Disculus selandicum ( Ravn, 1902 )

Hansen, Thomas, 2019, Gastropods from the Cretaceous-Palaeogene boundary in Denmark, Zootaxa 4654 (1), pp. 1-196 : 162

Disculus selandicum ( Ravn, 1902)

Figs 34 O–S

1902 Solarium selandicum n. sp. Ravn: 216, pl. I, figs 5–6.

1903 Solarium selandicum Ravn—Ravn : 380, 394–395.

1940 Solarium selandicum Ravn—Rosenkrantz : 513.

Diagnosis. Protoconch heterostrophic, submerged with around 2 ½ smooth, rounded convex whorls. Teleoconch H/W ratio approximately 0.55. Peripheral keel strongly developed, without sculpture. Adapical side of teleoconch whorl distinctly concave with fine but distinct spiral and radial sculpture; spiral threads strongest close to sutures; growth lines strongly posteriorly turned; beading on umbilical keel coarse.

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

Material. Of the eight type specimens used for the description of the species ( Ravn 1902: p. 216), only the two figured, MGUH 79 and MGUH 80, are registered and known today. The most complete of the two, MGUH 79, figured by Ravn (1902: pl. I: 5) is here selected as lectotype. MGUH 33304, MGUH 33305, 35 moulds with informal sample numbers SO.184, SH.149.A, SH.190.A–B, SH.195, SH.261, SH.286.A–B, SH.294.A, SH.339, SH.380, SH.415.A, SR.142, SR.145, SR.146, SR.152.A (2 specimens), SR.170.A–B, SR.286, SR.292, SR.295 (2 specimens), SR.359, SR.366.A, SR.391, SR.394, SR.467, SR.479, SR.492.A–B, SR.519.A, SR.519.A–B, SR.571. A–B, SR.578.A–B, SR.629.A, SR.682.A–B, SR.699 and DN.54.A, and around 30 specimens without numbers are deposited in the collections of the Natural History Museum of Denmark. The largest measures 9.0 mm in width.

Occurrence. Cerithium Limestone Member at Stevns Klint and the contemporaneous ‘dead layer’ in Dania and Vokslev quarries, Northern Jutland.

Description. Shell low cone-shaped, height/width ratio approximating 0.55. Protoconch heterostrophic, submerged with around 2 ½ smooth, rounded convex whorls. Large, width about 1.0 to 1.1 mm. Transition to teleoconch marked by beginning of sculpture. Teleoconch thick-walled with distinctly concave spire whorls transitioning into rounded peripheral keel at marked abapical suture. Last whorl with sharply rounded peripheral keel and convex base. Aperture rounded to sub-pentagonal, slightly wider than tall, acuminate laterally towards peripheral and umbilical keel. Umbilicus moderately wide, around 25 % of shell width, bordered by strong umbilical keel. A secondary pronounced keel occurs 1/3 the distance between umbilical and peripheral keel from umbilicus. Base between two abapical keels strongly concave.

Teleoconch sculpture on spire consisting of fine spiral threads and ribs and radial ribs or growth increments; suture bordered on both sides by stronger and weakly beaded spiral rib, beads formed by growth increments, numbering close to 40 on initial teleoconch whorl, increasing gradually on later whorls. Weak closely spaced spiral threads between subsutural ribs, often around six on early whorls, sometimes more on later whorls. Growth lines sigmoid, strongly prosocline on concave part, becoming more orthocline at sutural margins. Peripheral keel and base with slightly prosocline growth lines and no other sculpture. Keel abaxially to umbilical keel beaded, beads rounded, around 21 on last whorl of the 7.1 mm wide paralectotype MGUH 80. Umbilical keel bearing coarse, oblique posteriorly directed nodules or ridges, nodules numbering 12 to 13 on last whorl of the 7.1 mm wide paralectotype MGUH 80 .

Measurements. MGUH 33305 is 5.9 mm wide and 3.5 mm high, consisting of protoconch and three teleoconch whorls. The species may reach widths of at least 9 mm and have up to 4 ½ whorls.

Remarks. D. selandicum is easily distinguished from the Eocene type species D. disculus ( Philippi, 1846) from France by the more strongly posteriorly directed growth lines, the more concave upper surface of the whorls, the coarser beading on the umbilical keel and by the stronger collabral and spiral sculpture but lacking an outer spiral furrow on the abapical side of the whorl.

Figures

FIGURE 34. A–E. Admetula sp. A–D. Silicone cast and internal mould of MGUH 33297. Widths 6.6, 7.2, 7.0 and 6.1 mm respectively. E. Internal mould of MGUH 33298. Width 5.6 mm. F. Kroisbachia sp., Silicone cast of MGUH 33299. Width 2.4 mm. G. Mataxa s.l. sp., Silicone cast of MGUH 33300. Width 3.9 mm. H–I. Neogastropoda genus et sp. indet., Silicone cast of MGUH 33301. Width 0.7 mm. J–N. Ravniella danica (Ravn, 1902). J–K. Silicone cast of MGUH 33202. Widths 4.7 and 4.3 respectively. L. Silicone cast of lectotype MGUH 126. Width 4.3 mm. Photograph by S.L. Jakobsen. M–N. Silicone cast of MGUH 33303. Width 4.1 and 3.8 mm. O–S. Disculus selandicum (Ravn, 1902). O. Silicone cast of MGUH 33304. Width 8.7 mm. P–Q. Silicone cast of paralectotype MGUH 80 figured by J.P.J. Ravn (1902, Pl. I: 6). Width 7.1, 6.9 mm respectively. R. Silicone cast of lectotype MGUH 79. Width 6.0 mm. Photograph by S.L. Jakobsen. S. Silicone cast of MGUH 33305. Width 5.9 mm.