Bellardithala, Harzhauser & Landau, 2021

Harzhauser, Mathias & Landau, Bernard, 2021, An overlooked diversity-the Costellariidae (Gastropoda: Neogastropoda) of the Miocene Paratethys Sea, Zootaxa 4982 (1), pp. 1-70 : 14-16

Genus Bellardithala nov. nom.

Type species: By typification of replacement name: Voluta obsoleta Brocchi, 1814 , Pliocene, Italy [for figures of holotype see Rossi Ronchetti (1955: fig. 129) and Pinna & Spezia (1978, pl. 68, figs 2, 2a).

Diagnosis. Medium-sized Thala - and Mitromica -like shells with moderately slender fusiform shells, cancellate, weakly beaded or strongly reduced sculpture. Differing from Mitromica in their episodic growth, indicated by the formation of denticles in juvenile specimens and from Thala in the predominant axial sculpture on early spire whorls. Occasionally with elongate denticles within outer lip.

Description. Shell medium-sized, moderately slender fusiform with conical to weakly cyrtoconoid spire. Protoconch high conical, mammillate, consisting of about three smooth whorls; teleoconch of 4.5 to 5.5 weakly convex whorls. Sculpture of early spire whorls dominated by axial ribs crossed by weak or subordinate spiral cords, often confined to axial interspaces. Axial sculpture prominent or largely reduced on late spire whorls and last whorl, crossed by numerous spiral cords forming cancellate or weakly beaded sculpture. Aperture narrow to moderately narrow; anal sinus indistinct. Inner lip indistinct, narrow; columella with four folds. Outer lip weakly to distinctly thickened, dentate only in adults; elongated denticles occur at least in several species (own data B.L). Siphonal canal moderately long, bent weakly to left with shallow siphonal notch.

Synonyms. Micromitra Bellardi (1888) [non Meek, 1873].

Etymology. Combination of the surname of Luigi Bellardi (1818–1889), Italian malacologist, who has establish the costellariid genus Micromitra (name preoccupied) for the species placed here in Bellardithala , and the genus Thala .

Included species. Voluta obsoleta Brocchi, 1814 (for non-Paratethyan species see Bellardi 1888, Sacco 1904, Rossi Ronchetti 1955; Pinna & Spezia 1978, Giannuzzi Savelli & Reina 1883; 1978; Ferrero-Mortara et al. 1984, and Landau et al. 2019), Bellardithala baluki nov. sp., B. dacica nov. sp., B. fedosovi nov. sp., B. kovaci nov. sp., Micromitra abbreviata Bellardi, 1888 , M. granosa Bellardi, 1888 , M. intermedia Bellardi, 1888 , M. mangeliaeformis Bellardi, 1888 , M. propinqua Bellardi, 1888 , M. pusilla Bellardi, 1888 , M. seminuda Bellardi, 1888 , M. taurina Bellardi, 1888 , Mitra (Micromitra) boehmi Boettger 1906 , Mitra (Thala) kostejana Boettger 1906 , M. lapugyensis Hoernes & Auinger 1880 , M. laubei Hoernes & Auinger 1880 , M. partschi Hörnes 1852 , M. pupa Dujardin, 1837, Thala angustolonga Sacco, 1904 , T. elatocostata Sacco, 1904 , T. inflatoparva Sacco, 1904 , T. subnullecostata Sacco, 1904 .

Stratigraphic and geographic range. Early Miocene to Pliocene. The earliest representatives of Bellardithala appeared during the Burdigalian in the Proto-Mediterranean Sea, recorded from the Colli Torinesi in Italy (e.g. B. taurina Bellardi, 1888 , B. propinqua Bellardi, 1888 , B. seminuda Bellardi, 1888 ). Middle Miocene occurrences are documented from the Badenian (Langhian, Serravallian) of the Central Paratethys Sea as described herein. There, the genus displays its maximum diversity in the southern basins and during the early/middle Badenian (= Langhian). From the northeastern Atlantic the genus is documented from the Langhian to the Messinian ( Landau et al. 2019). The last occurrences are described from the Pliocene of the Mediterranean Sea (i.e. B. obsoleta ) ( Landau et al. 2019).

Palaeoenvironment. The occurrences suggest a distribution from shallow marine, inner neritic sand environments down to middle neritic soft bottom habitats.

