Dienerian (Early Triassic) ammonoids from Spiti, Himachal Pradesh, India

Introduction The Northern Indian Margin is known since the end of the 19th century as a classical region for the study of Early Triassic ammonoid faunas. In a companion work (Ware et al. 2018), Dienerian ammonoids of the Salt Range (Pakistan) were thoroughly revised. A new high resolution biostratigraphical scheme was proposed for the Salt Range and correlation with other recently established biozonations (Tozer 1994 for Arctic Canada; Dagys & Ermakova 1996 for northern Siberia; Shigeta & Zakharov 2009 for Primorye, Russia) was discussed. The Spiti region (Himachal Pradesh, India) is a classical area for studying Early Triassic ammonoids of the Northern Indian Margin. Ammonites were described in two monographs first by Diener (1897) followed by von Krafft & Diener (1909). These two pioneering works unfortunately lack a detailed stratigraphic context, and many new species are only indicated as coming from the ‘lower division of the lower Trias’, a very vague indication, which does not provide the basis for an accurate and precise biozonation. Until very recently, no other papers concerning Early Triassic ammonoids from Spiti have been published. In the mid-1990s, Leopold Krystyn started the investigation of various sections in Spiti, which led him to propose a section near Mud as a Global Boundary Stratotype Section and Point (GSSP) candidate for the Induan–Olenekian boundary (equivalent to the Dienerian–Smithian boundary; Krystyn et al. 2007a, b). Only a few ammonoids were illustrated and described in these studies, as the focus was on the boundary interval. After the work in India by Krystyn et al. (2007a,b) the research group at the University of Z€urich conducted two intensive seasons of fieldwork in 2008 and 2009. This lead to the discovery of typical Smithian ammonoids below the boundary (Br€uhwiler et al. 2010) proposed by Krystyn et al. (2007a,b), thus questioning the proposed definition of the Induan–Olenekian boundary (see the foreword, Ware & Bucher 2018a for details). Recently, Br€uhwiler et al. (2012) revised and described the middle and upper Smithian ammonoids from Spiti district. The present work focuses on the abundant and fairly well preserved ammonoid faunas from a detailed bedrock controlled sampling in the Mud, Guling and Lalung areas of the Spiti District. The great similarity with the Salt Range, Pakistan, allows the authors to establish a detailed and laterally reproducible but informal biozonation for the Dienerian of the Northern Indian Margin. Based on these results, a formal biozonation for the Northern Indian Margin was constructed by Ware et al (2015).

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