Brachiopods of the Mengkarang Formation (Early Permian, Sumatra) and their palaeobiogeographical significance

The position and hence the palaeolatitude of the West Sumatra Block in the Early Permian is still object of discussion by several authors. Two opposite positions have been delineated: according to the first, the West Sumatra Block has strong Cathaysian affinity and it is located at low southern latitudes in Early Permian times; the second asserts that West Sumatra Block shows a Gondwanan affinity and so it should have been located at higher southern latitudes. No palaeomagnetic data are available from West Sumatra, to constrain the position of the block; for this reason the study of palaeobiogeographic affinity of fossils is, for the moment, the only method available to discuss its location in the Early Permian. Following a taxonomic study, a palaeobiogeographical analysis of brachiopods from the Lower Permian Mengkarang Formation, West-Central Sumatra (Indonesia) has been done, comparing the Indonesian fauna with coeval assemblages. The Mengkarang Formation is a succession of alternating epiclastic and volcanoclastic deposits, containing a very rich flora (roots and tree trunks, plant fragments, palynomorphs) and fauna (fusulinids, brachiopods, ammonoids and crinoids). It thus offers the opportunity to test the Gondwanan versus Cathaysian affinity of the West Sumatra Block. The systematic study of the brachiopod fauna has led to the identification of six genera, two species and an undetermined genus and species, which have been used for the palaeobiogeographical analysis. This indicates that Lower Permian brachiopods from Sumatra are best grouped with warm taxa from boreal and Peri-Gondwanan region rather than with cold taxa from Gondwana, suggesting that the Indonesian brachiopod fauna is not influenced by the extensive glaciation and cold currents that affected much of Gondwana in Pennsylvanian-Early Permian times. Evidence from other fossil groups seems to confirm this finding, suggesting a low latitude position for the West Sumatra Block in the Early Permian. The presence of Cathaysian fusulinids and of oncoids suggest that the limestones of the Mengkarang Formation were deposited in a tropical environment. Studies on the Mengkarang flora, in particular on the Lower Permian Jambi flora and on palynomorphs, indicate a Cathaysian affinity with Gondwanan elements totally absent. Furthermore, the fossil wood in the Mengkarang Formation has no growth rings and thus should have grew in the tropical zone. The palaeobiogeographical analysis of the brachiopods of the Mengkarang Formation is a further evidence which, together with previous data from the biota of the same formation, indicates that the West Sumatra Block was probably located at low southern latitudes, where it benefitted from the warm surface current gyre that affected the palaeoequatorial region in the Early Permian.