Bathrotomaria jakobseni, Hansen, 2019

Hansen, Thomas, 2019, Gastropods from the Cretaceous-Palaeogene boundary in Denmark, Zootaxa 4654 (1), pp. 1-196 : 67

Bathrotomaria jakobseni n. sp.

Figs 21 C–H

2014 Bathrotomaria sp. Hansen & Surlyk: 335, fig. 5: 20, tab. 2.

Diagnosis. Species of Bathrotomaria characterized by a strong, thin peripheral keel, a low spire with sharp, well separated threads and by the fine growth lines changing to transverse ribbing at transition to wide umbilicus.

Derivation of name. In honour of conservator Sten Lennart Jakobsen at the Geological Museum of Copenhagen, Denmark, who has been extremely helpful throughout this project.

Type material. Holotype MGUH 33098 is a nearly complete internal and external mould but for extensive early diagenetic dissolution on the apical side . Paratypes MGUH 33099 and MGUH 33100 from the lithified top of the Tor Formation (probably lower Højerup Member ) at Holtug, Stevns, represent very fragmentary external moulds .

Type stratum and type locality. Slightly lithified upper ten centimetres of the Maastrichtian Sigerslev Member, Tor Formation, at Kulsti Rende, Stevns Klint.

Occurrence. Top Sigerslev Member to basal Højerup Member of the Maastrichtian Tor Formation at Holtug and Kulsti Rende, Stevns Klint.

Description. Protoconch not known. Teleoconch broadly conical, nearly twice as wide as high. Whorls sigmoid in outline with wide, only slightly inclined adapical ramp divided by weak angle from concave face. Selenizone located at mid-whorl on angle between adapical ramp and face of whorl, terminating in exhalant slit a quarter of a whorl from aperture, the width of which takes up nearly 20 % of whorl width. Periphery with very broad, attenuated keel delimiting strongly convex base. Transition from base to wide umbilicus gradual; width of umbilicus taking up 20 % of shell width. Aperture subrectangular, width taking up 44 % of shell width on holotype, corresponding to 142 % of aperture height.

Whorl sculpture consisting of fine spiral threads crossed by deeply opisthocyrt collabral threads with V-shaped sinus at selenizone; small tubercles occur at connecting points between spiral and collabral threads. Ramp carrying around four spiral ribs, while six to seven are observed on whorl face. Two additional but stronger ribs border the selenizone. Base with fine opisthocyrt growth lines on abaxial part, changing to low transverse ribs at transition to umbilicus. Spiral threads may occur.

Measurements. Holotype MGUH 33098, 11.1 mm high and 20.9 mm wide.

Remarks. Bathrotomaria jakobseni n. sp. differs from the Late Cretaceous B. praepyrenaica Astre, 1935 from southwestern France by the distinctly lower and less step-like spire. It is separated from the Senonian B. regalis (Roemer, 1841) and B. perspectiva ( Mantell, 1822) from Germany and Great Britain, respectively, by the strong, thin keel and the transverse ribbing at the transition to the umbilicus. Bathrotomaria jakobseni n. sp. strongly re- sembles the Early Albian Pleurotomaria shenleyensis Cox, 1960b , but differs in the main generic character, namely the location of the selenizone at the ramp angle and not well below it. Nevertheless the strong resemblance suggests that there may be problems with the present generic division.

Figures

FIGURE 21. A–B. Leptomaria meyeri Pacaud, 2004. Silicone cast and inner mould of MGUH 33097. Width of cast 14.5. Width of inner mould 11.4 mm. C–H. Bathrotomaria jakobseni n. sp. C–F. Silicone cast and inner mould of holotype MGUH 33098. Width of exposed shell 21.3, 21.3, 15.6, 17.3 mm respectively. G. Outer mould of paratype MGUH 33099. Preserved radius 4.4 mm. H. Silicone cast of last whorl of paratype MGUH 33100. Width 16.2 mm. I. Bathybembix? sp., Silicone cast of MGUH 33101. Width 3.65 mm. J–K. Danilia sp., Silicone cast of MGUH 33102. Width 1.25 mm. L–Q. Chilodonta rudis (Binkhorst, 1861). L–M. Silicone cast of MGUH 33103. Width 6.0 mm for complete specimen, 0.25 mm for protoconch. N–O. Silicone cast of MGUH 33104. Width 7.35 mm. P. Silicone cast of aperture MGUH 33105. Height 4.8 mm. Q. Silicone cast of MGUH 33106. Width 4.65 mm. R–T. Eucycloscala gracilicostatus n. sp. R. Silicone cast of paratype MGUH 33107. Width of fragment 1.3 mm. S–T. Silicone cast of holotype MGUH 33108. Width 2.3 mm. U. Phorcus sp., Silicone cast of MGUH 33109. Height 3.2 mm, exposed width 2.2 mm.