Moirechlamys actinodes (G. B. Sowerby I, 1846)
SOWERBY I, G. B. 1846. Descriptions of the Tertiary Fossil Shells from South America. In C. Darwin: Geological Observations on South America. Appendix: 249-264. London, 1846. [p. 253, pl. 3, fig. 33]
1846 Pecten actinodes G. B. Sowerby I, 1846
1921 Chlamys theresinae Rovereto, 1921
2010 Zygochlamys rizzoloi Reichler, 2010
1921 Chlamys theresinae Rovereto, 1921
2010 Zygochlamys rizzoloi Reichler, 2010
C. Darwin, 1846, plate 3.
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«P. testa suborbiculari, convexiuscula, tenui, valvis subæqualibus, radiis principalibus subelevatis circa 36, intermediis 3-7 minoribus, omnibus squamuliferis; auriculis inæqualibus, radiatim squamuliferis, altera magna, porrecta, altera parva, declivi.
Remarkable for its principal ribs being numerous and only slightly elevated, and for its intermediate ribs being very numerous. San Josef, Patagonia.» GEORGE BRETTINGHAM SOWERBY I, 1846
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«Remarks.— Sowerby (1846, pl. 1, fig.1) illustrated a left valve collected by Charles Darwin from the Puerto Madryn Formation exposed at Puerto San José (Chubut Province). It was identified as the holotype by del Río (1992, p. 29, fig. 7a, b), who provided a diagnosis of the species. However, Griffin and Nielsen (2008) recognized that the type material consisted of three syntypes, so Sowerby’s (1846) illustrated specimen was designated as the lectotype and is accompanied by two paralectotypes.
Moirechlamys actinodes n. comb. is restricted to late Miocene time, being represented in the Puerto Madryn Formation, the upper part of the Gran Bajo del Gualicho Formation, and the Barranca Final Formation. Ortmann (1902) was the first author to mention its occurrence in the late Miocene at ‘Cabo Buentiempo Beds’ (= Cape Fairweather Beds) (coast north of Río Gallegos, Santa Cruz Province). Although del Río (1992) considered that Ortmann’s (1902) material could not be Pecten actinodes, the present analysis confirms that it does belong in M. actinodes n. comb. because shells from Cabo Buentiempo have flabelliform discs with an umbonal angle of 100°–115° as in M. actinodes n. comb., and the shape and size of the auricles with a shallow byssal sinus, the length of the hinge margins, and the sculptural pattern allow us to place the material from Cabo Buentiempo in M. actinodes n. comb. This species was mentioned by Ihering (1897, 1907) as occurring in the early Pliocene ‘Araucanense Beds’ exposed at Bajo La Pava, Estancia Santa Rosa, and Monte Espejo. However, those specimens, as del Río (1992) suggested, do not belong in M. actinodes n. comb. and are included here in Moirechlamys aurorae n. comb. (Feruglio, 1954) (see the following). Reichler (2010) proposed Zygochlamys rizzoloi for specimens from Salinas del Gualicho (Río Negro Province) (upper part of the Gran Bajo del Gualicho Formation, late Miocene) (Fig. 10.7–10.9), but the presence of opisthocline, flabelliform, and left-convex shells with a shallow byssal sinus, ornamented with 26–42 plicae, each covered with up to five riblets, indicates that Z. rizzoloi is a synonym of M. actinodes n. comb. Del Río (1992) and del Río et al. (2016) demonstrated the presence of two well-differentiated morphotypes, one with valves strongly ornamented with high plicae (Fig. 10.4–10.6) from the Transgressive Phase of the Puerto Madryn Formation, and the other with low plicae (Fig. 10.1–10.3) recorded from the Highstand and Regressive Phases. This intraspecific variation is associated with changes in hydraulic energy. The material corresponding to Moirechlamys actinodes n. comb. (Sowerby, 1846) from ‘Cabo Buentiempo Beds’ has low plicae.» SANTELLI, M. B. & C. J. DEL RÍO. 2019. Neogene Pectinidae (Bivalvia) of tribe Chlamydini Teppner, 1922 in Patagonia (Argentina): Zygochlamys Ihering, 1907 and three new genera. Journal of Paleontology, 93 (2): 312-336, figs. 1-11. [p. 331]
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Moirechlamys actinodes (Sowerby, 1846) n. comb.; M. B. Santelli & C. J. del Río, 2019,Neogene Pectinidae (Bivalvia) of tribe Chlamydini Teppner, 1922 in Patagonia (Argentina): Zygochlamys Ihering, 1907 and three new genera, figures 10.1-10.11.
