Chlamys (Argopecten) gibbus antecessor Weisbord, 1964
WEISBORD, N. E. 1964. Late Cenozoic Pelecypods from Northern Venezuela. Bulletins of American Paleontology, 45 (204): 1-564, pls. 1-59. [p. 148, pl. 15, figs. 7-12; pl. 16, figs. 1-4]
«Chlamys (Argopecten) gibbus antecessor, new subspecies, PI. 15, figs. 7-12; PI. 16, figs. 1-4
1925. Pecten {Plagioctenium) gibbus (Linnaeus), Maury, Bull. Amer. Paleont., vol. 10, No. 42, pp. 238-239, pl. 14, fig. 2; pl. 16, fig. 1. Not of Linnaeus ? Cabo Blanco shell of moderate size and thickness, subequilateral, the valves a little unequal, varying in convexity from slight to full, occasionally gently humped medially. Beaks small, extending slightly beyond the hinge line. Ventral margin evenly rounded, the dorsal margins usually slightly concave, the posterior a little longer than the anterior and thus imparting a slight obliqueness to the valves, the divergence averaging about 95 degrees. Dorsal slopes short and steep, the submargins of the disk sharply defined. Hinge line of left valve sharp and straight, the hinge line of the right hardly bowed down toward the beak. Auricles of left valve rather high, a little unequal, the posterior one scarcely concave at the margin, the anterior one gently convex above, incurved below. A shallow radial furrow bisects the left anterior ear a little above the middle, and above the furrow the ear is smoothed and bears two widely spaced inconspicuous radial folds; below the furrow there are four or five closely spaced coarse radial riblets. The left posterior ear is also radiately ribbed but the riblets are finer than on the anterior ear. On the right, or lower valve the auricles are unequal, the posterior one triangular, more or less truncate at the margin, and scalloped along the edge, the anterior one subrectangular, convex at the margin, and subtruncate at the base. On the anterior ear of the right valve there are about five low unequal radial folds, and on the posterior ear there are about six subequal riblets on the lower two-thirds and two inconspicuous riblets on the upper third. Traversing the exterior of the auricles of both valves are concentric laminae which are sharper in the interspaces of the riblets than on the riblets themselves. Internally, the hinge plate is thickened by arcuate cardinal ridges, the ridges of the right valve more robust and more downswept than those of the left valve. On the left valve there is a fairly well-developed cardinal crura along the inner margin of the anterior ear, and below that there are two irregular nodulations representing the termini of the minor crurae. Chondrophore, or resilifer triangular and deep, the sides thickened and merging with the cardinal ridges. There are three to five pectinidial teeth on the byssal area below the right anterior ear, and, when well preserved, the teeth are hooked. Both valves are sculptured by 19 to 21 external ribs, and there are usually two ribs less in the interior than on the exterior. The external ribs are high and regular, flattened or slightly rounded at the crest, and wider than or the same width as the interspaces but occasionally slightly narrower than the interspaces. There seems to be a tendency for the ribs of the right valve to be a trifle wider than those of the left. The exterior is crossed by regularly spaced concentric laminae which are sharp in the interspaces of the ribs but smoothed off on the crest of the ribs. On the short steep dorsal slopes the radial ribs are faint or wanting, and the concentric markings, though crowded, are less conspicuous than on the disk proper. The internal ribs are flat low and broad, the interspaces narrow and shallow except at the ventral margin proper where the interspaces are in the form of short deep triangular flutings, so designed that they interlock with the termini of the ribs of the opposite valve. The internal ribs are thickened slightly at the sides, and they extend to the body cavity toward which they become progressively fainter. The body cavity is situated well within the interior, and the muscle impression is large and rudely orbicular. Dimensions.