BIPOLAR MOLLUSKS AND THEIR EVOLUTIONARY IMPLICATIONS

The phenomenon of bipolarity, one of the major disjunct distribution p atterns on the face of the earth, has been investigated repeatedly sin ce the mid-nineteenth century. Running through the many hypotheses tha t have been put forward to account for its occurrence, it is possible to detect two persistent themes: it is usually interpreted within a di spersal framework, and it is generally believed to be of comparatively recent origin. To many authors, the phenomenon is intimately linked t o the Plio-Pleistocene glaciations. Recent palaeontological investigat ions have established that bipolarity can now be traced back to at lea st the Early Jurassic period (i.e. 200 m.y.a.). Here it is well marked in the Pliensbachian stage by a variety of pectinacean bivalve taxa. Further bivalves indicate probable Middle Jurassic examples, but the p henomenon is more clearly seen in the Late Jurassic, especially in the Tithonian stage. At this time, inoceramid, buchiid and oxytomid bival ve occurrences at northern hemisphere localities such as arctic Canada , N.W. Europe, Siberia, N.E. USSR and Japan can be matched with those in southern South America, Antarctica and Australasia. A striking Earl y Cretaceous (Aptian-Albian) bipolar pattern for the oxytomid Aucellin a may be complemented by several infaunal bivalves, brachiopods and at least one gastropod. There is strong circumstantial evidence to sugge st that bipolar molluscs continued to develop through the Cenozoic era . Such is the level of generic and subfamilial differentiation within certain living forms as to suggest that they must be the product of a considerable evolutionary history. It is likely that present-day distr ibution patterns of prosobranch gastropod groups such as the whelks (B uccinidae), together with certain fissurellids, littorinids, naticids and turrids, can be related to a late Paleogene-early Neogene phase of bipolarity. Many amphitropical taxa, in both the marine and terrestri al realms, have probable late Neogene-Pleistocene origins. It is possi ble to set the Jurassic and Cretaceous examples of bipolarity within a largely vicariant framework based upon the disintegration of the Pang ean supercontinent. In this way the widespread ranges of putative Tria ssic ancestors were disrupted by tectonic processes in low latitude re gions, although it should be emphasized that major climatic and oceano graphic changes were almost certainly involved too. Similarly, it is p ossible to view late Paleogene-early Neogene bipolarity as a vicariant event, but this time with climatic change identified as the single mo st important agent. Widespread or cosmopolitan distributions are held to have formed during global cool phases (such as the late Eocene-earl y Miocene) only to be disrupted by global warming (such as in the late early Miocene). It is even possible to view Plio-Pleistocene patterns as, at least in part, the products of vicariant events caused by rapi d temperature and sea level shifts. Clearly, there is an urgent need h ere for more critical taxonomic data to test these various hypotheses. Phylogenetic studies of groups such as the Mesozoic bivalve superfami ly Monotoidea and the Cenozoic Buccinidae, in particular, should const itute future rigorous tests. In so doing, they should also provide muc h useful information on the relative roles of dispersal and vicariance in promoting global disjunction in marine faunas. Repeated formation of bipolar patterns through geological time may have had important imp lications for modes of speciation and phenomena such as the origin of taxonomic diversity gradients.