Euvola gurabensis 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. 100, pl. 13, figs. 4-10]
2011 Euvola gurabensis Waller, 2011
T. R. Waller, 2011, plate 13.
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«Diagnosis.— Euvola with RV of low to moderate convexity, LV nearly flat to slightly concave, broadly flaring with umbonal angle reaching 120º; both auricles of RV with only weak costae or none; rib crests of disk of RV bearing 2-4 low, non-scabrous costellae in late ontogeny; ribs of LV simple, rounded, without medial riblets in interspaces.
Description.— Thin shell of small size, reaching ca. 40 mm Ht, acline, equilateral, large specimens with L exceeding Ht, broadly flaring, umbonal angle reaching 120°; RV cvx low to moderate, ca. 15% Ht; LV flat to slightly concave with raised shoulders; disk gapes probably present but not observed directly. Disks with 25-28 low ribs approximately equal in width to interspaces on RV, narrower on LV; crests of right ribs becoming flattened, then bifid or subdivided by 3 or 4 low non-scabrous costae in late ontogeny; crests of left ribs remaining rounded; lateralmost ribs on RV much narrower than others, increasing in width toward central sector; lateralmost ribs on shoulders of LV lower than others except central rib on posterior shoulder higher than ribs adjacent to it. Commarginal lamellae passing straight across interspaces and rib crests, weakly developed on RV, more prominent and more widely spaced in interspaces of LV. Disk flanks of both valves narrow, curved in cross section, of approximately same width as neighboring rib interspace, steeper on LV, and without radial costellae. Prismatic stage of RV ending at at least 5 mm Ht. Auricles approximately equal in length, total hl ca. 40% L; right anterior auricle with byssal notch becoming obsolete late in ontogeny, anterior margin rounded, byssal fasciole not swollen, suture along out-turned flange of anterior disk flank, surface of auricle with few very weak radial costellae in central sector in early ontogeny, fading later; free margins of left anterior auricle and both posterior auricles straight or slightly convex, with overall trends forming obtuse angles with dorsal margin, right posterior auricle commonly without radial costae, left auricles commonly with 1 weak costa or 2 wellseparated weak costae in dorsal sector. Hinge dentition of RV dominated by single dorsal tooth on each side of resilifer; resilial teeth very weak or absent, intermediate teeth absent, hinge plate bearing vertical microcrenulations. Interior surface of RV not observed; that of LV having foliated-calcite re-entry extending ventrally to pallial line and extending posteriorly along ventral margin of adductor scar. Edges of ribs on shell interior carinate near valve margins; ribs on shell interior extending well inward from pallial line, crossing adductor scar. Etymology.— Name derived from the Gurabo Formation of the Dominican Republic. Holotype and measurements.— USNM 541004, 1 RV with matrix-filled interior, 29 mm Ht, 32 mm L (Pl. 13, Figs 4-5). Type locality.— Locality TU 1338: Road cut, 0.3 km west of the ford over Río Gurabo, on the Los Quemados-Sabaneta road, or 2.9 km west of Los Quemados, northern Dominican Republic. Other material.— 12 specimens from 5 localities in the northern Dominican Republic (Table 20). Two of these are fragments; the others are all single valves. Remarks.-- Euvola gurabensis n. sp. does not co-occur with E. soror at any of the sampled localities, although the limited amount of material of the new species leaves open the possibility that this might be due to chance. The single locality in the Río Gurabo section from which E. gurabensis n. sp. was collected is within the stratigraphic range of E. soror in the same section (compare Tables 20-21). The closest extant relative of E. gurabensis is E. chazaliei (Dautzenberg, 1900), a relatively uncommon species that lives in the tropical western Atlantic region at depths from 20-100 m, although single valves are sometimes found in sediments at shallower depths. Comparisons.— Euvola gurabensis n. sp. differs from E. soror in having a thinner shell, more flaring and less convex right valve, and more numerous ribs that on the right valve are subdivided in late ontogeny by secondary radial sculpture, either a single medial groove or 2-4 low costae. The extant species E. chazaliei resembles E. gurabensis n. sp. in having a thin shell, left auricular margins forming obtuse angles with the dorsal margin, weakly developed auricular costae, and commonly > 25 ribs on its right valve. The fossil species, however, has right disk ribs that are less variable in spacing and width and that more commonly have secondary radial grooves and costae on their rib crests. The ribs of the left disk of E. gurabensis n. sp. are also more uniform in size and spacing and somewhat broader relative to interspaces. Euvola coralliphila (Olsson, 1922), from the Moin Formation, late Pliocene (Cotton, 1999: 74), of Costa Rica is very similar to and possibly a junior synonym of the extant species E. chazaliei. Euvola reliquua (Brown & Pilsbry, 1913), from the middle part of the Gatun Formation of Panama (Woodring, 1982: pl. 108, figs 1-3) of Late Miocene age (Collins et al., 1996), has a relatively much more convex right valve with stronger, more rectangular ribs. Evolution.— Euvola gurabensis n. sp. is the first definite fossil member of a subclade of thin-shelled, low-convexity Euvola that includes the extant species E. chazaliei and E. turtoni (E. A. Smith, 1890), the latter of St. Helena Island in the central South Atlantic. Based on the close similarity between E. gurabensis n. sp. and E. soror, the latter species could be representative of the ancestral group from which E. gurabensis n. sp. evolved. If this is the case, then morphological change involved decreasing convexity of the right valve, increasing irregularity in rib spacing and width, and thinning of the shell, possibly as adaptations to a change in habitat to deeper water. The time of origin of E. gurabensis n. sp. is unknown, although its first occurrence in the upper Gurabo Formation suggests that it was within the Pliocene. If so, the duration of the species was short, assuming that the first appearance of the extant species E. chazaliei in the latest Pliocene Moin Formation marks an anagenetic change from E. gurabensis n. sp. Occurrence.— In the northern Dominican Republic, Euvola gurabensis n. sp. has thus far been found only in the upper Gurabo Formation and the upper Mao Formation in deepwater facies of Early Pliocene age (based on determination of nannofossil zone NN 15 for highest outcrops of the Mao Formation on the Río Gurabo by Saunders et al., 1986: 19). Distribution.— Euvola gurabensis n. sp. is thus far unknown outside the Dominican Republic.» THOMAS RICHARD WALLER, 2011
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