Acesta cf. saginata Marshall, 2001

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 : 9

Acesta cf. saginata Marshall, 2001

( Fig. 5 D)

Material. One specimen from Rocky Knob (Y16/f0642), medium sized, L4652, UOA.

Description. Shell small for genus (H ≈ 70.0 mm, I ≈ 20 mm; L ≈ 55.0 mm; see remarks below), shell length ca. 80% of shell height, single valve inflation ca. 20% of shell length (see remarks below); dorsal margin straight (running for short distance behind umbo before being eroded), nature of passage to posterior margin unknown; anterodorsal margin running at ca. 45° from umbo, gently convex, evenly rounded; other margins unknown; external sculpture unknown; finely spaced, subtly wavy, radial striae on shell interior, especially strong in anterior half of medial region, mostly obsolete elsewhere, a few very faint striae visible in posterior half.

Remarks. The single specimen is an internal mould of a left valve, with the ventral portion and auricle only partially preserved, leaving any attempt at specific identification tenuous. The fossil seep species is morphologically closest to Acesta saginata Marshall, 2001 , based upon the broadness of the beak in comparison with the other modern New Zealand species, Acesta maui Marshall, 2001 , and the four New Zealand fossil species (all narrower except Acesta regia , in which it is broader). Also, of the two modern species, A. saginata has the most strongly internally reflected radial striae, which closely resemble the narrowly spaced striae on Acesta cf. saginata . The nature and extent of internal reflections on the other New Zealand fossil species are unknown.

Estimations of the size of our specimen from the curvature of the antero- and posterodorsal flanks (either side of the narrow beak) and from the position of a faint muscle scar suggest that it was considerably smaller than the other New Zealand species, fossil and modern. The specimen also appears to be considerably less inflated than A. saginata notwithstanding their similar height vs. length ratios. Nonetheless, owing to a lack of complete specimens, these dimensions can only be regarded as approximate, especially inflation vs. length. The fine radial striae in the medial portion probably reflect a moderate to strong external radial sculpture, and their subtle waviness may indicate where weaker concentric elements cross the radials.

All four New Zealand fossil species are older than those from the Rocky Knob deposit, the youngest records being from the Waitakian period (late Oligocene–early Miocene), but the two Recent species suggest that the genus has been present in New Zealand throughout this time.

Figures

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.).