Plesiotrochidae Houbrick, 1990

Family Plesiotrochidae Houbrick, 1990 View in CoL

Genus Hemicerithium Cossmann, 1894 View in CoL

Type species. Cerithium imperfectum Deshayes, 1833 , original designation by Cossmann (1894: 302). Eocene , France .

Diagnosis. “ Cerithium imperfectum as the type of a section, exclusively fossil in the Tertiary terrains, is composed of relatively smaller species, and characterized by the numerous varices of the surface, by the hardly excavated columella, by the short and not very oblique canal. […] In the interior of the aperture one generally notices some tubercular swellings opposite the last varix, the labrum is more or less vertical, without the slightest inflection, and differs consequently from the characteristic arrangement of Cerithium adansoni .” ( Cossmann 1894: 302, translated from French).

Synonyms. Conocerithium Sacco, 1895 , type species Cerithium (Conocerithium) tauroconicum Sacco, 1895 , original designation by Sacco (1895: 22); Burdigalian, Italy. The type species is broad conical, stocky with almost straight sided, conical whorls, bearing drop-shaped axial ribs, which terminate at the slightly undulating abapical suture. The last whorl is strongly constricted (see Ferrero Mortara et al. 1984, pl. 35, fig. 2). Morphologically, it is highly reminiscent of Hemicerithium resectum (de Basterot, 1825) (= H. fallax Grateloup, 1832 ) from the Early Miocene of France and there is little reason to assume that they represent two different genera. Therefore, we consider Conocerithium Sacco, 1895 as subjective junior synonym of Hemicerithium Cossmann, 1894 and place Hemicerithium tauroconicum in Plesiotrochidae . Houbrick (1990) did not mention Conocerithium in his paper on Plesiotrochus .

Discussion. Several Plesiotrochidae have been described from the Oligocene and Miocene of the Northeastern Atlantic and the Central Paratethys Sea, e.g., Hemicerithium adultum ( Grateloup, 1845) (= H. subtrochleare d’Orbigny, 1852 ), H. banaticum ( Boettger, 1902) , H. dissitum ( Sandberger, 1858) , H. evae ( Boettger, 1902) , H. olgae ( Boettger, 1902) , H. resectum (de Basterot, 1825) (= H. fallax Grateloup, 1832 ). Some of these Tertiary European species were placed in Plesiotrochus Fischer, 1878 by Lozouet et al. (2001), Lozouet & Maestrati (2012), and Thivaiou et al. (2019), probably following Houbrick (1990) who treated Hemicerithium as junior synonym of Plesiotrochus . The type species of Plesiotrochus , Plesiotrochus souverbianus Fischer, 1878 , is an extant species from the Indo-West Pacific Region and is characterized by a moderately slender trochiform shell with prominent angulation close above the abapical suture and sculpture of several delicate spiral cords without varices. Other extant species, currently placed in Plesiotrochus , such as P. fischeri Smith, 1909 , P. luteus ( Gould, 1861) , P. pagodiformis Hedley, 1907 , P. penetricinctus ( Cotton, 1932) and P. unicinctus (A. Adams, 1853) , follow this scheme.An exception is Plesiotrochus monachus (Crosse & Fischer, 1864) , which differs in its mid-whorl angulation and the presence of blunt varices and axial sculpture. This species is closer to the Tertiary Hemicerithium species than to extant Plesiotrochus . Cotton (1932: 539) introduced Hypotrochus for P. monachus , but Houbrick (1990) suggested that this was a junior synonym of Plesiotrochus based on anatomical features. Molecular data will be needed to decide on the status of Hypotrochus . Already Pacaud (2019) criticized synonymizing Hemicerithium and Plesiotrochus by Houbrick (1990) based on conchological differences especially of the last whorl. Herein, we prefer to place the European Tertiary species in Hemicerithium . In our opinion, the IWP genus Plesiotrochus did not occur in the Paratethys Sea.

The Plesiotrochidae were not very diverse in the Paratethys Sea and are represented only by the genus Hemicerithium . Four species are recognized herein in the Central Paratethys, and one species is recorded from the Eastern Paratethys. No Plesiotrochidae passed the Badenian/Sarmatian boundary. In the Eastern Paratethyan, the Plesiotrochidae were confined to the Middle Miocene Tarkhanian and Chokrakian stages ( Iljina 1993).