The value of a single character: the Paleogene European land snail Ferussina Grateloup, 1827 is likely a cyclophorid (Gastropoda, Caenogastropoda)

Abstract Ferussina Grateloup, 1827 is a European Paleogene land snail genus, which is currently classified in its own family, the Ferussinidae Wenz, 1923 (1915), in the superfamily Cyclophoroidea. The shell of this genus is remarkable by its last quarter whorl turning towards the apex instead of away from it, which is an unusual trait in terrestrial snails. We show, however, that this trait has evolved at least nine times in terrestrial Eupulmonata and Caenogastropoda, and it does not justify distinction at the family level in any of the reported cases. This observation suggests the systematic position of Ferussina should not be based on the apexward-turning last quarter whorl alone but instead on the general morphology of the shell. As a result, we re-evaluate the systematic position of the Ferussinidae and treat it as a subfamily of the Cyclophoridae.


Introduction
Ferussina Grateloup, 1827(and its synonym Strophostoma Deshayes, 1828see Wenz 1923;Kadolsky 2008) is a genus reported from middle Eocene (Lutetian) to upper Oligocene (Chattian) deposits of France, Germany, Italy, and Switzerland (Fig. 1);  Table S1 for more information). The map was created with ESRI ArcGIS 10.4. a dubious record comes from presumably lower Miocene strata of southern France (Degrange-Touzin 1892). It is currently classified in its own family, the Ferussinidae Wenz, 1923Wenz, (1915 (Bouchet et al. 2017) in the superfamily Cyclophoroidea Gray, 1847. Ferussina is characterized by a relatively large (ca 1-3 cm), depressed-globular shell with an obtusely conical spire, a round aperture, and a last quarter whorl turning towards the apex (Sandberger 1870(Sandberger -1875Roman 1899;Rey 1968;Kadolsky 2008;Salvador et al. 2016). As a result, the aperture opens in the adapical direction of the shell, orientating the umbilicus of the shell upwards while the animal was crawling.
An "upright" turning last whorl (termed "anostomy" by Nordsieck 1986) is unusual in terrestrial snails but has repeatedly evolved in both the Eupulmonata ("pulmonates") (at least six times) and Caenogastropoda (at least three times) (Schileyko 1998(Schileyko , 1999(Schileyko , 2000Egorov 2009Egorov , 2013, and even in a Devonian marine gastropod (Braun 1838). This trait has not been considered a justification for the distinction at the family level in any of the reported cases. This observation suggests the systematic position of Ferussina should not be based on the apexward-turning last quarter whorl alone but instead on the general morphology of the shell. As a result, we re-evaluate the systematic position of the Ferussinidae and treat it as a subfamily of the Cyclophoridae Gray, 1847.

Results
While the apexward-turning last whorls are unique among fossil European land snails, we found this trait in a number of unrelated extant and fossil clades, including 12 pulmonate (Wenz 1940;Schileyko 1998Schileyko , 1999Schileyko , 2000 and four caenogastropod genera (Egorov 2009(Egorov , 2013, representing at least nine independent events. Table 1 summarises the key information. Table 1. Summary of key information of extant land snail genera with apexward turning body whorl. In addition, we provide information on shell shape of relatives within the same family to assess the relevance of shape traits for systematic placement. Information derives from Wenz (1940) and Schileyko (1998Schileyko ( , 1999Schileyko ( , 2000.

Genera
Size ( Remarks. These genera were included in the family Hypselostomatidae by Schileyko (1998), which was recognized as a subfamily of Gastrocoptidae Pilsbry, 1918 by Bouchet et al. (2017). Other genera of the same (sub)family are variable in shape, ranging from ovoid and conic to lenticular and globular. The direction of the aperture is variable even in the same genus. Some Hypselostoma and Gyliotrachela species have even normally coiled shells. The shells are small (2-4 mm). All the species with detached last whorl inhabit limestone rock areas and spend a considerable time of their life tightly attached to rock surfaces (Panha and Burch 2005).
(2) Genus Campolaemus Pilsbry, 1892 Remarks. This genus was classified in the Hypselostomatidae by Schileyko (1998). However, this species more probably belongs to the Streptaxidae (Páll-Gergely 2020). Nevertheless, its position within that family is questionable. Shell height is ca 2 mm. No information on its habitat preference is known. However, it is probably not a rockdwelling species, because streptaxids typically occur among leaf litter, in decaying plant material, and under logs and stones (Páll-Gergely pers. obs.). Remarks. Anostoma was classified in the tribe Odontostomini (Bulimulidae, Bulimulinae) by Schileyko (1999), which was recognized as a distinct family by Bouchet et al. (2017). According to Schileyko (1999), there are 11 high-spired genera and 3 lowspired/globular genera in the Odontostomini, all of which comprise relatively large snails (

(4) Genus Hendersoniella Dall, 1905
Remarks. This genus was classified in the Urocoptidae, Holospirinae (Schileyko 1999), where many high-spired genera belong. Shell diameter is 11-13 mm. Hendersoniella are obligate rock-dwelling, as the other members of the family ("live snails were found under limestone slabs that were spalding from the underlying rock"; Thompson and Correa 1991: 15).

