, which otherwise would be difficult due to faunal provincialism. Recent discoveries of Permian bipolar taxa from Argentina have also allowed us to re-assess the relative strength of the various existing scenarios invoked to explain the origin and historical development of the Permian. Apart from those reviewed and critically evaluated by Shi and Grunt (2000), herewith we propose yet another possible interpretation after a detailed taxonomic study of two separate, but morphologically similar, lineages of Late Palaeozoic productoid brachiopods. It appears likely that the morphological similarities between a lineage of Late Palaeozoic Argentinean brachiopod genera, ranging from Lanipustula (Viséan) through Verchojania (Moscovian-Kasimovian) to Jakutoproductus (Asselian-Sakmarian) (e.g. Taboada 2008), and those of the same lineage from Siberia (Verchoyansk Mountains), could be a consequence of parallel evolution, although trans-oceanic dispersal as a mechanism of initiating and sustaining Late Palaeozoic biogeographic bipolarity remains a viable scenario. If parallel evolution was indeed the mechanism for initiating Late Palaeozoic bipolarity between Argentina and Siberia, it must have commenced during the late Viséan at a time when the Earth was experiencing a global cooling phase allowing the trans-oceanic and global dispersal of some cool-water brachiopod genera from lower to higher latitudes. Although as a general biogeographical feature bipolarity was ubiquitous among many marine invertebrate taxa between Gondwana and Eurasia,  there are bipolar taxa shared only between Argentina and Siberia, which are absent from Australia. The reason for this remains little understood, although several possibilities could be discussed. Obviously, the role of Antarctica is critical in this discussion due to its position between South America and Australia during the Late Palaeozoic; however, the scarcity of Late Palaeozoic marine fossils found in Antarctica presents a great challenge for any further studies in this regard.