Megastomia sp. 2, Monterosato, 1884

Thivaiou, Danae, Harzhauser, Mathias & Koskeridou, Efterpi, 2019, Early Miocene Gastropods from the Felli Section (Proto-Mediterranean Sea NW Greece), Geodiversitas 41 (8), pp. 323-366 : 352

Megastomia sp. 2

(Fig. 8 D1-D3)

MATERIAL EXAMINED. — Sample F11: AMPG ( IV) 2563-2569 (seven specimens).

DIMENSIONS. — Maximum height: 2.20 mm.

DISTRIBUTION. — Early Miocene. Proto-Mediterranean Sea: Greece (this paper).

DESCRIPTION

Moderately slender, solid shell; apical angle c. 27°. Partially immersed, type B protoconch. Teleoconch comprising four feebly convex, smooth whorls, with inconspicuous microscopical orthocline growth lines separated by impressed sutures. Last whorl attaining about 60% of total height; slightly angulated at periphery. Columella concave with narrow columellar fold; aperture ovate; outer lip with five observable spiral lines. No umbilicus.

REMARKS

Two protoconch whorls are visible on the shells (including the nucleus). Type B protoconch and columellar tooth place this shell in the Odostomia group, and the spiral striations of the inner lip in the Megastomia genus. It is also separated from ‘ Odostomia ’ sp. 2 of the present material, in having a more immersed protoconch and more rounded whorls.

The overall morphology is similar to M. conoidea with the main difference being that the protoconch of the Greek specimen is of type B a slightly more rounded last whorl. The size and protoconch type coincides with that of Odostomia sp. from the stratotype region of the Aquitanian (Aquitaine Basin, SW France) ( Lozouet et al. 2001). Nevertheless, the Greek specimen has a more conical shape and spiral sculpture on the inner lip that differentiates it.

Figures

FIG. 8. — A1, A2, ‘Odostomia’ sp. 2, AMPG(IV) 2558; B1-B3, Megastomia sp. 1, AMPG(IV) 2560; C1, C2, Brachystomia sp., AMPG(IV) 2570; D1-D3, Megastomia sp. 2: D1, AMPG(IV) 2563;D2, D3, AMPG(IV) 2564;E1-E4, Pyramistomia aliakmoni n. sp.: E1, E2, holotype, AMPG(IV) 1500;E3, paratype 1, AMPG(IV) 1501 (SEM image); E4, AMPG(IV) paratype 1, AMPG(IV) 1501, protoconch (SEM image); F1-F4, Parthenina sp. 1: F1, F2, AMPG(IV) 1573; F3, AMPG(IV) 1574 (SEM image); F4, AMPG(IV) 1574, apex (SEM image); G1, G2, Parthenina sp. 2: G1, AMPG(IV) 2575; G2, AMPG(IV) 2575, apex (SEM image); H,?Syrnola sp., AMPG(IV) 2577 (SEM image). Scale bars: A1, A2, B1-B3, D1, G1, H, 500 µm; C1, C2, D2, D3, E1, E2, 1 mm; E3, F1, F2, 400 µm; E4, F3, 200 µm; F4, G2, 100 µm.