Obsoletiformes obsoletum obsoletum (Eichwald, 1830)

Frolov, Pavel D., Danukalova, Guzel A. & Osipova, Eugenia M., 2018, Late Miocene molluscs of the Morskaya 2 site (Azov Sea region, Russia), Palaeontologia Electronica (a 20) 23 (1), pp. 1-32 : 22-23

Obsoletiformes obsoletum obsoletum (Eichwald, 1830)

Figure 3.9 -10

1830 Cardium obsoletum Eichwald , p. 208.

1850 Cardium obsoletum Eichwald ; Eichwald, p. 61, pl. 4, fig. 19.

1976 Cerastoderma (Obsoletiforma) obsoletum obsoletum (Eichwald) ; Iljina, Nevesskaya, and Paramonova, pl. 1, fig. 37-48, pl. 2, figs. 25-29, pl. 3, figs. 3-26

1993 Obsoletiforma obsoleta obsoleta (Eichwald) ; Nevesskaya, Goncharova, Paramonova, Popov, Babak, Bagdasaryan, and Voronina, p. 247, pl. 56, figs. 26-29.

2001 Obsoletiformes obsoletus obsoletus (Eichwald) ; Nevesskaja, Paramonova, and Popov, p. 180.

Material. 12 specimens from layer 1 (8 left valves, 12 right valves).

Description. Round-triangular shell (LS up to 10.5 mm; HS up to 9.5 mm; Con. up to 2.8 mm), equivalve. Closure of the valves is dense. Umbo is pointed, bent forward with a central position. The anterior branch of the dorsal edge is gently sloped and without noticeable angles merging into a widely rounded anterior edge. The posterior branch of the dorsal edge forms a well-defined blunt angle with a slightly convex (almost straight) posterior edge. Ventral edge slightly convex. Valves are almost equilateral, convex. The shell surface is covered with well-developed, triangular-squared ribs. With a gentle front slope and a steep back. The number of ribs is approximately 25. The intercostal distance is approximately equal to the width of the ribs. Scaly-like outgrowths are often preserved on the first ribs (up to the 6th) in the anterior slope of the shell. They are also present on the three posterior ribs. They are often developed more prominently on the posterior ribs than on the other part of the shell, taking the form of triangular spines. Growth lines are clearly visible, becoming coarse downward.

There are two cardinal teeth, two anterior lateral teeth (the lower one is more developed), and one posterior tooth in the hinge of the right valve. There are one cardinal tooth and two lateral teeth in the left valve. The short nymph is behind the umbo. The pallial line is whole. The scars of the anterior and posterior muscles are round-oval, equal in size. There are grooves corresponding to the ribs on the inner surface. On the edge of the shell they take the form of “serration.”

Remarks. Species epithet has been changed from obsoletus to obsoletum (Nevesskaya et al., 2013) . There are only juvenile shells in our collections. Adult specimens can reach 25, rarely 32 mm in length (Nevesskaya et al., 1993)

Region of distribution and age. Early and Middle Sarmatian (Volhynian and Bessarabian) of the Eastern Paratethys (Nevesskaya et al., 1993).

Figures

FIGURE 3. Assemblage of the Middle Sarmatian (Bessarabian) marine bivalves (layer 1). 1-2, Polititapes ponderosa (d’Orbigny, 1844). 3-4, Polititapes vitaliana (d’Orbigny, 1844). 5-6, Sarmatimactra fabreana (d’Оrbigny, 1844). 7-8, Obsoletiformes obsoletum ingratum (Kolesnikov, 1929). 9-10, Obsoletiformes obsoletum obsoletum (Eichwald, 1830). 11, Plicatiforma fittoni fittoni (d’Orbigny in Murchison et al., 1845). 12-13, Ervilia dissita dissita (Eichwald, 1830).