Cyclopecten acuminatus 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. 18, pl. 1, figs. 1-3]
2011 Cyclopecten acuminatus Waller, 2011
T. R. Waller, 2011, plate 1.
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«Diagnosis.— Small Cyclopecten with tiny prisms in very narrow commarginal rows, apposition zones of hinge with transverse ridges, smooth left exterior with anterior disk flank forming sharp angle at intersection with disk; right anterior auricle narrowly tapering with prominent thin dorsal scroll, left anterior auricle bearing coarse commarginal ridges along its base; posterior auricles pointed, with posterior auricular margins forming acute angles with dorsal margins.
Description.— Cyclopecten of small size, not known to exceed 5 mm Ht; disk unevenly rounded with posterior margin slightly flattened, equivalved, acline, convexity moderate; auricles relatively large, anterior auricle narrowly tapering with deep byssal notch, posterior auricles pointed, with posterior auricular margins forming acute angles with dorsal margins. Exterior of left disk smooth and glossy, exterior of right disk with very fi ne commarginal ridges separating prism bands; right anterior auricular surface crossed by raised commarginal lamellae outlining its narrowly tapering form, byssal fasciole set off by single weak radial costella; left anterior auricle with prominent raised commarginal lirae on its ventral part; posterior auricles lacking radials, bearing only fi ne commarginal growth lines; disk flanks of RV low and rounded, but anterior disk flank of LV steep, forming sharp angle with disk surface. Hinge lacking discrete teeth, but with ventrally convex apposition zone on each side of hinge plate crossed by regular vertical ridges that incline dorsally toward beak; internal ribs absent except for weak rib developed from internal aragonitic layer on posterior side. Shell microstructure consisting of thin prismatic outer layer on RV with tiny irregularly rectangular prisms arranged in narrow commarginal bands reaching maximum width of only ca. 20 μm; thin outer non-prismatic calcitic layer on LV; inner layer of crossed-lamellar aragonite present on disk and auricles of both valves, appearing opaque white as seen through gray, translucent outer calcitic layers. Adductor scars not fully exposed, but left adductor scar apparently not deeply impressed. Pigment patterns rare, observed as evenly distributed white spots on one LV.
Etymology.— Referring to the acutely pointed auricles and angular intersection of the left anterior disk flank with the disk surface.
Holotype and measurements.— NMB G17491 (Pl. 1, Figs 1-2), partial LV missing periphery of disk, height from dorsal margin to midventral fracture 2.7 mm, length from anterior margin to posterior fracture 3.1 mm, hl 2.6 mm.
Type locality.— NMB 15829, Río Gurabo, Mao Formation, 758-761 m above base of section (Saunders et al., 1986: text-fig. 4), northern Dominican Republic.
Other material.— Thirteen paratypes: NMB 15823 (1 partial LV); NMB 15828 (1 RV, 8 LV, poorly preserved on blocks of matrix); NMB 15829 (1 LV, 2 RV), all from the Mao Formation on the Río Gurabo, spanning a stratigraphic interval from 670-761 m above the base of the section as plotted by Saunders et al. (1986, text-fig. 4).
Remarks.— The concept of Cyclopecten acuminatus n. sp. is based mainly on the holotype (Pl. 1, Figs 1-2) and the single right valve from locality NMB 15828 (Pl. 1, Fig. 3). The other specimens are too poorly preserved to be of much use in assessing variation or in expanding the description. Because specimens that are free of matrix are incomplete and those that are embedded in matrix have only their exteriors accessible, the presence and extent of lateral internal ribs are uncertain. There is little doubt, however, that these are not the juveniles of much larger Parvamussium. Left valves are readily identifiable by directing light at a low angle from the posteroventral side. This brings out the sharpness of the intersection between the disk and the anterior disk flank as well as showing the coarseness of the commarginal ridges along the base of the left anterior auricle.
The species clearly lived in deep water, as evidenced by its association with Dimya sp. and Chagrepecten paracactaceus n. gen., n. sp. Saunders et al. (1986: 16) commented on the common occurrence of turbidity-flow deposits between 600 and 700 m above the base of the section on the Río Gurabo and inferred, on sedimentological grounds, that depths of deposition higher in the section, from ca. 700-770 m, were still probably >100 m. Comparisons.— Cyclopecten acuminatus n. sp. is morphologically closest to C. zalaya n. sp. but diff ers in having much more acute auricles, particularly the right anterior one, and in having a left anterior disk flank that forms a sharp angle with the disk surface. See the section on C. zalaya n. sp. for other comparisons.
Evolution.— Cyclopecten zalaya n. sp., which stratigraphically precedes but broadly overlaps C. acuminatus n. sp., is possibly the ancestral species. If this is the case, then evolutionary trends involved the loss of more centrally located internal ribs and an increase in the acuteness of auricles and disk flanks.
Occurrence.— In the northern Dominican Republic, Cyclopecten acuminatus n. sp. is known only from the lower part of the Mao Formation at the three localities listed above.
Distribution.— Cyclopecten acuminatus n. sp. has not been recognized outside of the Dominican Republic.»
THOMAS RICHARD WALLER, 1011
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