GNS Science

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

New Zealand Cenozoic Mollusca

Phialopecten thomsoni Marwick, 1965


(Pl. 33b): holotype of Phialopecten thomsoni Marwick, 1965 (right valve only), Raukawa Limestone, Pukeora Hill, Waipukurau, Hawke's Bay, Mangapanian (TM2682, GNS)

Beu & Maxwell (1990): Chapter 14; p. 275; pl. 33 b.

Synonymy: Phialopecten thomsoni Marwick 1965, p. 22; Phialopecten triphooki (in part), Beu & Maxwell 1990, p. 275

Classification: Pectinidae

Description: Large for family (130-175 mm high), calcitic, most specimens slightly longer than high, left valve markedly more inflated than right; ears large, almost equal, symmetrical in the 2 valves, almost square in shape to triangular and contracting strongly to disc at base. Sculpture of 17-26 (18-22 on most specimens) prominent radial costae with wide, gently convex surfaces, each costa subdivided by 4-5 weak grooves; costae wide and relatively closely spaced on right valve, narrow and relatively widely spaced (higher than wide, on most specimens) on left valve. Internal features visible on very few specimens.

Comparison: The sections on Pliocene Pectinidae in Beu & Maxwell (1990) were written before the groups were clarified by Beu (1995), following extensive collecting and mapping of North Island east coast Pliocene limestone beds. The real taxonomy and phylogeny of Phialopecten and of Towaipecten Beu, 1995 became apparent only during this mapping, when Phialopecten marwicki and Towaipecten ongleyi were collected together at a few localities. Beu (1978a) regarded Phialopecten thomsoni as part of the variation of P. triphooki, but larger collections demonstrated that they could be separated, in the manner proposed by Marwick (1965, pp. 20-23) when naming P. thomsoni. Early Nukumaruan specimens from Napier (typical P. triphooki) and from limestone in the same stratigraphic position in central and southern Hawke's Bay are recognisable by their moderate rather than huge size (height 100-150 mm, most specimens not over about 130 mm), their relatively large number of primary radial costae (18-28, most specimens with 20-25), and the regular, even subdivision of each costa by a single obvious median groove, with little subsidiary sculpture. Phialopecten triphooki is common at Napier in excavations from near the base of the Scinde Island Limestone section, and also occurs at Pakipaki quarry (south of Hastings) and in the lower beds of Mason Ridge. The Mangapanian (and very rarely, earliest Nukumaruan) species P. thomsoni is illustrated here. It differs from P. triphooki in its larger size (height 130-175 mm), fewer primary radial costae (17-26, mostly 18-22), and in the broader surfaces of the costae, with numerous subdivisions of the rib crests rather than the single prominent median groove of P. triphooki. The ears also are larger on many specimens, and are more triangular than in P. triphooki, contracting sharply to the disc at the base. This is the species that is common in many localities in the widespread Mangapanian Te Aute facies limestone (Raukawa Limestone) in eastern Hawke's Bay and the Puketoi Range in southern Hawke's Bay, and in Te Waka Limestone along the western edge of Hawke's Bay. It also occurs in many shellbeds, shallow-water sandstone and conglomerate beds in Hawke's Bay and Wanganui Basin. It evolved abruptly from the Opoitian-Waipipian species P. marwicki (Beu, 1970), in a speciation event preserved in Mangapani Shellbed in Wanganui Basin, at the base of the Mangapanian succession.

Phialopecten marwicki is easily recognised by almost all specimens having only 3 obvious riblets on the crest of each radial costa (tripartite or fasciculate ribs), and in having more numerous, narrower, more closely spaced radial costae (28-37, mostly 30-33) than in P. thomsoni. It also reached a smaller size; although a single huge, little-inflated valve was collected from Rotookiwa Limestone on Te Aute Trust Road, near Lake Poukawa, south-central hawke's Bay (175 mm high), few others exceed 130 mm in height and many are smaller. The main development in this species throughout Opoitian-Waipipian time is an increase is size, and Beu (1995) suggested that the separation of the Opoitian and Waipipian Stages by this criterion should be drawn at 110 mm for large populations; specimens larger than 110 mm are Waipipian, whereas smaller ones are Opoitian. The Kapitean species P. tolagaensis (Marwick, 1931) also has tripartite costae, and differs from Opoitian specimens only in its still smaller size and finer and more numerous ribs, so it is uncertain whether a distinction from P. marwicki should be maintained in this apparently anagenetic series. The specimens illustrated here as P. marwicki (Waipipian) and P. marwicki (Opoitian) were illustrated by Beu & Maxwell (1990, pl. 33a, e) as P. marwicki and P. ongleyi, respectively, but are now realised (following Beu 1995) to be earlier (smaller) and later (larger) specimens in the anagenetic evolution of P. marwicki.

The genus Towaipecten differs from Phialopecten in its much lower, wider and more numerous radial costae, and specimens of the more extreme end-members of each genus, T. katieae Beu, 1995 and P. thomsoni, which occur together at a few localities in Mangapanian limestone in Hawke's Bay, are very strongly distinct. However, the earliest known species of Towaipecten, T. ongleyi (Marwick, 1965), is similar enough to finely ribbed Phialopecten specimens to have confused everyone from Marwick (1965) to Beu & Maxwell (1990). The very low, closely spaced nature of the radial costae makes the distinction obvious when specimens are collected together with P. marwicki, however. The only species of the Towaipecten lineage illustrated here is the youngest, T. mariae (Finlay, 1927) (Nukumaruan; Pl. 38h, i), which introduced a new character into the Towaipecten lineage by developing radial folds in the shell (plicae).

Distribution: Mangapanian and, very rarely, earliest Nukumaruan; Raukawa Limestone, Pukeora Hill, SE of Waipukurau, Hawke's Bay, type of Phialopecten marwicki; common in Raukawa and Te Waka Limestone in Hawk'es Bay, not uncommon in many shellbeds and conglomerate beds in Hawke's Bay and Wanganui.

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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