Temnotropis stevniensis, Hansen, 2019

Hansen, Thomas, 2019, Gastropods from the Cretaceous-Palaeogene boundary in Denmark, Zootaxa 4654 (1), pp. 1-196 : 64-65

Temnotropis stevniensis n. sp.

Figs 20 K–M

2014 Temnotropis sp. Hansen & Surlyk: tab. 3.

Diagnosis. Temnotropis with selenizone located immediately above abapical suture. Last whorl well rounded with convex base. Sculpture fine with granulated spire whorls and basal spiral ribs.

Derivation of name. Refers to the famous Stevns Klint coastal exposure at which it has been found.

Type material. The holotype MGUH 33093 is an internal and external mould, while paratype MGUH 33094 is a well preserved external and internal mould from the lithified top of the Højerup Member at Rødvig, Stevns Klint.

Additional material. ØSM.10042-244-a. Additionally eight informally numbered and fragmentary specimens preserved as external and internal moulds are found in the collections of the Natural History Museum of Denmark (SR.754, SR.781, SR.786, SR.787.D and F, SR.905.A–B, SR.910, SR.934.A, SR.1045.A–C).

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

Occurrence. Uppermost part of the Maastrichtian Tor Formation at Rødvig and Holtug Quarry, Stevns.

Description. Shell dextral, umboniiform, with 2 ½ fairly quickly widening whorls. Spire low, flatly rounded with shallow suture lines between whorls. Protoconch consisting of around 1 ¼ whorls, seemingly smooth. Teleoconch whorls wide, weakly convex adapically, becoming increasingly more convex laterally; midwhorl with slight angle on which upper side a narrow, slightly raised and gently convex selenizone is found; abapical whorl ramp short and straight, distinguished from lower midwhorl by strong angle. Selenizone partly covered by adapical wall of succeeding whorl. Base convex, with central narrow umbilicus. Aperture oblique, rounded.

Teleoconch sculpture on first half whorl consisting of faint transverse threads just inside selenizone; following whorls with beaded fine spiral threads, the beading formed by the crossing of even finer and denser collabral threads describing a moderately opisthocyrt course from adapical suture and out to selenizone. Number of spiral threads above selenizone approximately 27 on last whorl. Base covered by evenly spaced, fine, unbeaded spiral threads spaced much farther from each other than on apical side of whorl; number of spiral threads at least 16 on second whorl, increasing with growth. Growth lines nearly effaced, slightly opisthocyrt while strongly opisthocline.

Measurements. Largest specimen 3.4 mm wide and approximately 1.8 mm high.

Remarks. The latest Maastrichtian age of the Danish species makes it the youngest of its genus described so far, although a few as yet undescribed specimens have been found in mid-Danian limestone deposits from Denmark as documented by Lauridsen & Schnetler (2014). They appear morphologically very similar to the present species and could possibly be conspecific. The stratigraphically and morphologically closest Cretaceous relative is the Spanish Temnotropis frydai Kiel & Bandel, 2000 from the Campanian. Temnotropis stevniensis n. sp. differs from the latter by the more inflated whorls carrying a finer, denser sculpture with distinctly beaded spiral threads on apical whorl surface and by the convex base covered by at least 16 spiral threads on second whorl. Both Temnotropis frydai and T. stevniensis n. sp. seems with regard to the general shape of the shell, the low selenizone and the presence of a distinct umbilicus more similar to the extant genus Sasakiconcha Geiger, 2006 from the Western Pacific than to the Triassic type of Temnotropis . On the other hand both species seem to lack the foramen formed by converging margins of the slit, and the umbilical wall on at least T. frydai appears to connect with the previous whorl. No preserved apertures and umbilical troughs have so far been uncovered for the Danish taxon, although the mid-Danian taxon do seem to have a rather wide umbilicus (see Lauridsen & Schnetler 2014, Fig. 5 ). Better material may thus suggest a reassignment of the species to Sasakiconcha or related genera.

Figures

FIGURE 5. Sketch of a 16 m high profile near Stevns Fyr at Stevns Klint. Note how the lithified horizon below the early Danian hardground topping the Rødvig Formation only reaches down into the Maastrichtian Højerup Member on the bryozoan mound crests. The flint bands nicely outline the large bryozoan mounds of the Korsnaeb Member.

FIGURE 20. A–B. Palaeoloxotoma sp., inner mould of MGUH 33087. Length 17.2 mm. C–I. Anatoma sp. A. C–E. Silicone casts of MGUH 33088. Width 1.3 mm. F–G. Silicone cast of MGUH 33089. Width 1.9 mm. H. Silicone cast of MGUH 33090. Preserved width 1.35 mm. I. Silicone cast of MGUH 33091. Preserved width 1.35 mm. J. Anatoma? sp. B, Silicone cast of MGUH 33092. Preserved width 1.05 mm. K–M. Temnotropis stevniensis n. sp. K–L. Silicone cast of holotype MGUH 33093. Width 3.5 mm. M. Umbilical view of paratype MGUH 33094. Width 2.1 mm. N–P. Leptomaria niloticiformis Schlotheim, 1820. N–O. Inner mould of MGUH 33095. Width 44 mm. P. Silicone cast of MGUH 33096. Middle Danian Coral Limestone from Faxe, Denmark. Height nearly 11 cm. Q. Leptomaria meyeri Pacaud, 2004. Silicone cast of MGUH 33097. Cast 14.5 mm wide, 12.5 mm high.