BIOGEOGRAPHICAL ANALYSIS OF LATE SILURIAN BRACHIOPOD FAUNAS, CHIEFLY FROM ASIA AND AUSTRALIA

Shallow-water benthic marine invertebrates (such as brachiopods from B enthic Assemblages [BA] 1-3) have usually played a much more important role than deeper ones (BA 4-5 or even deeper) in evaluating biogeogra phical provincialism in geological history. The Silurian brachiopod Re tziella Fauna, characterized by the common presence of Retziella in as sociation with various provincial taxa and many common North Silurian Realm genera, is known from southwest Tienshan, North China, South Chi na, North Vietnam, and East Australia. It is possibly also present in North Korea, the central Pamirs, Afghanistan, and New Zealand. The coe val Tuvaella Fauna occurs only in the southern marginal belt of the Si berian Plate. Splecologically, both faunas inhabited a normal, shallow -water, level-bottom environment, usually with al ow-diversity communi ty (commonly 3-8 genera); assignment to BA 2-3 is indicated. Their mut ual exclusiveness is of biogeographical significance: subdivisions of the Uralian-Cordilleran Region can be based on them, with the Tuvaella Fauna bring included in a redefined Mongolo-Okhotsk Province. A Sino- Australian Province is established and defined herein for the area occ upied by the Retziella Fauna during the Ludlow-Pridoli and probably th e Wenlock. Two subdivisions of the province can be recognized, a Sino- Central-Asian Subprovince and an Australian Subprovince, based on diff erent endemic brachiopods and separate geographical positions. The pre sence of a number of more cosmopolitan genera in both the tropical-sub tropical Sino-Australian and subtropical-temperate Mongolo-Okhotsk Pro vinces during the Late Silurian testifies to oceanic surface current c irculation patterns adequate for the distribution of planktic larvae c apable of long-distance dispersal while maintaining reproductive commu nication. This contrasts with the dispersal potential of endemic compo nents of the newly defined Silurian biogeographical units.