Amaea stevniensis, Hansen, 2019

Hansen, Thomas, 2019, Gastropods from the Cretaceous-Palaeogene boundary in Denmark, Zootaxa 4654 (1), pp. 1-196 : 130

Amaea stevniensis n. sp.

Fig. 29W

Diagnosis. Shell slender with strongly convex whorls. Protoconch consisting of four whorls. Teleoconch with flattened, smooth base. Spiral ribs four to six in number, slightly stronger than transverse ribs. Transverse ribs straight.

Derivation of name. Refers to the Stevns Klint exposure at which it was found.

Type material. The nearly complete external mould MGUH 33241 is selected as holotype.

Additional material. In addition to the type specimen another three fragmentary external moulds with informal sample numbers SH.386.A–B, SH.394 and SR.311.A have been examined.

Type stratum and type locality. Cerithium Limestone Member just north of Højerup Church , Stevns Klint .

Occurrence. Amaea stevniensis n. sp. has been collected from 25 to 30 centimetres above the base of the Rødvig Formation (base of the P1a Foraminiferal Subzone) in the Cerithium Limestone Member at Højerup Church, and from 30 to 40 centimetres above the base of the Rødvig Formation (Pα Foraminiferal Zone) at Rødvig, Stevns Klint.

Description. Shell high turriform. Protoconch multispiral, approximately 0.6 mm high and 0.35 mm wide, consisting of four smooth whorls. Apex somewhat blunt with low convex whorls; convexity increasing gradually to the last and strongly convex protoconch whorl. Teleoconch whorls strongly convex, with a height corresponding to 2/3 the width. Transition to slightly thickened base on last whorl marked by distinct cord. Aperture rounded ovoid with slight angle at transition from outer lip to columella.Aperture height corresponding to ¼ the shell height. Outer lip with weak varix.

Teleoconch sculpture cancellate, consisting of six sharp and moderately strong spiral ribs; the adapical weakest, sometimes completely effaced. Spiral ribs crossed by slightly weaker and less closely spaced, opisthocyrt to nearly straight, orthogonal or slightly prosocline transverse ribs. Number of transverse ribs per whorl 20 to 22 on first three whorls. Interconnecting points with weakly developed tubercles. Weak varices may occur. Base smooth.

Measurements. Holotype MGUH 33241 2.3 mm high and 1.0 mm wide, consisting of eight whorls.

Remarks. Amaea stevniensis n. sp. differs from the Selandian Acrilla fenestrata Ravn, 1939 by the smaller protoconch; the orthogonal transverse ribs and the more convex whorl outline. It differs from Amaea ? sp. C treated below by the lower number of transverse ribs per whorl and the absent to weakly developed and rare varices. It is distinguished from the Danish mid-Danian Acrilla sp. 1 of Lauridsen & Schnetler (2014: p. 66) by the slightly more convex whorls and seemingly also by the transverse ribs not being stronger than the spirals. It is distinguished from Acrilla sp. 3 of Lauridsen & Schnetler (2014: p. 67) from the same place and horizon as the one before, by the higher number of spirals.

Figures

FIGURE 29. A–B. Ageria skeldervigensis n. sp., Silicone cast of holotype MGUH 33226. Width 3.8, 4.5 mm respectively. C–E. Ageria laxa n. sp. C–D. Silicone cast of holotype MGUH 33227. Width 4.6 mm for teleoconch, 0.3 mm for protoconch. E. Silicone cast of paratype MGUH 33228. Width 6.7 mm. F–G. Ageria sp. F. Silicone cast of seemingly mislaid specimen MGUH 33229. Width 3.7 mm. G. Silicone cast of MGUH 33230. Width 0.6 mm. H–K. Coniscala umbilica n. sp. H. Silicone cast of paratype MGUH 33231. Width 1.4 mm. I–J. Holotype MGUH 33232. Width 19.6 mm. K. Paratype MGUH 33233. Width 12.4 mm. L–N. Opalia sp. L–M. Silicone cast of MGUH 33234. Width 1.3 mm. N. Silicone cast of MGUH 33235. Width 0.5 mm. O–Q. Claviscala minor n. sp. O–P. Silicone cast of holotype MGUH 33236. Width of teleoconch 0.9 mm. Width of protoconch 0.4 mm. Q. Silicone cast of paratype MGUH 33237. Width 1.3 mm. R. Claviscala sp., Silicone cast of MGUH 33238. Width 1.4 mm. S–V. Amaea aliceae n. sp. S–U. Silicone cast and inner mould of holotype MGUH 33239. Width 2.7, 2.0 and 3.1 mm respectively. V. Silicone cast of paratype MGUH 33240. Width 1.7 mm. W. Amaea stevniensis n. sp., Silicone cast of holotype MGUH 33241. Width 1.0 mm. X. Amaea sp. A. Silicone cast of MGUH 33242. Width of teleoconch 1.1 mm.