Mesopeplum polymorphoides (Zittel, 1865)
ZITTEL, K. A. 1865. Fossile Mollusken und Echinodermen aus Neu-Seeland. In F. von Hochstetter, M. Hörnes & F.R. von Hauer (Eds.), Paläontologie von Neu-Seeland. Reise der Österreichischen Fregatte Novara, Geologischer Theil, 1 (2): 15-68, pls. 6-15. [p. 51, pl. 11, fig. 3]
1864 Pecten polymorphoides Zittel, 1865
K. A. Zittel, 1865, plate 11.
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«Char. Testa suborbicularis, aequilatera, compressa, margine inflexa, quadriplicata. Plicae obsolete striatae vel sublaevigatae. Margo inflexus striatus, auriculae parvae subaequales.
Höhe 16 Millim., Länge 15 Millim.
Schale klein, gerundet, gleichseitig, ungeföhr eben so hoch, als lang, mit vier starken Falten, die nach unten zu schwach gestreift, oben aber glatt sind. Der Stirnrand ist umgebogen und mit starker Streifuug bedeckt. Die Oberfläclie sehr schwach gewölbt, fast glatt; die Buckeln treten nur unmerklich vor, und ihre vertieften Seitenränder laufen in einem spitzen Winkel zusammen. Die Ohren sind verhältnissmässig klein, und beinahe gleich. Die Ähnlichkeit dieser Species mit dem P. polymorphus Br. aus dem Mittelmeere ist so auflallend, dass ich kaum gezaudert hätte beide zu identificiren, wenn mir eine grössere Anzahl von Exemplaren aus Neu-Seeland zu Gebote gestanden hätten. Pecten polymorphoides unterscheidet sich nur durch seine flachere Form und die schwächer gestreiften Falten von der europäischen Art. Vorkommen: Waikato Southhead.» KARL ALFRED ZITTEL, 1865
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«Type material. Pecten polymorphoides holotype NHMW 1959/335/42 (Fig. 11B), from locality 3, ‘Waikato Southhead’; plaster replica GNS TM4233. It is concluded under ‘Fossil localities’ that the holotype is from the Ahirau Sandstone Member of the Glen Massey Formation, of early Whaingaroan (latest Eocene-earliest Oligocene) age. The subsequent lack of recognition of Mesopeplum polymorphoides in New Zealand probably results from it being one of the few records of Mesopeplum from rocks this old.
Dimensions. Holotype of Pecten polymorphoides: H 16.4, L 14.5 mm; GNS TM4234, holotype of Pallium syagrus: H 39.8, L 40.5 mm.
Remarks. Mesopeplum polymorphoides is poorly understood, and the name has been used very little (at least, in New Zealand) since 1865, other than in a few faunal lists. Tate (1886, p. 113) and several later workers used the name for an Australian species, but Marwick (1924d) and Beu & Darragh (2001) showed that this was incorrect; the Australian species is M. incertum (Tenison Woods, 1865). Marwick (1924d, p. 321) noted that ‘No New Zealand examples of this species are in the Geological Survey collection’ and this seems still to be the case. However, it is possible that this is an earlier name for M. syagrus (Marwick) (1924d, p. 326 [under Pecten palmipes Tate, 1886], pl. 5, fig. 7). The holotype of M. polymorphoides has only four major radial plicae, with another weak one at each end of the disc, whereas that of M. syagrus has seven slightly narrower plicae, but the number of radial plicae is variable to some degree in all Mesopeplum species. A strongly and evenly plicate species of Mesopeplum with five to six plicae but virtually no radial costae, that is, the surface is superficially smooth, is common in Pliocene (mainly Opoitian, early Pliocene) limestone beds in northern Hawke’s Bay, particularly on Mahia Peninsula, where it occurs syntopically with but a little less commonly than the unusually prominently ribbed species M. waikohuense (Marwick, 1931). The range of variation of Mesopeplum species in New Zealand is still poorly understood, but it is likely that M. polymorphoides is an earlier name for M. syagrus, a species distinct from both M. convexum and M. waikohuense that occurs in rocks as young as Opoitian at Mahia Peninsula.
The holotype of Mesopeplum polymorphoides has been difficult to interpret because of the apparently smooth surface of much of the specimen, but examination under the microscope showed that it has been broken through the shell material, parallel to the surface, and most of the outer surface is missing. On the steeply inclined ventral rim, with radial costae still preserved, faint remnants are preserved of many closely spaced commarginal ridgelets between the costae. The specimen is a weakly opisthocline right valve, with the anterior margin of the disc about two-thirds the length of the posterior margin, and matrix largely filling the very narrow byssal notch. The anterior auricle bears four low radial costae, whereas the posterior auricle apparently is smooth. These characters confirm a position in Mesopeplum. The holotype of Mesopeplum syagrus is still more poorly preserved than that of M. polymorphoides. The holotype (GNS TM4234, from J.A. Thomson’s locality 11, ‘lower calcareous band, Mount Brown beds, southwest of Mt Brown’, Canterbury; Altonian, late early Miocene) is a severely weathered specimen with seven radial folds, with the surface chalky and abraded, and almost no original surface sculpture preserved. Cleaning off the deeply weathered, lichen-impregnated matrix in October 2011 showed that the specimen illustrated by Marwick (1924, pl. 5, fig. 7) is only about half the entire shell, and the little cleaning undertaken earlier had stopped at the margin of a prominent growth step. A wide (incomplete) area beyond the growth step is similarly weathered and abraded, but shows weak signs of three to four low, wide, closely spaced radial costae on each radial fold. However, it still reveals no commarginal sculpture to confirm a position in Mesopeplum. It is very difficult to relate this weathered fragment to any other named species, and it might well be merely a weakly sculptured, exceedingly poorly preserved specimen of M. burnetti, but it seems feasible that it is a larger specimen of the same species as the holotype of M. polymorphoides. Unfortunately, very little other Mesopeplum material of late Eocene-early Miocene age is available at present to help refine the taxonomy of this species, as the type of low sedimentation-rate environment in which Mesopeplum lives was rare in New Zealand over this age range. The other few early, Oligocene-early Miocene specimens available are all prominently ribbed and resemble the type material of M. burnetti.» BEU, A. G., S. NOLDEN & T. A. DARRAGH. 2012. Revision of New Zealand Cenozoic fossil Mollusca described by Zittel (1865) based on Hochstetter’s collections from the Novara Expedition. Memoirs of the Association of Australasian Palaeontologists, 43: 1-69, figs. 1-21. [p. 34, 35]
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Mesopeplum polymorphoides (Zittel, 1865); A. G. Beu, S. Nolden & T. A. Darragh, 2012, Revision of New Zealand Cenozoic fossil Mollusca described by Zittel (1865) based on Hochstetter’s collections from the Novara Expedition, figure 11B.
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