Rissoa costeiensis, Kowalke & Harzhauser, 2004

Kowalke, Thorsten & Harzhauser, Mathias, 2004, Early ontogeny and palaeoecology of the Mid-Miocene rissoid gastropods of the Central Paratethys, Acta Palaeontologica Polonica 49 (1), pp. 111-134 : 117-118

Rissoa costeiensis sp. nov.

Fig. 5B .

Derivation of the name: Named after the type locality (spelling according to Studencka et al. 1998).

Type horizon and locality: Grey clay from the Early Badenian of Coştei in western Romania; Banat Region, 50 km W of Deva, northern slopes of the Poiana Ruscă Moutains (see Studencka et al. 1998 for references and further data) .

Material.— The holotype ( NHMW 2002 z0030/0006) and four paratypes ( NHMW 2002 z0030/0007) from the Badenian of Coştei , Romania .

Diagnosis.—Small, slender Rissoa with whorls narrowing towards the aperture; prominent, sickle−shaped axial ribs crossed by fine spiral threads; small, rounded to subangular aperture; large protoconch with prominent subsequent incision of early teleoconch.

Description.—The small, slender shell comprises five rounded whorls measuring up to 3 mm in height. The whorls narrow towards the aperture and are separated by moderately deep sutures. Sculpture consists of 12–16 prominent, sickle−shaped axial ribs which are crossed by 12–14 fine spiral threads. The base is sculptured by 8–10 fine spiral threads. The last whorl forms 50–60% of total shell height. The small aperture is rounded to subangular. It is characterised by a thin peristome whose columellar portion is slightly bent. A very narrow slit−like umbilicus is present.

The large protoconch has three rounded whorls measuring up to 0.55 mm in height and up to 0.5 mm in maximum diameter. The width of the initial cap amounts to 0.03 mm and the first whorl measures 0.13–0.14 mm in maximum diameter. The transition from the embryonic to the larval shell is not obvious. The second whorl is characterised by remains of two very fine, granulated spiral threads above the abapical suture. The protoconch is terminated by closely spaced, sinuous, thickened growth lines. A subsequent incision of the whorl and the formation of regular adult growth lines indicate the onset of the teleoconch.

Remarksanddifferences.— R.costeiensis sp. nov. differs from the Recent Mediterranean species R. similis (fig. 5A) by its smaller size, the teleoconch sculpture having more axial ribs, and by its larger protoconch comprising one larval whorl more. Ferrero Mortara et al. (1984) reported Apicularia angulatacuta Sacco, 1895 and A. guerini var. antiqua Schwartz v. Mohrenstern, 1864 from the Pliocene of Italy, which are distinguished by more slender shells. According to the figure in Ferrero Mortara et al. (1984: pl. 38: 5a), Rissoa angulatacuta appeared to be a direct developer, without a free planktonic veliger stage during its early ontogeny.

The Badenian species Rissoa acuticosta from Steinebrunn/ Austria is characterised by a similar protoconch as R. costeiensis , but it is distinguished by comprising only 2.6 whorls measuring only up to 0.4 mm in height and in maximum diameter. It furthermore lacks the prominent subsequent incision following the sinusigera terminating the protoconch. The teleoconch of R. acuticosta differs by lacking the characteristic elongated shape, and by the teleoconch whorls narrowing towards the aperture.

Figures

Fig. 5. Two species of Rissoa Desmarest, 1814. A. Rissoa similis Scacchi, 1836. Specimen (NHMW 2002z0030/0005), Recent, from the Adriatic Sea (A1). Protoconch of the same specimen in lateral view (A2). B. Rissoa costeiensis sp. nov. Holotype (NHMW 2002z0030/0006) from the Badenian of Coştei, Romania (B1). Protoconch of the same specimen in lateral and apical views (B2, B3). The protoconch is terminated by closely spaced, sinuous, thickened growth lines. The onset of the teleoconch is indicated by a prominent incision of the whorl.