Gymnothilda Schröder, 1995

Gründel, Joachim & Nützel, Alexander, 2013, Evolution and classification of Mesozoic mathildoid gastropods, Acta Palaeontologica Polonica 58 (4), pp. 803-826 : 807

Genus Gymnothilda Schröder, 1995

Fig. 2A .

Type species: Gymnothilda levata Schröder, 1995 , original designation; Early Cretaceous (Valanginian); Wąwał, Poland .

Included species: Gymnothilda levata Schröder, 1995 , Valanginian; Mathilda tomaszina Schröder, 1995 , Valanginian; Gymnothilda dispiralis Gründel, 1997 , late Bathonian–early Callovian; Gymnothilda torallolensis Kiel and Bandel, 2001 , Campanian; Gymnothilda reesi Kaim, 2004 , Valanginian.

Diagnosis.—Shell broad, conical; protoconch transaxial to slightly medioaxial, smooth or with axial folds; early teleoconch with one or two primary spiral ribs; whorls keeled at one of primary spiral ribs; base sharply demarcated from whorl face by edge; additional spiral ribs absent or only present on keel; axial ornament lacking or only weak on early teleoconch whorls or only developed on the keel; micro−ornament may be present.

Remarks.—In its present composition, Gymnothilda is rather heterogeneous. However, only a few species are included so that a subdivision is not warranted. Gymnothilda contains species with one or two primary spiral ribs and with or without micro−ornament. Kaim (2004) restricts the genus to Early Cretaceous species (Valanginian). The oldest species assigned to Gymnothilda is the Bathonian G. dispiralis Gründel, 1997 (late Bathonian to early Callovian). However, Kaim (2004: 134) doubted this generic assignment and stated that it probably represents a species of Promathildia , and that its reduced axial ornamentation derived independently. We consider this as a possible interpretation but leave the Middle Jurassic G. dispiralis in Gymnothilda until we know more about this genus.

Stratigraphic and geographic range.— Gymnothilda as understood here, has its first occurrence in the late Bathonian and ranges with G. torallolensis Kiel and Bandel, 2001 into the Late Cretaceous (Campanian). Gymnothilda pagodoidea Kiel, 2006 (Albian) belongs to Bathraspira (see below). It is also possible that the only known specimen of G. torallolensis represents a juvenile of Bathraspira . The genus is known from Germany, Poland.

Figures

Fig. 2. Type species and examples for Cretaceous genera of the family Mathildidae. A. Gymnothilda levata Schröder, 1995, Wąwał, Valanginian; from Kaim (2004: fig. 113A1). B, C. Bathraspira pagodoidea (Kiel, 2006), Mahajanga Basin/Madagascar, Albian; from Kiel (2006: figs. 8.1, 8.2). Juvenile (B) and adult (C) specimens.