Chagrepecten paracactaceus 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. 67, pl. 8, figs. 8-20]
2011 Chagrepecten paracactaceus Waller, 2011
T. R. Waller, 2011, plate 8.
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«Diagnosis.— Chagrepecten n. gen. with 15-17 radial ribs with very narrow crests; rib crests on RV bordered by deep intercostal spaces.
Description.— Shell small, maximum known ca. 30 mm Ht, Ht exceeding L in early ontogeny, equilateral later, acline to slightly prosocline, nearly equivalved and of low convexity, with convexity of RV slightly greater than LV; umbonal angle ca. 105-113º; disks with 15-17 simple ribs originating from 2.1-2.4 mm Ht after early prismatic stage of RV and smooth pre-radial stage of LV; rib crests of RV very narrow, bearing projecting scales spaced further apart than commarginal lamellae on rib flanks; rib flanks concave in profile in early ontogeny, becoming more gradually sloping toward shallowly rounded rib interspace later, covered by merged commarginal lamellae producing secondary surface; enclosed vesicles on rib flanks commarginally elongated and nearly perpendicular to rib crest; rib interspaces with 4 or 5 very fine radial costellae beginning by 4.5 mm Ht, increasing by intercalation to 6 or 7 costellae at distal margin; spaces between costellae bearing finely cusped lamellae that commonly merge with succeeding lamellae; left disk with similar pattern of ribs, scales, costellae, and vesicles but rib crests even narrower than on right and with additional narrow intercostal space with vesicles immediately adjacent to rib crest beginning in mid-ontogeny and persisting to margin. Disk flanks low and steep, with very faint radial costellae in late ontogeny. Auricles small relative to size of disk; anterior auricles slightly longer than posterior, total hl slightly less than half length of disk; right anterior auricle with coarsely scabrous dorsal margin bordered ventrally by 4 more finely scabrous radial costae, with additional fine costellae intercalated between costae near margin; byssal fasciole fairly broad, nearly planar in cross section, crossed by commarginal lirae, depth of byssal notch becoming shallow in late ontogeny, with depth only ca. ⅓ length of auricle; ctenolium becoming weakly developed in late ontogeny, consisting of few teeth obscured in planar view of valve by overhanging flange at base of anterior disk flank; posterior margin of right posterior auricle sigmoidal with overall trend forming slightly acute angle with dorsal margin, dorsal margin bearing smaller scales than on anterior dorsal margin, surface with 3 primary radial costae, dorsal costa close to dorsal margin, other two widely spaced with fine radial costellae between them; left auricles with smooth, straight dorsal margins, one pair of dorsal costae close together, and 1 or 2 additional major costae ventral to these, separated by fine radial costellae. Hinge dentition of RV consisting of long, narrow dorsal tooth on each side of resilifer, bordered ventrally by long, narrow infradorsal socket and then obscure intermediate tooth of low relief; resilial teeth low and poorly demarcated; entire hinge plate crossed by irregular vertical microcrenulations. Inner foliated-calcite layer extending ventrally from dorsal margin to level of middle of adductor scar on anterior side, passing down anterior margin of striate muscle scar into re-entry between striate and nonstriate portions of adductor and along ventral margin of adductor scar; this ventral calcite not thickened to form raised adductor platform. Internal plicae with well-developed carinate edges near margin of shell. Etymology.— prefix para- from Greek, meaning "near," combined with the species name cactaceus, signifying close resemblance to Pecten cactaceus. Holotype and measurements.— NMB G17515 (Pl. 8, Figs 8-15), a pair of matching valves, 28.2 mm Ht, 28.2 mm L, 7.5 mm cvx of articulated shell. Type locality.— Locality NMB 15827, left bank of Río Gurabo, Mao Formation, 658-660 m above base of section (Saunders et al., 1986: text-fi g. 4). Other material.— Paratypes, 6 whole or partial RV and 7 LV from 8 localities on the Río Gurabo and Río Mao, upper Gurabo and Mao formations (Table 12). Remarks.— The rarity of this new species is probably related to its deep-water habitat in which the population was originally sparsely distributed. At TU 1292, in the upper Gurabo Formation on the Río Mao, Bold (1988: 14) noted the presence of deep-water ostracodes, and E. Vokes (1989: 18) referred to the beds at this locality as representing the deepest water of the Gurabo Formation when proceeding downstream (i.e., up the section). NMB 16016 is in the Mao Formation near its base on the Río Gurabo, where Saunders et al. (1986: 16) referred to "badly sorted material swept into a relatively deep-water environment, probably by turbidity flows." Comparisons.— Chagrepecten paracactaceus n. gen., n. sp. differs from C. cactaceus in having a greater number of ribs, usually 15 compared to 13, although rib counts can overlap. There is also a different pattern of secondary costation, particularly on the right valve. On the right valve of C. cactaceus (Pl. 8, Figs 6-7), the central rib crest is slightly wider and is not bordered on each side by an intercostal space that is significantly wider than the other intercostal spaces, and the costellae present in the rib interspaces of C. cactaceus are coarser and more minutely scabrous, with closed vesicles less common between the costellae. The sculptural pattern of left valves of the two species are more similar than on right valves, but in C. paracactaceus n. gen., n. sp., the rib crests are much narrower, and the costellae in the center of rib interspaces are finer, less scabrous, and begin much later in ontogeny (at ca. 20 mm Ht in C. paracactaceus n. gen., n. sp. compared to 5 or 6 mm in C. cactaceus). Closed vesicles that produce a secondary surface are more commonly present in C. paracactaceus n. gen., n. sp. then in C. cactaceus. Evolution.— The occurrences of Chagrepecten paracactaceus n. gen., n. sp. in the Pliocene upper Gurabo and Mao formations postdate the occurrences of C. cactaceus in the Chagres Formation of Panama, which is now regarded as Late Miocene in age (Collins et al., 1996; Collins & Coates, 1999), and the possibly correlative Coatzacoalcos Formation of southeastern Veracruz, Mexico. It is possible, therefore, that C. paracactaceus n. gen., n. sp. evolved from C. cactaceus. (See also the remarks under Chagrepecten n. gen.). Distribution.— Chagrepecten paracactaceus n. gen., n. sp. is known only from the upper Gurabo and Mao formations, Lower and Middle Pliocene, of the northern Dominican Republic.» THOMAS RICHARD WALLER, 2011
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