Jorgechlamys centralis (G. B. Sowerby I in Darwin, 1846)
SOWERBY I, G. B. 1846. Descriptions of Tertiary Fossil Shells from South America. In: C. Darwin: Geological Observations of South America: Being the Third Part of the Geology of the Voyage of the Beagle, Under the Command of Capt. Fitzroy, R. N. During the years 1832 to 1836. Smith, Elder, London. Appendix: 249-264. London, 1846. [p. 253 [610], pl. 3, fig. 31]
1846 Pecten centralis G. B. Sowerby I in Darwin, 1846
1897 Myochlamys praenuncius Ihering, 1897
1907 Myochlamys centralis var. ameghinoi Ihering, 1907
1981 Chlamys riggsi Zinsmeister, 1981
1897 Myochlamys praenuncius Ihering, 1897
1907 Myochlamys centralis var. ameghinoi Ihering, 1907
1981 Chlamys riggsi Zinsmeister, 1981
C. Darwin, 1846, plate 3.
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«Pecten testa subcirculari, depressa, radiis quinque squamuliferis centrali eminentiore, lineis radiantibus asperis numerosisque ornata; auriculis magnis, subaequalibus?
A single fragment of this remarkable species was found at Port S. Julian; two others were brought from Port Desire. These fragments are all of the same side, so that we are as yet but very imperfectly acquainted with the species. San Julian, Port Desire, S. Cruz, Patagonia.» GEORGE BRETTINGHAM SOWERBY I, 1846
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«TYPE MATERIAL. Specimen NHM-L27696 was designated lectotype (as ‘tipo’) by Morra & Erdmann (1989: 121), but as Sowerby’s original description clearly implies that there was more than one specimen, their lectotype designation by inference of holotype is invalid (ICZN 1999: Art. 74.5). This specimen is a highly incomplete left valve from ‘Port San Julian, Argentina’. Of the two other syntypes, NHM-L27697 (from Port Desire) is a fragment of the disc of a small specimen with very coarse scales on the costal crests, while NHML27698 (also from Port Desire) is a small, highly incomplete umbo from a large and flat specimen, i.e. a left valve, with weak surface sculpture and three prominent plicae. Del Río (2004a) has recently reviewed the large pectinids from the Patagonian Cenozoic and illustrated specimen NHM 27696, which she erroneously considered as the holotype (p. 696, fig. 6.10), probably following Morra & Erdmann (1989). Again, this type designation is invalid, as there are other syntypes, as explained above. As one of the other syntypes belongs in a different species, and in order to stabilise nomenclature and avoid further confusion with this species, we follow general usage and formally designate this specimen (NHM 27696) as lectotype.
TYPE LOCALITY. ‘Port San Julián, Port Desire’, province of Santa Cruz, Argentina. Darwin collected specimen NHM 27696 from the late Oligocene–early Miocene Monte León Formation, which in the area surrounding Port San Julián (albeit not at the exact location of the present city) overlies the San Julián Formation. The other two specimens come from Puerto Deseado (Port Desire). The exact locality and stratigraphical unit from which the latter come is uncertain, but they were probably from the Tertiary beds exposed some 15–20 km inland from Puerto Deseado, the nearest locality with rocks yielding this large pectinid.
REMARKS. The illustration provided by Sowerby appears to be a composite of all three specimens he had available. The width of the main radial costae and the prominence of the scales are greatly overemphasised on the figured specimen, which is the lectotype NHM-L27696. Conversely, paralectotype NHM-L27697 has much coarser scales than the figure, while paralectotype NHM-L27698 accounts for the exaggerated width of the main costae of the figured specimen. This is one of the large pectinids found in the late Oligocene and early Miocene of Patagonia and, as stated by Beu & Darragh (2001: 52–53 ) fits reasonably well in Equichlamys Iredale, 1929 [type species Pecten bifrons Lamarck, 1819]. It shares with other members of the genus the large inequivalve shells with few strong ribs and shagreen microsculpture covering the entire shell surface.
In her revision of this species, del Río (2004a) included it in the new genus Jorgechlamys del Río, 2004 [type species Myochlamys juliana Ihering, 1907], a viewwhich is followed here. Most of the material she described comes from rocks exposed along the southern coast of the San Jorge Gulf and included by her in the San Julián Formation, although the exact relationship of these rocks with the San Julián Formation exposed further south remains to be fully understood. Likewise, the material referred to this species and described and illustrated by Morra & Erdmann (1989), also comes from rocks exposed along the San Jorge Gulf, i.e. Cañadón El Lobo and Bahía Mazarredo. They also included these sediments in the San Julián Formation of Bertels (1970), the type section of which lies about 400 km to the south. These rocks are probably referable to other lithostratigraphical units (Bellosi, 1995).» 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. 263]
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Jorgechlamys centralis
(Sowerby, 1846), lectotype; M. Griffin & S. N. Nielsen, 2008, A revision of the type specimens of Tertiary molluscs from Chile and Argentina, plate 3, figure 4. |