Biostratigraphic implication of Olcostephanus Neumayr, 1875 (Ammonoidea) from the Lower Cretaceous Giumal Formation, Spiti Valley, Tethys Himalaya, India

The paper records new species of the ammonoid genus Olcostephanus Neumayr from the Lower Cretaceous Giumal Formation exposed near Gete village in the Spiti Valley (Himachal Himalaya), India. Olcostephanus is one of the most significant genera for intrabasinal to intercontinental bio-chronostratigraphic correlation in view of its episodic global expansion during the Valanginian. This is the first report of Olcostephanus (Olcostephanus) sakalavensis (Besairie), Olcostephanus (Olcostephanus) cf. salinarius Spath, Olcostephanus (Olcostephanus) cf. fascigerus Spath and Olcostephanus (Olcostephanus) sp. indet., from the Indian Cretaceous. Based on close resemblance with the well-known biostratigraphically precised Mediterranean and South American Olcostephanus species, the present taxa are placed in the Neocomites platycostatus Subzone of the Karakaschiceras inostranzewi Zone to the Saynoceras verrucosum Subzone of the Saynoceras verrucosum Zone interval of the Standard Tethyan Scheme developed in Mediterranean of Europe. A possible correlation with Olcostephanus bearing stratigraphic horizons of South Africa, Madagascar and Pakistan is also proposed. The findings also confirm a significant eustatic rise close to the lower/upper Valanginian boundary.