Cyclopecten zalaya Waller, 2011
WALLER, T. R. 2011. Neogene Paleontology of the Northern Dominican Republic. 24. Propeamussiidae and Pectinidae (Mollusca: Bivalvia: Pectinoidea) of the Cibao Valley. Bulletins of American Paleontology, 381: 1-197, pls. 1-18 [p. 23, pl. 1, figs. 9-12]
2011 Cyclopecten zalaya Waller, 2011
T. R. Waller, 2011, plate 1.
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Diagnosis.— Small Cyclopecten with tiny prisms of more or less uniform size in commarginal rows, broad apposition zones of hinge with prominent transverse ridges, single internal rib on each side of disk formed from inner aragonitic layer, smooth left exterior, and shallowly sigmoidal posterior auricular margins.
Description.— Cyclopecten of small size, not known to exceed 5 mm Ht; disk unevenly circular, with anterior margin more sharply rounded than posterior; equivalved, convexity moderate; acline, with anterior slightly more extended than posterior; auricles of moderate size relative to size of disk, approximately equal in length or posterior auricle slightly shorter than anterior; right anterior auricle with shallow byssal notch with rounded or blunt apex, anterior margin rounded, dorsal margin with narrow, very tightly folded, fragile scroll near distal end; anterior margin of left anterior auricle shallowly sigmoidal, its dorsal part intersecting dorsal margin at obtuse angle and ventral part nearly vertical, lacking byssal sinus, dorsal margin straight; posterior margins of posterior auricles straighter than margin of left anterior auricle and meeting dorsal margin at slightly acute angle; dorsal margin of right posterior auricle with shallow, fragile scroll near distal end. Exterior sculpture of right disk consisting of fine commarginal lines demarcating commarginal bands of rectangular calcitic prisms; right anterior auricle with commarginal lirae and single, slightly scabrous radial costa setting off narrow byssal fasciole that otherwise would not be distinguishable from surface of auricle; other auricles lacking radial costellae, crossed only by weak commarginal lirae; left disk nearly smooth except for fine commarginal lirae and even finer growth lines, surface glossy in well-preserved specimens; disk flanks of RV valve low, anterior one slightly concave, sloping toward auricle, posterior flank lower and steeper; disk flanks of LV low and moderately steep. Hinge lacking discrete teeth but having ventrally convex apposition zone on each side of hinge plate crossed by regular vertical ridges that incline dorsally toward beak; apposition zones dark gray in contrast to adjacent white inner shell layer. Interior of disk with single internal rib on each side near disk flank, posterior rib stronger and beginning earlier in ontogeny than anterior rib, rarely 1 or 2 additional incipient internal ribs present on medial side of posterior rib, rib termini attenuated or slightly raised; very thin rib or knob present on interior of base of each auricle, more developed on posterior side than anterior. Shell microstructure of RV consisting of very thin outer layer of simple columnar prismatic calcite extending to margin of disk, with tiny, radially elongate, rectangular prisms arranged in commarginal rows, maximum dimensions of prisms ca. 40 μm. Thin foliated calcitic layer inside outer prismatic layer of RV and forming outer surface of LV. Inner shell layer of both valves commarginal crossedlamellar aragonite, extending nearly to margins of disk but withdrawn slightly from distal margins of auricles; internal ribs forming within crossed-lamellar layer, lacking calcitic cores. Dorsal margins of adductor scars impressed, that of RV much higher in umbonal region than that of LV. Pigment patterns commonly well developed from beak to margin; RV with 12-17 broad, opaque white rays of somewhat uneven trend, with bifurcation and intercalation toward margin; LV with white spots commonly clustered in 4-7 indistinct, distally widening, radial bands.
Etymology.— Named for Arroyo Zalaya, northern Dominican Republic.
Holotype and measurements.— USNM 540939, 1 LV, 4.4 mm Ht, 4.3 mm L, 0.4 mm cvx (Pl. 1, Fig. 10).
Type locality.— TU 1227A, Arroyo Zalaya, "Turbidity flow lens (ca. 30" long, 6" thick) ca. two feet above base of outcrop at point approximately 75 feet downstream from highway bridge," northern Dominican Republic (Saunders et al., 1986: 65).
Other material.— Northern Dominican Republic: 178 specimens (84 RV, 94 LV) from five localities (Table 2); Jamaica, Bowden shell beds: 1 RV in Dall's type material of Cyclopecten guppyi, USNM 135779.
