Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 28f): Lectotype of Pecten sectus Hutton, 1873 (not of Goldfuss) = Pecten wollastoni Finlay, 1927, GS103, 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, GNS)
Beu & Maxwell (1990): Chapter 13; p. 255; pl. 28 f.
Synonymy: Pecten sectus Hutton 1873b, p. 30 (not of Goldfuss, 1836); Pecten wollastoni Finlay 1927b, p. 526 (new name for P. sectus Hutton, preoccupied)
Type species of Sectipecten Marwick, 1928
Description: 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, i.e., 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.
Comparison: 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 species 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 of adults, 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 (Pl. 28c) is an uncommon Tongaporutuan form occurring in shallow-water, near-shore 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 the 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 (Pl. 33c; Momoe-a-Toa Shellbed, Chatham Island), in which about 30% 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, Pl. 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 a little more deeply subdivided than most specimens in 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 forms is therefore:
1. The Waiauan S. diffluxus has about 30 narrow, square-edged costae with finely serrated edges; but almost unknown away from the Hinnites Shellbed, Weka Pass.
2. Early-late Tongaporutuan S. grangei is easily recognised, as no area of undivided costae occurs on either valve.
3. Early Kapitean (Globorotalia conomiozea zone) specimens of S. wollastoni occur in highly variable populations in which at least some right valves have smooth, flat-topped costae over the proximal third to half of the disc. No specimens are known from the G. sphericomiozea zone (late Kapitean).
4. Pliocene (Opoitian-Mangapanian) specimens are recognisable as S. allani by at least some members of the population having undivided costae on the right valve (but most material, other than at Momoe-a-Toa, consists of poorly diagnostic left valves resembling S. grangei). A deep, prominent, second subdivision of the right valve costae began in the Waipipian, but the genus became extinct during Mangapanian or early Nukumaruan time.
These forms all intergrade through time, so this is a classic example of an anagenetic series, and it is doubtful whether Tongaporutuan-Nukumaruan specimens can be segregated into the traditional "species". However, the particularly square, serrate edges of the costae in S. diffluxus show that this is a distinct species, so the lineage evidently commenced with at least one abrupt speciation event (punctuated equilibrium) and subsequently became anagenetic.
Distribution: 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 blue-grey mudstone, as well as in the shallower facies that other Sectipecten species seem to have preferred. After the beginning of Pliocene time, it seems to have become more and more restricted to near-shore facies.
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
(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)