Palliolum cibaoense 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. 40, pl. 4, figs. 1-4]
2011 Palliolum? cibaoense Waller, 2011
T. R. Waller, 2011, plate 4.
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«Diagnosis.— Small, thin-shelled Palliolini lacking antimarginal microsculpture; disks smooth except for microscopic commarginal growth lines; anterior auricles with radial costae; posterior auricles with weak radial costae or smooth, posterior margins of posterior auricles forming very obtuse angle with dorsal margins.
Description.— Shell small, not known to exceed 9 mm Ht, slightly prosocline; margin of disk nearly circular, Ht and L approximately equal; LV distinctly more convex than RV; disk gapes apparently absent; hl short, less than half length of shell, ahl longer than phl (ahl/phl ca. 1.4); right anterior auricle with shallowly rounded anterior margin and deep byssal notch floored by active ctenolium of 4 or 5 small teeth; left anterior auricle with sigmoidal margin leading to shallow byssal sinus; posterior margins of posterior auricles slightly outwardly convex and forming obtuse angle of ca. 130° with dorsal margin. Exterior of disks and disk flanks smooth except for faint, irregularly spaced commarginal lines and very fine, short antimarginal striae on anterior and posterior extremities of disk; right anterior auricle with broad, sunken byssal fasciole and 3 or 4 weak to moderately strong radial costae crossed by imbricated commarginal lamellae; left anterior auricle with 6 or 7 radial costae of variable strength, in some cases fading distally; posterior auricles separated from disk by low, rounded disk flanks and possessing only very weak, barely discernible costae. Shell translucent, lacking pigment patterns. Prodissoconch possibly of lecithotrophic type consisting mainly of PI stage with little or no PII, length 177 μm. Hinge dentition consisting mainly of dorsal and infradorsal teeth crossed by transverse microscopic ridges; resilial teeth weak or absent. Adductor scars poorly exposed but striate and nonstriate scars of RV well separated on anterior sides of scars. Foliated-calcite re-entry restricted to umbonal region on both valves. Outer columnar prismatic layer of RV not definitely detected and possibly absent. Etymology.— Named after the Cibao Valley of the northern Dominican Republic. Holotype and measurements.— USNM 540952 (Pl. 4, Fig. 1), 1 LV, 9.0 mm Ht, 9.0 mm L, 1.6 mm cvx. During the process of being handled, a large but very thin piece of the outer shell layer in the center of the disk was broken off and could not be reattached in precisely its original position. Type locality.— USGS 8516, bluff on right bank of Río Amina at ford near Potrero, Province of Santiago, Dominican Republic, Gurabo Formation, collected by T. W. Vaughan and C. W. Cooke, 02 May 1919. Saunders et al. (1986: text-fig. 34) indicated that USGS 8516 is from the same locality as TU 1219, and their text-fig. 35 shows the position of TU 1219 relative to NMB localities. Th eir discussion of these localities (1986: 33) indicates that TU 1219 is just below the Miocene/Pliocene boundary. Other material.— Five single valves from four localities, all from the Cercado Formation on the Río Mao at Bluff s 2 and 3 (Table 6). Remarks.— Based on its association with Argopecten eccentricus eccentricus, Leptopecten thompsoni, and Cyclopecten guppyi at Bluffs 2 and 3 on the Río Mao, Palliolum? cibaoense n.sp. was probably a shallow-water species that lived in associationwith marine grasses in clear, normal marine waters. The association of the type specimen with A. thetidis and Amusium papyraceum on the Río Amina suggests deposition in deeper water than at the Río Mao localities. The rarity of this species is possibly due to the diffi culty of fi nding and recognizing it because of its small size and very fragile shell. Comparisons.— The reason for the generic query is that all extant species and known fossil species of Palliolum from the eastern Atlantic have prominent antimarginal striae, at least on their disk flanks in early ontogeny. Although these prominent striae are absent on the Dominican Republic specimens, their shell shape greatly resembles that of the type species of Palliolum. Eburneopecten Conrad, 1865, a Paleogene palliolinine genus of the Atlantic Coastal Plain of the United States, tends to be smooth but has a shallower byssal notch and sharply crested right dorsal auricular margins. Palliolum? cibaoense n. sp. is readily distinguished from comparably sized propeamussiids in the genus Cyclopecten in possessing a ctenolium and lacking a persistent prismatic outer shell layer on the right valve. Palliolum? cibaoense n. sp. also has more prominent dentition on both valves as well as an aragonitic inner shell layer that is delimited by the pallial line rather than extending nearly to the shell margins as in Cyclopecten. Evolution.— If the new species is indeed a member of the genus Palliolum, it is possible that it might have an eastern Atlantic origin. The smoothness of its exterior would be expected to be a derived feature, because the fossil record of true Palliolum indicates more strongly sculptured ancestors. Occurrence.— In the Dominican Republic, Palliolum? cibaoense n. sp. is known only from the Cercado Formation at Bluffs 2 and 3 on the Río Mao and from the Gurabo Formation on the Río Amina, of late Miocene age. Distribution.— Palliolum? cibaoense n. sp. is known only from the Late Miocene of the northern Dominican Republic.» THOMAS RICHARD WALLER, 2011
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