Article: Middle Jurassic ammonites of Tibet and the age of the lower Spiti Shales

Middle Jurassic ammonites of China are known only from Tibet. Eighteen localities are described, some new. Only Lower Bajocian and Lower-Middle Callovian are established, whereas the other Middle Jurassic stages or substages are usually represented by hiati or non-marine facies. Two major ammonite faunas are distinguished: (1) Witchellia-Fontannesia Association, including Fontannesia kiliani ['Dorsetensia' auct.], Laeviuscula Zone, known from the central part of Tethyan Himalaya and with strong affinity to northern and western Australasia of the extreme south-east Tethys; (2) 'Grayiceras' Association, including Grayicerasl gucuoi n. sp., late Calloviense Zone (Subkossmatia opis Assemblage Zone of Kachchh), possibly widely distributed in the basal Spiti Shales (Belemnposis gerardi Beds) and equivalents of Tethyan Himalaya, to which it appears to be largely endemic, together with rare Indo-Madagascan (Ethiopian) and Mediterranean elements. In addition, the Early Bajocian Discites and ?Sauzei-Humphriesianum Zones, and the Middle Callovian Corona turn Zone are indicated locally.The 'Grayiceras' fauna was dated as Oxfordian by most previous authors, but the stage is missing at least locally in South Tibet, where Kimmeridgian lies conformably on Lower Callovian. Biogeographic affinities support an origin of the Tethyan Himalaya from the Gondwana margin, not too distant from northern Australasia, whereas the limited North Tibetan faunas are consistent with a Eurasian position.