Sectipecten wollastoni (Finlay, 1927)
FINLAY, H. J. 1927. New
specific names for Austral Mollusca. Transactions of the Royal Society of New
Zealand, 57: 488-533. [p. 526]
1873 Pecten secta Hutton, 1873
1927 Pecten wollastoni Finlay, 1927 [nomen novum pro Pecten secta Hutton, 1873]
1927 Pecten wollastoni Finlay, 1927 [nomen novum pro Pecten secta Hutton, 1873]
Sectipecten wollastoni (Finlay); A. U. E. Boreham, 1965, A revision of F. W. Hutton's pelecypod species described in the Catalogue of Tertiary Mollusca and Echinodermata, plate 7, figure 3, right valve of lectotype (TM 2761).
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«Pecten sectus Hutton, 1873:— Pecten wollastoni n. n.
(Cat. Tert. Moll., p. 30.) Not of Goldfuss, 1836 (Petref. Germ, vol. 2, lief. 5).» HAROLD JOHN FINLAY, 1927
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«Large for family (95-110 mm high), equidimensional to slightly longer than high; 2 valves almost equally inflated (right valve slightly more inflated than left in some specimens). Ears almost square; posterior edge of posterior ears slopes slightly backward, anterior edge of anterior ears weakly sinuous, with wide, shallow byssal sinus in right anterior ear. Right valve sculpture of 6 prominent, square-edged, flat-topped, radial costae, smooth or weakly grooved over umbonal 0.3-0.5 of disc but becoming progressively subdivided by widening grooves over outer 0.5-0.7 of disc, with lower, narrower costae on each end of disc and a prominent, narrow, median costa in each major interspace, each flanked over outer half of disc by narrow tertiary costellae. Left valve sculpture a mirror image of right, Le., prominent, narrow, deeply grooved major costae, with 2-4 narrow costellae strengthening out over disc in each wide interspace. Interior not seen; all specimens articulated.
Kapitean; Callaghan's Creek, a tributary of Kapitea Creek, Hokitika district, Westland (lectotype), and widespread from East Cape to southern Southland, in level-bottom environments in near-shore to mid-shelf depths. Most common in rock types with low deposition rates (particularly greensand) but, unlike other Sectipecten species, S. wollastoni occurs sparsely in normal shelf bluegrey mudstone, as well as in the shallower facies that other Sectipecten species seem to have preferred. Sectipecten wollastoni is the traditional Kapitean index fossil, but unfortunately its lineage evolved very gradually, and has proved difficult to use in biostratigraphy. The putatively ancestral S. diffluxus is a rare species occurring in only a few Waiauan shellbeds (mainly the Hinnites Shellbed at Weka Pass, North Canterbury); it has strongly unequal ears, a deep, perhaps functional byssal notch in the right anterior ear, five radial folds (like those of Mesopeplum (Borehamia) in the umbonal area of the disc, and about 30 similar, relatively narrow, raised, flat-topped radial costae with finely serrated edges; it seems likely to have evolved from M. (Borehamia). S. grangei (PI. 28c) is an uncommon Tongaporutuan species occurring in shallow-water, nearshore greensand, sandstone, limestone and shellbeds, but never in the widespread early Tongaporutuan "Hurupi facies" with diverse shelf molluscs. S. grangei has much more subdivided sculpture than S. wollastoni, early to late Tongaporutuan shells having no smooth-topped, undivided area of main costae. Through the Kapitean, the area of the disc of the right valve over which major costae remain undivided increased progressively over the disc (but quite irregularly in different proportions of different populations), culminating in the early Opoitian type population of S. allani (PI. 33c; Momoe-a-Toa Shellbed, Chatham Island), in which about 30070 of the population has completely undivided, smooth-topped costae over the whole disc of the right valve. However, the population is exceedingly variable (see under S. allani, PI. 33 c). The lectotype of Pecten sectus Hutton (= S. wollastoni), from Callaghan's Creek, a tributary of Kapitea Creek, has the major costae on the right valve the S. wollastoni bed in greensand at the base of the sequence in Kapitea Creek itself, suggesting that the lectotype may have come from a little lower in the sequence. The sequence of Sectipecten species is therefore:
Figured specimen (PI. 28f): Lectotype of Pecten sectus Hutton, 1873 (not of Goldfuss) = Pecten wollastoni Finlay, 1927, GSI03, K36/f6009, greensand, Callaghan's Creek, tributary of Kapitea Creek, Hokitika District, Westland, Kapitean ? (specimen is presumably from high in Callaghan's Greensand, and could be either latest Tongaporutuan or early Kapitean) (TM2761, 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. 255, 256]
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Sectipecten wollastoni (Finlay); A. G. Beu & P. A. Maxwell, 1990, Cenozoic Mollusca of New Zealand, plate 28, figure f.
