BIVALVE PROVINCES IN THE PROTO-ATLANTIC AND ALONG THE SOUTHERN MARGINOF THE TETHYS IN THE JURASSIC

Using multivariate methods. the distribution pattern of Jurassic bival ve genera and subgenera within the Proto-Atlantic and along the southe rn margin of the Tethys is analysed based on an extensive literature s urvey and examination of field collections. Four bivalve provinces are recognised within the Proto-Atlantic: (1) the Boreal Bivalve Province in the north. (2) the Northern Transitional Province: (3) the Souther n Transitional Province; and (4) the Mediterranean Bivalve Province. T he former two provinces are assigned to tile Boreal Faunal Realm, the latter two to the Tethyan Faunal Realm. The southwestern margin of the Tethys belongs to the Mediterranean Bivalve Province, the southeaster n margin to the Ethiopian Bivalve Province. The latter is only poorly defined by bivalve genera/subgenera; in particular, its northern bound ary is blurred, with the Arabian region occupying an intermediate posi tion between the Mediterranean and the Ethiopian bivalve provinces. Pr ovince boundaries based on bivalves do not always coincide with those based on ammonites. The reason for this is probably different modes of life: ammonites, as a rule, being active swimmers and bivalves being substrate-related and subjected to passive dispersal during the larval stage. Diversity is not suitable to characterize bivalve provinces. d ue to differences in outcrop size, preservation potential and sampling history, and due to differences in the degree of environmental hetero geneity within and between regions. Migration patterns of Jurassic biv alves either reflect changes in tie climatic condition (e.g. the north ward spread of Tethyan taxa in the late Early Jurassic and Middle Jura ssic in the course of amelioration of the climate) or prevailing curre nt patterns. The origin of faunal provinciality it; thought to be comp lex and a result of the interplay of several factors, the importance o f which varied throughout the Jurassic. (C) 1998 Elsevier Science B.V.