Ageria laxa, Hansen, 2019

Hansen, Thomas, 2019, Gastropods from the Cretaceous-Palaeogene boundary in Denmark, Zootaxa 4654 (1), pp. 1-196 : 124

Ageria laxa n. sp.

Figs 29 C–E

Diagnosis. An Ageria with approximately two smooth protoconch whorls. Teleoconch strongly and fairly densely sculptured; last whorl widening with base contour shifting from flat to strongly convex.

Derivation of name. Refers to the strong widening of the last whorl seen on large specimens.

Type material. The holotype MGUH 33227 is a nearly complete external mould with protoconch. Paratype MGUH 33228 is from the Cerithium Limestone Member at Stevns Klint.

Additional material. ØSM.10042-133 and ØSM.10042-388-a comes from the Crab Layer in the Korsnaeb Member. A mould in the informally catalogued sample SR.296.B–C and an additional three external and internal moulds without numbers are from the Cerithium Limestone Member and belong to the Natural History Museum of Denmark.

Type stratum and type locality. The Crab Layer , a hardground related lithified bed approximately two metres up in the Korsnaeb Member of the lower Danian Stevns Klint Formation at Stevns Klint .

Occurrence. The lower Danian Cerithium Limestone Member of the Rødvig Formation and up into the Crab Layer, located about two metres up in the Korsnaeb Member of the Stevns Klint Formation at Stevns. Stevns Klint: Rødvig, Korsnaeb, Skeldervig, Højerup and Holtug Quarry.

Description. Protoconch 0.3 mm high and 0.3 mm wide, consisting of around two smooth, slightly convex whorls forming a continuation of teleoconch spire.

Teleoconch very slender coeloconoid; initial whorls slightly convex, becoming gradually more flattened abapically. Whorls moderately low, height on teleoconch whorl 16 corresponding to approximately 45 % the width. Suture narrow, distinct, but not deep. Keel sharp, located at transition to base at abapical suture line. Base flat with short anterior canal. Last whorl on large specimens may increase dramatically in width, becoming more convex with the keel changing to a more central location on the whorl before disappearing just behind aperture. Aperture rounded subrectangular with smooth, slightly concave columella. Teleoconch strongly sculptured adapically, becoming smoother abapically with only weak spiral ribs on base of inflated final whorl. First whorl with five to six strong transverse ribs on a half whorl, crossed by two weaker spiral ribs, forming two weak tubercles on transverse ribs. A third, narrower adapical spiral rib is added on third whorl. Spiral ribs increase rapidly in number while becoming finer. Transverse ribs may increase slowly or faster in number abapically, while tubercles become more pronounced, arranged in up to four rows. Keel carrying narrow spiral rib, which may be slightly serrated due to densely spaced growth lines. Base with single primary spiral rib slightly adaxially of keel rib and succeeded by closely spaced, weaker secondary spirals. Growth lines closely and evenly spaced, nearly straight adapically of keel, weakly opisthocyrt.

Measurements. The holotype MGUH 33227 is 16.2 mm high and 4.8 mm wide with 19 whorls. Largest examined specimen is 15.4 mm wide.

Remarks. The Danish early Danian Ageria laxa n. sp. resembles the Maastrichtian A. weeksi Wade, 1926 from the Ripley Formation of southern United States, but is distinguished by the higher number of tubercle rows on the later whorls and the apparently two and not three smooth protoconch whorls. It differs from the Maastrichtian Ageria gankinensis Kaim et al., 2004 from northern Russia by having a more convex base, a stronger developed anterior canal and apparently by having only two protoconch whorls. It is furthermore distinguished from the two British late Albian species A. gaultina Abbass, 1973 and A. costata ( Sowerby, 1827) by the distinctly less pronounced spiral ribs and growth line sculpture and by a slightly more convex base. The differences between Ageria laxa n. sp. and Ageria skeldervigensis n. sp. are presented above in the remarks for the latter species.

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.