Chesapecten crassus 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 [p. 45-48, pl. 8, figs. 1-6, text.-fig. 22]
Chesapeeten crassus n. sp.
Plate 8, figs. 1-6: Text-fig. 22 «Holotype: CPBA 15.107, a right valve from Lobería Punta Pirámide (LPP8). Puerto Madryn Formation, Middle Miocene (DEL RÍO collection).
Material: Holotype, forty four right valves, twenty left valves and one articulated specimen from Lobería Punta Pirámide: CPBA 13.751-13.756, CPBA 13.767-13.771, CPBA 13.782-13.785, CPBA 15.093-15.106, CPBA 15.108, CPBA 15.109 (thirty two specimens (LPP8), CPBA 15.372-15.374 (LPP6). All material from Puerto Madryn Formation, Middle Miocene (DEL RÍO collection). Derivatio nominis: From the latin crassus: thick, because of the great thickness of valve. Diagnosis: Shell large in size and thick. Mature individuals as high as long or longer than high. Auricles long. Free margins of posterior auricles sloping forward. Byssal notch very wide and deep. Ctenolium presentduring all ontogenetic development. Resilial pit slightly higher than long. Description: Shell of large size, thick, of low inflation, left-convex, higher than long in young specimens and as high as long or a little longer than high in mature stages. Apical angle obtuse (115º). Dorsal margin of disk straight. Auricular dorsal margin straight and long (AL/L x 100 = 60-80%). Anterior auricles 25% longer than posterior auricles. Right valve with anterior auricle ornamented with 7 to 9 radial, wide and squamous rib lets; free margin rounded to straight. Byssal notch wide and deep with acute or subrounded apex. Posterior auricle sculptured with 10 to 17 fine scaly riblets; free margin straight and rather perpendicular to hinge margin. Resilial pit higher than long and flat. Two pairs of cardinal crura. Denticle feebly impressed. Ctenolium with 5 or 6 pectinidial teeth present throughout growth stages. Disk surface sculptured with 15 to 17 flat-topped plicae in young individuals and flattened to subrounded plicae in mature specimens, separated by flat and usually narrower interspaces. Secondary ribbing consisting of 3 to 7 fine, flat-topped scaly and of equal width ribs that are also longitudinally ornamented with 3 fine riblets being the central one commonly wider than the laterals. Left valve with auricles sculptured with 16 to 21 very fine squamous radial riblets or covered with growth lines. Free margin of posterior auricle sloping forwards. Byssal notch impressed. External sculpture of disk with 14 to 17 flat-topped, high in cross section with vertical sides plicae, separated by round-bottomed and wider interspaces. Secondary sculpture is highly variable (Text-fig. 22). On one end specimens present whole disk surface covered with fine, crowded concentric lamellae that in interspaces form a thin scaly riblet. In these cases there is a plicae sculptured with high, prominent and rather broadly spaced scales, that were originated by the lamellae, situated among two or three smooth-topped plicae. On other end plicae and interspaces are longitudinally costate with flat-topped, narrow and squamous secondary riblets that are mote prominent than the secondary sculpture of the opposite valve. Measurements in mm:
Comments: As it has been mentioned, specimens examined are distinguished by the presence of a different secondary sculpture on opposite valves. This feature may be due to a different response of the disk surfaces to the environmental conditions.
Presence of a wide and deep byssal notch throughout growth indicates that Chesapecten crassus n. sp. remained fixed to the substrate during its whole life. In this way, right valve would be very near of substrate while the opposite valve may face more stirred conditions than the right valve. Inferences regarding the variation of the left secondary sculpture are open to question until new fossil localities are found since up to date only one shelly bed is known bearing this taxon in Lobería Punta Pirámide. This type of variation may be related to ecologic modifications of abiotic factors and produced by different responses to bathymetric changes, just as it was noted too in Patinopecten healyi (ARNOLD) (MOORE 1979). Chesapecten crassus n. sp. clearly resembles Ch. nefrens WARD & BLACKWEWER, the type species of the genus, and the only features that set the Argentine new species apart from Ch. nefrens (WARD & BLACKWELDER 1975, PI. 2, figs. 4-6, PI. 3, figs. 4-7, PI. 4, figs. 1-2; GIBSON 1987, PI. 31, fig. 7; Chlamys (Lyropecten) madisonius CONRAD in GARDNER 1944, PI. 4, fig. 5, PI. 9, fig. 7) are a more convex left valve, presence of ctenolium throughout growth, wider resilial pit and free margins of auricles forming acute angles at junctures with outer ligament. Depth of byssal notch is highly variable in Ch. nefrens but it is always shallower than in Ch. crassus n. sp. Chesapecten coccymelus (DALL) (Calvert and Pungo River Formations) (DALL 1898, PI. 34, fig. 11; WARD & BLACKWELDER 1975, PI. 3, figs. 1-2; PI. 7, figs. 14-15; GIBSON 1987, PI. 27-30) differs in having smaller shells, anterior auricles about twice longer than posterior, free margin of posterior auricles sloping backwards and plicae have steeper sloping sides throughout growth stages.» CLAUDIA JULIA DEL RÍO, 1992 |
C. J. del Río, 1992, plate 8.
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