Gurabopecten uniplicatus 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. 72, pl. 9, figs. 6-16]
2011 Gurabopecten uniplicatus Waller, 2011
T. R. Waller, 2011, plate 9.
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«Diagnosis.— As for the new genus Gurabopecten.
Description.— Shell small, not known to exceed 23 mm Ht, distinctly prosocline with posteroventral region extended and posterior disk fl ank longer than anterior, Ht > L (Ht/L = 1.1-1.2), moderate cvx (cvx/L = 0.22-0.29), nearly equiconvex, short hl (hl/L = 0.5-0.6), ahl longer than phl (ahl/phl = 1.2-1.4); ventral margin commonly with slight inturning or ledging beginning in late ontogeny. Disks with radial ribs very uneven in width; central sector of RV with single, wide, nearly flat-crested, steep-sided plica bordered on each side by wide, deep interspace; anterior and posterior sectors each with 5 or 6 much smaller ribs, those next to central sector tending to merge in mid-ontogeny to produce single larger rib that is not as broad as central rib; central sector of LV with central wide, deep interspace bordered on each side by wide, round-crested plica; anterior and posterior sectors each with 4 to 6 much smaller ribs and a deeper interspace bordering each side of central sector; all ribs becoming secondarily costellate in early ontogeny but broad interspaces becoming costellate much later in ontogeny. Commarginal lamellae widely spaced, transverse across interspaces in early ontogeny but forming distally concave arcs on rib flanks and between costellae in later ontogeny. Disk flanks steep, smooth in early ontogeny but with fine radial costellae beginning in mid-ontogeny. Auricles small; right anterior auricle with rounded anterior margin and 4 or 5 radial costae; byssal notch of shallow to moderate depth bordered by active ctenolium of only 1 or 2 teeth obscured in lateral view by overhanging flange of right anterior disk flank; left anterior auricle with moderately deep byssal sinus and 5 or 6 radial costae; posterior auricles small, with concave posterior margins that meet dorsal margin at acute angle and 7 or 8 radial costae in early ontogeny, increasing in number by intercalation in mid-ontogeny. Hinge dentition strong, dominated on RV by resilial teeth, which can be centrally depressed and somewhat bifid; infradorsal teeth of LV prominent, fitting into deep sockets between resilial and dorsal teeth of RV; all teeth crossed by irregular vertical microridges. Striate and nonstriate adductor scars of RV deeply separated; adductor scar of LV with posterodorsal projection formed by scar of byssal retractor muscle. Foliated-calcite re-entry on RV completely penetrating separation between striate and nonstriate adductor scars and variably extending across valve along ventral margin of nonstriate adductor scar, remaining separated from midventral pallial line by zone of aragonite; foliatedcalcite re-entry of LV commonly extending to level of ventral margin of adductor scar but not extending posteriorly along ventral margin of scar. Pigment pattern, preserved in several specimens, consisting of broad, dark diagonal bands, observed mainly on posterior sector of LV, and narrower transverse bands on posterior disk flank (Pl. 9, Fig. 11). Etymology.— Named with reference to the single major plica in the central sector of the right valve. Holotype and measurements.— USNM 540976 (Pl. 9, Figs 6-10), 1 RV, 18.0 mm Ht, 16.1 mm L, 8.5 mm hl, 4.9 mm cvx. Type locality.— Locality TU 1211, left bank of Río Gurabo, second bluff below the ford on the [old] Los Quemados-Sabaneta road (= USGS 8546), ca. 381-385 m above base of section, Gurabo Formation, Upper Miocene, northern Dominican Republic (Saunders et al., 1986: text-fi g. 15). Other material.— 165 specimens from 21 localities in the northern Dominican Republic (Table 13). All but one of these are single valves. Remarks.