Das et al.—Oldest turritelline gastropods from the Upper Jurassic of India


Table 2. Similarity matrix showing clusters of the Jurassic species discussed in this paper and selected Cretaceous turritelline species (based on data from Table 1). Methods based on Raup and Stanley (1978, p. 144–145).


A B C D E


AB C D E F G H I 72 5436453627 3636


F45 G36


6464545445 5445 54454545 3636 364545 3627 4536 5454 45


H45 I


36 27


375


Table 3 . Similarity matrix showing clusters of the Jurassic species discussed in this paper and selected Mesozoic mathildidoid species (based on data from Table 1).


A B C D J


AB C D J K L M N 72


5436 6464 54


274554 276436 362718 27 18


K64 L64


6464 9 18


1827 3618 1827 0


5454 72


54 54


M45 N


are divided into several formations (Mitra et al., 1979). The major fossil-bearing units are the Patcham (Bathonian) and Chari (Callovian to Oxfordian) formations of the mainland of Kutch (Biswas, 1977). The fossils discussed here were collected from the Dhosa Oolite Member (Oxfordian) of the Chari Formation (23°24’47.57”N, 69°36’09.26”E) (Fig. 1).


Stratigraphic information.—Rocks of both the Patcham and Chari formations were the products of continental shelf margin sedimentation (Datta, 1992; Fürsich et al., 2001). The Dhosa Oolite Member is a condensed, time-averaged unit (Singh, 1989; Fürsich et al., 1992; Pandey et al., 2009; Roy et al., 2012). In sequence stratigraphic terms, it represents a Transgressive System Tract (TST; Fürsich et al., 2001) and Maximum Flooding Zone (MFZ; Fürsich and Pandey, 2003). The top part of the Dhosa Oolite is hard, conglomeratic, highly fossiliferous, and contains ferruginous oolite. The lower part contains shale and sandstone with sparse oolite (Fürsich et al., 2001; Roy et al., 2012) (Fig. 2). The present turritelline species were found in the lower part of the unit, mostly in shale, or occurring loosely, free of matrix. Mitra and Ghosh (1979) reported two ammonite species,


Peltoceras kumagunense Spath, 1931 and Paryphoceras rugosum Spath, 1928, in association with the Turritella specimens they described. According to them, these two species belonged to the Oxfordian and were found only in the Dhosa Oolite Member, which was overlain by layers of Kimmeridgian age. We also collected a single belemnite specimen tentatively assigned to Belemnopsis tanganensis (Futterer, 1894) (see Spath, 1927, p. 9, pl. 1, figs. 3a, b, 4) from one Turritella- bearing horizon. Despite the previous discovery of these cephalopod fossils,


determination of the precise age of the Turritella-bearing horizons remained unclear. Recently, however, Roy et al. (2012)


Figure 1. Geographic locations (solid circles) of the Dhosa Oolite Member. Present fossil collection has been made from a pond section near Jhura (*). Modified after Bardhan et al. (2012).


demonstrated that the Dhosa Oolite Member spans the entire Oxfordian. Peltoceras kumagunense ranges from upper Callovian to lower Oxfordian (Spath, 1931; A. Roy, personal communication, 2016). Spath, on the other hand, described all species of Paryphoceras from the Dhosa Oolite Member without specifying from which part of the unit the species came. Alberti et al. (2015) revised the species of Paryphoceras of Kutch, and concluded that the majority of the species are found in the middle Oxfordian (Cardioceras cordatum to Gregoryceras transversarium zones; ca. 159.4–161.4 Myr; Gradstein et al., 2012). The only species described by Alberti et al. (2015) come from the lower Oxfordian part of the Dhosa Oolite. From these faunal occurrences, it appears that the Turritella-bearing horizons are either early Oxfordian or middle Oxfordian in age. From the lithological point of view, Turritella-yielding horizons are mostly shale and sandstone, occasionally within limestone with sparse ooids (Fig. 3). This facies association is typical of the upper part of the lower Oxfordian part of the Dhosa Oolite Member.


Locality information.—The Dhosa Oolite Member is widely distributed in the mainland of Kutch as well as in the Jhura dome. The Turritella-bearing assemblages, however, are restricted to a pond section (23°24’47.57”N, 69°36’09.26”E) near the village of Jhura, which is 45 km northwest of Bhuj, the district town of Kutch (Fig. 1).


Materials and methods


The samples were collected following both bulk sampling (see Kowalewski, 2002) and random surface-sampling protocols (see Mallick et al., 2013). Eleven bulk samples were collected in four separate field trips (2012, 2013, 2014, and 2016). The tur- ritelline gastropods were separated in the laboratory. At several stratigraphic horizons, we collected all specimens encountered, including those small and large, broken, and intact. Turritelline specimens were identified at supraspecific levels following cri- teria and morphological terms used in the previous literature (e.g., Allison, 1965; Allmon, 1996; DeVries, 2007; Allmon and


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