GNS Science

Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)

New Zealand Cenozoic Mollusca

Phenatoma decessor Marwick, 1928


(Pl. 37w): GS12164, CH/f25B, tip of Tarawhenua Peninsula, Pitt Island, Chatham Islands; Whenuataru Tuff, Waipipian (GNS)

Beu & Maxwell (1990): Chapter 14; p. 302; pl. 37 w.

Synonymy: Phenatoma decessor Marwick 1928, p. 491

Classification: Conidae: Borsoniinae? ("Clathurellinae", clade 15?; Puillandre et al. 2008)

Description: Small for genus (to 21 mm high), with tall, straight-sided spire and relatively short last whorl; whorls lightly but clearly angled (at lower third on spire whorls), coarsely sculptured. Sculpture dominantly of spiral cords; a prominent, wide subsutural fold is followed below by shallowly concave sutural ramp bearing 1 or 2 narrow cords, then by a prominent peripheral cord forming the shoulder angle, then 2 more cords on sides of spire whorls and 7 below shoulder on last whorl; all crossed by many low axial threads, producing fine but conspicuous granules at sculptural intersections; axials sinuous over sutural ramp in conformity with deep U-shaped anal sinus in outer lip. Aperture with smooth, unthickened lips; anterior canal short, widely open, deeply notched, with low fasciole. Protoconch tall and narrowly conical, of 3.5 smooth whorls.

Comparison: Phenatoma decessor is very similar to the Castlecliffian-living P. rosea (Pl. 49m), differing in its smaller size (P. rosea reaches at least 34 mm high), its clearly angled whorls, its lack of threads between the spiral cords, and its fewer spiral cords (12 to 14 cords on the last whorl of P. rosea). P. precursor (Mangapanian-Nukumaruan) has many low, smooth, narrow, closely spaced spiral cords, and appears to have been ancestral to the weakly sculptured Castlecliffian-Recent species P. zealandica. The Altonian P. perlata and P. lawsi are small (8-9 mm high) with simple coarse sculpture, and P. lawsi has smooth cords that suggest a relationship to Tomopleura.

Distribution: Waipipian; Whenuataru Tuff, Tarawhenua Peninsula, northern Pitt Island, Chatham Islands (type), moderately common; also a few specimens from shallow-water sandstone with diverse Mollusca at Waihi Beach, Hawera, South Taranaki. (The ecology of the Whenuatara Tuff assemblages is discussed under Mitrolumna granum, Pl. 37v).

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)


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