Discussion. Bellardi (1888: 5) established Micromitra as new genus for “small subovate species with moderately inflated spire, long aperture with canal, thickened, varix-like outer lip with internal denticles and four columellar folds” (free translation of parts of the original diagnosis in Latin). Bellardi (1888: 5) designated Mitra obsoleta Brocchi, 1814 from the Pliocene of Italy as type species of Micromitra and placed several Burdigalian and Pliocene species from Italy in his new genus [ Coan (1966: 130) designated Mitra taurina Bellardi, 1888 as type species, obviously overlooking that Bellardi (1888) had already designated a type species]. The name Micromitra , however, was already preoccupied by Meek (1873: 479) for a brachiopod and the Neogene gastropod genus would have needed a new name. No replacement name was proposed subsequently because Micromitra Bellardi, 1888 was treated as subjective junior synonym of Thala H. Adams & A. Adams, 1853 by most subsequent authors ( Cossmann 1899; Peyrot 1903, 1928; Sacco 1904; Sphon 1969; Cernohorsky 1970; Giannuzzi-Savelli & Reina 1983). Only Boettger (1906) used Micromitra Bellardi, 1888 as subgenus of Mitra for his Mitra (Micromitra) boehmi , separating it from Thala species.

A further potential name for the Neogene European species is Mitromica Berry, 1958 , which was established for extant species from the Eastern Pacific (type species: Mitra solitaria C.B. Adams, 1852 ). Conchological differences between Mitromica and Thala are subtle and Rosenberg & Salisbury (2003) stressed the continuous growth of Mitromica versus episodic growth of Thala , reflected by the development of denticles in juvenile stages and growth stops on the shell surface. Furthermore, Rosenberg & Salisbury (2003) assumed a geographic separation of both genera. Thala is restricted to Indo-Pacific species whereas western African, western Atlantic and Eastern Pacific species are placed in Mitromica Berry , with overlap in distribution of the two genera only around South Africa. Separation of a ‘western’ Mitromica -clade from an Indo-Pacific Thala -clade is also supported by radular analyses by Fedosov et al. (2017; pers. comm. May 2020). Molecular data to support these clades is, however, not yet available.

Rosenberg & Salisbury (2003) observed episodic growth and denticles in juveniles in the Paratethyan ‘ Thala ’ partschi kostejana Boettger, 1906 and therefore, treated it as Thala . Herein, we document episodic growth also for Bellardithala kovaci nov. sp., B. pupa, B. boehmi , and B. kostejana . Thus, despite some morphological similarities, the Paratethyan species cannot be placed in Mitromica . Placement in Thala , however, is problematic as well. The middle Miocene Paratethyan species and all early Miocene to Pliocene species, originally placed in Micromitra by Bellardi (1888), are characterised by predominant axial sculpture during early stages of growth, whereas the spiral sculpture is subordinate. None of the species develops pointed nodes and prominent beads like Indo-Pacific Thala . In some species, such as M. kovaci , M. obsoleta and M. taurina , the sculpture is largely reduced on the last whorls, which would be an atypical feature for Indo-Pacific Thala (see Thala species described in Rosenberg & Salisbury, 2003, 2014). Therefore, we consider the Old World species to represent a clade distinct from Thala and Mitromica , and erect the replacement name Bellardithala nom. nov. for Micromitra Bellardi, 1888 , non Meek, 1873.

Early ontogeny. The protoconchs of Thala and Mitromica seem to have little taxonomic value above the species level. The extant species currently listed as Mitromica and Thala by Rosenberg & Salisbury (2003, 2014) and Fedosov et al. (2017) suggest two different modes of development. Paucispiral bulbous protoconchs with inflated whorls are found in both genera [e.g. M. solitaria (C.B. Adams, 1852) , M. foveata ( Sowerby, 1874) and Thala todilla ( Mighels, 1845) , T. merrilli Rosenberg & Salisbury, 2014 ]. Likewise, multispiral protoconchs occur in both genera [e.g. M. africana ( Rolán & Fernandes, 1995) and T. mirifica ( Reeve, 1845) , T. jaculanda (Gould, 1860) ]. The species of Bellardithala display high conical protoconchs. In Thala , this type of protoconch correlates with wide geographic distributions ( Rosenberg & Salisbury 2014) and the endemicity of Paratethyan species has to be treated with caution, because it may rather be a collecting artifact. We note that all protoconchs of Bellardithala species preserved are tall, multispiral, and in most species mammilate. Species with paucispiral protoconchs have not been found until now.

Key to Paratethyan Bellardithala nov. nom. species:

1. Shell very slender fusiform, convex spire whorls...................................................... B. laubei - Shell broader fusiform................................................................................. 2 2. Weakly oblique suture................................................................................. 3 - Strongly oblique suture........................................................................ B. kostejana 3. Sculpture subobsolete on late adult whorls................................................................. 4 - Sculpture well developed, beaded or tuberculate............................................................. 5 - Sculpture cancellate........................................................................ B. lapugyensis - Sculpture predominantly axial with broad flattened spirals............................................. B. partschi 4. Axial sculpture on early whorls strong, aperture low................................................... B. boehmi - Axial sculpture on early whorls weak, aperture tall.............................................. B. kovaci nov. sp. 5. Sculpture finely beaded................................................................... B. baluki nov. sp. - Sculpture coarsely tuberculate............................................................ B. fedosovi nov. sp. - Sculpture coarsely tuberculate early whorls, cancellate later whorl................................. B. dacica nov. sp.