Biogeographical and stratigraphical significance of the latest Turonian and Early Coniacian inoceramid/ammonite succession of the Manasoa section on the Onilahy River, south-west Madagascar

Recent, bed-by-bed fossil collections from Manasoa on the Onilahy River, south-west Madagascar, reveal the stratigraphical succession of the inoceramid and ammonite faunas of the topmost Turonian through Lower and possibly lower Middle Coniacian of the area. Six successive interval inoceramid zones are recognised; from bottom upward these are zones of Tethyoceramus modestoides, Tethyoceramus madagascariensis, Tethyoceramus basseae, Tethyoceramus ernsti, Inoceramus? nukeus, and Inoceramus sp. aff. pacificus. The T. modestoides Zone lies within the Turonian. The base of the T. madagascariensis Zone marks the base of the Coniacian; all higher zones also lie within the Coniacian. The succession identified correlates well with other areas of the Southern Hemisphere and, to a lesser degree, with the Euramerican succession. oceramid faunas of the topmost Turonian through lower Coniacian of Madagascar are very similar to those of Zululand and are closely related to other Southern Hemisphere faunas. The Madagascan faunas are, however, clearly distinct from Euramerican faunas. The rich ammonite assemblage of the Manasoa section is dominated by endemic species characteristic of the Malgascan province, with only few forms known from Japan, Alaska and Europe. All of the ammonites represent the Barroisiceras onilahyense-Kossmaticeras (K.) theobaldianum Zone, which is shown to correspond to possibly topmost Turonian and Lower Coniacian. mic descriptions of the inoceramid species are provided, with range data and photographic documentation.