Thereitis cretacea, Hansen, 2019

Hansen, Thomas, 2019, Gastropods from the Cretaceous-Palaeogene boundary in Denmark, Zootaxa 4654 (1), pp. 1-196 : 115

Thereitis cretacea n. sp.

Figs 28 B–D

2014 Thereitis sp. Hansen & Surlyk: 339, fig. 5: 19, tab. 3.

Diagnosis. Protoconch of five whorls with densely spaced oblique collabral threads and two spiral ribs after initial 1 ½ whorl. Teleoconch with flat whorls carrying four spiral ribs, the adapical one generally much stronger than the others while the fourth is covered by succeeding whorl. Siphonal canal strongly oblique.

Derivation of name. Refers to its status as the first representative from the Cretaceous.

Type material. The holotype MGUH 33206 is a protoconch and initial teleoconch whorl preserved as an external and internal mould . Paratype MGUH 33207 comes from Holtug Quarry , while paratype MGUH 33208 was collected near Stevns Fyr, Stevns Klint . The two paratypes are preserved as external moulds and come from the lithified top of the Højerup Member .

Additional material. Six moulds with the informal sample numbers SR.734.B, SR.741.B, SR.990.A–B, SR.997.B–C, SR.1090.A–B and SR.1109 were collected in connection with this study.

Type stratum and type locality. Lithified top of the Maastrichtian Højerup Member of the Tor Formation at Holtug Quarry, Stevns Klint.

Occurrence. Top of the Maastrichtian Tor Formation at Rødvig, Holtug Quarry and north of Stevns Fyr, Stevns Klint.

Description. Protoconch just over 0.5 mm long and 0.35 mm wide, multispiral, high conical with five moderately convex whorls separated by fairly deep sutures. First 1 ½ whorls smooth, the following covered by up to around 22 oblique collabral threads. Two sharply defined spiral ribs located close to the adapical and abapical suture, respectively, appearing together with the axial riblets. The adapical spiral rib moves down to the periphery on the last two protoconch whorls.

Teleoconch consisting of up to 12 flat whorls with four spiral ribs of slightly varying strength. Spiral rib 1 usually distinctly stronger than abapical ones and situated just below adapical suture. Spiral rib 2 and 3 separated by wide interspace; spiral rib 3 located immediately above abapical suture, in part merging with weak spiral rib 4 situated just below suture line. Whorl base flat. Aperture rounded rectangular with a short, smooth and straight columella terminated by short and strongly transverse canal. Aperture takes up less than 20 % of total shell length.

Measurements. The two largest specimens measure 4.3 mm in height (protoconch excluded) and approximately 1 mm in diameter and consist of 12 teleoconch whorls. Aperture height of MGUH 33207 is approximately 0.5 mm, while the lower whorl takes up around 0.9 mm of the teleoconch height.

Remarks. This species resembles the early Danian Thereitis carlsbergi n. sp. described above, but differs by the smaller size, by the more typical protoconch sculpture, by a stronger left-turn of the apertural canal and by the spiral ribs, where the adapical one is distinctly stronger than the others and situated immediately below the suture, while a wide interspace separates spiral rib 2 and 3, whereas spiral ribs 3 and 4 are nearly fused together due to the slightly lower location of spiral rib 3. It differs from the Selandian (Late Palaeocene) T. weinbrechti Schnetler, 2001 of Denmark by the fewer protoconch whorls with their slightly sparser axial sculpture.

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.