Bivalve provinces in the Proto-Atlantic and along the southern margin of the Tethys in the Jurassic
Abstract Using multivariate methods, the distribution pattern of Jurassic bivalve genera and subgenera within the Proto-Atlantic and along the southern margin of the Tethys is analysed based on an extensive literature survey 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 Southern Transitional Province; and (4) the Mediterranean Bivalve Province. The former two provinces are assigned to the Boreal Faunal Realm, the latter two to the Tethyan Faunal Realm. The southwestern margin of the Tethys belongs to the Mediterranean Bivalve Province, the southeastern margin to the Ethiopian Bivalve Province. The latter is only poorly defined by bivalve genera/subgenera; in particular, its northern boundary is blurred, with the Arabian region occupying an intermediate position between the Mediterranean and the Ethiopian bivalve provinces. Province 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, due to differences in outcrop size, preservation potential and sampling history, and due to differences in the degree of environmental heterogeneity within and between regions. Migration patterns of Jurassic bivalves either reflect changes in the climatic condition (e.g. the northward spread of Tethyan taxa in the late Early Jurassic and Middle Jurassic in the course of amelioration of the climate) or prevailing current patterns. The origin of faunal provinciality is thought to be complex and a result of the interplay of several factors, the importance of which varied throughout the Jurassic.