Antillipecten janicoensis 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. [p. 115, pl. 16, figs. 11-13]
2011 Antillipecten janicoensis Waller, 2011
T. R. Waller, 2011, plate 16
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«Diagnosis.— Antillipecten with equilateral, nearly equivalved shell of low biconvexity and umbonal angle of ca. 100°; disks with 15-17 simple, rounded, steep-sided ribs of uneven height and spacing, and 1 or rarely 2 secondary costae originating in interspaces at 11-12 mm Ht; close-set commarginal lamellae of two orders, with major lamellae crossing rib crests.
Description.— Shell small, maximum known ca. 23 mm Ht, equilateral, acline, L slightly exceeding Ht, umbonal angle 98-103°, valves approximately equal and low in cvx; hinge line short, ca. 60% L, ratio of anterior hl to posterior hl 1.29. Disks with 15-17 simple, rounded, steep-sided ribs originating near beak; ribs of uneven height and spacing, on LV 5 ribs slightly higher than intervening ribs, lateralmost rib on each side of LV doubling by bifurcation in mid-ontogeny; secondary costae originating at 11 or 12 mm Ht, commonly only one narrow costa per interspace, rarely two. Commarginal lamellae of close-set decatopectinine type present on both ribs and interspaces, in two distinct orders in early ontogeny in interspaces with major lamellae raised and passing straight across rib crests, in late ontogeny commarginal lamellae most prominent in interspaces extending up to edges of rib crests, lower on rib crests; projecting scales absent. Disk flanks low, rounded, lacking radial costellae. Auricles of moderate size relative to size of disk; right anterior bearing radial costae but strength and number unknown, byssal notch not preserved, active ctenolium with small teeth present; left anterior auricle with 8-10 radial costae, posterior auricles with 8-10 weaker costae, becoming obsolete in dorsal sector but remaining strong in ventral sector. Left anterior auricle and posterior auricles with free margins forming acute angles with dorsal margin. Hinge dentition weak, dominated by dorsal and infradorsal teeth, intermediate teeth absent. Adductor scars not preserved or partially inaccessible. Foliated-calcite re-entry not extending beyond level of striate adductor scar. Edges of ribs on shell interior carinate near shell margin. Etymology.— Named after the town of Jánico, with reference to the type locality on the Santiago-Jánico road, north of Jánico. Holotype and measurements.— USNM 541015 (Pl. 16, Figs 11-12), 1 LV, 21.5 mm Ht, 23.3 mm L, 3.4 mm cvx. Type locality.— Locality TU 1406, road cuts, Santiago- Jánico Road, 3-5 km west of junction with road to Baitoa, or 15-17 km from bridge over Río Yaque at Santiago, Dominican Republic. [The entry for this locality by Saunders et al. (1986: 66) incorrectly indicates "northwest of bridge." In fact it is south of the bridge.] According to the Vokes' entry in the Tulane Locality Register (at USNM), this is an exposure of the Cercado Formation. The assemblage of pectinids, however, suggest that these exposures could include some of the Gurabo Formation. The assemblage includes Argopecten eccentricus caimiticus, Leptopecten thompsoni, Lindapecten plurinominis, Lindapecten paramuscosus, Amusium papyraceum, Euvola soror, Antillipecten quemadosensis, and Nodipecten nodosus. Other material.— Locality NMB 16844, paratypes, 1 RV (23.9 mm Ht; Pl. 16, Fig. 13) and 1 LV (14.0 mm Ht), Gurabo Formation (Arca beds), 228 m above base of section, Río Cana, Dominican Republic, associated in same sample with Argopecten eccentricus caimiticus and Lindapecten plurinominis. Remarks.— The holotype is the most complete of the three specimens, although it is fragile and was broken and repaired during this study. The two specimens from NMB 16844 have better preserved shell structure and show pigment patterns not visible in the holotype. On both valves, the pattern consists of narrow white antimarginal lines on the disk flanks, some of which bifurcate just before reaching the margin of the valve. The left valve has a pattern on the disk consisting of narrow diagonal white lines that intersect to produce a chevron pattern. White pigment is also concentrated on the crests of the five high ribs. NMB 16844 is in the so-called "Arca beds" or "Scapharca patricia beds," which were placed by Saunders et al. (1986, text-fig. 16) in the Cercado Formation but by Maury (1917b, 1919) and Bold (1988) in the Gurabo Formation. Comparisons.— Antillipecten janicoensis n. sp. resembles A. cercadicus in having commarginal lamellae of two orders, with the major lamellae forming raised transverse ridges on the rib crests in early ontogeny; the two species also have similar auricular shapes and broad umbonal angles. There is a clear difference in ribbing, however, in that A. cercadicus has higher, broader ribs that are evenly spaced and of uniform height and lacking well-developed costae in the interspaces. Compared to A. quemadosensis n. sp., A. janicoensis n. sp. has more ribs (15-17 compared to 10-13) as well as stronger and more numerous secondary costae in the rib interspaces. Evolution.— Antillipecten janicoensis n. sp. is possibly ancestral to A. quemadosensis n. sp., which occurs stratigraphically higher in the Gurabo Formation and which, in turn, might have given rise to the extant tropical American species A. antillarum based on the elaboration of secondary radial costae and their spread from interspaces to rib crests. Antillipecten cercadicus appears to be an independent lineage that is a more plesiomorphic species than any of these, based on its lack of secondary costae. Occurrence.— In the northern Dominican Republic the new species is known only from two well-separated collecting sites, one on the Río Cana, the other in a road cut north of Jánico. The Río Cana site (NMB 16844), at 228 m above the base of the section, is below the fi rst occurrence of planktic foraminiferans at 300 m, which are in turn below the possible position of the Miocene-Pliocene boundary that is at least as low as 315 m (Saunders et al., 1986: 22). NMB 16844 is therefore likely to be late Miocene (NN11 Zone). Judging from the fossil assemblage, the other site (TU 1406) is likely of the same age. Distribution.— Antillipecten janicoensis n. sp. is known only from the Upper Miocene of the northern Dominican Republic.» THOMAS RICHARD WALLER, 2011
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