Theodoxus susuzianus, 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 : 359

Theodoxus susuzianus sp. nov.

Fig. 2 .

ZooBank LSID: urn:lsid:zoobank.org:pub:153CB896-6EE8-48F1-A330-80FCD3B1ED94

Etymology: Referring to the upper Oligocene–lower Miocene Susuz Formation.

Type material: Holotype: NHMW 2018 /0019/0001, moderately preserved, adult shell ( Fig. 2A ) . Paratypes: NHMW 2018 /0019/0002, moderately preserved, adult shell ( Fig. 2B ) ; NHMW 2018 /0019/0003, moderately preserved, adult shell ( Fig. 2C ) ; NHMW 2018 /0019/0004, moderately preserved, adult shell ( Fig. 2D ). All from type locality and horizon .

Type locality: Kömürlü (40°46’14.74” N, 42°18’21.05” E, WGS84 ), Oltu-Narman Basin , northeastern Turkey .

Type horizon: Marly silt and sand of the upper Susuz Formation; upper Oligocene or lower Miocene.

Material.— Type material and additional 21 specimens ( NHMW 2018 /0019/0005). All from type locality and horizon .

Diagnosis.—Depressed drop-shaped neritid with colour pattern of regularly spaced, continuous brown and white spiral stripes. Aperture and columellar shelf simple with smooth columellar edge. Sculpture of last whorl consisting of prominent (for genus), densely spaced, prosocline axial riblets, being most prominent in adapical half of whorl.

Description.—Medium-sized shell ( Table 1) with elevated spire; protoconch consisting of less than one smooth, low, moderately convex whorl of 500 μm diameter. Transition into teleoconch marked by faint rim and onset of delicate growth lines. Teleoconch comprising 2.5 whorls; first teleoconch whorl moderately high, weakly convex. Last whorl rapidly expanding; maximum diameter in lower third; whorl weakly convex to nearly flat above periphery, regularly convex below, resulting in depressed drop-shaped outline. Sculpture of last whorl consisting of densely spaced, prosocline axial riblets resulting from prominent growth lines, separated by narrower interspaces. Wide semi-circular aperture with thin outer lip. Columellar shelf broad, strongly expanding, well demarcated from base, forming thickened and convex callus in adults. Columellar edge nearly straight to faintly concave, slightly recessed behind basal lip; no denticles aside from very weak anterior swelling. Colour pattern consisting of 12–20 regularly spaced, continuous dark-brown spiral stripes separated by white stripes of about equal width. Operculum unknown.

Remarks.—The large protoconch, which consists of less than one whorl, places this species in Theodoxus and excludes a relation with other Neritininae genera, such as Clithon Montfort, 1810 , Neritina Lamarck, 1816 and Pictoneritina Iredale, 1936 (see Bandel 2001). This classification is also supported by the simple columellar edge that lacks denticles, as well as the fluvial-lacustrine depositional environment, which was completely isolated from the sea. Theodoxus susuzianus displays little variation in colour pattern but is somewhat variable concerning the convexity of the adapical half of the last whorl.

Only a small number of Theodoxus species have been described so far from the Oligocene and early Miocene of central and eastern Europe and Turkey. Theodoxus supraoligocaenicus Báldi, 1973 , from the Egerian (= Chattian/ Aquitanian) of Hungary, differs in the presence of axial stripes on the spire and the smooth shell surface ( Báldi 1973). Moreover, Báldi (1973) discusses similarities of the aperture of T. supraoligocaenicus with that of Vitta picta ( Férussac, 1823) , which might cast doubts about the generic placement. Theodoxus hassiacus ( Wenz, 1919) sensu Esu and Girotti 2010 ), from the Chattian of southern Italy, is readily distinguished by its broader last whorl, slightly gradate spire and the speckled colour pattern. Theodoxus crenulatus ( Klein, 1853) , frequently reported from Oligocene and Miocene deposits of Europe and Turkey (e.g., Klein 1853; Báldi 1973; Kojumdgieva and Sapundgieva 1981; Harzhauser et al. 2012), is distinguished by its denticles on the columellar edge and the blunt speckles. The colour pattern of Theodoxus susuzianus is reminiscent of the late Miocene Neritina vetranici Brusina, 1902 ) sensu Harzhauser et al. (2002), which differs from Theodoxus susuzianus in the presence of two protoconch whorls. Similarly, Theodoxus eugenii Jekelius, 1944 sensu Bandel (2001) , from the late Miocene of Hungary, is highly reminiscent of the Turkish species concerning pattern and outline, but differs in the presence of columellar denticles. Another quite similar species in terms of colour pattern is Theodoxus bukowskii ( Oppenheim, 1919) from the late Miocene of Turkey as described by Wesselingh et al. (2008). This species can be distinguished from T. susuzianus in its lower spire. None of the discussed species develops a marked axial sculpture on the last whorl.

Figures

Fig. 2. Neritid gastropod Theodoxus susuzianus sp. nov. from the upper Oligocene–lower Miocene Susuz Formation, Kömürlü, Turkey. A. NHMW 2018/0019/0001, holotype in apertural (A1), oblique apical (A2), and dorsal (A3) views. B. NHMW 2018/0019/0002, paratype in apertural (B1) and oblique apical (B2) views; detail of protoconch (B3), arrowhead indicates transition into teleoconch. C. NHMW 2018/0019/0003, paratype in apertural C1) and oblique apical (C2) views. D. NHMW 2018/0019/0004, paratype in apertural (D1) and oblique apical (D2) views; details of sculpture (D3). Scale bars 2 mm, if not stated otherwise.