Cerithiella selandica, Hansen, 2019

Hansen, Thomas, 2019, Gastropods from the Cretaceous-Palaeogene boundary in Denmark, Zootaxa 4654 (1), pp. 1-196 : 118-119

Cerithiella selandica n. sp.

Figs 28 J–O

Diagnosis. Protoconch with four and a half whorls, with collabral threads but no or only very weakly developed spiral ribs on last whorl. Teleoconch whorls with strong keel; columellar plate not strongly developed; columella with weak fold; base carrying only one primary spiral rib. Transverse sculpture fairly dense.

Derivation of name. Name of the Danish island Sealand from which it derives.

Type material. The holotype MGUH 33212 is an external mould presenting the last protoconch whorl and 13 teleoconch whorls . Paratype MGUH 33213 , consisting of an external and fragments of the internal mould, comes from the same locality and horizon, while paratype MGUH 33214 , an external mould of protoconch and teleoconch, was collected from Vokslev Quarry, Northern Jutland . Paratype MGUH 33215 is an external spire mould from the Cerithium Limestone Member at Holtug, Stevns Klint .

Additional material. ØSM.10042-178-a–c, eight specimens collected in connection with this study and found in the informally catalogued samples SH.105.A–B, SR.296.B, SR.476.A–C, SR.565, SR.620.A–B, SO.93, DN.1 and DN.52.A–B, and 15 external and in a few cases internal moulds deposited at the Natural History Museum of Denmark. An additional specimen from Dania Quarry is deposited in the private collection of K. I. Schnetler.

Type stratum and type locality. Cerithium Limestone Member at Skeldervig, Stevns Klint.

Occurrence. Cerithiella selandica n. sp. ranges from the lower part of the lower Danian Cerithium Limestone Member and up into the lower part of the Korsnaeb Member. It is moderately common in the southern to northern part of the Stevns Klint exposure and is also found in the corresponding layers at Vokslev and Dania in Northern Jutland.

Description. Protoconch turriform multispiral with approximately 4 ½ convex whorls; initial whorl seemingly smooth, the following carrying around 14 to 18 fine, strongly opisthocline and somewhat opisthocyrt collabral threads per half whorl. Last whorl with two faintly developed spiral threads centrally. Transition to teleoconch very gradual, showing a continuation of spiral ribs and densely spaced collabral threads, the former becoming much stronger and adjoined by weak abapical one located slightly adapical to suture. Protoconch 0.9 mm high and 0.5 mm wide.

First teleoconch whorls high with marked keel and concomitant deeply V-shaped sutural groove, suture narrow and shallow. Succeeding whorls flat sided with a pronounced, sharp keel located slightly above narrow abapical suture; height of whorls corresponding to nearly 50 % of whorl width. Base weakly convex. Aperture rounded subrectangular to pentagonal, abapically continuing out into relatively long siphonal canal; siphonal canal strongly oblique; outer lip thin with broad and fairly deep sinus just above keel; parietal lip strongly developed but thin; columellar plait low and broad, located on lower part of slightly concave columella, accentuating weak canal.

Whorl sculpture initially weakly reticulate with densely spaced collabral threads and three spiral ribs, two centrally on the whorl and a weaker abapical one located slightly above suture. Collabral threads describing forwardly directed sinus around abapical spiral thread. Later whorls strongly reticulate, reticulation consisting of 12 to 16 transverse ribs per whorl on third or fourth teleoconch whorl, crossed by slightly stronger spiral ribs. Additional spiral ribs are added to three initial ones, which become the second to fourth spiral ribs with the fourth moving down below the suture line and the third and strongest situated on keel. Spiral ribs 2 and 3 closer than 1 and 2. Transverse ribs terminating just below keel. Number of transverse ribs increasing abapically to around 40 ribs on the 20 th teleoconch whorl, while the tubercles at the rib junctions and the transverse and adapical two spiral ribs become weaker. Third spiral rib becoming higher and more sharply delineated on later whorls. Base carrying a single primary spiral rib succeeded by a weak, but wide furrow and densely spaced weak spiral ribs covering the whole lower base.

Measurements. Largest specimen, MGUH 33213, is 31 mm high and 7.1 mm in diameter with 18 teleoconch whorls.

