Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
| | Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990) |
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  (Pl. 45j): GS4013, R22/f6353, Tainui Shellbed, Castlecliff, Wanganui, Castlecliffian (TM2015, GNS) |
Beu & Maxwell (1990): Chapter 16; p. 335; pl. 45 j.
Synonymy: Notovola tainui Finlay 1930b, p. 51; Pecten tainui aotea Fleming 1953, p. 344; Pecten novaezelandiae tainui; Pecten novaezelandiae aotea; Pecten tainui, Beu & Maxwell 1990, p. 335, pl. 45j.
Classification: Pectinidae
Description: Moderately large for genus (to about 110 mm high and 130 mm long), i.e., slightly longer than high when adult; left valve flat to weakly concave, concavity most marked near umbo; right valve moderately and evenly convex. Ears almost equal. Right valve with 20 to 22 extremely prominent, flat-topped, well raised radial costae with squarely raised or even slightly undercut edges, separated by deep, flat-bottomed interspaces, each about 0.2 width of 1 costa; interspaces bear many fine commarginal lamellae on most specimens. Left valve with about 20 high radial costae of almost square section, or in some specimens with slightly undercut edges, varying from 0.7 interspace width to equal in width to 1 interspace.
Comparison: Pecten novaezelandiae is recorded earliest in New Zealand at two localities on the east coast: a fragment from Cape Kidnappers (inland section exposed in the Maraetotara River) is from a short distance below the Kidnappers Tuff, i.e., Potaka Tephra (1.0 Ma; Shane 1994). Several more complete specimens were collected from Castlecliffian fossiliferous sandy mudstone (Leader siltstone lithofacies; Warren 1995) in the Leader River on "Mendip Hills" station, Parnassus district, North Canterbury. Potaka Tephra occurs in the overlying Albyn conglomerate lithofacies, several sedimentary cycles above Leader siltstone lithofacies (Warren 1995, figs. 19, 21). Both these localities, therefore, are more than 1 m yrs old.
Pecten lives today on intertidal sand flats in large bays, and on the shelf down to about 30 m (rarely to about 120 m), lying on the convex (right) valve with the flat valve at the sediment-water interface, and swims when necessary to avoid predators (mainly starfish); so Pecten occurs fossil in shallow-water beds only. The absence of Pecten from beds older than Upper Westmere Siltstone (oxygen isotope stage 19) in Wanganui basin therefore conceivably results from their deposition in water too deep to be inhabited by Pecten. However, the lack of even a single valve from older Castlecliffian formations after more than 100 years of collecting indicates that Pecten arrived in eastern New Zealand well before it arrived in Wanganui basin. In Wanganui basin, the numerous forms described by Fleming (1957) were shown by Beu (2006) to be ecophenotypes of one species. However, the Pecten zones described by Fleming (1957) still offer some help with correlation within the basin. Obviously, the appearance of Pecten in oxygen isotope stage 19 is particularly useful for identifying the Brunhes-Matuyama magnetic reversal across the basin, as the reversal took place during glacial stage 20, which (like almost all other glacial periods) is represented by an unconformity, in this case beneath Upper Westmere Siltstone (oxygen istope stage 19). Secondly, the "tainui form" (illustrated), with flat rib crests and square-edged or even undercut rib margins, is virtually limited to Tainui Shellbed (oxygen isotope stage 13), and greatly helps its recognition, as well as of laterally equivalent units, across the basin. In a few places, Landguard Sand (oxygen isotope stage 9) contains a similar form, as despite the form at Landguard Bluff having only low ribs, extreme undercut ribs are present on some specimens from further east, in the Whangaehu Valley. Almost all other formations with Pecten contain only the evenly convex-ribbed "marwicki form" of Pecten novaezelandiae. However, recognition that these are ecological forms of a single species is helped by the occurrence of intermediate specimens, "Pecten jacobaeus toi" of Fleming (1957), in the upper part of Pinnacle Sand (oxygen isotope stage 14, i.e., a glacial period) in "the pinnacles" gully at Castlecliff.
Specimens resembling the northern Recent form of Pecten novaezelandiae, with low but convex-crested ribs, occur fossil at Ohope Beach and, in particular, Ohinekoau Stream, west of Matata, in the Bay of Plenty (Castlecliffian, about oxygen isotope stage 15), so this form certainly existed long before fossils of this type occurred at Wanganui. The abundant specimens at Te Piki, near East Cape (oxygen isotope stage 7) have flat rib crests and quite sharp rib margins, and are similar to Landguard Sand specimens at Wanganui, despite their much younger age. Taken altogether, the history of Pecten in New Zealand is very complex, but there is little doubt that it was controlled by ecological chages during the rapidly oscillating glacial-interglacial cycles of the last 1 million years.
Distribution: Late Castlecliffian-Haweran; Tainui Shellbed, Castlecliff, Wanganui, late Castlecliffian (type of Notovola tainui); Landguard Formation, Landguard Bluff, east of Wanganui River, early Haweran (type of Pecten tainui aotea). Pecten novaezelandiae is common in some beds at Wanganui, from oxygen isotope stage 19 onwards (Upper Westmere Siltstone), and also occurs fossil in all other shallow-water rocks of suitable facies of late Castlecliffian to Haweran age throughout New Zealand: Te Piki and Ohope Beach area in the Bay of Plenty, Cape Kidnappers (rare), and Parnassus district, North Canterbury; and in many Haweran terrace deposits.
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
ISBN
978-0-478-19705-1
ISSN 1177-2441
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