Phialopecten thomsoni Marwick, 1965
MARWICK, J. 1965. Upper Cenozoic Mollusca of Wairoa District, Hawke's Bay. New Zealand Geological Survey Paleontological Bulletin, 39: 1-83, pls. 1-11. [p. 23, pl. 5, fig. 1; pl. 6, fig. 1]
1965 Phialopecten thomsoni Marwick, 1965
J. Marwick, 1965, plates 5, 6.
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«Shell very large; left valve \vell inflated, right valve gently so. Sculpture of 18-20 main ribs, high and strong, but of uneven strength and spacing, those of left valve much narrower than interstices, but those of right valve mostly broader than interstices, tending to be weaker toward dorsal margins where flanked by numerous close, fine radial ridges; a few ribs divide deeply to become double at an early stage; the wider interstices and ribs bear several secondary or higher order riblets separated by linear grooves; whole surface, with fine, sharp, concentric ridges better preserved in interstices and lower ribs where they may fenestrate the finer radials. Ears subequal, very large, forming an exceptionally long hinge line (nearly two-thirds total length in holotype), posterior ones with posterior margin curving in to dorsal margin of disc, similar to anterior ears; right anterior one with byssal notch very deep until about half-growth then becoming progressively shallower. Ventral margin of left valve broadly dentate, projecting at ribs several millimetres beyond interstices of right valve which is thus less high than left valve.
HOLOTYPE: N.Z. Geological Survey, cOluplete individual, collected by J. Allan Thomson, 1922 (TM2682).
Length, 148 mm; height, left valve 135 mm, right valve 133 mm; inflation, left valve 34 mm, right valve 25 mm. LOCALITIES: Limestone, roadside, Pukeora, 31- miles south of Waipukurau (type). GS5430, Quarry, 1 mile at 32° from Trig 20, Waipukurau Survey District, N146/940794, T. L. Grant-Taylor, 1951. GS639, Ostrea ingens bed, Waitotara, J. Park, 1886. AGE: Waitotaran. The holotype was recorded by Thomson (1926, p. 352) as Pecten n.sp. aff. triphooki ZitteI.» JOHN MARWICK, 1965
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«REMARKS: The realisation that collections from Pakipaki quarry, south of Hastings, contain Zygochlamys delicatula (of which the incoming in North Island sequences is the main indicator of the base of the Nukumaruan Stage in shallow-water facies) has at last allowed the recognition that Pakipaki Limestone is Nakumaruan and distinct from the Mangapanian Te Onepu Limestone (Beu 1990a, p. 84). Typical large specimens of Phialopecten triphooki (sensu stricto) occur in the limestone at Pakipaki and, as this limestone was formerly assumed to be Mangapanian Te Onepu Limestone, made a distinction between a Mangapanian P. thomsoni and an early Nukumaruan P. triphooki seem insupportable. The recognition that all P. triphooki are early Nukumaruan allows the return to Marwick's concept of a separable Mangapanian P. thomsoni.
The two species are in fact quite markedly distinct, on the basis of several characters. P. thomsoni reaches a markedly larger mean and maximum height (c. 175 mm max.) than P. triphooki (c. 150 mm), has much more martedly discrepant costae on the two valves (nearly equal in width on the two valves in P. triphooki, but in P. thomsoni low and much wider than the interspaces on the right valve, and high and much narrower than the interspaces on the left valve), and has much more finely subdivided costal crests than in P. triphooki (a major median groove subdivides costae of both valves into two, with many very weak tertiary and finer treads, on P. triphooki, whereas primary costae of the right valve are subdivided into four or more equally prominent secondary costae and numerous finer ones on P. thomsoni). The auricles are also larger and are separated from the disc by shallower grooves in P. thomsoni than in most specimens of P. triphooki. In his original description, Marwick (1965, p. 23) pointed out a further unusual, distinctive character of P. thomsoni: the inegularity of the major costae. On the specimens from Wilkies Shellbed, at the mouth of Waitotara River (GS639), listed by Manrwick (1965, p. 23) as specimens of P. thomsoni, and on most Te Onepu Limestone and Te Waka Limestone specimens, a few major costae are markedly higher and wider than their neighbours, and on many specimens one or two main costae subdivide very early to produce a pair of narrower costae, separated by a groove that, towards the ventral margin, becomes as deep and wide as normal primary radial interspaces. However, some specimens from muddy shellbed facies (notably from Wilkies Shellbed in Wanganui basin; e.g., Fig. 18d) have regular costation, suggesting that the irregularity seen on many other specimens is, like auricle size and disc inflation, at least partly phenotypic. DISTRIBUTION: Te Onepu Limestone throughout the Raukawa Range-Hatuma area and the Puketoi Range; Te Waka Limesone from Te Waka Trig. to the southern Glenross Range, including the area around Kuripapango; a few muddy and gravelly shallow-water shellbeds elsewhere in Hawke's Bay (e.g. Gorge Stream, Patoka); Wilkies and Te Rama Shellbeds in Wanganui basin, i.e. all shallow-water shell concentrations of Mangapanian age. A few poor specimens are known also from the limestone in Buxton Creek, near Cheviot, North Canterbury (033/f47) mentioned below under Sectipecten allani. A very few specimens of most basal Nukumaruan age are identified as P. thomsoni rather than P. triphooki: several from Seconds Ridge Conglomerate on the crest of the northern Ruahine Range; one specimen from Sentry Box Limestone in the Ohara Depression (abraded and incomplete; remanié from underlying Kaumatua Formation?); one poor but very large specimen from Otope Limestone at Totara Road, Kumeroa. AGE: Virtually limited to Mangapanian; very rare in a few basal Nukumaruan formations.» BEU, A. G. 1995. Pliocene Limestones and their scallops. Lithostratigraphy, pectinid biostratigraphy, and paleogeography of eastern North Island late Neogene limestone. Institute of Geological & Nuclear Sciences Monograph 10. Institute of Geological and Nuclear Sciences, Ltd., Lower Hutt, New Zealand, 243 p. [p. 40, 42]
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Phialopecten thomsoni (Marwick); A. G. Beu, 1995, Pliocene Limestones and their scallops, figures 17a-c (above); figure 18b (below).
