Exelissa indiana, Szabó & Jaitly, 2019

Szabó, János & Jaitly, Anand Kumar, 2019, Additional Middle Jurassic gastropods from Kachchh (western India) in the collections of the Banaras Hindu University (Varanasi, India), Fragmenta Palaeontologica Hungarica 36, pp. 3-30 : 17-20

Exelissa indiana n. sp.

( Figs 43–59)

Holotype – Inv. No.: BHU 2020I 13.

Paratypes – Inv. Nos: BHU 2020I 14–20.

Type locality – Khadir Island near Gadhada village.

Type strata – Gadhada Formation, Lower Callovian.

Derivation of name – Modified from India.

Diagnosis – Small shells of turriculate spire with just detectible cyrtoconoidal outline. Full grown shells consisting of 9 to 12 whorls, separated by furrow of V-shape cross-section with suture in the deepest line. Outer face of whorls flattish between two sutural furrows. Last whorl downward and leftward tending, last peristome subcircular, trumpet-like. Ornament composed of sparse, collabral riblets and dense, differently strong spiral threads. Shell shape and ornament highly variable.

Material – 278 more or less incomplete specimens in addition to the types below (Inv. No.: BHU 2020I 21 –299).

Measurements – BHU 2020I 13; Holotype: H dam. = 7.7 mm, D = 2.2 mm ( Figs 43–45); BHU 2020I 14; Paratype 1: H dam. = 9.8 mm, D = 2.7 mm ( Figs 48–49); BHU 2020I 15; Paratype 2: H dam. = 10.1 mm, D = 2.5 mm ( Figs 56–58); BHU 2020I 16; Paratype 3: H dam. = 9 mm, D = 2.7 mm ( Figs 54–55); BHU 2020I 17; Paratype 4: H dam. = 9.5 mm, D = 2.5 mm ( Figs 52–53); BHU 2020I 18; Paratype 5: H dam. = 7.8 mm, D = 2.4 mm ( Figs 46–47); BHU 2020I 19; Paratype 6: H dam. = 7.5 mm, D = 2.1 mm ( Figs 50–51); BHU 2020I 20; Paratype 7: D = 2.4 mm ( Fig. 59).

Description – Shells are rather highly turriculate with slightly pupiform outline. The teleoconch whorls are separated by deeply impressed suture, running in V-shaped furrow with angular rims. The wide outer face between the angulations is barely convex to barely concave. Base as a whole is convex with just convex wall; it joins to the outer face through the lower angulation that becomes rounded for the last whorl. The former coiling mode changing during formation of the last whorl; growing turns downward while the sutural furrow widens. The peristome, which is sub-quadrangular in earlier growth phases, changes into subcircular and its plane turns abaxially about 70° from the former position beside increasing in its prosocline orientation. In its final form, the peristome is trumpet-like, and far extending left from the axis. The latest parietal and columellar lips are most commonly detached from the basal shell parts.

The shape of the growth lines is variably opisthocyrt between the almost straight to clearly arched degrees; their orientation is also changing between slightly prosocline to slightly opisthocline. This variability appears mainly between different specimens but sometimes also between whorls of a single specimen. The last whorl modifications result every time in marked prosocline growth line orientation. The teleoconch whorls are ornamented by sparse collabral ribs and differently strong spiral threads. Following the apparently smooth protoconch, not found in entire form, the ornament starts with single threads on angulations, which limit the outer face; they are crossed by few (7–8) riblets/ whorl; third spiral thread appears on the next whorl between the former ones. The number of the spiral threads increase for the last whorl to ten or little more; the number of the ribs on the same whorl reaches 12 ± 1–2. The two primary threads, running on the angulations, become cord during the growth; the other threads remain thinner but variable in strength. The base is covered only with spiral threads. The ribs are nodosed at intersections with the primary threads/ cords and sometimes also at the other threads. Frequently, the ribs are rudimentarily developed but in a highly variable way: like nodes only at rims of the outer face, as shallow undulation, or fully lacking from shell parts, etc.

Remarks – In distinction of the early cerithiform genera, key importance has been attributed to the morphology of the protoconch. Amongst the available abundant specimens, no entire early shell has been found. However, the observable earliest whorl, which may be part of the protoconch, is bicarinate in several specimens. By GUZHOv (2004) modified diagnosis, the last protoconch whorls in Exelissa are bicarinate. Since the shape of the teleoconch also corresponds to this genus, the Indian species is regarded also as Exelissa , in spite of the spiral ornament that has been said characteristically sparser.

The most similar species is Exelissa africana Cox, 1965 (?Bajocian); its description ( COx 1965, p. 157) matches well also to this new Indian species; their close phylogenetical relation is probable. The most prominent differences are in their shape: Exelissa indiana n. sp. has more turriculate shell with only slight trace of cyrtoconoidal coiling while, at least in the holotype, E. africana has marked, cyrtoconoidally coiled early shell that gives bullet-like outline for the shell. Specimens, similar to the figured paratypes of E. africana do not occur in the Kachchh material. The shape and measurements of Exelissa (?) preralpina Cossmann, 1905 as figured by COSSMANN (1913, p. 120, Pl. 5, Figs 15–16) are similar to those of Exelissa indiana but differ in having a third marked and nodosed cord in mid-whorl position.

Occurrence – Khadir Island, Gadhada Formation, Lower Callovian.