Leopecten cocosensis Waller, 2007
WALLER, T. R. 2007. The evolutionary and geographic origins on the endemic Pectinidae (Mollusca: Bivalvia) of the Galapagos Islands. Journal of Paleontology, 81 (5): 929–950. [p. 946, figs. 2.11-2.14]
2007 Leopecten cocosensis Waller, 2007
T. R. Waller, 2007, figure 2.
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«Diagnosis.— Leopecten with overall curvature of left valve in dorsoventral plane slightly convex in dorsoventral profile, with change in curvature from early concave to later convex occurring at valve height of around 18 mm. Shell height and length about equal or with height slightly exceeding length. Free margins of auricles forming obtuse angles with hinge; byssal notch very shallow or obsolete.
Description.— Shell outline: Shell of moderate size, up to about 60 mm in height, equilateral, acline, with length and height about equal. Right valve moderately convex; left valve slightly convex in dorsoventral plane, with early flat or slightly concave stage changing to convex later stage at shell height of about 18 mm. Margin of disk circular, anterodorsal and posterodorsal margins of disk slightly outwardly concave in lateral view. Disk flanks of both valves very low but steep, those of right valve slightly undercut; right anterior disk flank curving outward to form flange parallel to plane of commissure; ctenolium consisting of very small teeth present on edge of recurved flange in very early ontogeny only. Narrow disk gapes present between anterodorsal and posterodorsal margins of disks ventral to leading edges of auricles; byssal gape small, commonly at end of arched byssal fasciole bounded ventrally by disk flange; auricular gapes narrow. Auricles about equal in size; dorsal margins of right anterior auricles only slightly scrolled; free margins of all auricles convex, forming obtuse angles with dorsal margins; byssal notch very shallow to obsolete in late ontogeny. Ornament: Right disk with 22–25 plicae, broader than interspaces; plicae low, trigonal, and medially carinate in mid-ontogeny, those of central part of disk becoming tricarinate in late ontogeny while lateral plicae become bicarinate, two or three lateralmost plicae finer than others. Closely spaced (about 80 in 5 mm distance beginning 40 mm from beak), punctate, commarginal lamellae present in interspaces and extending over tops of plicae in late ontogeny; each lamella commonly extending parallel to shell surface and merging with following lamella to form secondary external surface (‘‘pseudo-surface’’); commarginal growth lines absent from interspaces between projecting lamellae; prismatic stage of right valve smooth and glossy, extending distally to origins of radial plicae and terminating at valve height of about 7 or 8 mm. Auricles covered by closely spaced commarginal lamellae; right anterior auricle with two or three radial costae in early ontogeny, absent in later ontogeny; right posterior lacking costae. Left disk with plicae interlocking with those of right disk, narrower than interspaces and rounded to trigonal with only obscure medial carination; second-order very low medial costa intercalated in some but not all interspaces in late ontogeny (at height of about 40 mm); commarginal lamellae in interspaces less prominent than those on right valve; pre-radial stage of left valve smooth proximally and with very fine antimarginal striae distally. Left anterior auricle with two low, widely spaced costae in dorsal half and commonly with third costa in lower half; left posterior auricle with three low, widely spaced costae. Color: Right valve white, with umbonal region yellowish or pink; left valve overall yellowish, with somewhat darker maculations or banding but without high contrast. Interior: Right valve with prominent dorsal tooth bordering ventral side of ligament groove on each side of hinge; intermediate teeth absent; resilial teeth moderately sharp and narrow; left valve with outer ligament forming dorsal margins of auricles, infradorsal teeth bordering ventral sides of right dorsal teeth, and with shallow sockets to accommodate weak resilial teeth of right valve; fine irregular vertical striae present on apposition zones of hinge plate, crossing dorsal teeth. Resilium high and narrow; ventral borders of both resilifers nearly flush with floors of umbonal cavities or slightly elevated. Basal auricular buttress present at base of each right auricle, ending abruptly in low denticle where free margin of auricle meets disk; basal auricular buttresses less evident on left valve except for raised termini higher and more prominent than on right valve. Interior plicae with well-developed marginal carinae near disk margin. Adductor scar of left valve slightly impressed on dorsal side. Pallial line well inset from margin of disk. Shell microstructure: Thin, outer, finely columnar prismatic calcitic outer layer of right valve extends from prodissoconch boundary to shell height of 7 or 8 mm, extending over proximal ends of disk plicae and terminating just before start of commarginal lamellae. Outer ostracum foliated calcite; inner layer of aragonitic structure transgressed during ontogeny by innermost layer of foliated calcite. On left valve this transgression extends to pallial line in late ontogeny; on right valve foliated transgression less extensive in early ontogeny, extending only to about level of ventral margin of adductor scar. Prodissoconch: Prodissoconch umbonate and of planktotrophic type, with small PI and large PII stage; length 234 m (measurable on only one of the specimens examined). Etymology.— Named after Isla del Coco (Cocos Island), Costa Rica. Types.— Holotype: USNM 764999, pair of matching valves, Chatham Bay, Cocos Island, Costa Rica, depth 86 m, R/V Velero III, Station 780b–38, 14 January 1938, sand. Paratypes: USNM 1081784, pair of matching valves, same data as preceding; USNM 122864, pair of matching valves, near Cocos Island, Costa Rica, 05º32'N, 86º54.5'W, 121 m, bottom temperature 14.5C, rocky, R/V Albatross, Station 3368, 24 February 1891. Measurements of holotype.— Height 56.8 mm, length 57.5 mm, hinge length 21.0 mm, convexity 10.0 mm. Occurrence.— Known only from the vicinity of Cocos Island, Costa Rica, Holocene. Discussion.— Leopecten cocosensis most closely resembles L. sericeus, a species with which it was confused by Grau (1959, p. 142). Leopecten cocosensis differs from L. sericeus in being less elongate relative to height, in having more prominent radial costae on the left auricles (particularly in early ontogeny), right plicae that become trigonal earlier in ontogeny, and a left valve that is convex in its overall curvature rather than concave as in L. sericeus. So far as known, L. sericeus does not occur at Cocos Island, although this species is present southwest of Buenventura, Colombia (USNM 753249 and 753240), and at Isla de la Plata, Ecuador (USNM 765005).» THOMAS RICHARD WALLER, 2007
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