Pecten benedictus tepungai Fleming, 1957
FLEMING, C. A. 1957. The genus Pecten in New Zealand. New Zealand Geological Survey Paleontological Bulletin, 26: 1-69, pls. 1-15. [p. 33, pl. 4, figs. 1-4]
«Pecten benedictus tepungai Fleming, n. subsp. Pl. 4, Figs. 1 to 4.
1951. Notovola marwicki Finlay: Te Punga, N.Z. Geol. Surv. Mem. 8: 2 (in part). 1952. Notovola marwicki Finlay: Te Punga, N.Z.J. Sci. Tech. B34 (3): 158 (in part; not of Finlay). HOLOTYPE: New Zealand Geological Survey Collection (GS 3082.4), right valve. Length, 61; height, 57; inflation 21 mm. DIAGNOSIS: A population of high, inflated, benedictus with weak primary ribs but with strong persistent secondary radial sculpture and concentric lamellae. MATERIAL: Of 20 valves, mostly imperfect, from the type locality, 7 fragmentary and worn specimens of P. b. marwicki are stained, rolled or bored by sponges and are probably derived from an underlying bed. The remainder (with one exception) are also stained bluish-grey, but some are less severely worn, and they are distinguished as a group by their high inflated shells, weak primary ribbing, and conspicuous secondary radial sculpture, crossed by concentric lamellae. The exception is a young shell with both valves preserved, which shows sufficient secondary sculpture to suggest that the ornamented population existed during the deposition of the Rangitawa shellbeds. It is possible, however, that the population was dimorphic, and included shells indistinguishable from marwicki as well as those with the diagnostic characters of tepungai. In that case the sample suggests that about 75 per cent of the population can be distinguished from the majority (if not all) of the population of the underlying Upper Marwicki Zone. The following description is based on the fourteen ornamented specimens. DESCRIPTION: Shell reaching a large size (c. 140 mm. judged from incomplete valves); high and inflated (see Table 1); beaks rather higher and stronger than in marwicki. Ribs generally 16 (one more than the average in marwicki), very low and gently rounded in most specimens, somewhat higher and angled in a few (14 per cent). Secondary radial sculpture either of 4 to 6 spaced raised narrow threads on each rib and 2 to 3 in the interspaces between ribs, or of linear grooves separating broad flat-topped interspaces, representing broader threads. Concentric lamellae generally well-developed, spaced, and continuous across summits of ribs in young unworn specimens, intersecting the radial threads to form a distinctive reticulate surface. Very young shells (15 mm.) probably lack secondary sculpture; lamellae and secondary radials are developed chiefly on flanks of a 25 mm. shell and extend to the ventral margin of a 30 mm. shell; at 40 mm. the characteristic sculpture is fully developed, and persists to at least 60 mm. On some larger shells, lamellae are lost from rib summits and threads from interspaces but they persist in some of the largest specimens available. Although most specimens have weak rounded primary ribs, 3 valves show tendencies to square ribbing. A 30 mm. valve has square-cut narrow subdivided ribs, thus approaching toi and tainui, but differing in the greater width of interspaces relative to ribs. A 93 mm. valve and a fragment of a 120 mm. valve have subangular ribs, closely resembling toi, but with shallower broader interspaces bearing secondary radials. Left valve, of a 25 mm. juvenile, strongly concave (15 per cent); not separable from marwicki. LOCALITY: Rangitawa shellbeds (Te Punga, 1952) exposed in banks of Rangitawa Stream, Ongo Survey District, Rangitikei Section (GS 3082, 4264). The horizon of the Rangitawa shellbed and the pumice tuff which overlies it is probably between the Putiki Shellbed and the Landguard Formation at Wanganui. AFFINITIES: This form has some characters of the benedictus group and others of the jacobaeus group suggesting hybrid origins. It is classed as a subspecies of benedictus on account of its inflation, high beaks, and generally low rounded ribs. In its strong beak and relatively great inflation, P. b. tepungai resembles the living New South Wales form P. b. fumatus. Its other distinctive characters (secondary radials, well-developed lamellae) are apparently due to gene flow from a geographically adjacent population of the jacobaeus group. A form of the latter group (aotea) succeeds tepungai in the Rangitikei section. It seems that populations formed by hybridization between the two groups tend to be more highly ornamented than pure populations of either group. Hybrid populations are characterized by great variability (Sylvester-Bradley, 1951: 90). The hybrid subspecies of Pecten in New Zealand, formed independently, at three different times in the Castlecliffian, have characters in common, and, although they differ in their average characters, they overlap in such a way that some individuals of one population can scarcely be distinguished from individuals of another. The high shells, strong beaks, and weak ribs of tepungai generally allow separation from kupei, few of which have lamellae crossing the ribs. The ribs of the righly ornamented form of toi are higher, their interspaces deeper, and their sides more nearly vertical than those of tepungai. Outside New Zealand, the Pectens closest to tepungai seem to be shells like Reeve's hypotype of P. sinensis Sowerby (= P. b. excavatus Anton) (Plate 1, Fig. 4) that have secondary radials on low rounded ribs.» CHARLES ALEXANDER FLEMING, 1957
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C. A. Fleming, 1957, plate 4.
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