Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 36i): GS14088, U20/fll6, Taruarau River, Ngamatea plateau, northern Ruahine Range, Opoitian (GNS)
Beu & Maxwell (1990): Chapter 14; p. 293; pl. 36 i.
Synonymy: Struthiolaria obesa Hutton 1885b, p. 329
Description: Small for subgenus (42-58 mm high), with spire equal to or shorter than height of aperture, simple suture, strongly bisinuate, varicate outer lip meeting heavily, widely callused inner lip at relatively deep anterior notch, and very large, strongly inflated last whorl. Whorls roundly angled at a shoulder that is medial on spire whorls; shoulder bears low, pointed nodules on early whorls of most specimens; below shoulder, whorls are only weakly convex above a gently rounded, obscure peribasal angulation; sutural ramp straight, base weakly concave. Spiral sculpture of many very fine, closely spaced threads, obscure on many specimens. Inner-lip callus low, wide and smooth, ascending at least to shoulder, and above suture on some specimens; anteriorly, heavily callused individuals have a nodule to left of aperture, separated from last whorl by a deep groove. Protoconch not known, presumably similar to that of S. papulosa.
Comparison: Specimens from diverse shallow-water molluscan faunules of Opoitian age in northern New Zealand (Kaawa Creek, southwest Auckland; Oweka Creek, between Cape Runaway and Hick's Bay; and in the extensive Opoitian outcrops in the Tauwhareparae area, inland from Tolaga Bay) consistently differ from southern specimens of Struthiolaria obesa by bearing prominent, pointed nodules around the shoulder angle, and are the form named S. arthritica by Bartrum & Powell (1928). As the nodules are the sole differentiating character, and as the character seems to have a clear geographical separation, it is possible that S. arthritica was a geographical subspecies of S. obesa. The apparently ancestral species S. callosa (Pl. 29d, e) differs from S. obesa in its larger size, taller spire, much heavier callus, and in consistently bearing peripheral nodules. Intermediate forms occur in middle-late Tongaporutuan rocks at a few localities (Tirangi Stream, North Taranaki; Mt Bruce, Wairarapa). The extinction of the subgenus at the end of Opoitian time is a particularly useful biostratigraphical index.
Distribution: (Upper Tongaporutuan?) Kapitean-Opoitian; "Shepherd's Hut, Waipara" (type), presumably in Middle Waipara River region, North Canterbury (probably of Opoitian age) but "the type locality of this species and of Xymene monilifera (Hutton), 'Shepherd's Hut, Waipara', has never been satisfactorily located" (Fleming in Wilson 1963, p. 62). Fleming thought the type specimens of S. obesa resemble in preservation the specimens in GS3865, Omihi Creek, a northern tributary of Waipara River west of Weka Pass, and suggested "possibly the shepherd's hut referred to was on Glenmark Station". S. obesa occurs widely in New Zealand in shallow-water, near-shore, soft-bottom facies of Kapitean and Opoitian age, in the northern Rangitikei River tributaries (eastern Wanganui basin) where shallow-water Neogene rocks lap onto the Ruahine-Kaimanawa ranges, in the Glenross area of the northeastern Ruahine range (near the Napier-Taihape Road), in the upper Wanganui River sequence near Retaruke, near Tarata in inland Taranaki, at several localities in Dannevirke district, southern Hawke's Bay, and in the Manawatu Gorge saddle sequence, very commonly in the Awatere Valley (Upton Brook and Stace Stream sections), and sporadically but widely in North Canterbury (Weka Pass-Omihi area, and in the Kowai Syncline). A particularly interesting record is the specimens found on beaches between Cape Wanbrow and Gee's Point, south of Oamaru, presumably washing ashore from submarine Opoitian exposures (the late J. Graham, Oamaru, pers. comm.), possibly the source of Finlay's "Awamoa" specimen cited by Marwick (1924b, p. 184). These and other Pliocene molluscs typically have a distinctive pale orange colour that distinguishes them from the Altonian fossils that are also cast up on these beaches.
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)