Argopecten gratus del Río, 1992
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. 42, pl. 3, fig. 4]
1992 Argopecten gratus del Río, 1992
C. J. del Río, 1992, plate 3.
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«Holotype: CPBA 13.788, a right valve from Punta Norte, from the Puerto Madryn Formation, Middle Miocene (DEL RÍO collection).
Material: Holotype, nine right valves and four left valves from Punta Norte: CPBA 13.789-13.801 (PN1). All material from Puerto Madryn Formation, Middle Miocene (DEL RÍO collection). Derivatio nominis: From the latin gratus: gentle, for the gently wavy exterior surfaces of valves. Diagnosis: Shell of medium size. Disk sculptured with slightly rounded and low plicae. lnterspaces and sides of plicae covered with fine, sharp, squamous, sometimes moderately projecting lamellae becoming feebly to fairly impressed on top of plicae. Description: Shell of medium size, thin, longer than high, lightly inflated with left valve more convex than right valve. Apical angle obtuse (115º). Auricular margin and dorsal margin of the disk straight. Right valve with anterior auricle longer than posterior, covered with 6 or 7 wide and distinct rib!ets and growth lines; free margin rounded. Byssal notch triangular with apex angular. Posterior auricle sculptured with 11 to 13 fine riblets; free margin straight and forming right-angled junctures with outer ligament, with a slight basal inflection. Resilial pit shallow, higher than long. Two pairs of slightly impressed cardinal crura. Denticle present. Ctenolium with 4 or 5 pectinidial teeth. Sculpture consisting of 16 to 18 very low, rounded plicae omamented with very fine and feeble radial secondary riblets and trend of concentric, scaly lamellae distally concave on top of plicae and convex across interspaces. Disk flanks sculptured with fine, sometimes scaly rib!ets. Left valve with anterior auricle omamented by 15 or 16 riblets. Byssal sinus distinct. Free margin of posterior auricle sloping backwards, sculptured with 11 to 13 riblets. Disk ornamented with plicae narrower and higher than those on the opposite valve. Measurements in mm:
Comments: Argopecten gratus is a rare taxon and it is only known from Punta Norte where it is associated with glycymerids, arcids and heterodontids.
This taxon is rather similar to A. charltonius (MANSFlELD) which occurs in the Middle Miocene strata of the United States of America (WALLER 1969, PI. 4, figs. 3-8). This species differs from A. gratus n. sp. in having concentric lamellae only present in interspaces, in lacking scales and by the presence of a shallower byssal notch. The features that set this new species apart from Argopecten insuetus n. sp., the other Middle Miocene species from the Puerto Madryn Formation, are the shallower byssal notch (Text-fig. 20), the shorter auricles (Text-fig. 21 ), the interspaces without radial sculpture, the presence of concentric lamellae more crowded and the plicae lower and more rounded in cross section». CLAUDIA JULIA DEL RÍO, 1992
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«Conchilla más larga que alta, poco convexa y con la valva izquierda más convexa que la derecha. Aurícula anterior más larga que la posterior, con escotadura bisal triangular y ápice anguloso; ctenolio compuesto por 4 o 5 dientes pectidinales pequeños. Margen libre de la aurícula posterior derecha formando un ángulo recto con el margen dorsal y margenes libres de las aurículas de la valva izquierda inclinados hacia atrás. Valva derecha con 2 pares de cruras cardinales y dentículos presentes. Ornamentación externa dada por 16 a 18 pliegues bajos redondeados, tendidos y recorridos por costulitas secundarias apenas visibles y por numerosas lámelas comarginales.
Distribución: Formación Puerto Madryn aflorante en Punta Norte.» 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. 59]
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Argopecten gratus del Río, 1992; C. J. del Río, C. J. & S. A. Martínez Chiappara. 1998, Moluscos marinos miocenos de la Argentina y del Uruguay: Clase Bivalvia, plate 4, figures 6, 7.
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