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Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)

New Zealand Cenozoic Mollusca

Anomia trigonopsis Hutton, 1877


(Pl. 18a): Oneroa, Waiheke Island, Auckland, Otaian (GNS, ex Fleming Colln)


(Pl. 18b): Oneroa, Waiheke Island, Auckland, Otaian (GNS, ex Fleming Colln)

Beu & Maxwell (1990): Chapter 11; p. 179; pl. 18 a,b.

Synonymy: Anomia alectus of Hutton 1873a, p. 83 (not of Gray, 1850); A. cytaeum of Hutton 1873a, p. 83 (not of Gray, 1850); Anomia trigonopsis Hutton 1877b, p. 598; Anomia walteri Hector 1895, p. 292; A. huttoni Suter 1913c, p. 843; A. undata of Suter 1913c, p. 843 (not of Hutton 1885); A. poculifera Marshall 1918, p. 20

Classification: Anomiidae: Anomiinae

Description: Moderately large for family (length up to 90 mm, height to 76 mm), subcircular, weakly inflated; right valve thin and fragile, not often found fossil. Left valve thicker but of lamellar-calcitic structure, most fossils flaky and fragile; attached to hard substrate (shells, or more usually rocks) by a calcified byssal plug passing through a foramen (a constricted byssal notch; see description under Pododesmus incisurus, below) near umbo of right valve; byssal plug smaller than foramen, therefore remaining attached to substrate on death. Sculpture on left valve only, exceedingly variable, from almost smooth or irregularly hummocky, or with very fine radial costellae to quite coarse, nodulose costae. (Illustrated shell has moderately coarse sculpture). Many specimens show additional xenomorphic sculpture (copying that of the substrate). Ligament internal, dorsal, attached to a prominent but relatively small, simple crus with flat dorsal face in right valve, and to matching pit below dorsal margin in left valve. White aragonitic central area of right valve with large, smooth, circular byssal adductor muscle scar; left valve with 3 scars arranged in a triangle, a large subcircular one above 2 much smaller, similar ones.

Comparison: The presence of three muscle scars in the left valve easily distinguishes Anomia trigonopsis from most other New Zealand Anomiidae (the large, uppermost scar and small, central one are scars of the divided byssal retractor muscle, and the lowermost small scar is of the valve adductor; Yonge 1977). The exception is Patro undatus (Pl. 39d,f), which differs in having a prismatic (rather than lamellar) right valve, and in having the three muscle scars in the left valve arranged in a nearly straight line, the lowest (valve adductor) being the largest. All New Zealand (and probably Australian) Anomia specimens examined appear to be conspecific, and in view of the great range of variation in shape and sculpture it is difficult to find meaningful specific criteria in the genus; most bivalve taxonomists now agree that there are very few living species, and there are certainly no obvious shell differences between A. trigonopsis and the type species A. ephippium Linne, 1758 (Recent, North Atlantic and Mediterranean).

Distribution: Bortonian?, Duntroonian-Recent; Southburn Sand, White Rock River, Altonian (type locality of A. trigonopsis); Pakaurangi Formation, Pakaurangi Point, Otaian or Altonian (type locality of A. poculifera); Paihia, Bay of Islands, Recent (type locality of A. walteri). Widely recorded from throughout 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
ISBN 978-0-478-19705-1
ISSN 1177-2441
(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)


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