Bithynia erzurumensis, Harzhauser & Neubauer & Hoşgör, 2018

Harzhauser, Mathias, Neubauer, Thomas A. & Hoşgör, İzzet, 2018, Oligocene-Miocene freshwater gastropods from the Oltu-Narman Basin in eastern Turkey, Acta Palaeontologica Polonica 63 (2), pp. 357-369 : 361-362

Bithynia erzurumensis sp. nov.

Fig. 3 .

Material.— Type material and additional 13 specimens ( NHMW 2018 /0019/0011) and 37 opercula ( NHMW 2018 / 0019/0012–0014) ( Fig. 3F, G ). All from type locality and horizon .

Diagnosis. —Small, stout shell with low conical spire, large globular last whorl, low, rapidly contracting base and deeply incised suture. Maximum diameter of last whorl in mid-height; aperture markedly angulated with posterior tip below periphery of last whorl.

Description.—Protoconch comprises very low initial part and about one strongly convex whorl of c. 950 μm diameter; transition into teleoconch unclear due to poor preservation. Low spire ( Table 2) consisting of two convex teleoconch whorls, rapidly increasing in width with maximum diameter in lower third; suture deeply incised. Last whorl globular, regularly convex with maximum diameter in mid-height; base moderately convex, rapidly contracting. Aperture distinctly angulated at posterior tip, contact with last whorl below its periphery. Umbilicus narrow, partly covered by thin and narrow inner lip. Operculum solid, posteriorly angulated. Outer surface with central nucleus, prominent concentric growth lines, distinctly concave. Inner surface convex with shallow, irregular dents; height/width ratio c. 1.4.

Remarks.—The available material is only moderately preserved, but the characteristic outline of this stout globular species allows a clear separation from other Bithynia species and justifies its description as new species. A specimen with the operculum preserved in situ ( Fig. 3D ) proves that shells and opercula belong to the same species.

A globular shell shape and a low spire are also found in some species of the bithyniid genus Pseudobithynia Glöer and Pešić, 2006 , e.g., in its type species Pseudobithynia irana Glöer and Pešić, 2006 . As only genital features allow a separation of both genera ( Glöer and Pešić 2006), we place the fossil species tentatively in Bithynia .

Although the genus was diverse during the Miocene and Pliocene in Europe ( Bouchet et al. 2018 list 34 fossil species), only few have been documented from Oligocene and lower Miocene deposits (see Wenz 1928). Of these, only Bithynia globuloides ( Sandberger, 1873) , from the Oligocene of England, is similar to the Turkish species in outline. It differs in its ovoid aperture, which lacks a marked angulation. The middle Miocene Bithynia jurinaci Brusina, 1884 , from the Dinaride Lake System, as well as B. glabra ( Zieten, 1832) and B. dunkeri Gude, 1913 , both from southern Germany, differ in their higher shells and less incised suture (see Schlickum 1966; Neubauer et al. 2016). The fossil species is reminiscent of the extant Turkish Pseudobithynia pentheri ( Sturany, 1904) concerning the low spire and large last whorl, but differs from this species in its even more globular outline and the narrower umbilicus (see Glöer and Yildirim 2006). The extant Turkish Pseudobithynia guldeni Gürlek, 2018 lacks an umbilicus and has a low spire but differs in its higher last whorl and the smaller size (see Gürlek 2018). Similarly, the extant Bithynia mostarensis von Möllendorff, 1873 , from Skadar Lake, has a rather globular outline but develops a higher last whorl, and its base is only slowly contracting compared to the fossil species ( Glöer and Pešić 2007). The extant Bithynia radomani Glöer and Pešić, 2006 , also from Skadar Lake, has a globular last whorl with low base but differs in its higher spire (see Glöer and Pešić 2007).

Figures

Fig. 3. Bithyniid gastropod Bithynia erzurumensis sp. nov. from the upper Oligocene–lower Miocene Susuz Formation, Kömürlü, Turkey. A. NHMW 2018/0019/0006, holotype in apertural view. B. NHMW 2018/0019/0007, paratype in apertural view. C. NHMW 2018/0019/0008, paratype in dorsal view. D. NHMW 2018/0019/0008, paratype in apertural view showing in situ operculum. E. NHMW 2018/0019/0010, paratype in apical view. F. NHMW 2018/0019/0012, outer surface of operculum. G. NHMW 2018/0019/0013, outer surface of operculum. H. NHMW 2018/0019/0014, inner surface of operculum. Scale bars 0.5 mm, if not stated otherwise.