Leopecten isabelensis 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. 945, figs. 2.6-2.10]
2007 Leopecten isabelensis Waller, 2007
T. R. Waller, 2007, figure 2.
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«Diagnosis.— Leopecten with overall curvature of left valve in dorsoventral plane very slightly convex, with inflection from early concave stage to later convex stage occurring at valve heights of about 30–35 mm. Plicae of right valve broader than interspaces, with gently rounded crests and steep but not undercut sides.
Description.— Shell outline: Shell of moderate size, up to 70 mm in height, equilateral, acline, with length exceeding height. Right valve moderately convex, left valve nearly flat, with early slightly concave stage followed at shell height of 30–35 mm by slightly convex to flat stage. Margin of disk nearly circular, anterodorsal and posterodorsal margins of disk slightly outwardly concave in lateral view. Disk flanks of both valves very low but steep; right anterior disk flank curving outward to form flange parallel to plane of commissure; ctenolium consisting of very small teeth present on this flange in early ontogeny, disappearing in late ontogeny. Narrow gape present between closed valves on anterior and posterior sides of disk ventral to auricles; byssal gape small, at end of radial arch on right anterior auricle adjacent to disk suture; auricular gapes narrow. Auricles about equal in size, anterior auricles slightly less broad than posterior. Dorsal margins of right auricles slightly scrolled, causing these margins to rise distally and to form broad ‘‘V.’’ Free margin of right anterior auricle descending ventrally at obtuse angle with dorsal margin, then curving sharply inward at dorsal edge of byssal arch, then forming shallow concavity over byssal gape. Dorsal third of free margins of left anterior and both posterior auricles forming obtuse angle with dorsal margin, then descending ventrally at right angle to dorsal margin. Ornament: Right disk with 23–26 plicae, strong, rectangular, and broader than interspaces, with gently rounded to nearly flat crests and steeply sloping but not undercut sides, three anterior and three posterior plicae finer than more central plicae; plicae of right disk lacking raised medial carina on crest and not known to become distinctly grooved or tricarinate in late ontogeny. Closely spaced, punctate, commarginal lamellae present in interspaces (about 27 lamellae per 5 mm at distance of 40 mm from beak in center of disk), rising up and extending across ribs in later ontogeny. Five or six commarginal growth lines present between each pair of projecting lamellae in rib interspaces in mid-ontogeny. Prismatic stage of right valve smooth or with low, irregular commarginal undulations, extending distally to origins of radial plicae and terminating at valve height of about 8–10 mm. Right anterior auricle with three radial costae dorsal to byssal fasciole in very early ontogeny, becoming smooth except for commarginal lamellae in later ontogeny; right posterior auricle with very faint radial costae and commarginal lamellae. Left disk with plicae interlocking with those of right disk, narrower than interspaces and rounded to trigonal, tending to become medially carinate; interspaces flat to gently rounded in profile; commarginal lamellae in interspaces and on rib flanks less prominent and projecting than on right valve; pre-radial stage of left valve smooth or with very fine antimarginal striae visible under high magnification. Left auricles with from one or three costae, dorsal one strongest, others very weak. Color: Right valve white, with faint tint of yellow or pink on umbo and with brownish red flange or right anterior disk flank; left valve light reddish brown, common with narrow dorsally pointing wedges of lighter pigment. Interior: Right valve with dorsal tooth bordering ventral side of ligament groove on each side of hinge; intermediate teeth absent; resilial teeth bordering resilifer very weak; left valve with infradorsal teeth bordering ventral sides of right dorsal teeth and with shallow sockets to accomodate 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. Basal auricular buttress present at base of each auricle, ending abruptly where free margin of auricle meets disk. Interior plicae with well-developed marginal carinae near disk margin. Adductor scar of left valve slightly inset, its outline extended on posterodorsal side probably as result of insertion of pedal retractor. Pallial line well inset from margin of disk (midventral inset of 23 mm on a left valve with height of 70 mm). Shell microstructure: Thin, outer, finely columnar prismatic calcitic outer layer of right valve extends from prodissoconch boundary to shell height from 7.5 to nearly 10 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 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 265–295 m. Etymology.— Named after Isla Isabela (Albemarle Island), Galápagos Archipelago, Ecuador. Types.— All are from Archipie´lago de Colo´n (Gala´pagos Islands), Ecuador. Holotype: USNM 753317, right valve, northeast of Cabo Barrington, Isla Isabela (Albemarle Island), 0º37–34'S, 90º51–52'W), depth 78 m, R/V Anton Bruun, Cruise 188, Station 795, 24 September 1966. Paratypes: USNM 1081785, four right valves and three left valves, same data as for holotype; USNM 765000, Post Office Bay, Isla Santa Marı´a (Charles Island), depth 64-73 m, R/V Velero III, Station 197-34, 29 January 1934; USNM 765002, small pair of matching valves, between Isla Seymour and Isla Daphne, depth 100 m, R/V Velero III, Station 346-35, 13 December 1934; USNM 765020, pair of matching valves (juvenile), Tagus Cove, Isla Isabela, depth 55 m, R/V Velero III, Station 147-34, 13 January 1934; LACM 34-57, two right valves and three left valves, between Isla Albany and Isla San Salvador (James), 010.8'S, 90º52.13'W, 91-128 m, R/V Velero III, Station 183-34, 24 January 1934; LACM 34-64, right valve, east of south end of Isla Isabela (Albemarle), 0º55'S, 90º30'W, depth 106-110 m, sand, nullipores, R/V Velero III, Station 190-34, 26 January 1934; LACM 34-199, two right valves and one left valve, off Tagus Cove, Isla Isabela (Albemarle), 0º16.78'S, 91º23.02'W, depth 146 m, R/V Velero III Station 325-35, 10 December 1934; LACM 38-193, right valve, Academy Bay, Isla Santa Cruz (Indefatigable Island), approximately 0º45'S, 90º18'W, depth 18-37 m, R/V Velero III, Bottom Sample 483, 25 January 1938; LACM 66-207, juvenile articulated shell, west of Isla Baltra, off north end of Isla Santa Cruz, 0º26–27'S, 90º20–21'W, depth 95 m, R/V Anton Bruun, Cruise 188, Station 791-C, 21 September 1966; LACM 66-211, two right valves and four left valves, same data as for holotype; LACM 147-34, left valve, same data as for USNM 765020. Measurements of holotype.— Height 69.0 mm, length 72.5 mm, hinge length 30.0 mm, convexity 15.3 mm. Occurrence.— Known only from the Galápagos Islands, Holocene. Discussion.— Leopecten isabelensis most closely resembles L. stilllmani but differs from that species in the ontogeny of curvature of its left valve. In L. isabelensis the early concave stage is consistently larger, the inflection from concave to convex occurring at a shell height of about 34 mm as compared to about 18 mm in L. stillmani. The left valves of L. isabelensis are also more lightly pigmented with a less contrasty color pattern. The right valves of the two species are very similar in disk shape and plication and in the shapes and costation of the auricles. The right valves of L. stillmani commonly have a pale orange color, rarely completely white, whereas all observed right valves of L. isabelensis are white.» THOMAS RICHARD WALLER, 2007
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