Adamussium cockburnensis Jonkers, 2003
JONKERS, H. A. 2003. Late Cenozoic-Recent Pectinidae (Mollusca: Bivalvia) of the Southern Ocean and neighbouring regions. Monographs of Marine Mollusca, Nº 5: viii, 125 p., pls. 1-17. Backhuys Publishers. [p. 69, pl. 16, figs. i-n; pl. 17, figs. c-d]
2003 Adamussium colbecki cockburnensis Jonkers, 2003
H. A. Jonkers, 2003, plate 16.
H. A. Jonkers, 2003, plate 17.
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«Etymology. From Cockburn Island, the only known locality for this subspecies.
Type material. BAS, DJ.853.60 (internal mould of LV VH 64.5 mm; holotype, illustrated by Jonkers, 1998a, pl. 1, fig. 2, 1998b, fig. 3b; Plate16, fig. i), DJ.853.110 (internal mould of articulated pair, VH 50.7 mm; pararype; Plate 16, fig. m), DJ.854.25 (internal mould of RV VH 47.5 mm; paratype), DJ854.29 (internal mould of RV VH 59.9 mm; paratype; Plate 16, fig. n), DJ.854.41 (internal mould of RV VH 52.5 mm; pararype; Plate 16, fig. j), DJ.854.43 (internal mould of RV VH 51.6 mm; paratype), DJ.854.81 (external mould of RV VH c. 45 mm; pararype; Plate 16, fig. k), from the Pliocene Cockburn Island Formation, Cockburn Island, 64º12' S, 56º51' W, N Antarctic Peninsula, on lower terrace of basalt plateau in SE corner of island, c. 190-200 m above sea level, H.A. Jonkers leg., 20-21 Jan. 1996. Material examined. 6 pairs, 28 LV, 31 RV, 14 indeterminate valves (all internal and external moulds), from the Late Pliocene Cockburn Island Formation, Cockburn Island, northern Antarctic Peninsula; lodged at BAS (see Appendix). Description. Shell of moderate size (mean VH 56 mm; maximum observed VH 61.6 [DJ.853.107]), extremely thin (usually not preserved); valves opisthocline (VHA/VHP and VLA/VLP 0.91; but there is a large error in the latter value [Fig. 41]); weakly convex, with LV more so than RV umbonal angle wide (c. 134°). Dorsal disc gapes present. Radial sculpture of c. 15 broadly undulating, rather low plicae, with narrower and lower ones intercalated between main plicae. Commarginal sculpture formed by regularly and widely spaced lirae. Microsculpture of narrow antimarginal ridges as in A. c. colbechi. Auricles highly asymmetrical (OLA/OLP 1.4); LV anterior auricle with free margin curved to anterior (Plate 17, fig. d); RV anterior auricle with comparatively deep byssal notch. Measurements and counts.VH 55.9 ± 1.2 mm (n= 24), VHA 28.0 ± 2.8 mm (n= 4), VHP 29.9 ± 1.2 mm (n = 2), VL 55.5 ± 4.3 (n = 5), VLA 29.2 ± 1.5 mm (n = 3), VLP 33.5 ± 2.1 mm (n = 3), C (LV) 9.7 ± 0.6 mm (n = 9), C (RV) 7.0 ± 0.4 mm (n = 13), OL 15.1 ± 0.3 mm (n = 6), OLA 8.6 ± 0.3 (n = 8), OLP 6.4 ± 0.2 mm (n = 9), AHA (RV) 8.0 ± 0.6 mm (n = 4), AHA (RV) 6.9 ± 0.3 mm (n = 5), AHP 7.8 ± 0.7 mm (n = 8), BND 3.1 ± 0.3 mm (n = 3), UA 133.5° ± 0.7 (n = 4), AAPD 131.3° ± 2.2 (n = 8). Stratigraphical age and distribution. This new subspecies is known only from the Late Pliocene Cockburn Island Formation (c. 3 Ma [Jonkers & Kelley 1998]; Fig.25), Cockburn Island, northern Antarctic Peninsula (Figs. 3, 6), where it occurs together with abundant Austrochlamys anderssoni (Hennig, 1911). Remarks. Adamussium colbecki cockburnensis subsp. nov. differs from subsp. colbecki in its flatter valves, fewer and less pronounced ribs (compare Plate 17, figs. c and e), a straight hinge throughout ontogeny, highly asymmetrical auricles (mean OLA/OLP 1.45; OLA/OLP of A. c. colbecki is 1.02; Fig. 46), and a deeper byssal notch; its valve shape is possibly more opisthocline. A further difference is seen in its commarginal sculpture, which is more regularly spaced and much wider (0.4-0.7 mm) apart than in modern A. colbecki (Plate 16, fig.k).» H. A. JONKERS, 2003
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