Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 45a): GS6665, Q29/f8504, concretions trawled off Cape Campbell, Cook Strait, late Haweran (C14 age: 19 000 years) (TM4179, GNS; specimen illustrated by Boreham and Fleming in Pantin 1957, fig. 4)
Beu & Maxwell (1990): Chapter 16; p. 340; pl. 45 a.
Synonymy: Mytilus magellanicus (not of Lamarck) of Suter 1913c, p. 865; Mytilus maorianus Iredale 1915, p. 484; Aulacomya ater maoriana, Beu & Maxwell 1990, p. 340, pl. 45a.
Classification: Mytilidae: Mytilinae
Description: Moderate-sized for family (to 120 mm long), a coarsely ribbed "mussel". Radiating, weakly anastomosing costae cover most of exterior, costae extremely variable in number and prominence; the narrow antero-ventral area bears much lower, narrower, closer costellae than remainder of shell, or is almost smooth in some specimens. Beak narrowly pointed and umbo anterior, as in Mytilus; a prominent ridge extends from umbo along junction of antero-ventral area with remainder of shell. Pallial line prominent, but single large adductor scar very little impressed, not visible in most fossils; a low, short tooth and groove below umbo of each valve is eroded from some specimens.
Comparison: Aulacomya maoriana is much more coarsely ribbed than the superficially similar Early Miocene Trichomya species. It is larger and much more coarsely ribbed than Miocene Septifer species, which also differ in having a shelf inside the umbo. Also, it is much larger than the Paleocene Hormomya(?) willetsi (Marwick). In the modern fauna, A. maoriana lives byssally attached to hard substrates in the intertidal zone, and down to about 50 m. Although considered to be a geographical subspecies of the circum-subantarctic species A. atra (Molina) by Dell (1964b, p. 176), the New Zealand form differs from the South American one in its smaller size, finer radial sculpture, and in retaining a small anterior adductor muscle scar that is not present in A. atra (Beu 2004, p. 147; H. Zaixso, Universidad Nacional de la Patagonia, Argentina, pers. comm.) and is now seen to be a distinct species. It appeared suddenly in New Zealand during Waipipian time, and occurs mostly in cool faunas, so is presumably a neoaustral immigrant from South America, or from the southern islands of New Zealand.
Distribution: Waipipian-Recent; Recent, New Zealand (type). Widespread but uncommon in New Zealand before Nukumaruan time, but abundant in shallow-water shellbeds of Nukumaruan age, and younger; particularly common in near-shore shellbeds at Wanganui (Castlecliffian) and in the many Haweran terrace faunas around New Zealand.
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)