Zamorapecten 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. 110]
«Genus ZAMORAPECTEN n. gen.
Type species.— Pecten (Amusium) zamorensis F. & H. Hodson in Hodson et al., 1927, Punta Gavilán Formation, eastern end of point at Sabanas Altas, District of Zamora, State of Falcón, Venezuela. Lower Pliocene (Hodson et al., 1927: 38, pl. 22, figs 1, 3-6; Pl. 15, Figs 1-6).
Diagnosis.— Small to medium-sized Amussiopecten-like Pectinidae, nearly equivalved, with small auricles; radial plicae on umbones beginning as fi ne costae of approximately same strength as commarginals, later becoming high and round, with prominent commarginals in interspaces, before gradually disappearing by ca. 6 mm Ht, followed by smooth exterior; coarse antimaginal ridges present on early disk flanks and bases of auricles; dorsally facing edges of auricles of both valves flattened or concave; right anterior disk fl ank inturned, concave in cross section, meeting disk surface along sharp edge; inner shell surface with sharp radial carinae as in Amusium;
Etymology.— The prefix Zamora- refers to the District of Zamora, State of Falcón,Venezuela, where the type species occurs.
Remarks.— The importance of Amussiopecten as a circumglobal tropical pectinid in the Late Oligocene to Pliocene was emphasized by Masuda (1971b) in his review of species present in North and South America. More recent studies have recognized that what was formerly called Amussiopecten is in fact a polyphyletic assemblage of lineages that have evolved an amusioid form by becoming more streamlined, abandoning their ribbed exterior except on the umbones, and becoming externally smooth while retaining the internal carinate edges of the radial ribs. Concomitantly, the auricles become shortened and develop more obtuse angles between their dorsal and terminal margins; the byssal notch and ctenolium become obsolete in early ontogeny; and the hinge dentition tends to become simplified, with intermediate teeth becoming weak or disappearing. A spectacular example of convergence on Amusium is provided by "Amusium" cristatum (Bronn, 1828) of the Mediterranean Pliocene, which looks very much like extant western Indo-Pacific species of true Amusium except for the presence in the fossil species of large, projecting spines along the dorsal margins of its right auricles and remnants of coarse antimarginal microsculpture along the dorsal edges of its disk and disk flanks. The detailed morphology of some members of the group represented by this convergent "Amusium" were first brought to light by Freneix et al. (1982), with later contributions by Bongrain et al. (1994), Baldí et al. (1999), Mandic (2004), and Bongrain & Cahuzac (2004). The basal members of this group from the Early and Middle Miocene (Aquitanian and Langhian) of France that have scaly secondary radial costae in addition to spines on the right dorsal auricular margins were formally named Amussiopecten (Costellamussiopecten) by Bongrain et al. (1994). This group was later expanded by Bongrain & Cahuzac (2004) to include species that have lost the secondary disk costae but retain the dorsal auricular spines, including "Amusium" cristatum. Bongrain & Cahuzac (2004) elevated this enlarged group to genus rank and named it Cristatopecten. Unfortunately, this name was stillborn in the sense that it was immediately a junior synonym of Costellamussiopecten, the type species of which was included in the new genus by these authors.
Bongrain & Cahuzac (2004) regarded Amussiopecten as a junior synonym of Flabellipecten, thus reverting to the old position of Depéret & Roman (1910). This matter has long been contentious, with some authors maintaining that Amussiopecten, as represented by its type species, Pecten burdigalensis Lamarck, 1806, from the Lower Miocene (Aquitanian and Burdigalian) of Europe, intergrades with Flabellipecten, represented by its type species Ostrea flabelliformis Brocchi, 1814, from the Late Pliocene of Italy, whereas others (e.g., Cox, 1927; Masuda, 1971b; Waller, 1991: 38, 2006a: 25; Harzhauser et al., 2007: 149) have regarded these genera as distinct. In the present study, I continue to regard Amussiopecten as a valid genus applicable to a group of Oligocene to Pliocene pectinids with a circumtropical distribution as described by Masuda (1971b). This distribution was achieved before the end of the Early Miocene. With the closure of the eastern gateways of the proto-Mediterranean in the Burdigalian (Harzhauser et al., 2007), the Mediterranean representatives of Amussiopecten became isolated and failed to diversify, except to achieve large sizes and thick shells, such as in A. burdigalensis and A. expansus (G. B. Sowerby I, 1847), while tropical western Indo-Pacific and American Amussiopecten underwent extensive speciation. As matters now stand, the name Amussiopecten applies to those members of the former Amussiopecten s. l. that lack scaly costae on disk or auricles and have right dorsal margins that are rounded and not spiny. The name Costellamussiopecten applies to "Amussiopecten" bearing prominent spines on the right dorsal auricular margins, the more primitive members of the group having scaly secondary costae and prominent antimar ginal microsculpture in early ontogeny. Finally, Zamorapecten n. gen. applies to forms having dorsal auricular margins that in cross section are concave, with prominent commarginals present in the interspaces of early umbonal ribs. The early ontogeny of Zamorapecten n. gen. suggests that it, like its European counterpart Costellamussiopecten, probably originated from within the Oligocene group of Aequipecten-like species, but the two genera likely had different ancestors that were geographically widely separated, one in the western Tethys, the other in the tropical waters of the Americas. Geographic and stratigraphic ranges.— Pliocene, northern Venezuela and northern Dominican Republic.
THOMAS RICHARD WALLE, 2011
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Zamorapecten zamorensis (F. & H. Hodson in Hodson et al., 1927); T. R. Waller, 2011, Neogene Paleontology of the Northern Dominican Republic. 24. Propeamussiidae and Pectinidae (Mollusca: Bivalvia: Pectinoidea) of the Cibao Valley, plate 15, figures 1-6; Zamorapecten maoensis n. sp., plate 15, figures 7-10.
Pecten (Amusium) zamorensis F. and H. Hodson, n. sp.; F. Hodson, H. K. Hodson & G. D. Harris, 1927, Some Venezuelan and Caribbean mollusks, plate 22, figures 1, 3-6.
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