Jurassic ammonite faunas from Nepal and their bearing on the palaeobiogeography of the Himalayan belt

From the Upper Bathonian up to the Tithonian-Berriasian, six main faunas an d twelve basic faunal assemblages within them are distinguished in Nepal. T he successive faunas show (1) low taxonomic diversity and (2) the dominance of a small number of genera and the subordinate place of the associated ta xa. The assemblages include: (1) strictly Tethyan (e.g., Mediterranean or Europ ean Tethyan) species and/or genera, very few in number and occurring as iso lated individuals or discontinuous faunal horizons; (2) Indo-Malagasian com ponents, some scattered, others with a wide occurrence in the SW Pacific, s ome as far as Antarctica and/or Patagonia; (3) indigenous genera endemic fo r the Himalayas and the SW Pacific region. Faunas of the same age for the S ula Islands, Papua-New Guinea, Australia, New Zealand, Antarctica and South America are also considered. In spite of common components, the Himalayan faunas contrast with the relat ively higher diversity of the Indo-Malagasian faunas. Low diversify and dom inance of indigenous genera mean that the faunas extending from the Himalay as to Antarctica and Patagonia represent an actual biogeographical unit, th e Indo Pacific (faunas and) Realm. Indo Pacific and Tethyan faunas show a less marked contrast than the Tethya n and Boreal. Transitional or mixed faunas of subaustral type developed in the Indo-Malagasian and Andean regions. This is explained by the absence of a geographical trap comparable to the land-locked palaeogeography of the A rctic Basin. The palaeogeography of the Arctic amplified the role of the ot her environmental factors. Among these the high latitude seasonal effects a re likely to have resulted in environmental instability, controlling trophi c resources and therefore the structure of the ecosystems, for instance low diversity and high density of the high latitude ammonite faunas. (C) 1999 Elsevier Science Ltd. All rights reserved.