Palaeobiogeography of Tethyan ammonites during the Tithonian (latest Jurassic)

Abstract

This study deals with the palaeobiogeographic distribution of Tethyan ammonites in the Tithonian. In particular, the relationships between ammonites of the Mediterranean Tethys, East Africa, Caribbean and Southeast Pacific areas are discussed. The study of palaeobiogeographical raw data allows one to recognize palaeobiogeographic patterns and delimit provinces and to infer palaeobiogeographical processes and, in some cases, palaeogeographical (geological) processes. Biological and physical factors affecting the palaeobiogeographic distribution of the ammonites are reviewed prior to a discussion of the distributional patterns. The analysis is constrained by different factors such as taxonomy, sample size, accuracy of the biostratigraphic information, etc. The highly problematic taxonomy of the family Ataxioceratidae hampers the use of quantitative analyses, only allowing `phenetic' definitions of provinces. Some of the Tethyan palaeobiogeographic provinces already known in the literature are dealt with further. In particular the subdivision into Mediterranean and Sub-Mediterranean provinces (or sub-provinces), that is quite clear at the scale of the entire Jurassic period, is rejected in the Tithonian though a phase of faunal separation is recorded during the transition between the early and late Tithonian. The ammonite assemblages in the shelves were less diverse and different from those of the distal, pelagic areas. Dispersal is recognized as an important process in ammonite palaeobiogeography although objective location of centres of origin is impossible. Vicariant phenomena certainly occurred but in marine environments barriers that separate areas of distribution instead of portions of space cannot be delimited objectively. The `barrier' is certainly represented by the different palaeoenvironmental contexts (shelf or pelagic habitats). The palaeobiogeographic distributions observed were produced by a complex of concurrent biogeographic processes.