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«The subspecies is much more variable than the earlier subspecies. Shells such as the holotype of Phialopecten thomsoni Marwick and a large right valve from N124/f515, GS 5422, Gorge Stream, Patoka (Fig. 26 ) are larger, broader, and have fewer and more irregularribs than any other specimens of Phialopecten, and were regarded by Marwick (1965) as a species distinct from C. (P.) triphooki. Some of the variation described above is shown by shells from the Te Aute Limestone (sensu stricto) of the Raukawa Range that are of the same age as the holotype of P. thomsoni and were collected along the strike from it, in the same rock body. All the specimens from a Phialopecten shellbed (N141/ f1616, GS 11 285, Argyll Road, Otane) are right valves with broad but extremely low, flat ribs (Fig. 24) . Specimens from Amner's lime quarry at Pakipaki (N134/f502 and 502A, GS 4965 and 5424) have moderately broad, well raised ribs that are more coarsely dissected into riblets than on most other specimens seen (Fig. 22). Other Mangapanian collections from Wanganui basin (N138/f446, GS 4208, Wilkies Shellbed, Kauarapaoa Valley-Fig. 29; N124/f516, GS 4124, Wilkies Shellbed, mouth of Waitotara River-Fig. 28) and a Nukumaruan right valve from Napier (N134/f470, GS 576, Lower Scinde Island Limestone; Fig. 23) are very similar, having large, square-section ribs, only slightly less prominent than those of typical P. thomsoni.
Specimens from the Lower Scinde Island Limestone at Napier (topotypes of Pecten triphooki Zittel) are relatively small (83-119 mm high in 12 specimens; mean 106.4 mm) with narrower and slightly more numerous radial ribs than those of Mangapanian specimens (18-27 in 12 specimens; mean 22.1), but the range and average number of ribs are raised by a single aberrant left valve from Napier with 27 narrow, well-raised, closely spaced radial ribs. Other Napier specimens have fewer, broader ribs than this extreme variant, and most of them closely resemble Mangapanian specimens from Amner's quarry, Pakipaki. The left valve with 27 ribs was one of few well-preserved left valves from Napier available to Marwick (figured by Marwick 1965, pI. 2, fig. 1) and influenced his opinion that the coarsely ribbed Mangapanian form was a distinct species (J. Marwick, pers. comm.). In a single incomplete paired specimen from Napier (N124/f468, GS 194, "from the lower limestone on the seaward side of Scinde Island, Hawkes Bay County", A. McKay's MS notes) the right valve has 21 primary ribs that widen markedly near the ventral margin to bear four strong riblets on their outer surfaces (Fig. 25) and the left valve has a similar number of high, narrow, widely spaced, finely subdivided ribs, so that the specimen closely resembles Mangapanian shells of the thomsoni form from Waipukurau and from Wilkies Shellbed in Wanganui basin. In the scatter diagram comparing height with numbers of radial ribs (Fig. 1), Nukumaruan shells are shown to be almost entirely enveloped by the range of variation of Mangapanian ones, the one Nukumaruan specimen lying outside the Mangapanian field being the left valve with 27 ribs, described above. The number of ribs on Nukumaruan shells falls almost entirely within the range of variation of Mangapanian ones, and it is concluded that Nukumaruan shells are merely immature specimens of the same taxon as occurs in Mangapanian rocks. The variation encompasses both topotypes of Pecten triphooki and the holotype of Phialopecten thomsoni, and so the name Phialopecten thomsoni Marwick, 1965, is here synonymised with Chlamys (Phialopecten) triphooki triphooki (Zittel, 1864).» BEU, A. G. 1978. Taxonomy and biostratigraphy of large New Zealand Pliocene Pectinidae (Phialopecten and Mesopeplum). New Zealand Journal of Geology & Geophysics, 21: 243-269, figs. 1-30. [p. 254, 257]
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Chlamys (Phialopecten) triphooki triphooki (Zittel) forma thomsoni Marwick; A. G. Beu, 1978, Taxonomy and biostratigraphy of large New Zealand Pliocene Pectinidae, figures 28, 29.
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