High-resolution biochronology and diversity dynamics of the Early Triassic ammonoid recovery: The Smithian faunas of the Northern Indian Margin Thomas Brühwiler a, ⁎, Hugo Bucher a, ⁎, Arnaud Brayard b , Nicolas Goudemand a a Paläontologisches Institut und Museum, Universität Zürich, Karl Schmid-Strasse 4, 8006 Zürich, Switzerland b UMR CNRS 5561 Biogéosciences, Université de Bourgogne, 6 Bd. Gabriel, 21000 Dijon, France abstract article info Article history: Received 22 February 2010 Received in revised form 1 September 2010 Accepted 3 September 2010 Available online 9 September 2010 Keywords: Early Triassic recovery Smithian Ammonoids Northern Indian Margin Biochronology Diversity Based on new collections of abundant and well preserved material from the Salt Range (Pakistan), Spiti (Northern India) and Tulong (South Tibet), several recent studies focused on the taxonomic revision and detailed biostratigraphy of Smithian ammonoids. In this work, biochronological data for these three well- documented basins are analyzed by means of the Unitary Associations method, resulting in a biochronological scheme of unprecedented high-resolution for the Smithian of the Northern Indian Margin (NIM). Data for each basin are first processed separately, thus yielding three local biochronological zonations. Then, the three sequences are processed together as a regional three-section data set for the construction of an inter-basin sequence at the NIM level. The latter zonation comprises 16 Unitary Associations grouped into 13 zones for the entire Smithian. Analysis of ammonoid diversity dynamics based on this new highly resolved time frame highlights (i) a marked diversification during the early Smithian, (ii) a severe extinction during the late Smithian, and (iii) an overall very high turnover throughout the Smithian. At a global spatial scale and stage resolution, the diversity of Smithian ammonoid genera appears surprisingly high, as highlighted by a previous study. It is shown that at a smaller geographic scale and with the most highly resolved time frame, Smithian ammonoids of the NIM reached their explosive diversity peak essentially through extremely high turnover rates rather than through a classic diversification process of high origination rates coupled with low extinction rates. Based on recently published U/Pb ages, regional apparent total rates of origination and extinction of more than 100 species per My can be inferred for the Smithian ammonoids of the NIM. © 2010 Elsevier B.V. All rights reserved. 1. Introduction In the aftermath of the end-Permian mass extinction, ammonoids were among the fastest clades to recover. The recent analysis of a global diversity data set of ammonoid genera from the Late Carboniferous to the Late Triassic shows that Triassic ammonoid genera actually reached levels of diversity higher than in the Permian less than 2 million years after the Permian–Triassic Boundary (PTB) (Brayard et al., 2009). The evolution of Early Triassic ammonoids was neither a smooth, nor a gradual process. It is characterized by the following main features: (i) a very low diversity in the Griesbachian (early Induan), (ii) a moderate diversity increase in the Dienerian (late Induan), (iii) an explosive radiation in the early Smithian (early Olenekian), (iv) a late Smithian extinction event followed by (v) a second explosive radiation during the early Spathian (late Olenekian, Brayard et al., 2006), and (vi) another significant diversity drop around the Spathian/Anisian boundary (Bucher, 1989; Brayard et al., 2009). This generic diversification scenario is coeval with a marked biogeographical structuring and strong latitudinalisation of faunas (Brayard et al., 2007). Several localities on the Northern Indian Margin (NIM) such as the Salt Range (Pakistan), Kashmir, Spiti (Northern India) and Tulong (South Tibet) have long been known for their abundant Early Triassic ammonoid faunas (Mojsisovics et al., 1895; Waagen, 1895; Diener, 1897; Frech, 1905; Krafft and Diener, 1909; Diener, 1913; Spath, 1934; Schindewolf, 1953; Kummel, 1966, 1970; Wang and He, 1976; Guex, 1978; Bando, 1981). Among these, the Salt Range and Spiti have played central roles in the development of the Early Triassic time scale. However, the majority of Smithian ammonoids from the NIM areas were hitherto insufficiently known due to small numbers of individuals and/or poor preservation, all of which influencing the definition of taxa and as a consequence, their biostratigraphic and biogeographic distributions. Moreover, the exact stratigraphic occur- rences of many taxa remained either approximate or unknown. Based on new collections of abundant and well preserved material, a number of recent studies focused on the taxonomic revision and detailed biostratigraphy of Smithian ammonoids from the Salt Range (Brühwiler et al., in press-b), from Spiti (Krystyn et al., 2007a,b; Brühwiler et al., 2010b, in press-a) and from Tulong (Brühwiler et al., 2010a)(Fig. 1). These studies have resulted in the most complete and Palaeogeography, Palaeoclimatology, Palaeoecology 297 (2010) 491–501 ⁎ Corresponding authors. Tel.: +41 44 634 26 98; fax: +41 44 634 49 23. E-mail addresses: bruehwiler@gmx.net (T. Brühwiler), hugo.fr.bucher@pim.uzh.ch (H. Bucher). 0031-0182/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2010.09.001 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo