Austrochlamys natans walosseki Jonkers, 2003
JONKERS, H. A. 2003. Late Cenozoic-Recent Pectinidae (Mollusca: Bivalvia) of the Southern Ocean and neighbouring regions. Monographs of Marine Mollusca, 5: i-viii + 1-125 pp, 17 pls. Backhuys Publishers, Leiden. [p. 61, pl. 14, figs. a-e]
2003 Austrochlamys natans walosseki Jonkers, 2003
H. A. Jonkers, 2003, plate 14.
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«Etymology. Named after Dieter Waloβek, University of Ulm, Ulm, Germany, who collected this new taxon on the Patagonian shelf during the I978 RV "Walther Herwig" cruise to South America.
Type material. ZMH 2909 (LV, VH 68.8 mm; holotype; Plate 14, fig. d), from RV "Walther Herwig" Sta. 697/78,40°27.8' S, 56°31.8' W, edge of Argentine shelf, in 164 m water depth, D. Waloβek leg., 27 Jun. 1978; ZMH 2908 (3 LV, VH 65.8 mm, c.77 mm, 100.1 mm; pararypes; Plate 14, fig. c illustrates largest specimen), from RV "Walther Herwig" Sta. 631/78, 50°16.5' S, 61°34.5' W, NW of Falkland Is., in 160 m water depth, D. Waloβek leg., 12 Jun. 1978; ZMH 2910 (3 RV, VH c. 107 mm, c. 111 mm, c. 115 mm; pararypes; Plate 14, fig. e illustrates smallest specimen), from RV "Walther Herwig" Sta. 571/78, 50°31.6' S, 59°55.2' W, N of Falkland Is., in 140 m water depth, D. Waloβek leg., 24 May 1978. Material examined. 7 LV (including 1 umbonal part),7 RV, 1 disc fragment from the Argentine and Falkland Islands shelf, and from Burdwood Bank; 1 LV, 1 RV from off Auckland Islands, New Zealand region; all overgrown and discoloured (?Pleistocene fossils). Description. Shell large (mean VH c. 94 mm; maximum observed VH 116.7 mm [ZMH 2910]) and rather thick; slightly prosocline, with anterior part of disc somewhat longer than posterior part (VLA/VLP 1.05); LV strongly convex, and RV comparatively flat; umbonal angle 117°. Dorsal disc gapes present. Radial macrosculpture of 30-35 main plicae, with on LV one (occasionally two) smaller rib(s) intercalated between primary ribs; ribs on RV strongly bifurcate, interspaces without intercalated secondary riblets. Commarginal sculpture of very prominent frills; radial microsculpture not observed. LV anterior auricle with 5-6 costellae, free margin curved to anterior, or straight; RV anterior auricle with relatively shallow byssal notch (BND/VH 0.04), initially acute (Plate 14, fig. a), later becoming more arcuate (Plate 11, figs. a, e); mean number of byssal teeth in functional ctenolium 3.7. Measurements and counts. VH 93.9 ± 4.7 mm (n = 15), VHA 47.2 ± 2.7 mm (n = I4), VHP 46.0 ± 2.4 mm (n = 14), VL 88.7 ± 4.8 mm (n = 13), VLA 44.9 ± 2.4 mm (n = 14), VLP 43.7 ± 2.2 mm (n = 14), C (LV) 17.9 ± 1.4 mm (n = 6), C (RV) 15.3 ± 1.4 mm (n = 7), OL 41.1 ± 1.9 mm (n = 14), OLA 23.8 ± 1.2 mm (n = I4), OLP 17.3 ± 0.7 mm (n = 16), AHA (LV) 16.1 ± 0.5 mm (n = 7), AHA (RV) 13.3 ± 0.8 mm (n = 8), AHP 14.6 ± 0.6 mm (n = 15), BND 4.3 ± 0.4 mm (n = 6), UA 116.7º ± 0.6 (n = 15), AAAD 105.8º ± 1.9 (n = 8), AAPD 117.9º ± 1.0 (n = 16), BT 3.7 ± 0.3 (n = 3). Stratigraphical age and distribution. Late Pleistocene?, shelf areas off southeastern South America (Fig. 20) and off Auckland Islands, New Zealand (a single collection: M.118772 [NMNZ]; Fig.22). No living A. natans were captured in RV "Walther Herwig" trawls, in the area in which subsp. walosseki was recovered (Waloszek, 1984, p. 232). The discoloured, usually mechanically damaged, bored and overgrown shells, and absence of recovery of live specimens, all indicate a non-Recent age of A. natans walosseki. Near the hinge of most valves in a limited area (which in Recent natans is the most strongly coloured part of the