Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
| | Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990) |
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  (Pl. 9q): GS13416, G45/f61, Pomahaka River, 500 m downstream from Oyster Creek, Duntroonian? (GNS) |
Beu & Maxwell (1990): Chapter 10; p. 140; pl. 9 q.
Synonymy: Neritella nitida Hutton 1873b, p. 15; Nerita nitida; Nerita pomahakaensis Finlay 1924a, p. 100 (new name for Nerita nitida (Hutton), a secondary homonym of N. nitida Donovan, 1804); ‘Theodoxus sp.’ (rejected as not a New Zealand species; Finlay 1926c, p. 374); Neritoplica(?) pomahakaensis, Beu & Maxwell 1990, p. 140
Classification: Neritidae
Description: Rather small for family (height 8-10 mm), globose, spire depressed, corroded on most specimens. Protoconch smooth, cap-shaped. Teleoconch of 2-2.5 whorls, slightly flattened or weakly concave adapically, convex below; last whorl very capacious. No sculpture apart from growth lines and weak spiral striae. Some shells with well developed colour pattern of narrow dark brown or grey zig-zag stripes on paler background; other shells unicoloured. Aperture large, ovate; inner lip oblique, almost straight, produced laterally to form a moderately wide, thin septum, bearing a single low fold near adapical end, otherwise smooth. Septum callus not particularly thick, strongly depressed abapically. Outer lip thin, prosocline, inclined at about 20° from vertical. Operculum ovate; outer face smooth, paucispiral, with subterminal nucleus; inner face with curved apophysis emerging opposite nucleus.
Comparison: This species was expunged from the New Zealand fauna by Finlay (1926c, p. 374) who considered the type specimen — which is badly damaged — to be a Recent Pacific species of Theodoxus (although the genus Theodoxus is actually limited to Europe and the Near East). Finlay was particularly impressed by the retention of a colour pattern, even though such patterns are preserved on a number of European fossil neritids, even of Mesozoic age. Until recently only the holotype was known, but in 1978, Dr M.J. Isaac (GNS Science) collected a considerable number of specimens from submerged outcrops in the Pomahaka River, downstream from Oyster Creek. V. pomahakaensis was therefore reinstated by Beu & Maxwell (1990) as a bona fide New Zealand species.
Because of its rather "generalised" neritid shell characters, Nerita pomahakaensis is of uncertain affinities, but assignment to "Neritella Gray, 1848" (= Neritina Lamarck, 1816) or Nerita Linne, 1758 can be ruled out. Theodoxus Montfort, 1810 (sensu stricto) also is a very unlikely location, but there is some similarity to certain extant Pacific species usually assigned to subgenera of Clithon Montfort, 1810. Among the Recent Philippines Neritidae illustrated by Lozouet & Plaziat (2008) and Eichhorst (in Poppe 2008, pp. 266-280), the Pomahaka shells are most similar to Vittina species, and we tentaively assign them there. Pomahaka faunules are undergoing active research at present, and the neritid will be reassesed as part of this work. Suter (1914, p. 13) referred this species to Nerita, and Finlay (1924a, p. 100) provided the replacement name Nerita pomahakaensis because of the prior Nerita nitida Donovan, 1804. Finlay's replacement name must be retained under ICZN Article 59.3.
Neritids have a very poor fossil record in New Zealand, far worse than the haliotids. The only other records we are aware of are N. (Lisanerita) melanotragus from Kaawa Creek (Opoitian) (Laws 1936b, p. 105), a beautiful, coloured specimen of a new Neritina species collected by Phil Maxwell from Bortonian (Middle Eocene) greensand at Evans Ford, Pareora River, and two undescribed species from Parengarenga Harbour (Otaian) (Wakefield 1976). Two specimens of true Nerita also have very recently been collected from Duntroonian(?) Chatton Formation near Waimumu, Southland.
Distribution: Duntroonian(?), Pomahaka Formation, Pomahaka (type). Absent from most collections from the unit, but not uncommon at one locality.
Cite this publication as: "A.G. Beu and J.I. Raine (2009). Revised
descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990). GNS
Science miscellaneous series no. 27."
© GNS Science, 2009
ISBN
978-0-478-19705-1
ISSN 1177-2441
(Included with a PDF facsimile file
copy of New Zealand Geological Survey Paleontological Bulletin 58 in CD version
from: Publications Officer, GNS Science, P.O. Box 30368 Lower Hutt, New
Zealand)