Phylogenetic assessment of characters proposed for the generic classification of Recent Scissurellidae (Gastropoda : Vetigastropoda) with a description of one new genus and six new species from Easter Island and Australia

Abstract

Twenty-five nominal generic names have been proposed for the approximately 150 Recent species described in the family Scissurellidae (Mollusca : Gastropoda : Vetigastropoda). The generic diagnosing characters have not been uniformly applied and the value of these characters for scissurellid classification remains to be demonstrated. Here, the characters used are reviewed, including those of the shell, slit and selenizone, radula, soft tissue anatomy and histology. An overview of the state of our knowledge of these various character sets is given. Exploratory phylogenetic analysis reveals that scissurelline protoconch sculpture is highly homoplastic. The character conflict between protoconch sculpture and slit/foramen in Scissurella and Sinezona is resolved in favour of the latter characteristic. The closure of the slit to a foramen is governed by a driven trend. Other characteristics cited as diagnostic for certain genera are highly homoplastic, including the umbilical funiculus of Praescissurella Lozouet, 1998 and the smooth protoconch of Sukashitrochus Habe and Kosuge, 1964. Sukashitrochus, diagnosed by strong spiral sculpture on the base of the shell, is transferred from Anatominae to Scissurellinae and 'Scissurella' dorbignyi Audouin, 1826 is assigned tentatively to this genus. Coronadoa simonsae Bartsch, 1946 shows a typical scissurelline radula with five laterals, contrary to earlier reports. Several recently proposed genera are synonymised here: Anatoma (+ Hainella Bandel, 1998), Scissurella d'Orbigny, 1824 (+ Maxwellella Bandel, 1998; Reussella Bandel, 1998, Praescissurella), Sinezona (+ Daizona Bandel, 1998), Thieleella Bandel, 1998 (+ Pagodella Bandel, 1998: nomen nudum). The newly described Satondella Bandel, 1998, and Thieleella, are considered valid. Ariella Bandel, 1998, is conservatively retained. Scissurona is synonymised under Incisura. Cladistic analysis based on shell and radular characteristics, using autapomorphies to code inapplicables, revealed several patterns. The two main lineages in Scissurellidae sensu lato are Scissurellinae (Coronadoa, Incisura, Satondella, Scissurella, Sinezona, Sukashitrochus) plus Anatominae (Anatoma, Thieleella) and Sutilizoninae (Sutilizona) plus Temnocinclinae (Temnocinclis, Temnozaga) plus Larocheinae (Larochea, Larocheopsis, Trogloconcha). These two lineages are characterised by differences among lateral teeth on the radula. Depressizona n. gen. is placed in its own subfamily, Depressizoninae n. subfam. One new genus and five new species of scissurellids are described from Easter Island (Depressizona exorum n. gen. and n. sp., Anatoma rainesi n. sp., Scissurella alto n. sp., Sinezona zimmeri n. sp., Satondella senni n. sp.) and one new species from Australia (Trogloconcha christinae n. sp.). The calyptraeiform Depressizona n. gen. exhibits a new type of shell morphology for the family. The scissurellids from Easter Island show a stronger biogeographic affinity with Polynesia and the Central Pacific than with the eastern Pacific.