Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
| | Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990) |
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  (Pl. 27b): GS 4454, U25/f6483, tributary of Waikohi Stream, east of Waikohi Valley Rd, east of Puketoi Range, southern Hawke's Bay, Tongaporutuan (GNS) |
Beu & Maxwell (1990): Chapter 12; p. 244; pl. 27 b.
Synonymy: Pholadomya neozelanica Hutton 1885b, p. 330
Classification: Pholadomyidae
Description: Moderately large for family (to 80 mm long), highly inflated, with very thin, fragile, highly nacreous shell, and umbo close to anterior end. Sculpture of many rather indistinct commarginal ridges and, on central region, about 17 to 20 low, widely spaced radial costae, forming weak nodules at sculptural intersections; anterior eighth and posterior fifth of length lack obvious radial sculpture. Escutcheon weakly defined, a narrow, smooth, weakly concave area on each side of dorsal margin. Anterior end short, inflated, bearing very weakly defined, pouting lunule, occupying third of shell height, bounded by very shallow, wide depression. Interior not seen in any New Zealand material of the genus; modern species lack hinge teeth. Almost all specimens distorted in a variety of ways.
Comparison: Hutton's (1885b, p. 330) holotype of Pholadomya neozelanica, illustrated by Suter (1915, p. 62, pl. 7, fig. 1), is said to be from "Oamaru"; the sole specimen we are aware of subsequently collected in Oamaru district is from Mount Harris Formation in South Oamaru (Altonian) and this could well be the horizon of Hutton's type. P. neozelanica, of large size and very inflated form, with the umbo very near the front, is the most commonly encountered of at least five species of pholadomyoidean bivalves recorded from the New Zealand Cenozoic. Until recently these were all included in the Pholadomyidae and assigned to either Pholadomya itself or to Procardia (Pl. 9h, i), but some modification to this classification is necessary in the light of work by Morton (1982). He proposed a new family Parilimyidae for anomalodesmatans that resemble pholadomyids in most shell characters, but differ significantly in anatomical details. In particular, parilimyids have "taenioid" (i.e., tape-like) siphonal retractor muscles, which leave prominent scars on the inside of the shell (a single scar near the centre of each valve), whereas pholadomyids have only greatly reduced siphonal muscles. On the other hand, parilimyids lack pedal retractor muscles, whereas these are well developed in Pholadomyidae. Unfortunately, fossil pholadomyaceans are rarely well enough preserved to discern muscle scar details, so assignment to one or other of these families normally depends on their similarity to extant species.
Pholadomya neozelanica resembles the type species of the genus (P. candida Sowerby, 1823; Recent, Caribbean) in having umbones close to the anterior end, and in its sculptural plan, and is probably congeneric, but P. waitotarana (Opoitian-Waipipian) and P. warrenae (Tongaporutuan) have more centrally placed umbones and have much weaker radial sculpture than Pholadomya neozelanica. In these respects they are much closer to the extant New Zealand species P. maoria Dell, 1963, which Morton (1982, p. 167) assigned to Parilimya Melvill & Standen, 1889, and although there is only limited similarity to P. haddoni Melvill & Standen, 1889 (Recent, Torres Strait), the type species of the genus, Beu & Maxwell (1990) provisionally assigned them there. Thracia neozelanica (Pl. 46g, i) is also provisionally included in Parilimya, but like P. waitotarana and P. warrenae it almost certainly requires a new genus. Procardia, which is represented in the New Zealand Cenozoic by P. dolicha (Pl. 9h, i), has previously been regarded as a pholadomyid, but it is very similar to Panacca Dall, 1903, which Morton (1982, p. 161-163) included in the Parilimyidae.
"Parilimya" warrenae is considerably smaller than Pholadomya neozelanica (length up to about 55 mm), is lower and less inflated, has a less convex ventral margin, has umbones situated at about a third of the length from the anterior end, and has less prominent radial sculpture. There are few obvious differences, however, between P. warrenae and P. waitotarana, and their relationship needs further study; specimens of P. waitotarana are known from Waipipi, Waverley Beach (Waipipian; type only), and from GS1548, Mangaone Crossing, Wairoa district, Hawkes Bay (Opoitian). P. maoria differs from P. waitotarana in having stronger radial costae and a more convex ventral margin.
An unnamed pholadomyacean, probably a parilimyid, is represented by a specimen in NZGS from Whitewater Creek, Castle Hill Basin, Canterbury (Duntroonian) with a nearly central umbo, a relatively thick shell, and prominent radial costae with no obvious commarginal sculpture; this may be the second species illustrated by Suter (1915, pl. 6, fig. 2) as Pholadomya neozelanica as, although Suter's specimen is unlocalised, the preservation appears to match that of the Whitewater Creek shell. It is not a typical Pholadomya, but has considerable similarity to P. pacifica Dall, 1907 (Recent, Japan), referred by Morton (1982, p. 172) to Parilimya although, like the other species he assigned there, it has little similarity to the type species. Another possibly distinct species is represented by an internal mould of a small (length 40 mm), evenly oval specimen from "GS945, breccia below Kakanui limestone, Kakanui" collected and listed by Park (1918, p. 68); the specimen is almost certainly from the top of the Deborah Volcanic Formation (Whaingaroan). Procardia dolicha (Pl. 9h, i) is readily distinguished from all other New Zealand pholadomyaceans by its extremely short anterior end and almost flat anterior area.
Although pholadomyaceans are often considered to be characteristic of deep waters they are also recorded from much shallower environments. Pholadomya candida itself apparently lives at depths of about three to six metres in the Virgin Islands (Runnegar 1979) and at least one of the records of P. neozelanica (Tongaporutuan, Kaiwara River, North Canterbury) is from a very shallow-water assemblage. "Parilimya" maoria is known only from the upper bathyal zone, but P. warrenae is limited to shallow-water "Hurupi facies" faunules in Palliser Bay (Tongaporutuan). Powell (1931a, p. 90) and Fleming (1953) concluded that the fossiliferous beds at Waipipi — the type locality of "Parilimya" waitotarana — were deposited at inner-shelf depths.
Distribution: (Duntroonian?) Otaian(?) to Tongaporutuan; rare in bathyal siltstone and some shelf assemblages 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
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