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«Lectotype (TM 2761), a well preserved complete individual with closed valves.
TYPE LOCALITY. Callaghan's Creek, Waimea, Westland. Age: Kapitean (Upper Miocene) . DISTRIBUTION. Sectipecten wollastoni is widespread in the Kapitean Stage, to which it is confined. REDESCRIPTION (based on about 70 complete and fragmentary specimens, the majority from Taranaki and Westland, a few from Marlborough, Hawkes Bay, and East Cape Peninsula). Shell large, subequilateral, both valves inflated, right sometimes slightly more than left. Ears large, subequal, byssal sinus completely obsolete in adult. Sculpture of right and left valves discrepant: primary ribs of right valve are broad, flat-topped folds, squarecut in profile, relatively wider than their interspaces; primary ribs of left valve are high-rounded and narrower than their interspaces. Right valve folds entire in early growth stages, later divided repeatedly by deep grooves, a maximum of 6 to 8 narrow sharp riblets being produced per major rib. The first groove, or pair of grooves, per rib appears at about 25 mm to 40 mm from the apex, and further pairs of grooves appear at successive growth stages. Left valve primary ribs split by a deep median groove, which develops slightly earlier than grooves on right valve ribs; a pair of shallow grooves may develop on either side of the median groove at a later stage. On both valves the secondary ribs formed by division of the primaries remain grouped and raised above the interspaces. One to three riblets develop in each major interspace on right valve, and 2 to 5 riblets in the comparatively broader interspaces on left valve. On anyone individual tihe sculpture of right and left valve is almost the exact antithesis, so that the broad folds with distinctive pattern of dividing grooves on right valve are matched by the wide interspaces with corresponding arrangement of intercalary riblets on left valve. Fine, sharp, crowded, concentric ridges cover the disc except for the broad rib crests of right valve where they have probably been worn off by friction with the substratum. Ears sculptured by about 5 fine riblets, strongest on anterior ear of right valve, practically obsolete on posterior ears.
DIMENSIONS. Lectotype: height, 103 mm; length, 111 mm; inflation (two valves), 39 mm. The lectotype is the largest complete specimen collected. A shell from Taranaki (GS 7203) is estimated to have had a height of 115 mm.
DISCUSSION. The number of primary ribs (i.e., those initiated at the umbo) is the same on both valves of a single individual, but there is a small variation among individuals from anyone area and between groups from different provinces. Thus 20 specimens from WestIand have an observed range of 7 to 10 ribs, mean 7.7, while 34 specimens from Taranaki have a range of 8 to 10 ribs, mean 9.2. Such slight differences are to be expected between separate populations within a species. Five specimens from East Cape, 3 from Takapau and 2 from Marlborough have means of 9.0, 9.7 and 9.5 respectively and are more like the Taranaki than the WestIand population. A further difference between populations is observed in the time of onset of grooving on right valve folds: On Westland specimens the first groove appears at about 25 to 35 mm from the umbo, whereas on Taranaki specimens grooves appear at about 30 to 40 mm. On well preserved shells (for example the lectotype) it is often possible to see faint incipient grooving before a definite groove appears, matched on the opposite valve by fine intercalary riblets. Such partial suppression of grooves is seen to a greater extent on some specimens of the Chatham Island S. allani (for example, the holotype) where distinct grooves do not appear at any stage on the right valve but are represented on the left valve by quite strong intercalary riblets.
The species is distinctive and remarkably constant. There are slight variations in the number of major ribs and detailed pattern of grooving and secondary rib development, as mentioned above, but the species is nearly always unmistakable when it occurs. One specimen is notable, however, from Kapitea Creek, Westland (GS 2888). It is a fairly small shell (68 mm in height) with 7 primary ribs which begin to geminate by formation of a groove at about 20 mm from the apex on both valves. No further division occurs. Ribs of the right valve are only slightly wider than their interspaces. Compared to typical S. wollastoni, the sculpture is less elaborate, for at 68 mm from the apex on right valves of S. wollastoni the major ribs have produced anything up to 5 grooves per rib. On the Kapitea Creek shell the right valve ribs are narrower relative to their interspaces, and begin to groove at an earlier stage than on typical S. wollastoni. The specimen appears to be a primitive form of S. wollastoni, and it may be significant that it is found at the base of the Kapitean section in Kapitea Creek, whereas typical S. wollastani occur higher up the section (GS 2630, 2631).» 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. 662-664]
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Sectipecten wollastoni (H. J. Finlay, 1926); A. U. E. Boreham, 1961, The New Zealand Tertiary Genus Sectipecten Marwick, plate 48, figures 11, 12.
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