Zamorapecten maoensis 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. 111, pl. 15, figs. 7-10]
2011 Zamorapecten maoensis Waller, 2011
T. R. Waller, 2011, plate 15.
|
«Diagnosis.— Small Zamorapecten of nearly circular amussioid form known only from LV; umbonal sculpture limited to first 3 mm of growth, consisting of fine radial costellae crossed by commarginal lirae of nearly equal strength and with finer antimarginal microsculpture present in interspaces and on early disk flanks; radial carinae on inner surface evenly spaced.
Description.--Zamorapecten of small size, not known to exceed ca. 30 mm Ht, nearly circular in outline with Ht and L approximately equal, and of low cvx; umbo of LV with very fine cancellate sculpture disappearing at ca. 3 mm Ht, consisting of 18-20 fine radial costellae crossed by commarginal lirae of approximately equal strength, their rectangular intersections filled by fine antimarginal microsculpture that extends onto disk flanks and proximal parts of auricles; remainder of disk nearly smooth except for very low radial ridges; disk flanks low but steep, bearing antimarginal striae proximally but only commarginal growth lines distally; auricles small relative to size of disk and approximately equal in length, ratio of total hl to L ca. 0.43; anterior margin of left anterior auricle nearly straight, forming an angle of slightly > 90° with dorsal margin, lacking byssal sinus distally but bearing shallow byssal sinus in first couple of millimeters of ontogeny; proximal part of left anterior auricle with fine cancellate sculpture continued from umbo, consisting of 7-10 radial costellae crossed by somewhat elevated commarginal lamellae, both radials and commarginals fading away at distance of ca. 2 mm from beak; posterior margin of left posterior auricle nearly straight, forming an angle with dorsal margin somewhat greater than 90°, proximal part of auricle with delicate commarginals but no radials, distal part smooth; dorsal margins of both auricles flattened, becoming a dorsally facing concave trough distally. Interior with ca. 27-30 nearly equally spaced, strong radial carinae extending nearly to margin of valve; auricular buttress ending in low denticle well developed along base of each auricle; ventral margin of resilifer slightly undercut by umbonal cavity; adductor scar large relative to size of disk, its posterior margin confluent with posterior margin of aragonitic inner shell layer; extent of foliated-calcite re-entry not determined in holotype but extending to approximately level of dorsal margin of adductor scar in smaller specimen. RV unknown. Etymology.— Named after the Mao Formation, northern Dominican Republic. Type material.— Holotype (Pl. 15, Figs 7-8), 1 LV, NMB G17529, 30.3 mm Ht, 30.1 mm L. Type locality.— Locality NMB 16122, Mao Formation, right bank of Río Gurabo ca. 0.5 km south of Gurabo Afuero, northern Dominican Republic, 893 m above base of Río Gurabo section. See Saunders et al. (1986: text-fig. 4) for precise position of locality on the river and in the stratigraphic section. Other material.— Paratype (Pl. 15, Figs 9-10), 1 LV, 12.9 mm Ht, from locality NMB 15833, Mao Formation, right bank of Río Gurabo ca. 0.5 km due south of the type locality (NMB 16122), northern Dominican Republic, 895 m above base of the Río Gurabo section (Saunders et al., 1986: text-fig. 4). Remarks.— Only two specimens of Zamorapecten maoensis n. sp., both left valves, are present in the collections, both found by the NMB team. Although one specimen is less than half the size of the other, both show nearly identical cancellate sculpture patterns on their umbones (Pl. 15, Figs 8-9). The holotype adheres to a block of matrix, but the anterior part of the disk is broken away to reveal the imprint of internal carinae on the matrix (Pl. 15, Fig. 7). Comparisons.— The left valve of Zamorapecten maoensis n. sp. closely resembles that of Z. zamorensis in shape of disk and auricles and the spacing and number of internal carinae. The major difference is in the umbonal sculpture. In Z. maoensis n. sp., the pattern is finely cancellate and disappears after the first few millimeters of growth (Pl. 15, Figs 8-9); in Z. zamorensis, the pattern is dominated by discrete radial ribs much stronger than the commarginal lamellae between them, and the ribs do not fade out until ca. 6-7 mm Ht (Pl. 15, Figs 2, 5-6). Species of Costellamussiopecten differ from Z. maoensis n. sp. in having projecting spines on their dorsal margins and lacking prominent commarginal lamellae in the interspaces of the umbonal ribs (Bongrain et al., 1994: pl. 2, figs 1c-d). Species of Amussiopecten differ in lacking both commarginal and antimarginal microsculpture between the umbonal ribs (Masuda, 1971b: pl. 26). Evolution.— The earliest phase of umbonal sculpture on the left valve of Zamorapecten zamorensis, in the first couple of millimeters of ontogeny before the radial umbonal ribs begin to strengthen, is a cancellate pattern not unlike that found more extensively in Z. maoensis n. sp. The latter is therefore paedomorphic (ancestral juvenile characters appearing in adult descendant; McNamara, 1986) relative to the former and in this sense is more evolutionarily advanced. This is consistent with the stratigraphic occurrences of the two species. The Punta Gavilán Formation, which yielded Z. zamorensis, has been dated as Early Pliocene, Globorotalia margaritae Zone (Hunter, 1978), whereas the Mao Formation above 700 m above the base of the section on the Río Gurabo has been dated as Middle Pliocene, Globorotalia miocenica Zone (Saunders et al., 1986: 19). Occurrence and distribution.— Known only from the Mao Formation, Middle Pliocene, northern Dominican Republic.» THOMAS RICHARD WALLER, 2011
|