Orthosurcula, Hansen, 2019

Hansen, Thomas, 2019, Gastropods from the Cretaceous-Palaeogene boundary in Denmark, Zootaxa 4654 (1), pp. 1-196 : 147-149

Orthosurcula s.l. cerithiorum ( Ravn, 1902)

Figs 32 B–G

1902 Pleurotoma Cerithiorum n. sp. Ravn: 236–237, pl. III, fig. 4.

1902 Pleurotoma Steenstrupii n. sp.—Ravn: 237, pl. II, figs 23–24.

2014 Turricula ( Orthosurcula s.l.) cerithiorum ( Ravn, 1902) —Hansen & Surlyk: 339, fig. 6: 2, tab. 3.

Diagnosis. Protoconch smooth with no terminating part carrying transverse riblets. Teleoconch whorls weakly convex with shallow shoulder sulcus. Siphonal canal moderately short and wide. Sculpture development variable but shells typically carries fine spirals and around 10 to 12 transverse nodes below sinus.

Material. Lectotype MGUH 123 , MGUH 119 , MGUH 33275 , MGUH 33276 , MGUH 33277 , MGUH 33278 , ØSM.10042-109-a–b, ØSM.10042-116, 42 moulds with the informal sample numbers SH.146, SH.147, SH.189, SH.194.A, SH.196, SH.203.A–B, SH.259, SH.276, SH.280, SH.281.C–D, SH.287.B, SH.287.B–C, SH.288, SH.301.A–B, SH.304.A–B, SH.385.A, SH.385.B, SH.451, SS.190.A–B, SR.291, SR.293, SR.306, SR.310.B, SR.353, SR.366.A–B, SR.367, SR.368, SR.416, SR.475, SR.503, SR.506, SR.514, SR.542.B, SR.564, SR.620. A–B, SR.739, SO.108.A–B, SO.112.A–B, SO.113, SO.128, DN.27.A–B and DN.53.A–B, and around 60 specimens without number, which are deposited in the collections at the Natural History Museum of Denmark.

Type stratum and type locality. The original work by Ravn (1902) was based on four specimens from the Cerithium Limestone Member at Stevns Klint, but only the one figured ( Ravn 1902: pl. III: 4) is known to this day. This specimen, MGUH 123, is here selected as lectotype. The type material of Pleurotoma steenstrupii Ravn, 1902 , here regarded as a subjective synonym, derives from the same locality and stratum.

Occurrence. Common in the Cerithium Limestone Member at Stevns Klint and contemporaneous ‘dead layer’ from Northern Jutland. This species may also occur sparingly in the lithified top of the Cretaceous Højerup Member.

Description. Protoconch 0.8 to 1.0 mm high and 0.8 to 0.9 mm wide; high conical with 3–3½ smooth whorls expanding slightly faster than succeeding teleoconch whorls.

Teleoconch high fusiform with weakly convex whorls with rounded to moderately sharp shoulder slightly abapical of whorl center; steep shoulder slope weakly concave to nearly straight. Aperture narrow, taking up around 45 % of shell height. Sinus broadly, but moderately deep V-shaped with rounded apex located on lower half of shoulder slope.

Teleoconch sculpture of generally 10–12 moderately distinct to completely effaced transverse nodes or short ribs below sinus, crossed by normally distinct but fine spiral cords. A more pronounced spiral rib typically borders adapical suture; rib often granulate due to growth lines.

Remarks. Orthosurcula s.l. cerithiorum differs slightly from the typical Orthosurcula species by the generally weakly developed shoulder sulcus, the typically distinct spirals and transverse sculpture and by the lack of transverse sculpture on the last protoconch-whorl. Except for the last feature all of these characters are found to vary within the same species as outlined below.

This species is highly variable with regard to sculpture and shape of the shoulder slope. Most specimens have very weakly developed shoulder sulcus, but some feature a more concave slope outline better resembling the typical deep sulcus so characteristic of the genus. Transverse ribs, when present, are mostly found on the early teleoconch whorls, fading out on succeeding whorls. The fine, sharp spiral cords are generally strongly developed, but may become nearly effaced, resulting in a rather smooth surface especially on older specimens. It has thus not been possible to find any differentiating characters between this species and Pleurotoma steenstrupii Ravn, 1902 , which is here regarded as a subjective synonym of T. (O.) cerithiorum ( Ravn, 1902) .

A few very fragmentary specimens of a turriculid closely resembling the present species have been found in the latest Maastrichtian Højerup Member at Stevns Klint. Until better material has been uncovered they are here regarded as conspecific.

Figures

FIGURE 32. A. Ptychatractidae genus et sp. indet. Silicone cast of MGUH 33274. Width 1.8 mm. B–G. Orthosurcula s.l. cerithiorum (Ravn, 1902). B. Silicone cast of lectotype MGUH 123. Height 11.4 mm. Photograph by S. L. Jakobsen. C. Silicone cast of MGUH 33275. Height 4.8 mm. D. Silicone cast of MGUH 33276. Height 13.7 mm. E. Silicone cast of MGUH 33277. Width 6.4 mm. F. Silicone cast of MGUH 119, holotype of Pleurotoma steenstrupii Ravn, 1902. Height 23.6 mm. G. Silicone cast of MGUH 33278. Height 2.4 mm. H–K. Epalxis? rosenkrantzi n. sp. H–J. Silicone cast of holotype MGUH 33279. Widths 2.7, 2.8 and 2.8 mm respectively. K. Silicone cast of paratype MGUH 33280. Width 2.5 mm. L–N. Pseudocochlespira transversa n. sp. L–M. Silicone cast of holotype MGUH 33281. Widths 5.9 and 6.0 mm respectively. N. Silicone cast of paratype MGUH 33282. Width 8.2 mm. O–S. Pseudotoma danica n. sp. O–P. Silicone cast of holotype MGUH 33283. Width 5.7 mm. Q. Silicone cast of paratype MGUH 33284. Width 7.0 mm. R. Silicone cast of paratype MGUH 33285. Width 6.3 mm. S. Silicone cast of ØSM.10042-272-b. Width 5.8 mm. T–U. Mangeliid genus et sp. indet., Silicone cast of MGUH 33286. Widths 3.3 and 4.8 mm respectively.