Remarks.— Cyclopecten zalaya n. sp. is remarkably constant in shape and external sculpture. The pigment patterns, other than the general plan of opaque radial rays on the right valve and less distinct clustering of white spots into radial rays on the left valve, are much more variable. Among the abundant specimens from TU 1227A, a few have no pigment pattern, whereas on others, the patterns are limited to the umbonal region. The great majority of specimens have only a single internal rib on the interior of the disk on each side, but a few have one or two additional incipient ribs on the medial side of the posterior rib. This was observed on four right valves from
TU 1227A and on another right valve from TU 1250. In the Dominican Republic, Cyclopecten zalaya n. sp. cooccurs with C. guppyi only at two localities where gravity-flow deposits are present (E. Vokes, 1989: 12; E. Vokes & D'Attilio, 1980: 52). In such mixed deposits, C. guppyi would likely be the shallower-water species that was swept into the deeper-water habitat of C. zalaya n. sp. The evidence for this is afforded by the records of these species in the Dominican Republic. Cyclopecten guppyi occurs in the shallow-water facies of the Cercado Formation, whereas C. zalaya n. sp. is restricted to the deeper-water facies of the upper Gurabo Formation (Tables 1-2). Comparisons.— Cyclopecten zalaya n. sp. resembles C. acuminatus n. sp. very closely. The right valves are not distinguishable on the basis of shape, auricular sculpture, or the configuration of prisms in the outer prismatic shell layer. Left valves, however, are distinguishable on the basis of the sculpture
and shape of the left anterior auricle. In C. zalaya n. sp., the byssal sinus is very shallow and the shape of the anterior margin is shallowly sigmoidal, intersecting the dorsal margin at an obtuse angle. In C. acuminatus n. sp., the left anterior auricle is pointed, with a prominent byssal sinus that begins in early ontogeny as evidenced by the trends of coarse commarginal ridges. In addition, C. zalaya n. sp. has more prominent lateral internal ribs. In the Dominican Republic Neogene, Cyclopecten zalaya n. sp. is readily distinguished from C. guppyi by the characters listed in Table 3. Cyclopecten almirantensis (Olsson, 1922), known only from a right valve from the Bocas del Toro basin of Panama (probably from the Pliocene Caya Agua Formation; Collins, 1993: 708) is small in size and has a single internal lateral disk rib on each side, but differs from C. zalaya n. sp. in many other characters: costate auricles; radially aligned nodes on the prismatic calcite layer of the right valve; commarginal prism bands that alternate between tiny polygonal prisms and larger, radially elongate prisms; lateral internal aragonitic ribs that have calcitic cores (because their termini are within the foliated-calcite marginal secretion band on the shell interior); and less well-developed vertically ridged apposition zones on its hinge plates. Cyclopecten thalassinus (Dall, 1886), an extant species with a broad geographic range in the tropical and temperate western Atlantic and a depth range from 40- >500 m (Abbott, 1974: 449; Theroux & Wigley, 1983), is a close living counterpart of C. almirantensis and differs from C. zalaya n. sp. in the same manner. In addition, C. thalassinus commonly has sculpture of high relief on its left valve. In the eastern Pacific region, the extant species Cyclopecten cocosensis (Dall, 1908), from Cocos Island, Costa Rica, and the Gulf of Panama at depths from 95-113 m, has single internal lateral disk ribs as well as a pigment pattern on its right valve consisting of whitish rays (Grau, 1959: 30). But it also has radial costellae and early reticulation on its left disk as well as commarginal prism bands alternating in prism size, more like the outer prismatic layer of C. thalassinus than like that of C. zalaya n. sp. Evolution.— In the Dominican Republic, the stratigraphic distributions of Cyclopecten zalaya n. sp. and C. guppyi are clearly facies controlled. Although the latter occurs stratigraphically lower, in the Cercado Formation, there is no reason to think that its acutal stratigraphic range precedes that of C. zalaya n. sp. Th is is because no comparable deep-water facies in which C. zalaya n. sp. would be expected to occur are present in the Cercado Formation. Elsewhere the relationships of Cyclopectens of very small shell size are still too poorly known to understand their evolutionary relationships.
Occurrence.— In the northern Dominican Republic Cyclopecten zalaya n. sp. is known only from the Pliocene part of the Gurabo Formation and the lower Mao Formation at 5 localities (Table 2).
Distribution.— Outside the Dominican Republic, the new species is known only from the Bowden shell bed, Bowden Formation, Jamaica, now regarded as late Middle Pliocene in age (Kohl & Robinson, 1998; Donovan, 1998: 6).
THOMAS RICHARD WALLER, 2011
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