Description and classification of Late Triassic Neritimorpha (Gastropoda, Mollusca) from the St Cassian Formation, Italian Alps
Titel:
Description and classification of Late Triassic Neritimorpha (Gastropoda, Mollusca) from the St Cassian Formation, Italian Alps
Auteur:
BANDEL, K.
Verschenen in:
Bulletin of geosciences
Paginering:
Jaargang 82 (2007) nr. 3 pagina's 215-274
Jaar:
2007
Inhoud:
Neritopsis represents the only surviving genus of an ancient group of the Neritimorpha that has no internal dissolution of its shell walls. While the two known living species have lecithotrophic early development without larval shell, Neritopsis aqabaensis n. sp. from the Gulf of Aqaba, Jordan, has a planktotrophic veliger. These living species of the genus differ in their protoconch shape as well as teleoconch morphology and ornament from the Triassic species that can be considered related to Neritopsis. Neritopsidae with the modern Neritopsinae is distinguished from the Triassic Cassianopsinae n. subfam. based on the genus Cassianopsis n. gen. with three species by features of their protoconch as well as the different characters of the operculum. Zardiniopsis n. gen. differs from these by higher shell shape and a smaller more complexly ornamented protoconch. Fossariopsis has a more angular shell shape. Colubrellopsinae n. subfam. with Colubrellopsis n. gen. resembles Cassianopsis n. gen. in respect to protoconch and features of the aperture of the teleoconch, but the former has rounded whorls and an ornament of axial ribs. Among the Fedaiellidae n. fam. with smooth shells two species of Fedaiella are redefined. The characters of the inner lip of their aperture connect them with the Neritopsidae, whereas the operculum in the Fedaiellidae with concentric structure on the outside distinguishes them from neritopsids.In distinction to the groups of the Neritopsoidea members of the Dephinulopsidae have a smooth inner lip of the aperture. Here Delphinulopsinae and Platychilininae n. subfam. differ from each other in the shape and ornament of their teleoconch. Schwardtopsis n. gen. resembles a juvenile Delphinulopsis grown to a larger size. The large concave callus of the inner lip and almost open coiling defines Delphinulopsis. Rows of nodes, low initial shell and rapid growth in shell diameter to an almost limpet shape characterises Platychilina, and lamellar growth increments on an almost limpet-like shell with flat initial part is present in Marmolatella. The Palaeonaricidae n. fam. contains two species of Palaeonarica, which have a Nerita-like shell with simple aperture. The Naticopsidae of the Carboniferous and Permian has its continuation in the Ampezzonaticopsinae n. subfam. of the Triassic, being connected to each other by the sinuous ornament of ribs on their larval shells. The genera are distinguished by teleoconch shape and differences in the ornament of their larval shell. Ampezzonaticopsis n. gen. has whorls well separated by deep sutures, Cortinaticopsis n. gen. has a simple aperture and a callus covering the umbilicus. The Hologyrinae n. subfam. with Hologyra have a ridge in the columellar furrow and an operculum that resembles that attributed to Carboniferous Naticopsis. Their protoconch has a characteristic chevron ornament on its larval whorls. The new family Tricolnaticopsidae is proposed to hold Tricolnaticopsis n. gen. with a smooth shell and convex inner lip and color pattern of dots. Into the aperture an operculum of bean-like shape may be fitted and this operculum has quite independent characters. It would also fit with Pachyomphalus with shallow sutures and two species, one with short rounded conical shape and the other with elongate shell. Both resemble Tricolnaticopsis n. gen. as does Rinaldopsis n. gen. with its wide flat inner lip. Laubopsis n. gen. has an open umbilicus and has no clear connection with similar species. The new family Scalaneritinidae is based on Scalaneritina n. gen. with an elongate shape and axial ribs. The protoconch is rounded with its larval shell having an ornament of low, fold-like collabral ribs. Ladinaticella n. gen. has three species: two have similar shell shape, in one case with a few strong ribs as ornament, while in another case there are many, and a third has a more rounded apex. In the case of Lancedellopsis n. gen., the shell is low, has a rapid increase in diameter and a low tooth in its aperture. Its relation to the other Neritimorpha remains undetermined. Among the species of Neritoidea from St Cassian Formation the members of the Neritariidae are arranged into the genera Neritaria, Ruganeritaria n. gen., Dentineritaria n. gen., Oncochilus, and Cassianpisulina n. gen. The genus Neritaria has a complexly ornamented protoconch, and the related Ruganeritaria n. gen. differs by the protoconch having only axial ribs. The operculum has the general shape of that found in modern Nerita and its relatives. Dentineritaria n. gen. has a central tooth on its inner lip and its protoconch is relatively large with straight axial ribs. The Oncochilinae is distinguished notably by projections of the inner lip into the aperture, which are two in Oncochilus and single larger one in Cassianpisulina n. gen. The latter is defined and compared with modern Pisulina. The Trachineritariinae n. subfam. with Trachyneritaria n. gen. is distinguished by characters of the teleoconch that has the inner whorls dissolved. Its relation to Trachynerita remains problematic.
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