Acirsa torensis, Hansen, 2019

Hansen, Thomas, 2019, Gastropods from the Cretaceous-Palaeogene boundary in Denmark, Zootaxa 4654 (1), pp. 1-196 : 132

Acirsa torensis n. sp.

Figs 30 F–G

Diagnosis. Protoconch conical multispiral with close to four whorls. Spire moderately broad with relatively convex whorls, smooth base and eight to nine coarse and slightly opisthocline transverse ribs per half whorl. Spirals few and very weak.

Derivation of name. Refers to the upper Maastrichtian Tor Formation at Stevns Klint from the top of which it was collected.

Type material. The nearly complete external mould MGUH 33245 is selected as holotype . Paratype MGUH 33246 is from the same locality and horizon.

Type stratum and type locality. Lithified top of the Højerup Member, Tor Formation, at Rødvig, Stevns Klint.

Occurrence. Lithified top of the upper Maastrichtian Højerup Member of the Tor Formation at Rødvig, Stevns Klint.

Description. Shell relatively broad conical in outline. Protoconch multispiral, conical, consisting of 3 ½ to four smooth and moderately convex whorls. Protoconch height 0.5 mm and width 0.5 mm. Transition to teleoconch sharp, marked by the appearance of coarse transverse ribs.

Teleoconch whorls weakly to moderately convex with periphery at or slightly above abapical suture; height corresponding to 2/5 the width. Transition to flattened base more or less sharp. Aperture rounded subrectangular with straight columella bearing narrow and smooth callus; aperture height taking up 30 % of shell height on holotype.

Teleoconch sculpture dominated by fairly coarse, slightly sharp-ridged transverse ribs numbering eight to nine per half whorl on first teleoconch whorl; ribs slightly opisthocline, adapically terminating in weak spiral rib bordering adapical suture. Abapical termination at sharp transition to base. Spiral ribs on first teleoconch whorl crossed by four to five weak, but moderately coarse spiral ribs, the most abapical located on transition to smooth base.

Measurements. Holotype MGUH 33245 1.3 mm high and 0.8 mm wide, consisting of the initial five whorls of the spire.

Remarks. Acirsa torensis n. sp. differs from the Selandian Acirsa elatior ( von Koenen, 1885) from Denmark by the less convex whorls with fewer and weaker transverse ribs. It is distinguished from the Late Cretaceous Acirsa clathrata Sohl, 1964a from the U.S.A. by a relatively wider spire with a smooth base and stronger spiral sculpture.

Figures

FIGURE 30. A–B. Amaea sp.A. Details of protoconch on external mould of MGUH 33242 shown in natural view and mirrored with inverted colours. Width of protoconch 0.6 mm. C–D. Amaea sp. B. C. Silicone cast of protoconch MGUH 33243. Width 0.6 mm. D. Silicone cast of protoconch MGUH 33244. Width 0.4 mm. E. Amaea? sp. C, Silicone cast of ØSM.10042-373. Width 5.5 mm. F–G. Acirsa torensis n. sp. F. Silicone cast of holotype MGUH 33245. Width 0.8 mm. G. Silicone cast of paratype MGUH 33246. Width 1.1 mm. H. Acirsa sp., Silicone cast of MGUH 33247. Height 6.2 mm. I–K. Opaliopsis carlsbergi n. sp. I. Silicone cast of holotype MGUH 33248. Height 2.2 mm. J. Silicone cast of paratype MGUH 33249. Height 1.7 mm. K. Silicone cast of paratype MGUH 33250. Height 3.0 mm. L–M. Fusinus schnetleri n. sp. L. Silicone cast of holotype MGUH 33251. Height 23.9 mm. M. Silicone cast of paratype MGUH 33252. Height 13.4 mm. N–O. Streptochetus sp. A. Silicone cast of MGUH 33253. Width of figure N 2.7 mm. Width of teleoconch 3.5 mm. P–Q. Streptochetus? sp. B. Silicone cast of MGUH 33254. Teleoconch width 2.6 mm.Width of protoconch 0.6 mm. R–T. Latirulus? sp. R–S. Silicone cast of MGUH 33255. Width of protoconch 0.7 mm. Height of teleoconch 10.5 mm. T. Silicone cast of MGUH 33256. Width 3.7 mm. U–W. Dolicholatirus lorenzi n. sp. U. Silicone cast of holotype MGUH 33257. Height 30.8 mm. V–W. Silicone cast of paratype MGUH 33258. Width of teleoconch 5.5 mm. Width of protoconch 0.7 mm. X. Vexillum tenestolidum n. sp., Silicone cast of holotype MGUH 33259. Width of teleoconch 4.3 mm.