— Holotype (T344a), left valve, height 32 mm.; width 33.5 mm., thickness 10.2 mm. Paratype (G344b), left valve, height 22.3 mm.; width 22.3 mm.; thickness 6.3 mm. Paratype (G344c), right valve, height 18.7 mm.; width 18.7 mm.; thickness 6.8 mm. Paratype (1344a) right valve, height 17.5 mm.; width 17.8 mm.; thickness 4 mm. Paratype (1344b), left valve, height 18.9 mm.; width 18.3 mm.; thickness 5.1 mm. Paratype (J344a), left valve, height 12.5 mm.; width 11.8 mm.; thickness 2.2 mm. Paratype (J343a), left valve, height 21.4 mm.; width 21 mm.; thickness 5.8 mm, Paratype (L340a), left valve, height 12.2 mm.; width 11.8 thickness 2.1 mm. Paratype (L340b), right valve, height 17 mm.; width 17 mm.; thickness 3 mm. Type locality.— Upper Mare formation, in stream 250 meters south-southwest of the mouth of Quebrada Las Pailas; Two left valves, the larger (T344a), the holotype. Other localities.—Mare formation at W-14, on hillside above west bank of Quebrada Mare Abajo. Six specimens including three left valves, two right valves, and one fragment. Mare formation at W-25, south flank of Punta Gorda anticline. One left valve. Lower Mare formation at W-13, on hillside above west bank of Quebrada Mare Abajo. Four specimens including two left valves and two right valves. Lower Mare formation, in small stream west of Quebrada Mare Abajo. Seven specimens including five left valves and two right valves. Playa Grande formation (Maiquetia member) at W-26, in Quebrada Las Bruscas, approximately 125 meters upstream from junction with Quebrada Las Pailas. One right valve. Playa Grande formation (Catia member), south side of Playa Grande road about 20 meters west of W-15. Four left valves. Playa Grande formation (Catia member), in bluff 125 meters west of the intersection of the Playa Grande Yaching Club road and coast road. One left valve. Remarks.— The new subspecies so closely resembles the Miocene to Recent C. gibbus gibbus (Linneaus) that a near relationship is indicated. C. gibbus antecessor, n. subsp. is differentiated from C. gibbus gibbus in both its much steeper dorsal slopes and thicker, more arcuate cardinal ridges. On many examples of the Recent C. gibbus gibbus the disk and submargins merge with but little change in the convexity, whereas on C. g. antecessor the dorsal slopes steepen so abruptly that the disk proper is subcarinate at the shoulders. I have seen variants of C. gibbus, notably C. gibbus versicolor (Lermond) from New Smyrna Beach, Florida, with fairly steep dorsal slopes, but on that form the cardinal ridges are not so arcuate or so thick as on the Cabo Blanco fossil. C. g. antecessor seems to be the same as Maury's C. gibbus from the Matura Pliocene of Trinidad, but the latter is probably not the C. gibbus of Linnaeus. Comparisons.— C. gibbus antecessor, n. subsp. exhibits affinity to the following American species: Pecten irradians irradians Lamarck (1819, An. sans Vert., vol. 6, p. 173). Pleistocene and Recent, east America. Larger and thinner than C. g. antecessor, and with fewer (17 to 18) ribs. Ostrea nucleus Born (1780, Testacea Musei Caesarel Vindobonensis, p. 107, pl. 7, fig. 2) (see Aequipecten gibbus nucleus (Born), Abbott, 1954, American Seashells, p. 368, pl. 34h). Recent, southeast Florida to northern South America. This is a thin form with 21 to 23 ribs and has smaller ears than