High-resolution biochronology and diversity dynamics of the Early Triassic ammonoid recovery: The Dienerian faunas of the Northern Indian Margin

Highlights

• A new ammonoid biochronological zonation for the Dienerian of the Northern Indian Margin is constructed, with 12 zones

• We propose to subdivide the Dienerian into three parts (early, middle and late) instead of two

• Ammonoids underwent a moderate recovery in the early Dienerian followed by low diversity before the early Smithian recovery

• Low diversity in mid-late Dienerian is associated with anoxic conditions and warm temperatures

• mid-late Dienerian times now appear as a distinct extinction phase in the Early Triassic recovery

Abstract

A new high resolution biozonation based on the Unitary Association (UA) method is constructed for the Dienerian ammonoid succession of the Northern Indian Margin. It includes 12 UA-zones and leads to the subdivision of the Dienerian into three parts (early, middle and late). The corresponding diversity analyses, coupled with results previously obtained for the early Smithian ammonoids of the same regions, highlight the four following phases: (1) a first modest peak of diversity in the early Dienerian; (2) a very low diversity persisting throughout the middle Dienerian; (3) a slow increase of diversity during the late Dienerian, and (4) a marked diversification in the early Smithian. Turnover rates are very high during this entire time interval, and the boundaries between early–middle and middle–late Dienerian are emphasised by complete renewals of the ammonoid faunas. The low diversity values in the middle and early late Dienerian are concomitant with an anoxic event on outer continental shelves and coincide with warmer temperatures than those of the early Dienerian and early Smithian. This diversity pattern stands in strong contrast with the credo of a protracted or stepwise recovery following the end-Permian mass extinction. Together with the end-Smithian extinction, the middle and early late Dienerian diversity crises were likely both radical setbacks in the recovery of Early Triassic ammonoids. However, these two diversity crises do not necessarily imply identical environmental triggers that ultimately led to anoxic bottom waters on outer continental platforms in both cases.

Introduction

Modes and rates of biotic recovery following the end-Permian mass extinction are currently attracting a lot of efforts. Ammonoids have been documented to be one of the fastest clades to recover and even to largely overshoot their previous Permian record highs (Brayard et al., 2009). At the genus level, ammonoids show a low diversity in the Griesbachian, a slight increase during the Dienerian and an explosive radiation in the early Smithian (Brayard et al., 2006, Brayard et al., 2009). Such a pattern provides the general outline of diversity trends, but is also influenced by uneven taxonomical practices across authors, relatively coarse time bins and the absence of consensus about some stage and sub-stage boundaries (i.e. Induan–Olenekian ill defined boundary; Brühwiler at al., 2010a). More recently, a significant advance toward a refined diversity analysis (Brühwiler et al., 2010b) hinged on a new, highly resolved biozonation of the Smithian from the Northern Indian Margin (NIM).

The NIM has long been recognised as a key area for the establishment of the Early Triassic time scale (Jenks et al., 2015). The Salt Range (Pakistan) and Spiti District (Northern India; Fig. 1) are especially notorious for their abundant and well preserved ammonoid faunas since the pioneer works of Waagen (1895) in the Salt Range and of Diener (1897) and Krafft and Diener (1909) in Spiti. However, until recently, no thorough and modern revisions of the taxonomy and biostratigraphy of the ammonoids of these two regions have been published. The understanding of most taxa described in these pioneering works is hampered by the small sample sizes and their approximate stratigraphic positions. Following the revision of the Smithian ammonoids from the Salt Range (Brühwiler et al., 2012a) and Spiti district (Brühwiler et al., 2012b), new abundant and well-preserved material allowed us to thoroughly revise the taxonomy and biostratigraphy of the Dienerian ammonoids from these two basins (Ware et al., submitted-a, Ware et al., submitted-b). As for the Smithian, they represent the most complete and detailed Dienerian ammonoid records known worldwide, with 12 Dienerian local maximal horizons in the Salt Range and 10 in Spiti, compared to only four horizons in Canada (Tozer, 1994), three in South Primorye (Shigeta et al., 2009) and four in Siberia (Dagys and Ermakova, 1996).

Here we present a new high-resolution ammonoid zonation for the Dienerian of the NIM based on a synthetic biochronological analysis of the Salt Range and Spiti basins at the species level. This new biostratigraphic scheme is based on bed by bed extensive collections in order to produce a reliable taxonomy reflecting intraspecific variation as well as the best possible resolution in time. Correlation of the Dienerian ammonoid succession of the NIM with other regions will need additional, similarly detailed work. Possible correlations are discussed in Ware et al. (submitted-a). This succession of the NIM provides a robust reference scheme for Dienerian times and further correlations at larger geographical scales. The hitherto poorly know Dienerian faunas can now contribute to an improved understanding of the Early Triassic recovery. The new highly resolved biostratigraphic framework allows the analysis of the biodiversity dynamics of the Dienerian ammonoids from the NIM with unprecedented detail, and to compare it with palaeoenvironmental proxies obtained from the same sections.