Structure and palaeogeography of the western Tethys during the Jurassic: tests based on ammonite palaeobiogeography

Abstract

Abstract Comparisons between the recently proposed palaeogeographical reconstructions for the Tethys and the palaeobiogeography of Jurassic ammonites are important in the study of the structural evolution and the palaeogeography of the Tethys and its margins. The geographical distribution of Jurassic ammonites is considered to be due to a combination of historical and ecological factors, with the existence of biota and bioma ( sensu Croizat 1958). ‘Boreal’ and ‘Tethyan biota’ are used and preferred to the ‘Boreal’ and ‘Tethyan realms’ (or provinces), as previously used to designate ammonite communities with similar geological and evolutionary history; several ‘Bioma’ are defined for faunal communities with similar morphofunctionary adaptations (for instance on very shallow platforms or in deep oceanic regions). The overlap of biota and bioma better explains the compound ammonite faunas. The recently proposed new maps of the Tethys show that in the west the oceanic areas are much more restricted in area than in previous reconstructions; a large oceanic region is constantly extended throughout the Jurassic, only in its eastern part. Subsequently, the total area west of the so-called Apulian Plate, and in Western Europe, takes a prominent part in the faunal ammonite distribution. The Arctic Basin sometimes acted as an evolutionary trap, where Boreal faunas were totally isolated from the Tethyan ones; sometimes an important eustatic transgression allowed faunal exchanges to be made between Boreal and Tethyan stocks. Several examples, taken from the Pliensbachian to the Callovian, serve to demonstrate the influence on ammonite distributions of either the possible deep-oceanic areas or very shallow platforms and emergent areas.