C. g. antecessor. Pecten circularis Sowerby (1835, Zool, Soc. London, p. 110) (see Arnold 1906, pp. 125-126, pl. 42, figs. 3-6; pl. 44, figs. 6-7). Upper Pliocene to Recent, west America. The typical Recent form is larger, more convex, and more oblique than C. g. antecessor. Pecten (Plagioctenium) eldridgei Arnold (1906, p. 87, pl. 25, figs. 3-6). Etchegoin formation (Pliocene), California jide Grant and Gale (1931, p. 214). The interspaces are only one-half as wide as the ribs. Pecten (Chlamys) santarosanus Bose ( 1906, Inst. Geol. Mexico, Bol. No. 22, pp. 23-24, pl. 1, figs. 1,4; p. 27, pl. 1, figs. 2,5; pl. 6, figs. 3-4). Pliocene, States of Veracruz and Oaxaca, Mexico. The right anterior ear of the Mexican forms illustrated by Bose is elongate-triangular, whereas that of C. g. antecessor is subrectangular. Pecten levicostatus Toula ( 1908, K.-K. Geol. Reichs. Jahr., vol. 58, p. 713, pl. 26, figs. 4-6). Middle Miocene of the Isthmus of Panama and Costa Rica. The right anterior ear of this species is also subtriangular. It has ben suggested by Olsson (1922, pp. 371-373, pl. 16, figs. 5-7) that the Costa Rican C. levicostata may be the same as Bose's C. santarosana (p. 73, pl. 6, figs. 3-4) from Barranca de Santa Maria de Tatetla, State of Veracruz, Mexico. Pecten circularis venezuelanus F. and H. Hodson (1927, pp. 25-26, pl. 14, fig. 6; pl. 15, figs. 2,4,5; pl. 17, fig. 1) from the Miocene and Pliocene of the State of Falcon, Venezuela, and P. circularis cornellanus F. and H. Hodson (1927, pp. 27-28, pl. 14, fig. 2; pl. 15, figs. 3, 10; pl. 16, fig. 3) from the Miocene of the State of Falcon, Venezuela. These have a somewhat smaller angle of divergence of the dorsal margins than the Cabo Blanco C. g. antecessor. Chlamys (Plagioctenium) amelea Woodring (1925, Carnegie Inst. Washington, Publ. No. 366, pp. 70-71, pl. 8, fig. 9). Middle Miocene of Jamaica. On some examples of this species a slight groove lies on the extreme lower part of a few ribs. No such grooves have been noted on C. g. antecessor. Also the fasciolar radial cord on the right anterior ear next to the margin of the disk is more elevated and much coarser than on the new Venezuelan subspecies. Chlamys (Plagioctenium) nicholsi Gardner ( 1926, U. S. Geol. Sur., Prof. Paper 142-A, p. 48, pl, 12, figs. 5-6). Shoal River formation (middle Miocene), Florida, U.S.A. This abundant and widespread form from northwest Florida has an average of 17 radial ribs contrasted with the 19 to 21 on the Venezuelan C. g. antecessor. Gardner was of the opinion that C. nicholsi may have been an ancestral type of the C. gibbus group. Recent Pectinidae of the group exemplified by C. gibbus under the subgenus Argopecten are listed by Grau (1960, Nautilus, vol. 74, No. 1, pp. 17-18). Pecten (Plagioctenium) gibbus portusregii Grau (1952, Nautilus, vol. 66, No. 2, p. 69), new name for P. gibbus carolinensis Grau, not Conrad (1952, Nautilus, vol. 66, No. 1, pp. 17-19, pl. 1, figs. 2-5,7). Recent, two miles off Port Royal, South Carolina, in about 13 fathoms. This species has scales on the radial riblets of the right anterior ear, and five or six pectinidial teeth on the ctenolium. C. gibbus antecessor, n. subsp. lacks scales on the right anterior ear, and the ctenolium or byssal area below the right anterior ear bears three to five pectinidial teeth. Pecten (Plagioctenium) evergladensis Mansfield (1931, U. S. Geol. Sur., Prof. Paper 170-D, pp. 47-48, pl. 17, figs. 1,2,4,5). Tamiami limestone (upper Miocene) of southern Florida. This has a more inequilateral shell than C. gibbus antecessor, n. subsp.» NORMAN EDWARD WEISBORD, 1964
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N. E. Weisbord, 1964,
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