Argopecten parathetidis 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. 55, pl. 6, figs. 15-22]
2011 Argopecten parathetidis Waller, 2011
T. R. Waller, 2011, plate 6.
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«Diagnosis.— Small, right-convex Argopecten with 19-21 ribs having central keel and parabolic cross section; disk flanks lacking radial costellae.
Description.— Shell not known to exceed 15 mm Ht, either equilateral or slightly extended posteroventrally, length and height approximately equal or with length slightly exceeding height. Convexity moderate, with RV commonly more convex than LV in late ontogeny. Disks with 19-21 simple, nonbranching, interlocking radial ribs separated by deep interspaces of approximately same width; rib profi les of both valves parabolic in late ontogeny, with narrow raised keel in center of crests of medial ribs or off set to anterior or posterior side of anterior or posterior ribs respectively. Commarginal lamellae in rib interspaces of both valves prominent, forming distally convex arcs across interspaces, distally concave arcs on rib flanks, and very narrow distally pointing loops on rib keels; projecting scales and secondary radial costae absent. Disk flanks moderately steep and smooth except for fine commarginal lamellae. Auricles of moderate size; anterior auricles commonly longer than posterior; right anterior auricle with 4 prominent slightly scabrous radial costae, slightly scrolled dorsal margin, rounded anterior margin, and moderately deep byssal notch and short ctenolium of 2 or 3 teeth commonly hidden beneath anterior flange of adjacent disk flank; other auricles commonly with 6 or 7 radial costae that do not fade distally, left anterior auricle with byssal sinus, posterior margins of posterior auricles with deep sinus and overall trend forming acute angle with dorsal margin. Right hinge with single strong resilial tooth on each side of resilifer, each bordered dorsally by deep socket to accommodate dorsal tooth of LV; irregular cross ridges prominent on all hinge teeth. Extent of foliated-calcite re-entry on interior of both valves very extensive in late ontogeny, filling space between striate and nonstriate adductor scars of RV and extending along ventral margin of adductor scar of LV. Etymology.—Prefix para-, meaning "near," combined with species name thetidis to indicate a close relationship of the new species to Argopecten thetidis. Holotype and measurements.— USNM 540962, 1 RV, 13.3 mm Ht, 14.0 mm L, 5.3 mm cvx (Pl. 6, Figs 15-18). Type locality.— Locality TU 1224, road cut 6.1 km west of Los Quemados, or 3.1 km west of Río Gurabo, on road to Sabaneta, Gurabo Formation, northern Dominican Republic. This locality was not plotted by Saunders et al. (1986). The Vokeses, in their entry in the Tulane Locality Register, assigned it to the Gurabo Formation. Other material.— 114 specimens (62 RV, 52 LV) from 9 localities in the northern Dominican Republic (Table 10). Remarks.— Argopecten parathetidis n. sp. exhibits a consistent morphology among the nine localities at which it was found, perhaps because these localities fall within a narrow stratigraphic range within the Lower Pliocene part of the Gurabo Formation, suggesting that there was insufficient time for evolutionary change. Pectinids associated with the new species (Table 10) include both shallow-water species (A. eccentricus caimiticus, Lindapecten plurinominis) and deeperwater forms (A. thetidis, Gurabopecten uniplicatus n. gen., n. sp.). Dimya sp., a deep-water indicator, as well as pycnodontinine oysters are present at the type locality, TU 1224. Comparisons.— Argopecten parathetidis n. sp. compares most closely with the variants of A. thetidis that have sharply trigonal ribs and smooth disk flanks (conforming to the morphology of Pecten oxygonum G. B. Sowerby I, 1850, now regarded as a junior synonym of A. thetidis). These forms, however, attain a much larger size, and their rib profiles are trigonal rather than parabolic and lack a distinctly diff erentiated keel. At the six localities (Table 10) at which A. parathetidis n. sp. is associated with A. thetidis, the latter are all of the advanced form with distinctly costellate disk flanks and grooved rib crests commonly with prominent scales along the edges of the crests. The left valve of Argopecten parathetidis n. sp. (Pl. 6, Fig. 19) somewhat resembles some left valves of A. uselmae in which the ribs in late ontogeny are parabolic in cross section with a narrow keel on the rib crest. These also have an extensive foliated-calcite re-entry that passes along the ventral margin of the adductor scar. In these specimens of A. uselmae, however, the commarginals are much less prominent and do not form the tight distally pointed loops on the keels that are present in A. parathetidis n. sp. Furthermore, the right valves of A. uselmae have rounded or flattened ribs without keels, very unlike the right valves of A. parathetidis n. sp. The extant species Argopecten noronhensis (Smith, 1885) as keeled parabolic ribs on its left valve similar to those of A. parathetidis n. sp., but the right valve of the former, like that of A. uselmae, has ribs with smooth, broad, non-keeled crests (Waller, 1973: figs 5-8). Evolution.— Argopecten parathetidis n. sp., like A. thetidis, is apparently endemic to Hispaniola and is assumed to have originated there. The new species is possibly an off shoot of A. thetidis, which has a similarly shaped posterior auricle and extensive foliated-calcite re-entry. Some A. thetidis individuals have a short "parathetidis" phase in their ontogeny, wherein the ribs are trigonal with a central keel and with prominent looped commarginal lirae crossing the keel. Argopecten parathetidis n. sp. is possibly ancestral to the A. noronhensis lineage, a relationship that is suggested by similarities in the sculpture and profi les of the ribs of left valves. Just as A. thetidis developed expanded rib crests possibly in response to life on fi ne sediment, so also A. noronhensis, an extant deep-water species, might have expanded the rib crests of its right valve, retaining plesiomorphic parabolic keeled ribs on the left valve. Occurrence.— Argopecten parathetidis n. sp. occurs in the Río Cana section (NMB 16825) above the Miocene-Pliocene boundary as determined by Saunders et al. (1986: 23). In the Río Gurabo section, the species is known only from TU 1211 (= USGS 8546). According to Saunders et al. (1986, compare text-figs 4-5, and 14), this collecting station spans a stratigraphic interval that includes the Miocene-Pliocene boundary based on either the NN11-NN12 zonal boundary or the first occurrence of Globorotalia margaritae Bolli & Bermúdez, 1965. Along the Río Yaque del Norte, A. parathetidis n. sp. occurs at La Barranca in beds that were determined by Saunders et al. (1986: 30) to be in the upper part of the G. margaritae zone dated as late Early Pliocene. Localities TU 1206 and TU 1207, which also yielded A. parathetidis n. sp., are presumably also in the G. margaritae zone. The road cuts sampled by the Vokeses (TU 1222, 1224, and 1299) are not tied into stratigraphic sections, but if their elevation is higher than nearby river canyons, then they can be expected to be fairly high stratigraphically. This is corroborated by the co-occurrence of A. parathetidis n. sp. with advanced A. thetidis as well as A. eccentricus caimiticus at TU 1222. The same association occurs in the Santiago area at TU 1205. Saunders et al. (1986: 30) determined that beds in this area, as at La Barranca, are in the Pliocene. In all cases where A. parathetidis n. sp. is associated in the same samples with A. thetidis, the latter are of the advanced form with strongly costate, scabrous disk flanks. Distribution.— Argopecten parathetidis n. sp. is thus far known only from the Cibao Valley, northern Dominican Republic.» THOMAS RICHARD WALLER, 2011
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