Simplexollata, Harzhauser & Landau, 2023

Harzhauser, Mathias & Landau, Bernard, 2023, The Architectonicidae and Mathildidae (Gastropoda, Heterobranchia) of the Miocene Paratethys Sea-victims of the Miocene Climatic Transition, Zootaxa 5370 (1), pp. 1-74 : 13-14

Genus Simplexollata nov. gen.

Type species. Simplexollata anticollata nov. sp., Langhian (early/middle Badenian), Central Paratethys Sea .

Diagnosis. Comparable in shape to Architectonica , Basisulcata and Psilaxis , but base with two prominent cords at periphery (LPC and IPC), almost smooth basal field with weak axial folds. No PUC but prominent spiral groove delimiting prominent UC ( Fig. 2 ).

Description. Shell medium-sized to large, solid; relatively flattened lenticular or moderately elevated conical to cyrtoconoid. Periphery angled, profile weakly convex above, flattened, very weakly convex below. Protoconch bulbous or moderately convex of <1 visible whorls. Spiral cords on first teleoconch whorls almost smooth or with more or less prominent subquadratic beads, fading on later whorls. SSC and two peripheral cords always developed. MCs moderately prominent or absent. LPC forming angled periphery with moderate keel, serving as upper point of whorl attachment. Base weakly convex with prominent, convex LPC and slightly narrower rounded IPC. Basal field wide, smooth, except for occasional shallow irregular spiral grooves and axial folds strengthening towards umbilicus. Axial folds originating approximately mid basal field. No PUC. UC separated from basal field by deep, narrow groove, UC overhanging umbilicus. Umbilicus moderately wide (~20–25% of total diameter); umbilical wall bearing fine axial growth lines. Aperture relatively small, rounded, angled at LPC, with deep CG and distinct PG.

Etymology. Composed of simplex and carocollata , referring to the two species, which are included in the new genus.

Included species. Simplexollata anticollata nov. sp., Solarium carocollatum Lamarck, 1822 , Solarium simplex Bronn, 1831 , Solarium gratteloupi d’Orbigny 1852 , Solarium stephanense Cossmann & Peyrot, 1919 , Solarium carocollatosimplex Sacco, 1892 , Architectonica ariei Wienrich, 2007 . The ‘varieties’ of ‘ Solarium carocollata’ and ‘ Solarium simplex ’ introduced by Sacco (1892) are all included here. Some of these may have species status, e.g., Solarium antiquoelata Sacco, 1892 , Solarium infernesulcata Sacco, 1892 , Solarium semitypica Sacco, 1892 .

Stratigraphic and geographic range. The oldest records are specimens from the Rupelian of Dego ( Italy) described by Sacco (1892) as Solarium infernesulcata and Solarium antiquoelata . The genus became widespread in the Circum-Mediterranean Region during the Early Miocene, when it is recorded from the Northeastern Atlantic and the Proto-Mediterranean Sea (Sacco 1892; Cossmann & Peyrot 1919). Early Miocene occurrences in the Central Paratethys are likely but based on poorly preserved material (Steininger 1973). During the Burdigalian/Langhian it also reached the North Sea Basin ( Janssen 1984; Stein et al. 2016) and the Loire Basin (Langhian, Glibert 1949). During the Serravallian and Tortonian it retreated from the northern regions and persisted in the Proto-Mediterranean Sea and adjacent Northeastern Atlantic into the Pliocene ( Chirli 2013). Last occurrence seems to have been in the eastern Mediterranean Gelasian Early Pleistocene of Rhodes Island (Chirli & Linse, 2011). It is tempting to consider the present-day Mediterranean Philippia lepida C. Bayer, 1942 a descendent of this group. However, as discussed above, we consider similarities between the shells of that species and Simplexollata superficial, and maintain it in the monotypic genus Basisulcata , a genus of unknown origins or affinities.

No representative of Simplexollata is so far known from the early Indo-Pacific Region. Architectonica affinis (J. de C. Sowerby, 1840) is comparable, but that species is a true Architectonica with a well-developed PUC and complex mid-whorl sculpture (J. de C. Sowerby 1840, pl. 16, fig. 5; Harzhauser et al. 2009: figs 5n, o; Kulkarni et al. 2010: fig. 2i).

Paleoenvironment. Inner neritic to outer neritic environments.

Discussion. Species included herein in Simplexollata have been previously placed in Architectonica R̂ding, 1798 [type species Architectonica perspectiva ( Linnaeus, 1758) , present-day, Indo-West Pacific], Psilaxis Woodring (1928) [type species Psilaxis krebsii (Mörch, 1875), present-day, West Indies and Florida] and Basisulcata Melone & Taviani, 1985 [type species Basisulcata lepida ( Bayer, 1942) , present-day, Mediterranean Sea]. Stein et al. (2016) recognized the similarity between ‘ Architectonica ’ carocollata and ‘ A.’ simplex , and doubting that placement in distinct genera was justified, placed them both in Basisulcata .

Comparison with Basisulcata lepida does not support this treatment. Simplexollata differs from the monotypic genus Basisulcata in the more convex base, the presence of a pair of very prominent spiral cords at the periphery in ventral view (formed by the LPC and the IPC), the less prominent keel and the deep furrow separating the UC from the base. The crenal groove and the parietal groove are indistinct in Basisulcata ( Fig. 2 ), whereas in Simplexollata both grooves are well developed. The basal field is almost smooth in Simplexollata , whereas Basisulcata develops distinct spiral cords and threads up to a groove in the middle of the basal field from where the axial folds start. These axial folds are more prominent in Basisulcata , and the UC are slightly weaker. Placement in the genus Psilaxis is rejected due to the comparatively higher base and relatively more rounded periphery, the weaker LPC and IPC on the base, the presence of a narrow PUC and the slightly less prominent UC (see also revised description of Psilaxis in Bieler 1985a: 238). Moreover, Psilaxis lacks a distinct parietal groove.

Placement in Architectonica is also unlikely as that genus differs in the higher number of mid-cords, the more prominent sculpture of the cords on the dorsal side, the presence of a PUC and in its wider umbilicus. Architectonica develops an even deeper crenal groove. In addition, the groove delimiting the UC is deeper and more prominent in Simplexollata .

Adelphotectonica Bieler, 1987 [type species Solarium reevei Hanley, 1862 ; present-day, Indo-West Pacific] is thin shelled and lacks a prominent SSC ( Bieler 1987: 208).

Figures

FIGURE 2. Whorl cross-sections of Architectonicidae genera. All modified after Bieler (1993) except for Simplexollata, Basisulcata and Ammotectonica. Solatisonax? refers to Solatisonax? kostejana (Boettger, 1907), Bold genera are represented in the Paratethys Sea. SSC: subsutural cord, MC: mid cord(s), UPC: upper peripheral cord, LPC: lower peripheral cord, IPC: infraperipheral cord; UC: umbilical crenae; PUC: proxumbilical cord.