shell, particularly in RVs) a purplish colour is present, which is thought to be a remnant of the taxon's original shell colour (incidentally the specimen in Plate 74, fig. c, is still purple-coloured on the outside), indicating that shells cannot be very old. The fact that A. natans walosseki was recovered in bottom trawls only, a situation similar as for late Pleistocene Psychrochlamys delicatula instar in New Zealand (e.g. in the Auckland Islands area, where both have been recorded; see Appendix), suggests that this taxon is of late Pleistocene age too (Last Glacial Maximum; Fis.25). Remarks. Austrochlamys natans walosseki subsp. nov. differs from subsp. natans natans in its larger overall size, more convex LV but flatter RV (hence a much higher contrast in valve convexiry; Fig. 41), RV radial sculpture solely consisting of bifurcate ribs, much stronger commarginal sculpture (see also Waloszek, 1984, p.232), and absence of any noticeable radial microsculpture, a shallower byssal notch, and a lower number of functional byssal teeth. Although byssal teeth could be reliably counted in three specimens only (VH 67.9, 115.4 and 116.7 mm), the low numbers in these (4,3, 4, respectively) could suggest that in subsp. walosseki there is no trend towards more teeth in larger individuals, as seen in subsp. natans (r = 0.356 [α <0.01; n = 130]; mean BT of adults 4.8 ± 0.1 [n = 68]). This suggestion would seem to be supported by the relatively much shallower byssal notch of subsp. walosseki, which in other pecinids analysed in this study is associated with fewer functional byssal teeth (Fig. 19b; Table 1). The bifurcating ribs on the RV of A. natans walosseki are much reminiscent of those of sub-Antarctic ?Late Miocene A. heardensis (Fleming, 1957) (Plate 12, fig. e); such ribs are also seen in some individuals of the Antarctic Pliocene A. anderssoni (Hennig, 1911) (e.g. DJ.851.5;Jonkers, 2000, fig. 2A), but in that species ribbing is more complex, with secondary and tertiary costellae in rib interspaces (see below). LV radial costation similar to that of A. natans walosseki is found in A. anderssoni too (Plate 13, fig. e). Shells of the New Zealand Psychrochlamys delicatula instar have a smaller maximum size, are opisthocline (with VLA/VLP <1) rather than slightly prosocline, have a less convex LV, a shorter outer Iigament, and a relatively lower LV anterior auricle than A. natans walosseki; such shells have fasciculate ribs and lack the strong commarginal sculpture of A. natans walosseki (compare Plate 11, figs. b, c and Plate14, figs. a, b).» H. A. JONKERS, 2003
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«Type material
Holotype (lv) ZMH 2909; off Argentina, 40°27.8’S, 56°31.8’E, 164 m, 27 Jun. 1978, RV Walther Herwig (not seen). Material examined SE of Auckland Islands, 51°10.0’S, 166°51.0’E, 490-510 m (2 v, M.118772). Distribution South-eastern South America and off Auckland Islands, southern New Zealand, 125–960 m (shells only) (Fig. 35). Remarks The two valves from off the Auckland Islands were trawled together with a quantity of similarly bleached, oldlooking shells and presumably date from the last glacial maximum.» DIJKSTRA, H. H. & B. A. MARSHALL. 2008. The recent Pectinoidea of the New Zealand region (Mollusca: Bivalvia: Propeamusiidae, Pectinidae and Spondylidae). Molluscan Research, 28 (1): 1-88, figs. 1-70. [p. 50]
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Austrochlamys natans walosseki Jonkers, 2003; H. H. Dijkstra & B. A. Marshal, 2008, The recent Pectinoidea of the New Zealand region, figure 37H.
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