Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 35c): holotype, GS3528, R11/f7014, Otahuhu Brewery well, Auckland, Waipipian (TM1280, GNS)
(Pl. 35i): holotype, GS3528, R11/f7014, Otahuhu Brewery well, Auckland, Waipipian (TM1280, GNS)
Beu & Maxwell (1990): Chapter 14; p. 285; pl. 35 c,i.
Synonymy: Mactra (Mactrona) mula Marwick 1948, p. 22; Beu & maxwell 1990, p. 285, pl. 35c, i
Type species of Mactrula Marwick, 1948 (not of Risso, 1826) = Mactrona Marwick, 1952; = Pseudocardium Gabb, 1866 (Beu 2004, p. 164)
Classification: Mactridae: Mactrinae
Description: Large for genus (74 mm high, 84 mm long), thick and solid, smooth except for weak growth ridges. Strongly trigonal, with high narrow umbo, rounded anterior extremity, and curved ventral and posterodorsal margins meeting; at posterior angle at termination of a sharply angling umbonal ridge. Hinge with a central, deep, rather narrowly triangular resilifer beneath umbo, and in front of resilifer a large, bilobed cardinal tooth and a small cardinal lamella in left valve and a pair of large cardinal teeth in right valve (similar to the large, slightly oblique cardinals of Cyclomactra); left valve with a single stout, relatively short, prominently pustulose or pitted lateral tooth on each side of hinge, right valve with a corresponding pair of short, stout, pustulose teeth and a deep, wide socket on each side of hinge. Hinge with a short spur above top of resilifer, but no "mactroid ridge" separating resilifer from dorsal ligament. Adductor scars oval, relatively small, near valve margins; pallial sinus small, semicircular, immediately below posterior adductor scar.
Comparison: Marwick (1948) pointed out that Mactrona is an unusual group combining the spisuloid ligament and large, slightly oblique cardinal teeth of Cyclomactra with coarsely pustulose lateral teeth and a shell similar to that of Mactra (sensu stricto). Beu & Maxwell (1990, p. 392) could see few differences between Mactrona Marwick and Pseudocardium Gabb, 1866 (Moore 1969, p. N601, fig. E94.10; Uozumi et al. 1986, pl. 9, fig. 2, 3, 5, pl. 10, fig. 1, 5), previously recorded only from Oligocene to Recent faunas of western North America and northern Japan, and Beu (2004, pp. 164-166) regarded Mactrona as a synonym of Pseudocardium. This seems, then, to be a genus that extended its range to New Zealand during mid-Pliocene time, from a formerly North Pacific range.
Distribution: Waipipian-Nukumaruan; Otahuhu Brewery well, Auckland, Waipipian (type, only specimen found). A huge, thick shell (101 mm high) from the Wanganui River road (Nukumaruan) and a suite of 15 specimens from Kaiwaka South Road, near Devil's Elbow, north of Napier (Nukumaruan) show a huge range of variation in shape (larger shells are taller and more angled than smaller ones) and indicate that the same species occurs in Nukumaruan rocks. All occurrences are in very shallow-water, near-shore facies (the Kaiwaka South Road locality is estuarine, with common Barytellina, Limnoperna, Cyclomactra, and Zeacumantus) which may explain the apparent rarity of the species.
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)