Cyclotopsis praecursor, Neubert & Damme, 2012

Neubert, Eike & Damme, Dirk van, 2012, Palaeogene continental molluscs of Oman, Contributions to Natural History 20, pp. 1-28 : 10-12

Cyclotopsis praecursor sp. nov.

( Figs. 8–9 )

Type specimens: Holotype NMBE 5018970 ; paratypes NMBE 5018971–5018973 , MNHN/25 , SMF 340184/25 ; paratypes Wadi Darbat , NMBE 5018974 , 5019038– 5019043 .

Type locality: Thaytiniti , Dhofar, Oman .

Stratum typicum: Paludal biomicritic limestones of the Zalumah Formation.

Age: Late Priabonian.

Material: This species is very abundant at Thaytiniti and Wadi Darbat.

Etymology: praecursor from Latin for predecessor, a noun in apposition.

Diagnosis: A fossil species of Cyclotopsis with a slightly elevated spire and an operculum with a lamella-like coiled spiral.

Description: Shell much broader than high, spire slightly elevated; protoconch relatively large, consists of 2 whorls, surface sculpture not preserved; teleoconch of 3.5–4 rapidly increasing whorls; surface sculpture of 3–5 strong spirals (only preserved on the upper whorls); last whorl only slightly descending before the aperture; aperture almost circular, with a slightly thickened lip; umbilicus very large, broad dish-like, umbilical walls very probably smooth; operculum multispiral, spiral consisting of a broad raised lamella of at least 3 whorls.

Measurements: Holotype ( Fig. 8 ): H = 6.73 mm; W = 9.18 mm; h = 4.42 mm; w = 4.25 mm; Wh = 5.5.

Remarks: Roger & al. (1994) probably refer to eroded specimens of this species as 'cf. valvata '. The preservation state of the specimens from the type locality in Thaytiniti is quite poor and mainly consists of internal casts. However, in a few cases, remains of the shells themselves are preserved, allowing reconstructing of some details of the surface sculpture. These specimens display the same teleoconch sculpture as the specimens from Wadi Darbat ( Fig. 9 ), where the preservation state is much better (but no specimen with an operculum could be traced there).

This fossil species is placed in the extant genus Cyclotopsis , because it is almost indistinguishable from the few species from India (for comparison see the syntype of Cyclostoma semistriatum SOWERBY, 1843 ( Fig. 10 ), the type species of Cyclotopsis ). It shares autapomorphic details of umbilicus and operculum together with the spiral sculpture of the teleoconch. It differs from most of the Soqotran species of Dioscopoma NEUBERT, 2009 , which usually have a reticulate surface sculpture, a more narrow umbilicus, and an operculum with a flat sutural line and not a raised spiral; also, most of the species of Dioscopoma display a spiral sculpure on the inner umbilical wall, which is very probably smooth in C. praecursor sp. nov. Today, the Dhofar area is inhabited by two species of the genus Rochebrunia BOURGUIGNAT, 1881 , but these have larger shells with a closed umbilicus and an almost smooth operculum. For a more detailed discussion of the pomatiid genera and species of the area see Neubert (2009), and for pomatiid operculum structure Wilmsmeier & Neubert (2012).

Figures

Figs. 8–10: Cyclotopsis praecursor sp. nov. Fig. 8: Thaytiniti, holotype NMBE 5018970 in frontal, apical and ventral view; Fig. 9: Wadi Darbat, paratype NMBE 5018974 in frontal, apical and ventral view; Fig. 10: Cyclotopsis semistriatum (SOWERBY, 1843), syntype NHMUK 20030591, India, Poona.