The Late Tithonian ammonite-bearing horizons have very limited geographic occurrence in Kutch, being restricted only to the western part of the mainland. A 15 m thick sequence consisting mainly of oolite-shale alternations and coarse grained sandstone yields the terminal Tithonian faunal assemblages. The previous comprehensive report comes from Spath's work (1927-1933) where he described some Early Tithonian ammonites from farther east within the mainland, but the typical Upper Tithonian assemblages are restricted only to the westernmost part, i. e., towards the paleoshoreline (Fig. 17.1).
Spath described altogether seven genera of Late Tithonian age and these include mostly species of the genus Virgatosphinctes Uhlig. Stratigraphic knowledge of the Kutch ammonites was very poor during Spath's time and the collections were made mostly by other workers. Our repeated field investigations and systematic collections revealed that the Upper Tithonian assemblages are restricted mainly to the top part of the Umia Member (Mitra et al., 1979) (Fig. 17.2). The stratigraphic distribution of all species and genera described by Spath (1927-1933) has been firmly established on the basis of additional collection. In addition, our recent collection has unearthed many other genera of various
2 Upper Tithonian Assemblages of Different Faunal Provinces..
3 Affinity of Kutch Assemblage
4 Migrational Routes and Paleolatitudinal Disposition of Kutch
5 Paleobiogeography of Mass Extinction
N. H. Landman et al. (eds.), CephalopodsPresentandPast: NewInsights andFresh Perspectives, 375-395. © 2007 Springer.
provinces and different levels within the Upper Tithonian. These include Durangites Burckhardt, Corongoceras Spath, Tithopeltoceras Arkell, Himalayites Uhlig, and Pterolytoceras Spath (Shome et al., 2004; Shome et al., 2005; Shome and Roy, 2006). Durangites and Micracanthoceras micracanthum (Oppel) are the zonal indices of tropical Europe (see Cecca, 1999). It now appears that the Upper Tithonian assemblage in Kutch is represented by at least ammonites of three Mediterranean zones (cf. Tavera et al., 1986; Cecca, 1999).
The state-of-the-art of the Upper Tithonian ammonite assemblages of the world has been reevaluated in the light of new data and the nature of endemism and diversity patterns have been studied in the present endeavor. The Jurassic-Cretaceous boundary arguably marks a mass extinction event and ammonites evidently show a high extinction signal. We believe it is critical to understand the biogeography of the Late Tithonian ammonites in order to evaluate the patterns and extent of the end-Jurassic extinction event.
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