Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 18c): GS9520, J41/f8029, Awamoa Creek, Oamaru, Altonian (GNS) (left valve interior)
(Pl. 18f): GS9520, J41/f8029, Awamoa Creek, Oamaru, Altonian (GNS) (right valve exterior)
Beu & Maxwell (1990): Chapter 11; p. 180; pl. 18 c,f.
Synonymy: Placunanomia incisura Hutton 1873b, p. 34; Monia zelandica (Gray) of Boreham 1965, p. 46 (in part)
Classification: Anomiidae: Placunanomiinae
Description: Size moderate for family (height 35-57 mm), suboval, a little higher than long, relatively thick-shelled and robust. Most specimens reasonably flat, but some weakly folded near margin; inflation slight (only 7 mm in an articulated shell 57 mm high). Attached to hard substrates (usually molluscan shells) by a thick, calcified byssal plug passing through a foramen (a deep byssal notch that becomes surrounded by shell margin during growth, leaving a shallow groove at the junction of shell edges) in right valve a short distance below dorsal margin. Byssal plug pale brown in all specimens, coarsely striated on internal face. Sculpture of regular, high, narrow radial costellae on both valves (except for a small attachment area near right valve umbo), very variable in prominence, but in most specimens relatively coarse, about 2 mm apart distally, and bearing many small, semitubular spines. Ligament internal, attached in right valve to a prominent, high, triangular, bilobed crus; in left valve to a matching pit below dorsal margin. Most of shell calcific; a central white (aragonitic) layer is irregularly oval in left valve but hooks around byssal plug in right valve. Within aragonitic area, a small, smooth oval adductor muscle scar in each valve, and a small, oval, radially wrinkled byssal retractor scar in left valve.
Comparison: Pododesmus incisurus is easily distinguished from Anomia (e.g., A. trigonopsis, above) by having only two muscle scars in the left valve (three in Anomia), by the byssal adductor scar being radially wrinkled, by the large, bilobed ligamental crus (lower and with flat dorsal face in Anomia), by the very distinctive, coarsely striated byssal plug (not constricted by the shell and so left attached to the substrate in Anomia) and by the coarse, narrow, spinose radial costae (low and irregular in Anomia). A very few specimens of P. incisurus have been seen with xenomorphic sculpture (reflecting the high relief of the substrate); this is much more common in Anomia. Other species of Pododesmus are P. maxwelli (Whaingaroan-Duntroonian, Aotea Harbour and Otago- Southland), with a smaller byssal plug, much finer sculpture, a more strongly triangular resilial crus, and a thinner shell which is thrown into two or three very deep radial folds; and P. paucicostatus (Clifdenian-Waiauan, Canterbury and Southland), which is closely similar to P. incisurus but has much coarser radial sculpture.
Yonge (1977, p. 459) found no anatomical differences between Pododesmus and Monia zelandica, the type species of Monia Gray, 1849, and considered Monia to be a synonym of Pododesmus. However, species assigned to Monia have a thinner right valve than Pododesmus (sensu stricto), a simpler ligamental crus like that of Anomia, and a much larger byssal plug that is not constricted by the shell margins, and this group is worthy of subgeneric rank under Pododesmus. P. (Monia) zelandicus (Opoitian-Recent) resembles Anomia in most shell characters, but has sculpture like that of P. incisurus and only two muscle scars in the left valve.
Distribution: Waitakian-Altonian; "Pareora Beds, Rangitata", Otaian? (type); recorded widely from shallow shelf faunules, particularly common at some Altonian localities near Oamaru.
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)