(5) Genus Tonkinia Mabille, 1887
Remarks. This genus was classified as a member of the Streptaxidae, Streptaxinae by Schileyko (2000), and in the Diapheridae in MolluscaBase (2020) following Dance (1970), who mentioned that Tonkinia and its probably closest relative, Platycochlium Laidlaw, 1950, are most similar to juvenile shells of Diaphera Albers, 1850 and Sinoennea Kobelt, 1904. With the exception of Platycochlium and Tonkinia, all other diapherids are high-spired. The shell is 4.3-5 mm wide (Schileyko 2000). We have not found any published information about its habitat preference, but it probably lives among decaying plant material and under logs and stones as other Diapheridae.

Caenogastropoda (1) Genus Anosycolus Fischer-Piette, C.P. Blanc, F. Blanc & Salvat, 1993
Remarks. This taxon was classified in the Hainesiidae by Egorov (2009) and in the Cyclophoridae in MolluscaBase (2020). However, a current investigation suggests it is a relative of Boucardicus, which includes conical-globular and high-spired species and may deserve its own family within Cyclophoroidea (Páll-Gergely unpublished information). Shell does not exceed 12 mm in maximum diameter.
(2) Genus Laotia Saurin, 1953 Remarks. This genus was classified in the Diplommatinidae by Egorov (2013) and in the Alycaeidae in Do et al. (2015). Recent investigations corroborate placement in Alycaeidae, where it will be classified in a separate new subfamily together with Messageria Bavay & Dautzenberg, 1904 (Páll-Gergely unpublished information). Shell diameter is 2.2-4.4 mm (Páll-Gergely 2014). Nothing is known about its habitat preference, but Laotia is probably not an obligate rock-dwelling genus, since the aperture is not flat in front profile to allow attachment to rock surfaces.

Discussion
The list above shows that shells with the last whorl turned apexward are present in numerous unrelated lineages of pulmonate and operculate terrestrial snails. In all cases, the species and genera with this peculiar shape have normally coiled relatives. Similarly, the fossil Ferussina certainly evolved from normally coiled ancestors, and we should not give too great importance to this trait when determining its systematic position. Moreover, the closest relatives of these genera are often species with high-spired shells. This suggests that we cannot exclude high-spired cyclophoroideans from the possible relatives of Ferussina.
We can exclude the Pomatiidae as possible relatives, as members of this family have calcareous opercula that are often found as fossils. No such opercula have been documented for Ferussina. The Cochlostomatinae, also with numerous extant and fossil members, are smaller than Ferussina and are characterized by high, conical shells, and some members have calcareous opercula (Fehér 2004;Zallot et al. 2015). The cyclophoroid family Craspedopomatidae, represented by several fossil species in Europe, comprises only very small, globular forms of only a few millimetres in diameter (Wenz 1923;Harzhauser and Neubauer 2018).
The most probable group of relatives is the Cyclophoridae. Most members of this family have broadly conical shells similar to that of Ferussina, except for the apexwards turn of the last quarter whorl. Extant Cyclophoridae have non-calcareous opercula, which are not preserved as fossils. So far, 14 species of Cyclophoridae are known from the Cenozoic sedimentary record of Europe (Wenz 1923;Steklov 1966;Schütt 1991Schütt , 1997Stworzewicz 1995). The oldest records derive from the upper Paleocene (Thanetian) of France. Earlier mentions of European cyclophoroids from the Jurassic and Cretaceous belong to the families Diplommatinidae, Megalostomatidae, and Pupinidae, or are unassigned cyclophoroids (Hrubesch 1965;Bandel 1991Bandel , 1993Neubauer et al. 2019). The genus Ventriculus Wenz in Fischer & Wenz, 1914 was classified in the family Cyclophoridae, subfamily Pupinellinae by Wenz (1923), a group now included in Pupinidae (Bouchet et al. 2017). The Pupinidae presently inhabits Asia from India to the oceanic islands (Egorov 2013).
Cyclophoridae are otherwise mostly restricted to south-eastern Asia, and the European fossils represent a rare exception of biogeographic affinity between both regions. Only a few other taxa that are widespread in East Asia today are also found in the European Cenozoic fossil record, such as Diplommatinidae, Strobilopsidae, and Pupinidae (e.g. Wenz 1923;Manganelli et al. 2008;Páll-Gergely et al. 2015;Harzhauser and Neubauer 2018).
In summary, we suggest a revised systematic position of the genus Ferussina in the Cyclophoridae. Given the distinct biogeographic and stratigraphic setting and morphological differences to extant Cyclophoridae, we suggest to maintain the genus in a distinct subfamily, Ferussininae.
The extant genera with apexward-turning body whorl listed above inhabit various habitats, with about half of them being obligatory rock-dwellers, indicating that this peculiar trait can be developed under various environmental conditions. Ferussina lived in a period when the regions it occurred in central and western Europe (France, Switzerland, Germany, Austria, and northern Italy) were dominated by warm-temperate to subtropical evergreen forests (Pound and Salzmann 2017). The Late Oligocene Ferussina tricarinata was thriving in semiarid conditions in the Mainz Basin on the shores of a brackish to hypersaline lake (Kadolsky 1989). Other Ferussina species may have dwelled in more humid climates among leaf litter and under decaying logs, but we have insufficient data about the taphonomy and paleoecology of their occurrences.