Melvilloceras rotaii

Dernov, Vitaly, 2022, Late Bashkirian Ammonoids From The Mospyne Formation Of The Donets Basin, Ukraine, Fossil Imprint 78 (2), pp. 489-512 : 498-499

Melvilloceras rotaii (LIBROVITCH in POPOV, 1979)

Text-fig. 8

1979 Lutuginoceras rotaii ; Popov, p. 86, pl. XI, figs 1, 2, text-fig. 10I.

2021a Melvilloceras rotaii ; Dernov, p. 13, text-fig. 1A–O.

H o l o t y p e. Specimen VSEGEI- 73 in the Russian Geological Research Institute (St. Petersburg, Russia); figured by Popov (1979) in pl. XI, figs 1 and 2.

Ty p e l o c a l i t y a n d s t r a t i g r a p h i c h o r i z o n. Rus’ko-Orlovka on Krynka River (Donetsk Region, Ukraine); Mospyne Formation, G 4 limestone layer (late Bashkirian).

D i a g n o s i s. Melvilloceras has a discoidal and subinvolute conch with narrow convex venter and weakly convex flanks covered with delicate concavo-convex growth lines; umbilical margin almost rectangular with small, sharp nodes.

M a t e r i a l. 13 specimens with conch diameter between 7 and 32 mm from stratigraphic levels No. 2–6 of the Mospyne Formation.

D e s c r i p t i o n. The largest available specimen IGSU-7/8040 ( Text-fig. 8a, b ) with diameter 32.1 mm is subinvolute (uw/dm = 0.19), thinly discoidal (ww/dm ~ 0.34) with a weakly compressed whorl profile (ww/wh ~ 0.73). The venter is strongly convex and narrow; flanks weakly convex and slightly converging towards the ventrolateral shoulder. The umbilical margin is almost rectangular. The umbilical wall is vertical and flattened. The surface of the conch is covered with frequent and clearly visible lamellar lines and delicate growth lines, forming a narrow, deep external sinus, high ventrolateral projection and shallow, wide lateral sinus. On the midflank the growth lines are spaced about 0.4 to 0.5 mm apart. The umbilical margin has sharp elongated nodes.

Specimen IGSU-7/468 is an almost fully limonitized specimen with a diameter of 23.4 mm ( Text-fig. 8g –i ) and weakly compressed whorl profile (ww/wh = 0.91). The conch is thickly discoidal (ww/dm = 0.48) with narrow umbilicus (uw/dm = 0.18) and high aperture (WER = 2.14); the umbilical wall is flat and narrow; and the umbilical margin is almost rectangular. The venter is strongly convex; flanks are broad and very slightly convex. Ornamented with small, sharp hardly noticeable umbilical nodes.

Specimen IGSU-7/472 is a steinkern of 20.5 mm diameter ( Text-fig. 8c, d ). The conch has a narrow umbilicus (uw/ dm = 0.23); a flattened, narrow umbilical wall and almost rectangular umbilical margin. The venter is strongly convex and narrow; the flanks are slightly convex (nearly flattened) and ornamented with small, sharp umbilical nodes (one node per 1 mm of the umbilical margin). Four narrow concavoconvex constrictions with high ventrolateral projection and deep ventral sinus are prominent on the whorl.

R e m a r k s. Melvilloceras rotaii differs from Melvilloceras sabinensis in the coarser umbilical nodes, which don’t disappear during ontogeny and absence of the longitudinal lirae. The conch form of the studied specimens differs from the single specimen of the type material, studied by Popov (holotype VSEGEI–73): ww/dm = 0.60 at conch diameter 15 mm in the holotype in comparison with ww/dm = 0.48 at conch diameter 14 mm (specimen IGSU-7/496).In addition, the specimens studied lack the longitudinal lirae noted by Popov (1979) in the original description. It is possible that longitudinal lirae are characteristic of young conchs; they may be absent on mature conchs (most of the specimens studied have a diameter of about 20 mm or more). The conch shape of the described specimens is more closely related to the species Melvilloceras sabinensis (ww/dm = 0.47 at 19.5 mm diameter in M. sabinensis and ww/dm = 0.47 at 19.7 mm diameter in M. rotaii ).

O c c u r r e n c e. Late Bashkirian, Mospyne Formation (shale 100 m below the G 1 2 limestone layer to the G 4 limestone layer; see Text-fig. 6 ); Donets Basin ( Ukraine).

Figures

Text-fig. 8. Ammonoids Melvilloceras rotaii (LIBROVITCH in POPOV, 1979) from the Mospyne Formation. a, b: A slightly laterally compressed specimen IGSU-7/8040, lateral view (a), ventral view (b). c, d: A slightly laterally compressed specimen IGSU-7/472, lateral view (c), ventral view (d). e, f: Specimen IGSU-7/540, lateral view (e), ventral view (f). g–i: Specimen IGSU-7/468, dorsal view (g), lateral view (h), ventral view (i). j: Specimen IGSU-7/6538, dorsal view. k, l: Specimen IGSU-7/620, lateral view (k), ventral view (l). m–q: Lateral views of the conchs showing the different relief of the umbilical nodes; m – specimen IGSU-4/4148, n – specimen IGSU-7/522, o – specimen IGSU-7/468a, p – specimen IGSU-7/472a, q – specimen IGSU-7/316. Scale bars 10 mm.

Text-fig. 6. Stratigraphic distribution of ammonoids in the Mospyne Formation. The lithological symbols are the same as in Text-fig. 2.