Branneroceras undefined-a

Branneroceras sp. A

Text-fig. 12a–u

2018 Branneroceras branneri; Dernov, p. 11, pl. 5, fig. 5, pl. 6, figs 2, 5.

M a t e r i a l. 22 specimens between 13 and 40 mm conch diameter from stratigraphic levels No. 3, 5b and 6 of the Mospyne Formation.

D e s c r i p t i o n. Specimen IGSU-4/315 is an almost fully limonitized specimen with 26.4 mm diameter (Textfig. 13a–c). The conch is thickly discoidal (ww/dm = 0.52) with wide umbilicus (uw/dm = 0.44) and moderate coiling rate (WER = 1.96) and moderately depressed (ww/ wh = 1.52) whorl profile; the venter is broad and weakly convex; ventrolateral shoulder is broadly rounded. The flanks are weakly convex, they slightly converge towards the ventrolateral shoulder. The umbilical margin is narrowly rounded and almost rectangular. The umbilical wall is flat and at an angle of 45º to the plane of symmetry of the conch.

The surface of the conch is covered with weak elongated umbilical nodes (~35 on the whorl), growth lines and welldeveloped lirae on the venter and ventrolateral shoulders. The growth lines form a shallow, broad sinus on the venter and a low projection on the ventrolateral shoulder; they are straight on the flanks. On the umbilical margin the transverse ribs are spaced about 0.60 to 0.75 mm apart. The number of lirae on the ventrolateral shoulder is two per 1 mm; on the midflank, the growth lines are spaced about 0.15 mm apart. Three weak concavo-convex constrictions with a low ventrolateral projection and a very shallow ventral sinus are prominent on the whorl.

Specimen IGSU-4/338 (Text-fig. 13d, e) is discoidal (ww/dm = 0.45) with a wide umbilicus (uw/dm = 0.46) and a weakly depressed whorl profile (ww/wh = 1.33) at 27.3 mm diameter. The venter is broad and weakly convex. The ventrolateral shoulder is broadly rounded. The flanks are weakly convex, they converge slightly towards the ventrolateral shoulder. The umbilical margin is narrowly rounded and almost rectangular. The umbilical wall is flat and at a 45º angle to the plane of symmetry of the conch. The surface of the conch is covered with weak elongated umbilical nodes and growth lines; the lirae on the venter and ventrolateral shoulders are not preserved. Two weak concavo-convex constrictions with a low ventrolateral projection and a very shallow ventral sinus are prominent on the whorl.

The suture line of specimen IGSU-4/725 (Text-fig. 12n, o, q) has a broad, sharply pointed lateral prong of the external lobe, a low, narrow ventrolateral saddle and V-shaped rectangular pointed adventive lobe. The suture line is not preserved on the umbilical wall and dorsum.

R e m a r k s. Branneroceras sp. A differs from Branneroceras branneri (SMITH, 1896) in a wider ventrolateral saddle, a narrower umbilicus (uw/dm = 0.43 at 15.5 mm diamener in Branneroceras sp. A and uw/dm = 0.54 at 15.7 mm diamener in Branneroceras branneri) and in weak constrictions, which are absent in Branneroceras branneri. Branneroceras sp. A differs from Branneroceras nicholasi NASSICHUK in a broader umbilicus (uw/dm = 0.55 at 18.0 mm diameter vs. uw/dm = 0.33 at 18.0 mm diameter, respectively) and a low whorl profile. The suture lines of Branneroceras sp. A and Branneroceras nicholasi NASSICHUK are similar, but the adventive lobe of Br. sp. A is more pointed. The morphology of the conch and weak umbilical nodes make Branneroceras sp. A similar to Inzeroceras bellum RUZHENCEV, 1974; the differences between Branneroceras sp. A and Inzeroceras bellum are in the narrower ventrolateral saddle and Y-like adventive lobe in Inzeroceras bellum.

O c c u r r e n c e. Late Bashkirian, Mospyne Formation; Donets Basin (Ukraine).