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


377


Figure 3. Schematic stratigraphic section of the pond section from where Turritella spp. were collected. Associated cephalopod taxa indicate the late early Callovian age.


Type species.—Turbo terebra Linnaeus, 1758 (original desig- nation), Recent, Indo-Pacific.


Turritella jadavpuriensis Mitra and Ghosh, 1979 Figures 4.1–4.4, 5.1, 5.2, 6, 7.1–7.6, 8.1–8.4


Type.—Neotype: specimen no. ISI/g/Jur/T 1.


Occurrence.—This species is known only from the Oxfordian horizons near a pond ~ 1km southeast of the village of Jhura, Kutch (23°24’47.57”N, 69°36’09.26”E).


Description.—Shell medium to large in size with 15–16 whorls. Maximum observed whorl diameter 23.5mm and maximum observed height 64.5mm. Apical angle 19–29°; apical angle and pleural angles not equal, pleural angle slightly less than apical angle. Suture moderately to deeply incised; incision increasing with ontogeny. Protoconch unknown (Figs. 4, 5). Apical sculpture formulaA1B1C1d2. Primary spirals A, B, andC


develop simultaneously; spiral A appears in the upper 1/3 of the first teleoconch whorl, spiral B in the middle of the whorl and spiralCin the lower 1/3 of the whorl. SpiralCquickly becoming prominent in the later whorls and forming an angulation near the anterior part of whorls. In inflated variant, spiral C changes during ontogeny from having a rounded edge to a well-marked,


blade-like angulation. Spiral A becomes prominent in later whorls, but less strong with respect to C and B spirals. In second teleoconch whorl, a prominent D spiral appears between spiral C and suture. Strength of all spirals (A, B, C, and D) increases during ontogeny. Order of relative strength is C, D, B, and A, which remains unchanged on adult whorls. Secondary spirals appear between all primary spirals. Spiral A and spiral B sepa- rated by one or two secondary spirals. Spiral C and spiral D also separated by two fine secondary spirals. One secondary spiral developed between spiral A and posterior suture, between spiral B and spiral C, and between spiralDand anterior suture (Fig. 6). Shell basally bicarinate, with carinae thinner toward its periph- eral (outer) part. Shape ranges from forms with high apical angle ~ 29° (i.e., “inflated” variants) to elongated forms with apical angle ~ 19° (i.e., “slender” variants). These two extreme variants are connected by intermediate forms forming overlapping morphotypes (Fig. 7.3, 7.4). All of them occur side-by-side in the same horizons. Shape of whorls in slender variants is imbricate and campanulate in robust variants. Spiral ornament in inflated variant forms a prominent keel-like angulation at whorl periphery. Both variants with convex early teleoconch whorls. Lateral aspect of growth line trace prosocline; lateral sinus is moderately deep (Fig. 8). Sinus with apex below mid- whorl is common while rarely occurring above mid-whorl. Single inflection point on top developed in some variants. Basal


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