Phialopecten marwicki (Beu, 1970)
BEU, A. G. 1970. Descriptions of new species and notes on taxonomy of New Zealand Mollusca. Transactions of the Royal Society of New Zealand, Earth Sciences, 7: 113-136 [p. 115, pl. 1, fig. 1, 6]
1970 Chlamys (Phialopecten) triphooki marwicki Beu, 1970
«Distinction between Phialopecten marwicki and P. thomsoni: Beu (1978) pointed out the most easily recognised and most abrupt change in Phialopecten phylogeny: large late Waipipian P. marwicki (which, as in Mangapanian P. thomsoni, reach a maximum height of 175 mm) all have about 28-32 (frequency maximum at 29-30) primary radial costae, whereas specimens of P. thomsoni (identified by Beu (1928) as P. triphooki all have about 18-22 costae (frequency maximum at 21). In other words, the mean rib number decreased abruptly to two-thirds of its previous value at the stage boundary.
As described above, the single known intermediate population is in the Mangapani Shellbed in Waitoara Valley, and represents a geologically instantaneous population precisely located at the stage boundary. The other major distinction between P. marwicki and P. thomsoni is in subdivision of the costal crests, and in the relative width of costae on the two valves. Specimens in Mangapani Shellbed have relatively narrow costae, and in all smaller specimens (up to about 120-130 mm high) most costal crests on most specimens are subdivided into the typical P. marwicki fasciculate profile by two grooves. At a variable distance from the umbo, and to some extent it appears to be earlier on the right valve than the left, subdivision becomes more complex through the development of further shallow grooves. Left valve costae remain narrow, whereas right valve costae become wider distally than higher up on the same specimen and, near the outer margins of the few large, broken right valves, have wide, relatively low costal crests subdivided into 4-6 riblets of about equal prominence. All later Mangapanian populations have costae as on the ventral margin of large Mangapani Shellbed specimens, from umbo to ventral margin: relatively few (about 18-22) costae, high, narrow, widely spaced and finely grooved on the left valve but relatively low, wide, closely spaced, and prominently subdivided into 4-6 equal riblets on rhe right valve. Specimens of P. thomsoni from eastern North Island (nearly all from Te Waka or Te Onepu Limestones) also differ from P. marwicki in having very wide, disproportionally large auricles, cut in to the base to produce a triangular shape, but this is nowhere near so marked in other P. thomsoni populations and the comparison of auricle shape (above) suggests that Te Onepu Limestone specimens have unusually large auricles for largely (entirely?) ecological reasons. DISTRIBUTION: Phialopecten marwicki ranges widely throughout New Zealand in shallow-water facies of Opoitian and Waipipian age, from Auckland (Oahuhu brewery well) to Marlborough (Starborough Formation) and North Canterbury (Greenwood Formation, Lower Waipara Gorge). It is particularly abundant in shellbeds in Taranaki and in Wanganui basin, and in Te Aute lithofacies from East Cape to South Wairarapa. A single specimen (Fig. 14c) of great interest has recently been collected by M.K. Eagle from the Kaawa Shellbed in a fallen block on the coast southof Kaawa Creek, south of Port Waikato, southwest Auckland (R13/f263, in colln M.K. Eagle). This specimen is 71.7 mm high and has 40 narrow but strongy raised radial costae, low commarginal lamellae in all radial interspaces, and many costae fasciculate in section (some have only one groove on costal crests). The age of the Kaawa Formation is well known from a variety of lines of evidence (e.g. presence of Struthiolaria (Callusaria), not known post-Opoitian) to be Opoirian in age, and the Phialopecten specimen indicates the geographically isolated evolution of Phialopecten that was continuing elsewhere in New Zealand while only Towaipectenn ongleyi occupied eastem North Island. AGE: Opoitian and Waipipian; stage boundary adopted at a maximum height of 115 mm in large P. marwicki populations, although few specimens of Opoitian age exceed 100 mm in height.» 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. 38, 40]
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Phialopecten marwicki (Beu), Opoitian specimens; A. G. Beu, 1995, Pliocene Limestones and their scallops, figures 14b-d (above); figures 15a, b, e (below).
Phialopecten marwicki (Beu), Opoitian specimens; A. G. Beu, 1995, Pliocene Limestones and their scallops, figures 16a, d (left and right Waipipian especimen), c, e (holotype).
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«The subspecies Chlamys (Phialopecten) triphooki marwicki attains a much larger size (height 51-175 mm in 40 specimens; mean 109 08mm) than does C. (P.) triphooki ongleyi. It has only slightly fewer major radial ribs (25-35 ribs on 40 specimens; mean 28'9) than does C. (P.) triphooki ongleyi. The primary radial ribs are higher than in C. (P.) triphooki ongleyi although lower than in C. (P.) triphooki triphooki, and are relatively narrow, finely subdivided, and widely spaced, with a single low, narrow secondary riblet in the centre of every interspace on almost all specimens. Most characteristically, specimens from Hawera, the Waipipi Shellbeds, and most Te Aute Limestone localities have primary ribs of the right valve initially high, narrow, square-sided and undivided, dividing into two narrow, well-raised riblets on the rib surface after about one third to half the height of the disc. Primary ribs of the left valve are similar but divide into three narrow, wellraised riblets on the rib surface. The valves are unequal, the left valve being more strongly convex than the right, and the ears relatively larger and much more nearly equal than those of C. (P.) triphooki ongleyi, Radial folds have not been observed on this or later forms.
Even small fragments of left valves that show the rib surfaces clearly are immediately identifiable as Chlamys (Phialopecten) triphooki marwicki because of their trifid ribs. TYPE LOCALITY: N129/f501, GS 5240, Waihi Beach,vHawera. Holotype (TM 2714) in New ZealandvGeological Survey. Known also from many other localities in the coastal section between Hawera and the Waitotara River; from a few localities further inland; from limestone at many localities in Wairarapa District; from Lower Waipara Gorge, North Canterbury; from sandstone at a few localities in Hawkes Bay (e.g., GS 1541, Te Reinga Falls, Wairoa); and from Te Aute limestone facies at 64 localities from Te Reinga and Nuhaka, northern Hawkes Bay, to Eketahuna. TIME RANGE: All known specimens are of Waipipian age (late Pliocene).» BEU, A. G. 1978. Taxonomy and biostratigraphy of large New Zealand Pliocene Pectinidae (Phialopecten and Mesopeplum). New Zealand Journal of Geology and Geophysics, 21: 243-269, figs. 1-30. [p. 252]
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Chlamys (Phialopecten) triphooki marwicki
Beu, 1970; A. G. Beu, 1978, Taxonomy and biostratigraphy of large New Zealand Pliocene Pectinidae (Phialopecten and Mesopeplum), figs. 8, 13, 15. |