Biogeographical analysis of Late Silurian brachiopod faunas, chiefly from Asia and Australia



Shallow water benthic marine invertebrates (such as brachiopods from Benthic Assemblages [BA] 1–3) have usually played a much more important role than deeper ones (BA 4–5 or even deeper) in evaluating biogeographical provincialism in geological history. The Silurian brachiopod Retziella Fauna, characterized by the common presence of Retziella in association with various provincial taxa and many common North Silurian Realm genera, is known from southwest Tienshan, North China, South China, North Vietnam, and East Australia. It is possibly also present in North Korea, the central Pamirs, Afghanistan, and New Zealand. The coeval Tuvaella Fauna occurs only in the southern marginal belt of the Siberian Plate. Synecologically, both faunas inhabited a normal, shallow-water, level-bottom environment, usually with a low-diversity community (commonly 3–8 genera); assignment to BA 2–3 is indicated. Their mutual exclusiveness is of biogeographical significance: subdivisions of the Uralian-Cordilleran Region can be based on them, with the Tuvaella Fauna being included in a redefined Mongolo-Okhotsk Province. A Sino-Australian Province is established and defined herein for the area occupied by the Retziella Fauna during the Ludlow-Pridoli and probably the Wenlock. Two subdivisions of the province can be recognized, a Sino-Central-Asian Subprovince and an Australian Subprovince, based on different endemic brachiopods and separate geographical positions. The presence of a number of more cosmopolitan genera in both the tropical-subtropical Sino-Australian and subtropical-temperate Mongolo-Okhotsk Provinces during the Late Silurian testifies to oceanic surface current circulation patterns adequate for the distribution of planktic larvae capable of long-distance dispersal while maintaining reproductive communication. This contrasts with the dispersal potential of endemic components of the newly defined Silurian biogeographical units. □ Late Silurian, Brachiopoda, Retziella, Fauna, Tuvaella Fauna, Synecology, Biogeography, Asia, Australia