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«TYPE MATERIAL. Lectotype: NHM-L27960, a left valve, less complete than suggested by the figure (Sowerby 1846: pl. 3, fig. 33), with a fragment missing from the anteroventral margin, with two small oysters attached to the disc on the postero-dorsal margin and with the anterior margin of the anterior auricle sloping strongly forward and almost completely straight, from San José; paralectotype: NHML27983, a juvenile right valve from San José; paralecotype: NHM-L27982, a large (height 115 mm, width 120 mm), complete and well preserved right valve, with the umbo and auricles a little hidden in the soft fawn-weathered sandstone, from San José. The identity of this specimen is doubtful but it certainly belongs in a different genus.
TYPE LOCALITY. ‘San José, Patagonia’. As pointed out by del Río (1992: 26), the type locality of this species must remain uncertain, as the reference given by Darwin (1846) suggests it may come from anywhere in the Valdés Peninsula area, in northeastern Chubut, Argentina. The exact stratigraphical provenence of the material is also uncertain, but it certainly comes from a bed within the late Miocene Puerto Madryn Formation, where this species is quite common.
REMARKS. Del Río (1992: text fig. 7) illustrated a cast of the left valve labelled NHM-L27960, which she referred to as the ‘holotype’. Apparently she was unaware of the existence of the other syntypes (see del Río 1992: 26). Sowerby’s original description does not clearly imply that there was more than one specimen. Therefore, this ‘designation by inference of holotype’ is valid according to Article 74.6 of the ICZN (1999).»
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. 257]
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Zygochlamys actinodes (Sowerby, 1846), lectotype; M. Griffin & S. N. Nielsen, 2008, A revision of the type specimens of Tertiary molluscs from Chile and Argentina, plate 1, figure 1.
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«Stratigraphical age and distribution. Abundantly present in the Middle Miocene Puerto Madryn Formation, Península Valdés, Chubut Province (Figs. 2, 25), where it is found in sympatry with Aequipecten paranensis (Orbigny, 1842) and Amusium paris del Río, 1992, less often with Flabellipecten pyramidesius (Ihering, 1907) and Chesapecten crassus del Río, 1992, and in the "Cape Fairweather Beds", Santa Cruz Province, Argentina. The age constraints on this species are rather poor; there is, for example, no micropalaeontological evidence to support a Middle Miocene age assignment of the Puerto Madryn Formation (del Río, 1988).
Remarks. Except for the two type specimens, no further material from Península Valdés was available for study. Ortmann (1902) reported the species from Cabo Buen Tiempo ("Cape Fairweather"), Santa Cruz Province, from deposits of presumably Pliocene age. According to del Río (1992), specimens from Cabo Buen Tiempo are not conspecific, and differ from typical Z. actinodes in having fewer and wider, sharp plicae and thicker shells. A plaster cast of a LV from Cabo Buen Tiempo in the BAS collections (ROW.22.1; Plate 6, fig. b) has c. 35 plicae, which is only slightly lower than the lower figure (40) given by del Río for Z. actinodes. Both this valve and a RV (RO\(2.2; Plate 6, fig. a) are very similar in outline and sculptural characteristics to the material illustrated by del Río, and are here considered to represent Z. actinodes. A.G. Beu and M. Griffin revisited Cabo Buen Tiempo and recovered some large pectinids from blocks that had fallen down the high cliff; in Beu's opinion the rocks from which the blocks had originated (mainly a conglomeratic oyster bed) appear to be a Pleistocene terrace deposit (A.G. Beu, pers. comm. 2001; see also Beu, 1985, p. 7).The pectinid was said to look very like the abundant Península Valdés material (i.e. Z. actinodes), but also like Psychrochlamys patagonica (King & Broderip, IB32). The Cabo Buen Tiempo material at BAS is very dissimilar to P. patagonica (compare Plate 6, figs. a, b and Plate 8, figs. c-f), differing from that species in having a much larger valve, more fasciculate radial ribbing, coarser rib scales, a relatively longer outer ligament with a correspondingly much deeper byssal notch, and complete absence of antimarginal microsculpture.» 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. 40]
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Zygochlamys actinodes (Sowerby I); H. A. Jonkers, 2003, Late Cenozoic-Recent Pectinidae (Mollusca: Bivalvia) of the Southern Ocean and neighbouring regions, plate 6, figures a, b.
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«Conchilla grande, con la valva izquierda más convexa que la derecha. Especímenes juveniles subtriangulares, equilaterales a subequilaterales, más altos que largos; individuos adultos con contorno orbicular, ligeramente inequilaterales, con incipiente crecimiento oblicuo y más largos que altos. Aurícula anterior más larga que la posterior, con profunda escotadura bisal en la valva derecha. Un par de cruras cardinales en ambas valvas con un fuerte dentilio. Superficie del disco recorrida por 40 a 50 pliegues redondeados y ornamentados por finas costillas radiales, o por 60 a 70 costillas primarias muy ñnas, bajas, filosas, separadas por costillas secundarias ínás delgadas.
