Zygochlamys geminata (G. B. Sowerby I in Darwin, 1846)
SOWERBY I, G. B. 1846. Descriptions of the Tertiary Fossil Shells from South America. In C. Darwin: Geological Observations of South America. Appendix: 249-264. London, 1846. [p. 252, pl. 2, fig. 24]
1846 Pecten geminatus G. B. Sowerby I in Darwin, 1846
1985 Zygochlamys dominator Morra, 1985
1985 Zygochlamys dominator Morra, 1985
C. Darwin, 1846, plate 2.
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«Pecten testa aequivalvi, ovata, auriculis inaequalibus, costis radiantibus squamuliferis 22, geminatis; interstiis alternis latioribus, nonnunquam costa minore instructa; auricula altera magna radiatim costata.
In general form this species resembles Pecten textorius (Schlot.) Goldf. Tab. XC. f. 9, but it has very few more than half the number of ribs, which in the present species are disposed in pairs. The P. textorius moreover belollgs to the Lias and inferior Oolite, according to Goldfuss. San Julian, Patagonia.» GEORGE BRETTINGHAM SOWERBY I, 1846
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«TYPE MATERIAL. Holotype: NHM-L27695.
TYPE LOCALITY. ‘San Julian, Patagonia’. As pointed out by Morra (1985: 302), the exact locality is uncertain.
REMARKS. The holotype is a juvenile right valve. Morra (1985: 302) correctly pointed out that, as the exact locality and stratigraphical horizon from whence the holotype came are not clear, and many other species of Zygochlamys share the geminate ribs on young specimens, the name should be left in suspense. However, an adequate suite of specimens of all stages of other species of this genus confirms that the species is valid, and Zygochlamys dominator Morra, 1985 (p. 303–304, pl. 2, figs 1a–b) is the adult stage. Sowerby’s illustration (Sowerby 1846: pl. 2, fig. 24) is fairly accurate, but does not show the costae as clearly arranged in pairs as in the specimen itself. It has eight pairs of costae plus one largish posterior one and two largish anterior ones, with one very fine secondary costa in each radial interspace in
between the pairs. Therefore, Zygochlamys dominator is a junior synonym of Zygochlamys geminata. Pecten geminatus is the type species of Zygochlamys Ihering, 1907.» GRIFFIN, M. & S. N. NIELSEN. 2008. A revision of the type specimens of Tertiary molluscs from Chile and Argentina described by d’Orbigny (1842), Sowerby (1846), and Hupé (1854). Journal of Systematic Palaeontology, 6 (3): 251-316, pls. 1-24. [p. 268, 269]
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Zygochlamys geminata (Sowerby, 1846), holotype; M. Griffin & S. N. Nielsen, 2008, A revision of the type specimens of Tertiary molluscs from Chile and Argentina, plate 5, figure 1.
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«Stratigraphical range and distribution. Known only from the Late Eocene-Early Oligocene San Julián Formation ("juliense" in early South American literature; Figs. 2, 25), exposed in Gran Bajo de San Julián (Manassero et al., 1997), where it is common, and at various coastal localities in Santa Cruz Province, southern Argentina (Morra, 1985). It is the oldest known species of Zygochlamys.
Remarks.According to Waller and del Río (in Beu, 1995, p. 15), Morra (1985) redescribed the adult form of this species as Zygochlamys dominator. Indeed, the common, large pectinid recollected by M. Griffin at Gran Bajo de San Julián, the assumed type locality of Pecten geminatus (cf, Manassero et al., 1997), and housed at IGNS, is identical to Morra's figured specimens. Zinsmeister (1981) incorrectly identified the specimens in his pl. 1, figs. 3, 7-9, as this species. However, the coarse and prominently paired main ribs of these specimens demonstrate that they belong to Zygochlamys jorgensis (Ihering, 1907). Zinsmeistert specimens were obtained from deposits at Punta Casamayor Santa Cuz Province (this is the type localiry of Z. jorgensis), which form part of the Late Oligocene-Early Miocene Monte León Formation, and not of the San Julián Formation (Fig. 2).» JONKERS, H. A. 2003. Late Cenozoic-Recent Pectinidae (Mollusca: Bivalvia) of the Southern Ocean and neighbouring regions. Monographs of Marine Mollusca, 5: i-viii + 1-125 pp, 17pls. Backhuys Publishers, Leiden. [p. 38]
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Zygochlamys geminata (Sowerby I); H. A. Jonkers, 2003, Late Cenozoic-Recent Pectinidae (Mollusca: Bivalvia) of the Southern Ocean and neighbouring regions, plate 4, figures a-f.
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