— Specimens of Gurabopecten uniplicatus n. gen., n. sp. vary slightly in shape, although in all cases height exceeds length. The amplitude and width of the hypertrophied plicae of the central sector of each valve are fairly constant, but the spacing of the smaller ribs of the anterior and posterior sectors is more variable. These ribs can variously merge or cluster adjacent to the central sector, in some cases producing an additional enlarged plica on each side of the central sector of the right valve and a corresponding deep groove on the left valve. In rare cases (two specimens, including the holotype), a small primary riblet, originating at the boundary of the prismatic stage, can occur in the center of one of the large interspaces of the right valve (Pl. 9, Fig. 6). Its expression on the left valve is a groove down the center of the corresponding major rib (Pl. 9, Fig. 9). Gurabopecten uniplicatus n. gen., n. sp. is interpreted as a species adapted to life on fine sediment, well within the photic zone, probably near the outer depth range of Argopecten thetidis and clearly in deeper water than A. eccentricus. Gurabopecten uniplicatus n. gen., n. sp. is associated with A. thetidis at 17 of the 21 localities (Table 13) but with the shallower-water species A. eccentricus at only one locality (TU 1250 on Río Verde). This latter locality, however, has several gravity-flow deposits in which specimens originally deposited in shallow water have been transported into deeper water (E. Vokes, 1989: 12). This could explain the additional association at this locality of G. uniplicatus n. gen., n. sp. with Parvamussium, a propeamussiid normally associated with deep water. Preference for a fine silt sedimentary environment is indicated by the matrix adhering to many specimens. A depth of deposition within the photic zone (< 200 m) is indicated not only by the shell pigment patterns on many specimens, particularly on left (upper) valves), but also by the presence of abundant foraminiferans of the genus Amphistegina. Saunders et al (1986: 16) characterized the paleoenvironment of that part of the Río Gurabo section in which G. uniplicatus n. gen., n. sp. occurs (ca. 285 to nearly 400 m above base of section) as having a depth of deposition "probably greater than 40 m and open ocean circulation." The stratigraphically highest specimens of G. uniplicatus n. gen., n. sp. collected on the Río Gurabo occur in the Gurabo Formation ca. 400 m above the base of section (Table 13) where there is rapid deepening of the depth of deposition to > 200 m (Saunders et al., 1986: 16). Comparisons.— Gurabopecten uniplicatus n. gen., n. sp. is distinguished from all other Neogene Pectinidae of the western Atlantic-Caribbean-Panamic region by way of its huge central plicae and minor lateral ribs. The distinctly pitted left beak (preradial zone) and relatively far-set commarginal lamellae, characters shared by G. uniplicatus n. gen., n. sp. with other members of the subfamily Aequipectininae, distinguish the species from uneven-ribbed members of the subfamily Decatopectininae Waller, 1986, which have smoother preradial zones and very close-set commarginal lamellae. Evolution.—I have observed no stratigraphic sequence of morphologies leading to the form of Gurabopecten uniplicatus n. gen., n. sp. The species appears and disappears suddenly after its narrow stratigraphic range in the Gurabo Formation. The morphology of the species suggests a sister-group relationship with Argopecten thetidis, and the rare specimen of the latter with unevenly spaced ribs, mentioned above, lends support to this hypothesis. The absence of Gurabopecten n. gen. in other regions could be due to its narrow ecological range, such that similar conditions have not been preserved at localities thus far sampled, and also due to its narrow stratigraphic range, suggesting that precisely correlative deposits in nearby regions have not been sampled. Occurrence.--Gurabopecten uniplicatus n. gen., n. sp. occurs almost entirely in the Gurabo Formation exposed along the Río Gurabo from ca. 285 to 403 m above the base of the section (Table 13). Th is interval extends from the Upper Miocene into the Lower Pliocene, the Miocene/Pliocene boundary occurring just below 400 m above the base of the section (Saunders et al., 1986: 17). Distribution.--Gurabopecten uniplicatus n. gen., n. sp. is known only from Upper Miocene to Lower Pliocene of the northeastern Dominican Republic.» THOMAS RICHARD WALLER, 2011
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