Parvamussium cf. maorium Dell, 1956

Saether, Kristian P., Jingeng, Sha, Little, Crispin T. S. & Campbell, Kathleen A., 2016, New records and a new species of bivalve (Mollusca: Bivalvia) from Miocene hydrocarbon seep deposits, North Island, New Zealand, Zootaxa 4154 (1), pp. 1-26 : 10-11

Parvamussium cf. maorium Dell, 1956

( Fig. 5 E1)

Material. One specimen from Moonlight North (Y16/f0694),?adult, TM9241, UOA (borrowed from GNS).

Description. Incomplete internal mould; auricles absent; anterior half ca. 70% intact, posterior half ca. 50% intact of which posteroventral region almost totally absent, if shell assumed to be typical; only segments of margin that are (possibly) intact are short (ca. 1 mm) anterodorsal portion and even shorter (ca. 0.4 mm) anteroventral part; at least nine strong, wide internal radial ribs emanate from beak region, widely and more or less evenly spaced, anteriormost six with gentle curvature away from posterior margin, posteriormost three straight for their short continuation before they are cut off by erosion, two anteroventral ribs are preserved in entirety to show rounded terminations at ca. 90% of distance from beak to margin; small umbonal notch present.

Remarks. The specimen is a partial internal mould that does not yield sufficient information for confident specific placement. What remains of it is comparable to the Mangapanian (late Pliocene)–Recent Parvamussium maorium Dell, 1956 ( Fig. 3 B2) from several localities throughout New Zealand. The radial ribs of the fossil specimen are of similar size, spacing, and position relative to the umbo compared to the modern species. The rounded terminations of the ribs, at 90% of the beak–margin distance, in the fossil specimen also are typical of P. maorium , and the umbonal notch is of a similar size in both species. The portions of intact margin in the fossil species also are of a similar shape and position relative to the umbo and ribs to that of P. maorium , and the fossil specimen is of a similar size to adult P. maorium .

Figures

FIGURE 3. Bivalves of uncertain sub-familial identity. A. Small (? juvenile) protobranch bivalve from Bexhaven. Specimen L 4646. Internal mould showing taxodont dentition typical of this group. Scale bar is 0.5 mm. B. Solemyidae gen. indet. Specimen L 4649 from Moonlight North. Scale bar is 5 mm. B 1. Lateral view of right valve. B 2. Lateral view of right valve, faint radial striations highlighted. B 3. Lateral view of left valve. B 4. Dorsal view.

FIGURE 5 Non-seep-typical bivalves found at Miocene New Zealand hydrocarbon seeps. A – B. cf. Pulvinites sp. from Moonlight North. Scale bar is 10 mm. A. Specimen L 4650. A 1. External view of shell. A 2. Internal view of shell. B. Specimen L 4651. B 1. Internal view of shell. B 2. Lateral view, showing thin laminae. C. Image of modern Pulvinites exempla (Hedley, 1914) for comparison. Specimen M. 281853, NMNZ (photography by K. P. S.). C 1. External view of shell. C 2. Lateral view, showing thin laminae. C 3. Internal view of shell. D. Acesta cf. saginata Marshall, 2001. Specimen L 4652 from Rocky Knob. Scale bar is 10 mm. D 1. Lateral view of right valve. D 2. Anterior view. D 3. Dorsal view. E. Parvamussium cf. maorium Dell, 1956. Scale bar is 2 mm. E 1. Specimen TM 9421 from Moonlight North, anteroventral portion absent. Lateral view of right valve. E 2. Image of modern Parvamussium maorium Dell, 1956 for comparison. Specimen M. 138360, NMNZ (photography by K. P. S.).