Distribución: Formación Puerto Madryn aflorante en Eje Tentativo, Punta Logaritmo, El Doradillo, Cerro Prismático,Cerro Avanzado, Punta Fárdela, Puerto Pirámide, Lobería de Punta Pirámide, Lote 39, Salina Grande, San José Este, Punta Tehuelche, Punta Quiroga, Puerto Lobos, Salinas del Gualicho.» DEL RÍO, C. J. & S. A. MARTÍNEZ CHIAPPARA. 1998. Clase Bivalvia. In C. J. del Río (Ed.), 1998: Moluscos marinos miocenos de la Argentina y del Uruguay. Anales de la Academia Nacional de Ciencias Exactas, Físicas y Naturales, Buenos Aires, Monografía 15: 96 p., pls. 1-26. [p. 58]
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Chlamys actinodes (Sowerby, 1846); C. J. del Río, C. J. & S. A. Martínez Chiappara. 1998, Moluscos marinos miocenos de la Argentina y del Uruguay: Clase Bivalvia, plate 5, figures 1, 2; plate 6, figure 2.
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«The earliest record of this group in Patagonia corresponds to Chlamys actinodes (SOWERBY), a taxon identified by SOWERBY (1846) on the basis of an immature individual that was collected by DARWIN (1846) in Middle Miocene strata of Puerto San José (old name given to Valdés Peninsula region at the beginning of 19th century). The fossiliferous locality where this species was found may be either the port itself called San Jose, in San Jose Gulf (see Text-fig. 1) or any other locality from Valdes Peninsule. In this way, the rype locality of Ch. actinodes remains unknown.
It must be pointed out that ORTMANN (1902) and IHERING (1907); and more recently, FLEMING (1957) and BEU (1985) identified with Ch. actinodes specimens found in the Lower Pleistocene? "araucanian" levels cropping out in Sierra Laziar, Bajo La Pava and Cabo Buentiempo (Santa Cruz Province). The analysis of the material described and illustrated by ORTMANN (op. cit.) and of that collected by lHERING (1907) from the mentioned localities (MACN 235, IHERING collection) led to the conclusion that it would not belong to Ch. actinodes, a taxon only recorded from Miocene strata (see comment below the description of Ch. actinodes). According to BEU (1985) the group of species related to Ch. patagonica (KING) may be subdivided into two subgroups. The earliest one (Late Miocene-Middle Pliocene) includes Ch. geminata (SOWERBY) (of Tavera Jerez), Ch. tenuicostata (HUPE) and Ch. seymouri MARWlCK and would be characterized, besides other features, by higher than long shells with a deep byssal notch throughout their growth. The other subgroup would correspond to the species ranging from Late Pliocene to Recent and includes Ch. patagonica s. str. (KING), Ch. patagonica delicatula (HUTTON), Ch. patagonica anderssoni (HENNING), Ch. patagonica heardensis FLEMING, Ch. vidali (PHlLlPPI), Ch. hupeana (PHILIPPI) and Ch. actinodes (SOWERBY), since BEU believed this last taxon was of Pleistocene age. The most characteristic features of this subgroup is the allometric growth which affects outline of shells and development of byssal notch. Inmature individuals are similar to adult shells of the earliest group but mature individuals develop shells as high as long, subequal auricles and shallower byssal notch. The present analysis allows to determine that chlamys from Miocene to Recent shows a distinct evolutionary trend in reduction of shell size, thinning of valves, development of equiconvex shells, reduction of byssal notch depth and smaller and subequal auricles (Text-fig. 5). The Middle Miocene Ch. actinodes (SOWERBY) has left-eonvex mature shells, auricles are large and unequal throughout growth and byssal notch depth remains of equal size in young and mature stages. This species shows strong allometric changes in outline of disks and auricular length. Adults are suborbicular and then are longer than high. Young specimens are subtriangular, higher than long and the ratio between auricular length and length of valves is greater than in mature individuals (Text-fig. 6). According to BEU (1985) this type of ontogenetic variations in outline of disks may have been developed in the Late Pliocene but this study indicates that it appears earlier differentiating some Miocene species such as Ch. actinodes.» DEL RÍO, C. J. 1992. Middle Miocene bivalves of the Puerto Madryn Formation, Valdes Peninsule, Chubut Province, Argentina (Nuculidae-Pectinidae), Pt. I. Palaeontographica, Palaeontologie A, 225: 1-58, pls. 1-8, 25 text-figs. [p. 26]
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