Sectipecten grangei Boreham, 1961
BOREHAM, A. U. E. 1961. The New Zealand Tertiary Genus
Sectipecten Marwick (Mollusca). Transactions of the Royal Society of New
Zealand, 88 (4): 655-668, pls. 45-49. [p. 660, pl. 46, figs. 4-5; pl. 47, figs. 8-10]
1961 Sectipecten grangei Boreham, 1961
A. U. E. Boreham, 1961, plate 46.
A. U. E. Boreham, 1961, plate 47.
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«1927. Pecten n.sp. aff. sectus Hutton; Marwick in Morgan & Gibson, N.Z. geol. Surv. Bull. 29: 41.
The species is named after Dr. L. I. Grange, Director of the Geological Survey, 1952-1956, who collected the holotype in 1925. Holotype (TM 2755), a right valve. Figured paratype (TM 2756), a left valve. Figured specimens from Mount Bruce, Tararua (GS 3110), a right valve (TM 2757), a right valve (TM 2758), a left valve (TM 2759). TYPE LOCALITY. Public Works Department Quarry, Te Wera, Ngatimaru Survey District, central Taranaki (GS 1224, 7320). Age: Upper Tongaporutuan (Upper Miocene). DISTRIBUTION. Sectipecten grangei is found in the Upper Tongaporutuan at several localities in the North and South Islands, including central and north Taranaki, Mount Bruce, Tararua, and Callaghan's Creek, Westland. DESCRIPTION (based on the holotype, paratype, and 20 topotypes from Te Wera Quarry, GS 1224 and 7320, all single valves). Shell small to moderate in size, equilateral, both valves slightly inflated. Anterior and posterior ears unequal, with deep byssal notch, sculptured by about 5 fine riblets, stronger on anterior than on posterior ears. On both valves, the whole disc except rib crests is crowded with fine sharp close concentric lamellae. Sculpture of right and left valves discrepant: right valve ribs flat or concave topped, slightly wider than their interspaces. Right valve ribs divided by the development of grooves on their crests and only a few intercalary riblets are produced; left valve ribs increase in number by intercalation and only 1 or 2 of the thickest primary ribs show any tendency to groove. The new species is a difficult one to characterise concisely because it is so highly variable in rib number and arrangement. Twelve right valves have 8 to 11 major ribs (mode 10), more or less distinctly grooved from the apex. There is a range of variation from specimens such as the holotype (PI. 46, f. 4), on which the major ribs are only superficially grooved and remain unmistakeably grouped to the ventral margin, to others on which grooving is deep from the start and the riblets soon separate, so that the initial grouping becomes virtually unrecognisable. On specimens of the latter variety each riblet could equally well be interpreted as a primary rib, but it is not strictly analogous to what has been counted as a primary rib on other specimens such as the type. Left valves have about 10 to 16 ribs at the apex, increasing to more or less double that number by intercalcation. On some specimens, for example, the paratype (PI. 46, f. 5), ribs are very unequal, and thick ribs alternate irregularly with finer ones, the latter appearing as intercalaries at different growth stages. On other specimens the ribs are less uneven in width and nearly all could be counted as primaries, although the finer ones are mere threads near the apex. These ambiguities of interpretation on both valves preclude a consistent and accurate mode of rib counting. DIMENSIONS. Holotype: height, 60 mm; length, 62 mm; inflation, 11 mm. Largest topotype: height, 68 mm (est.). The largest specimen of S. grangei seen is from Callaghan's Creek (GS 4984), height, 98 mm. The topotype population consists of unusually small shells, possibly a phenotypic character, as other specimens of S. grangei are larger and more comparable in size to preceding and following species of Sectipecten. DIAGNOSIS. Differs from S. wollastoni in having a far more variable, and less distinctive and regular sculpture. Right valve ribs of S. grangei are narrower than those of S. wollastoni, and more or less markedly grouped in two or threes. Ribs may be grooved once or twice but never show the multiple grooving of S. wollastoni. Left valve primary ribs of S. grangei are seldom grooved at all and never develop deep grooves at an early stage as on S. wollastoni. Differs also from S. wollastoni in having a higher average number of primary ribs (i.e., those initiated at the apex); S. grangei has 10 to about 24, s. wollastoni 7 to 10. Differs from S. diffluxus in having a far more variable and less simple sculpture, and more