Ammonite faunas and palaeobiogeography of the Himalayan belt during the Jurassic: Initiation of a Late Jurassic austral ammonite fauna

New data obtained in Nepal and Spiti (India) on stratigraphy and Jurassic ammonite faunas form the basis for a new biogeographical interpretation of the peri-Gondwanan Faunas. Faunas of the same ages from New Guinea, New Zealand, Antarctica and South America are also considered. From the upper Bathonian up to the Tithonian-Berriasian, six main successive assemblages are distinguished in Nepal and are compared to the faunas described in the above-mentioned areas, the latter often without accurate stratigraphic support. The faunas of Nepal show: (1) low taxonomic and generic diversity of the ammonites; and (2) dominance of one genus or a small number of genera in the same family and the subordinate place of the associated taxa. With a few exceptions, the classical biogeographical schemes do not recognize the existence of an austral ammonite fauna and/or Austral Province. Bipolar distributions of several benthic or nekto-benthic invertebrates as well as intertropical distribution of calpionellids express a latitudinal pattern of marine faunal distribution, Austral and Tethyan ammonite faunas show less marked contrast than the Tethyan and Boreal Realms and transitional or mixed faunas of subaustral type, of relatively high diversity, are developed in the Indo-Malagasian trough (or seaway) and the Andean region. The austral fauna of low diversity was spread around East and South Gondwanaland, from the Himalaya to Patagonia. The lack of a geographical trap such as the Arctic basin almost completely enclosed by land explains why the austral faunas were never as distinct as the Tethyan faunas from the Boreal. In addition to the role in the differentiation of the boreal faunas, the land-locked geography of the Arctic basin magnified the role of the other environmental factors, especially those linked with a high-latitude situation. Seasonal fluctuations of illumination in high latitude resulted in environmental instability which promoted. eurytopic taxa with high-density and low-diversity populations, (C) 1997 Elsevier Science B.V.