Cylichna stevniensis, Hansen, 2019

Hansen, Thomas, 2019, Gastropods from the Cretaceous-Palaeogene boundary in Denmark, Zootaxa 4654 (1), pp. 1-196 : 166

Cylichna stevniensis n. sp.

Figs 35 Q–T

Diagnosis. Shell rather elongate with pointed posterior end, aperture distinctly higher than aperturally strongly convex penultimate whorl; apex hidden on adult specimens, not sharply truncated. Columellar fold very weak. Surface sculpture of fine spiral grooves.

Derivation of name. Refers to the type locality Stevns Klint.

Type material. Holotype MGUH 33317 is a nearly complete external mould . Paratype MGUH 33318 is an internal mould and comes from the Cerithium Limestone Member at Skeldervig . Paratype MGUH 33319 comes from the same horizon and locality as the holotype .

Additional material. Six specimens with the informal sample numbers SH.179, SH.192.B, SR.608 (2 specimens), SR.680 and SO.191, together with three uncatalogued external and internal moulds from Stevns and a single mould from Northern Jutland, which is found in the private collection of K. I. Schnetler. The Stevns material belongs to the Natural History Museum of Denmark.

Type stratum and type locality. Cerithium Limestone Member of the Rødvig Formation at Rødvig, Stevns Klint.

Occurrence. The Cerithium Limestone Member at Stevns Klint and the stratigrapically equivalent ‘dead layer’ in Dania Quarry, Northern Jutland.

Description. Protoconch unknown. Teleoconch moderately thick-shelled, involute, elongated in shape with weakly curved outer lip and aperturally strongly convex penultimate whorl; apex covered by last whorl on adult specimens. Aperture apically pointed, widened and rounded abapically. Width of last whorl, excluding aperture, corresponding to half the spire height. Columella with very weak plait partly hidden by callus.

Sculpture, if present, consisting of weak, fine spiral grooves without pitting, grooves numbering round 30 on holotype MGUH 33317.

Measurements. Largest specimen MGUH 33319 is 5.2 mm high and at least 2.8 mm wide.

Remarks. As noted by Kaim & Beisel (2005: p. 61) the species of Cylichna are extremely similar and only few features may be used for differentiation on fossil specimens. Among the stratigraphically and geographically comparable taxa Cylichna stevniensis n. sp. is distinguished from the Danian Roxania raristriata ( Briart & Cornet, 1887) from Belgium as figured by Glibert (1973: pl. XI: 1) by the distinctly higher aperture opening relative to the apex. It resembles somewhat Cylichna sp. 1 of Kollmann & Peel (1983) from the Danian of Greenland, but the columella is generally longer with a slightly better developed columellar plait.

Figures

FIGURE 35. A–B. Pseudomalaxis sp., Silicone cast of MGUH 33306. Width 4.3 mm. C–D. Climacopoma? sp., Silicone cast of MGUH 33307. Width 13.1 and 9.9 mm respectively. E–I. Mathilda dania n. sp. E. Silicone cast of holotype MGUH 33308. Width 1.9 mm. F. Silicone cast of paratype MGUH 33309. Width 2.0 mm. G. Silicone cast of paratype MGUH 33310. Width 0.8 mm. H. Silicone cast of paratype MGUH 33311. Width 0.7 mm. I. Silicone cast of paratype MGUH 33312. Width 1.7 mm. J–K. Clathrobaculus sp., Silicone cast of MGUH 33313. Width 0.5 mm. L–P. Ringicula sp. L. Silicone cast of MGUH 33314. Width 7.0 mm. M–N. Silicone cast of MGUH 33315 and a partly hidden second specimen. Width 7.3 and 6.6 mm respectively. O–P. Silicone cast of MGUH 33316. Width 5.6, 5.3 mm respectively. Q–T. Cylichna stevniensis n. sp. Q. Silicone cast of holotype MGUH 33317. Width 1.4 mm. R–S. Internal mould of paratype MGUH 33318. Width 1.5 mm. T. Internal mould of paratype MGUH 33319. Width 2.8 mm. U–V. Cylichna? sp. A. Cast of MGUH 33320. Width 2.3, 2.4 mm respectively. W–Y. Cylichna? sp. B. W. Wax cast of MGUH 33321. Width 2.4 mm. X–Y. Inner mould of MGUH 33322. Width 1.3 mm. Z–AE. Roxania sp. Cast of MGUH 33323. Width 3.2, 3.3 mm respectively.