Euvola soror (Gabb, 1873)
GABB, W. M. 1873. On the topography and geology of Santo Domingo. American Philosophical Society Transactions, n. s., 15: 49-259. [p. 257]
1873 Janira soror Gabb, 1873
Pecten soror Gabb; H. A. Pilsbry, 1922, Revision of W. M. Gabb's Tertiary Mollusca of Santo Domingo, plate 44, figures 1, 2.
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«Shell sub-orbicular, nearly equilateral, very inequivalve; ears nearly equal; right valve very convex, left valve concave; surface marked by abouttwenty-two square radiating ribs, crossed by fine squamose lines of growth. Length 1.9 inch.
Allied to J. Jacobaeus, J. maxima, and J. media, but different from them all in the greater number and minutely squamose character of its ribs.» WILLIAM MORE GABB, 1873
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«Remarks.— Many of the characters of Euvola soror are highly variable. The wide range of rib counts is because the weaker, lateralmost ribs have been included in the counts. These marginal ribs, particularly those on the shoulders of the left valve, are variable in number and subject to wear, meaning that determinations of their numbers could in some cases be subjective. Many left valves of E. soror have discontinuous patches of thin, porous, projecting commarginal lamellae that bend over toward the shell margin and merge with adjacent lamellae to form a sort of "pseudosurface." This phenomenon occurs in many genera of Pectinidae, e.g., Argopecten, Cryptopecten, Leopecten, and Pecten, where its presence is variable among and within species. The minute pores in these frills result from incomplete calcification within a narrow fold of periostracum, as demonstrated by the presence of periostracum on each side of a frill, including each end of a pore (Clark, 1974).
Another kind of variation that can have some biostratigraphic utility occurs among specimens from four localities, all in the upper Gurabo Formation: NMB 16818 and TU 1354 on the Río Cana, TU 1278 on the Río Gurabo, and TU 1339 in the Santiago area. On some but not all specimens from these localities, secondary radial costellae develop on rib crests of both valves beginning at heights ranging from 20-25 mm (Pl. 14, Figs 10-13). Six or seven costellae per rib develop in the central sector, fewer on the narrower anterior and posterior ribs. The commarginal lamellae that cross the ribs are distally looped over the costellae. Specimens of Euvola soror from TU 1206 from the upper Gurabo Formation of the Río Yaque del Norte display yet another variation. These specimens have thinner shells and lower ribs than others, and because of the shell thinnesss, the interior ribs extend further than usual inward from the margin. All 14 left valves from this locality have beaks that are hyaline, with commarginal lamellae beginning suddenly at a valve height varying from 4.5-7.5 mm. The surface of the hyaline phase is well enough preserved on some of the specimens to show that there is no antimarginal or pitted microsculpture, only faint growth lines and the very low undulations marking the beginnings of radial ribbing. Once the commarginal lamellae begin to form, they merge to form a pseudosurface, the radial ribs remaining lower than is normally the case for this species. Comparisons.--Euvola soror resembles E. ventonensis, originally described from the vicinity of Santiago in southern Cuba and probably from the La Cruz Formation of latest Miocene or Pliocene age. The latter species differs from E. soror in having more prominent and numerous auricular costae, particularly on the auricles of the right valve, and in having more strongly developed carinate edges on internal ribs. In addition, the ribs of the right valve of E. ventonensis are more squared, with growth lines on rib crests looping further toward the margin, and these ribs develop low transverse bulges, giving them a somewhat beaded appearance.
Compared to Euvola barretti (Woodring, 1925) (= E. jamaicensis n. n., herein), originally described from the Bowden Beds of Jamaica, E. soror has a right valve that is less convex relative to valve height, with ribs narrower relative to interspaces and weaker auricular costae. The left valve of E. soror has fewer riblets on the shoulders of its disk and more gently rounded ribs than in E. barretti. Euvola gurabensis n. sp., described above, has the same shape, size, and number of ribs as E. soror but differs in that it is even thinner shelled, and the rib crests of its right valve are subdivided by shallow grooves, causing the ribs to appear bifid or trifid. Other species of Euvola that have simple ribs, such as E. colpotus (Woodring, 1982) from the Gatun Formation of Panama and E. caribea from the Cabo Blanco Group of Venezuela, differ in having much more convex right valves with stronger ribs and more prominent auricular costae. Euvola urumaconis (Harris in Hodson et al., 1927) and E. codercola (Harris in Hodson et al., 1927), both originally described as subspecies of E. soror from the late Miocene of Venezuela, were elevated by Woodring (1982) to species rank. They differ from E. soror in reaching a much larger size (up to 70 mm height) and in having a more deeply convex right valve that is more flaring, causing length to exceed width through most of ontogeny. Both have more prominent auricular costae, particularly the dorsal costae of the left auricles. In E. codercola, the commarginal lamellae of the right valve remain prominent across the rib crests, and the species is unusual in developing radial costae in late ontogeny on the disk flanks of its right valve. Evolution.— The strong resemblance between Euvola soror and E. gurabensis n. sp. and their stratigraphic relationship with the latter species succeeding the former suggest an ancestor-descendant relationship between these species in the Dominican Republic. (See above for discussion of evolution of the latter species.) The weak auricular costae of E. soror, as well as its simple rib pattern without medial grooves or medial ribs, is typical of the earliest species of Euvola. Strong right and dorsal left auricular costae occur in later species, beginning in the latest Miocene and Pliocene.
Occurrence.— In the Dominican Republic, Euvola soror occurs in the Baitoa, Cercado, and Gurabo formations (Table 21) in beds ranging in age from the late Early or early Middle Miocene to Early Pliocene.
Distribution.— Euvola soror has not been observed outside of the Dominican Republic.»
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. 106, 107]
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Euvola soror (Gabb, 1873); T. R. Waller, 2011, Neogene Paleontology of the Northern Dominican Republic, plate 14, Figures 3-13.
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