Remarks. Cerithiella selandica n. sp. closely resembles the Palaeocene Cerithiopsis sp. 1 sensu Kollmann & Peel (1983) from West Greenland, but C. selandica n. sp. has fewer transverse ribs and the fourth primary spiral rib is not exposed before the final whorl. It differs from the Palaeocene Cerithium zigzag Grönwall & Harder, 1907 from Denmark by the less concave spire profile; a slightly sharper transition between base and canal and by a denser transverse ribbing. Furthermore the growth lines appear in general to be straighter with a weaker bend at transition to base.

This species may look somewhat similar to Cerithiella moltkianum treated above, but differs in that spiral ribs 2 and 3 are closer to each other than spiral ribs 1 and 2; the number of transverse ribs on the adapical teleoconch whorls is normally lower and the whorls are generally higher and with a more pronounced keel. It is furthermore distinguished from the Late Cretaceous Cerithium tectiforme Binkhorst, 1861 from Holland by the distinctly lower number of transverse ribs per whorl, though they are clearly closely related.

The teleoconch of Cerithiella selandica n. sp. closely resembles that of Orthochetus Cossmann, 1889 , which recently has been revised by Darragh (2011). Darragh (2011) assigned several Palaeogene taxa to this genus based solely upon their teleoconch morphology and prominent keel, although recognizing that some species among the Cerithiella strongly resembled them but for their multispiral protoconch. Orthochetus is characterized by a paucispiral protoconch. The type species Orthochetus leufroyi ( Michelin, 1825) from the Eocene of France and the morphologically most similar species Orthochetus pagoda ( Chapman & Crespin, 1934) from the Eocene of Australia are furthermore distinguished by their aperture with its narrow and relatively long anterior canal and prominent columellar plate. The aperture may actually be more important as a diagnostic character from Cerithiella than the prominent keel as a few recent species like Cerithiella amblytera ( Watson, 1880) also developes a keel. The Maastrichtian C. tectiforme Binkhorst, 1861 from Holland, the early Palaeocene C. sp. 1 sensu Kollmann & Peel (1983) from Greenland and the Palaeocene Cerithium zigzag Grönwall & Harder, 1907 from Denmark were reassigned to Orthochetus in spite of a lack of known protoconchs. They are all more likely belonging to Cerithiella sensu lato due to their stratigraphical and morphological closeness with the present species, but a definite generic assignment must wait until their protoconchs have been uncovered. Undoubted species of Orthochetus are restricted to the Eocene to perhaps the Oligocene of Europe and Australia (see Darragh 2011).

Figures

FIGURE 28. A. Thereitis carlsbergi n. sp., Protoconch of paratype MGUH 33205.Width of protoconch 0.4 mm. B–D. Thereitis cretacea n. sp. B. Holotype protoconch MGUH 33206. Width 0.4 mm. C. Silicone cast of paratype MGUH 33207. Width 0.9 mm. D. Silicone cast of paratype MGUH 33208. Width 1.1 mm. E. Thereitis sp., Silicone cast of MGUH 33209. Width 0.4 mm. F–I. Cerithiella moltkianum (Ravn, 1902). F–H. Silicone cast of MGUH 33210. Width of teleoconch 4.6 mm. Height of protoconch 1.0 mm. I. Silicone cast of MGUH 33211.Width 9.2 mm. J–O. Cerithiella selandica n. sp. J–K. Silicone cast of holotype MGUH 33212. Teleoconch width 2.6 mm. Preserved protoconch width 0.5 mm. L–M. Silicone cast of paratype MGUH 33213. Height 31 mm. N. Paratype protoconch MGUH 33214. Width 0.7 mm. O. Silicone cast of paratype MGUH 33215. Width 2.6 mm. P–Q. Cerithiella sp. P. Silicone cast of MGUH 33216. Width 6.7 mm. Q. Silicone cast of MGUH 33217. Width 5.1 mm. R. Eumetula sp., Silicone cast of MGUH 33218. Width 0.9 mm. S. Trituba obliquecostulata (Kaunhowen, 1898). Silicone cast of MGUH 33219. Width 1.2 mm. T–V. Vatopsis heinbergi n. sp. T. Silicone cast of holotype MGUH 33220. Width 1.2 mm. U. Silicone cast of paratype MGUH 33221. Width 3.1 mm. V. Silicone cast of paratype MGUH 33222. Width 4.0 mm. W. Vatopsis sp., Silicone cast of MGUH 33223. Width 2.5 mm. X–Y. Ageria skeldervigensis n. sp. X. Silicone cast of paratype MGUH 33224. Height 4.1 mm. Y. Silicone cast of paratype MGUH 33225. Width 7.4 mm.