Amusium papyraceum (Gabb, 1873)
GABB, W. M. 1873. On the topography and geology of Santo Domingo. Transactions of the American Philosophical Society [new series], 15: 49-259. [p. 257]
1873 Pleuronectia papyracea Gabb, 1873
Amusium papyraceum (Gabb, 1873); T. R. Waller, 2011, Neogene Paleontology of the Northern Dominican Republic, plate 12, figures 1-12.
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«Shell discoidal, sub-circular, very slighthy longer than wide; slightly inequivalve; ears nearly equal; surface perfectly smooth, or marked only by faint lines of growth; internal surface marked with small double radiating ribs. Length from beak to base 2.2 inch, width 2 inches.
A very distinct species, without any of the radiating lines of the living West Indian smooth Pecten.» WILLIAM MORE GABB, 1873
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«Comparison.— Species of American Neogene Amusium that lack external radial ribs on their umbones and are thus comparable to A. papyraceum are A. toulae (Brown & Pilsbry, 1911) [junior synonyms Pecten (Amusium) luna Brown & Pilsbry, 1913, and possibly A. bocasense Olsson, 1922], A. mortoni (junior synonym Amusium rexmaris Maury, 1925a), A. darwinianum (d'Orbigny, 1842), and A. paris del Río, 1992.
Based on a new examination of specimens from Panama studied by Woodring (1982), the species that he described as Amusium toulae from the La Boca, Alhajuela, Gatun, and Chagres formations of Panama is actually two species. One is restricted to the La Boca Formation and is identified herein as A. aguaclarense (F. & H. Hodson in Hodson et al., 1927). The latter species was described from the Agua Clara Formation of Venezuela, a stratigraphic unit that is apparently of approximately the same age as the La Boca Formation (late Early Miocene, Burdigalian). The second species included within "A. toulae" by Woodring is the true A. toulae, which does not occur in the La Boca Formation but is present in the Alhajuela, Gatun, and Chagres formations. Morphotypes A and B of Amusium papyraceum differ from A. aguaclarense, A. toulae, and A. mortoni in the shape of the free margins of the auricles and in the extent of foliated-calcite re-entry on the shell interior. In the latter three species, the dorsal part of the free margins of the auricles sweep dorsally toward the beak to form a more obtuse angle with the dorsal margin than in A. papyraceum. In Panamanian A. aguaclarense, A. toulae, and A. mortoni, the inner aragonitic sector is broader than in A. papyraceum, so that the posterior margin of the adductor scar of both valves is inset from the aragonite margin posterior to the scar, whereas in A. papyraceum the posterior margin of the adductor scar is fl ush with the margin of the aragonite sector. Also, A. papyraceum differs from these other species in having more extensive foliated-calcite re-entries, extending to below the level of the ventral margin of the adductor scar in mature specimens, whereas in the other species, this shell layer barely extends to the mid-adductorscar level. Amusium aguaclarense differs from all of the other species in the number and pattern of its internal carinae. Its right valve commonly has 8-10 broad pairs of carinae, with interpair spaces not more than approximately twice the width of intrapair spaces. Because the carinae of the left valve are on the outer sides of the paired carinae of the right valve, interand intrapair spaces of the left valve are the same width or nearly so, making pairs impossible to distinguish. The number of single carinae on the left valve of A. aguaclarense (16-20) is therefore twice the number of pairs on the right valve. Amusium mortoni differs from the other species in attaining a much greater maximum size (heights commonly > 100 mm, rarely exceeding 200 mm) and in having obsolete disk flanks on the left valve so that the auricular surface and adjacent disk surface are at the same level, separated only by a groove. Amusium darwinianum and A. paris are two closely related species of early Late Miocene age, respectively from the Paraná and Puerto Madryn formations of Argentina (del Río, 1991, 1992). In spite of their geographic separation from the Dominican Republic, they resemble A. papyraceum more than do the other species discussed above. As in A. papyraceum, the posterior margin of the adductor scar is close to or coincident with the posterior boundary of the inner aragonitic layer, the free margins of the auricles are not greatly slanted to form high obtuse angles with the dorsal margins, and the internal carinae are in narrow pairs. The Argentine species differ in having posterior auricular margins that are slightly concave rather than convex, and in having more rapidly flaring disks. In A. paris, the dorsal shoulders of the disk become nearly parallel to the hinge line. The folded dorsal margins of the right auricles of A. paris are higher and more tightly folded than in A. papyraceum, and the hinge dentition of A. paris is more dorsoventrally compressed, the dorsal teeth being shorter and with less separation from the resilial teeth. Evolution.— In the Miocene of the southern and western Caribbean, there appears to be an evolutionary sequence from Amusium aguaclarense (Early Miocene of Venezuela and Panama) to A. toulae (late Middle Miocene to early Pliocene of Panama) to A. mortoni (Early or Middle Pliocene to earliest Pleistocene of southeastern USA, Veracruz and Baja California Sur, Mexico, and Venezuela. The Mexican and Venezuelan records for A. mortoni have not previously been recorded. They are based on specimens from the following localities: USGS 13089, Tehuantepec area, Rodriguez Clara, Mexico; USNM 418203, hypotype of Smith (1991a: figs 3h, j, identified by her as A. luna from an unnamed sandstone, Punta Paredon Amarillo, northern Concepción Peninsula, Mexico); and USGS 24703, San Gregorio Formation, San Gregorio, Falcón, Venezuela. Amusium papyraceum is not a part of this sequence, standing apart with the Argentine species, A. darwinianum and A. paris, for the reasons given above. This relationship with Argentine species is not unexpected. Del Río (2004: 1114) provided a detailed account of dramatic biogeographic changes that occurred in the early Late Miocene, when many Caribbean and paratropical species dispersed to the Patagonian region. The morphological differences between A. papyraceum and A. paris, specifically the higher dorsal auricular folds and more fl aring shape of the latter, suggest that A. paris is the more evolutionarily derived species. Occurrence.— In the northern Dominican Republic, Amusium papyraceum occurs in the Cercado and Gurabo formations, with Morphotype A restricted to the Cercado and lower Gurabo formations and Morphotypes B (with a few exceptions) and C restricted to the upper Gurabo. Distribution.— Based on the extensive Neogene tropical American collections in the Smithsonian collections, Amusium papyraceum has not been positively identifi ed from outside 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. [pp. 98, 99]
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Amusium papyraceum (Gabb, 1873); T. R. Waller, 2011, Neogene Paleontology of the Northern Dominican Republic, plate 13, figures 1-3 (below).
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«Shell with one valve concavely flattened, the other slightly convex; resembling the Oriental living species in being entirely smooth externally, except for occasional concentric growth lines. Marked internally with paired radiating ribs. Our most perfect shell measures 60 mm. in altitude and approximately 58 in length, but we collected fragments of much larger specimens apparently 100 mm. in altitude. The Miocene-Recent A. Mortoni Ravenel is allied and possibly identical. A. papyracea is also found at Bowden, but Santo Domingo is the type locality. The shell has never before been figured.
Localities.— (Exp'd' 16) Samba Hills at an approximate altitude of 540 feet near Los Caobas; Zone F, Rio Gurabo at Los Quemados; Guayubin to Mao road, Rio Cana; Zone I, Rio Cana at Caimito.» MAURY, C. J. 1917. Santo Domingo Type Sections and Fossils. Part 1: Mollusca. Bulletins of American Paleontology, 5 (29): 166-415, pls. 1-39. [p. 354]
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Amusium papyraceum Gabb; C. J. Maury, 1917, Santo Domingo Type Sections and Fossils, plate 26, figure 22.
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