markedly discrepant ribs on opposite valves: right valve ribs of S. diffluxus are hardly grouped or grooved, and left valve ribs of S. diffluxus are subequal in strength and little differentiated into primary and secondary intercalaries. Differs also from S. diffluxus in having a lower average number of primary ribs: S. grangei has 10 to about 24, S. diffluxus 24-30. DISCUSSION. The topotype population comprises a series of forms ranging from those with a few (about 10), broad, composite ribs divided by superficial grooves on the right valve and a corresponding number of strong primary ribs alternating with finer intercalaries on the left valve, to those with up to about 24 riblets indistinctly grouped in nvo and threes on the right valve and a corresponding number of not greatly unequal riblets on the left valve. The former type of shell (having few, composite ribs) may be interpreted as advanced forms because morphologically they are similar to the following species of Sectipecten — namely, S. wollastoni, although nowhere is there a complete series of transitional forms between even the most advanced S. grangei and earliest S. wollastoni. A few S. grangei have been collected from the Toe Toe Conglomerate at Ratapiko (GS 1217, 7314), stratigraphically above the Autawa Limestone at Te Wera, but these shells are less like S. wollastoni than are the more advanced Te Wera shells. The less advanced forms of S. grangei at Te Wera (having several, more simple ribs) are similar to an earlier population of S. grangei at Pohokura Tunnel (GS 1137), from an unnamed horizon stratigraphically below the Autawa Limestone. Six specimens at Pohokura display 20-22 ribs, average 21, which on the right valve tend to be loosely grouped in pairs or threes, and on the left valve are subequal in strength. The more advanced type of sculpture found among Te Wera shells is completely unrepresented. In rib number and arrangement, and in stratigraphic position, the Pohokura shells are intermediate between the Waiauan S. diffluxus and the uppermost Tongaporutuan S. grangei, and there may well be place here for a further systematic division. However, at present there is an insufficient number of exactly stratigraphically located specimens on which to base such a division: shells outside the immediate central Taranaki area cannot be placed in sequence within the Upper Tongaporutuan, and Sectipecten was so highly variable and rapidly evolving during this period that specimens need to be located in finer than Stage or Substage divisions if the detailed evolutionary sequence is to be described. Three specimens from Mount Bruce, Tararua (GS 3110, mid to upper Tongaporutuan) are illustrated (PI. 47, figs. 8-10) for comparison with the Te Wera shells, to show the variation within S. grangei. All are large shells, measuring 86 to 95 mm in height. The two right valves exhibit a polymorphism similar to that at Te Wera: one valve (Fig. 8) has several subequal primary ribs which are separate from the apex and only indistinctly grouped, whereas the other (Fig. 10) has fewer, composite ribs.» ANNE URSULA ENID BOREHAM, 1961
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«Most characters as in S. wollastoni (above); differs from S. wollastoni in being a little smaller, in being slightly more obliquely elongate posteroventrally, in being a little less inflated, in having a slightly deeper byssal sinus and, principally, in having much more complexly subdivided sculpture, lacking any undivided, smooth-topped major costae.
Tongaporutuan; Te Wera Quarry, central Taranaki, late Tongaporutuan(type; Boreham 1961, p. 661); widespread in facies where fine sediment was being bypassed during deposition (greensand, conglomerate, a few limestone beds), mostly in very near-shore facies, in North Taranaki, the Ngamatea Plateau (Napier-Taihape Road), northern Ruahine Range near Kuripapango, northern Wairarapa near Mount Bruce, the Kaiwara River in North Canterbury, and in Callaghan's Greensand in Westland. Figured specimen (PI. 28c): GS9478, N33/f9804, Kaiwara River near "Athol Glen", North Canterbury, late Tongaporutuan (NZGS) x 0.67.» BEU, A. G. & P. A. MAXWELL. 1990. Cenozoic Mollusca of New Zealand. New Zealand Geological Survey Paleontological Bulletin, 58: 1-518, pls. 1- 57. [p. 256]
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Sectipecten grangei Boreham; A. G. Beu & P. A. Maxwell, 1990, Cenozoic Mollusca of New Zealand, plate 28, figure c.
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