Sinezona mechanica Geiger & Marshall, 2012, new species

Sinezona mechanica new species

(Figures 4–6)

Type material. Holotype (NMNZ M.174828: Fig. 4). 1.28 × 0.88 mm. Paratypes from type locality (NMNZ M.045402, 2: Fig. 5 A–B). Additional paratypes: Saunders Canyon, off Otago Peninsula, South Island, New Zealand, 45.767˚S, 170.900˚E, 360 m (NMNZ M. 160300, 2: Fig. 5 C); Aerial Cove, Macquarie Island, 54.483˚S, 158.950˚E (AMS C.404496, 20).

Type locality. Ocean Bay, Chatham Island, New Zealand, 43.833˚S, 176.783˚W, 12–15 m, 7 Dec. 2000, F.J. Brook, bryozoans, shell and sand from rock crevice.

Etymology. Latin adjective referring to the sculpture of shell and protoconch suggestive of mechanical gears.

Description. Shell small (to 0.89 mm. Figs 4–5), depressed trochiform globular. Protoconch of 1 whorl, many dense axial cords on centrifugal portion of whorls, thickened at top forming (near) continuous connecting spiral cord. Apertural varix terminal (Fig. 8 B), not connected to embryonic cap, apertural margin straight. Teleoconch I of 0.9–1 whorl, approximately 17–20 strong (occasionally barely perceptible) axial cords, no spirals. Teleoconch II of 0.6 whorl, suture little impressed. Shoulder slightly concave, same density and strength of axial cords as on teleoconch I, finest growth marks in interstices, no spirals. Base with weak constriction below selenizone, rounded, same density and strength of axial cords as on shoulder; approximately 4 weak, granular spiral cordlets in adumbilical 1/5 of base. Umbilicus of moderate width, at sharp angle to base, bordered by weak to moderate strength spiral cord, wall straight, smooth. Aperture rounded, roof overhanging. Selenizone above periphery; keels low, moderate strength; lunules distinct; slit open, margins somewhat converging.

Radula (Fig. 6). Rachidian trapezoid, central cusp largest, three cusps on each side. Lateral teeth 1–3 similar, apical cusps largest, 4–6 on outer cutting edge. Lateral tooth 4 reduced, hook shaped, with 1 minute point on each side of tip. Lateral tooth 5 enlarged by broadening, apical cusp largest, 5–7 cusps along inner edge. Inner marginal teeth triangular, apical cusp largest, 1–2 along inner edge, 4–6 along outer edge of tip. Outer marginal teeth with spoon shaped tip bearing many fine bristles. Radular interlock of central field moderate.

Distribution. Pleistocene to Recent of New Zealand, 12– 106 m.

Specimen records. New Zealand. Ranfurly Bank, East Cape, 37.546˚S, 178.812˚E, 94 m (NMNZ M.303294, 1); Ranfurly Bank, 37.551˚S, 178.825˚E, 89–94 m (NMNZ M.303293, 15); Ranfurly Bank, 37.553˚S, 178.838˚E, 71–76 m (NMNZ M.303295, 50); Ranfurly Bank, 37.556˚S, 178.805˚E, 103–106 m (NMNZ M.303327, 5); Ocean Bay, Chatham Island, 43.833˚S, 176.783˚W, 12–15 m (NMNZ M.174828, 1); Saunders Canyon, off Otago Peninsula, 45.766˚S, 170.900˚E, 360 m (NMNZ M. 160300, 2); NE of Cape Saunders, off Otago Peninsula, 45.833˚S, 170.933˚E, 105 m (NMNZ M.045402, 2); off Taieri, S of Dunedin, 46.250˚S, 170.483˚E, 91 m (NMNZ M.066283, 2); off Poutama Island, South Cape, Stewart Island, 47.266˚S, 167.383˚E, 55 m (NMNZ M.303296, 60). Macquarie Island. Aerial Cove, 54.483˚S, 158.950˚E (AMS C.404496, 20).

New Zealand, fossil. GS4057, Kaikokopu Shellbed (Castlecliffian, oxygen isotope stage 19, middle Pleistocene), coast E of Omapu Stream mouth, between Castlecliff and Kai-Iwi beaches, Wanganui, 39.907˚S, 174.941˚E (GNS, 1).

Remarks. Sinezona mechanica can be distinguished from New Zealand species as follows. Sinezona apudornata (Laws, 1935) from the Miocene has a similar overall shape of the shell and similar overall sculpture of the protoconch, but its spire is much more elevated and it has a greater number of teleoconch II whorls (0.6 vs. 1.1). Sinezona bandeli Marshall, 2002 has a more subquadrate shell, much stronger constriction of the base below the selenizone, and lacks the top spiral cord on the protoconch. Sinezona brevis has a shorter selenizone (0.1 vs. 0.4 whorl), a more distinct periumbilical cord, and fine spiral threads on the shoulder. Sinezona enigmatica has axial cords on teleoconch I not reaching the suture and lacks distinct axial cords on teleoconch II. Sinezona iota (Finlay, 1926) is much taller, has strong axial lamellae, and lacks spiral sculpture on the base. Sinezona laqueus (Finlay, 1926) has a subquadrate overall shell shape, strong prosocline axial folds, and strong lunules in the selenizone. Sinezona levigata has an umbilical callus, has an overall wider shell, and has spiral sculpture on the shoulder and the base. Sinezona pacifica (Oliver, 1915) has stronger spiral sculpture on shoulder and base, a narrower umbilicus, and lacks the top spiral cord on the protoconch. Sinezona pauperata Powell, 1933 lacks a selenizone, has a round foramen, and a very narrow umbilicus. Sinezona platyspira n. sp. has a flatter spire, the umbilicus is produced by the rounded portions of the previous whorls, and the protoconch lacks the top spiral as well as an apertural varix.

The widely distributed subantarctic species Sin. subantarctica (Hedley, 1916) has a callus over the umbilicus, a very short (or absent) selenizone, and the protoconch lacks the top spiral cord and an apertural varix.