Delectopecten neuquenensis del Río, Beu & Martínez, 2008
DEL RÍO, C. J., A. BEU & S. A. MARTÍNEZ. 2008. The pectinoidean genera Delectopecten Stewart, 1930 and Parvamussium Sacco, 1897 in the Danian of Northern Patagonia, Argentina. Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen, 249: 281-295, figs. 1-4. [p. 288, figs. 4.1-4.7]
2008 Delectopecten neuquenensis del Río, Beu & Martínez, 2008
C. J. Del Río, A. Beu & S. A. Martínez, 2008, figure 4.
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«2007 Pectinidae gen. and sp. indet. – DEL RÍO et al., figs. 3v-x.
Etymology: After the Neuquen Basin.
Types: Holotype GHUNLPam 22868 (right valve) and paratypes GHUNLPam 22869 (left valve) and GHUNLPam 22871 (right valve), all from Cerros Bayos (La Pampa Province); paratypes MACN-Pi 4870 (right valve) and MACN-Pi 4871 (left valve) from General Roca (Río Negro Province).
Type locality and horizon: Cerros Bayos, La Pampa Province, Argentina; Danian, Roca Formation.
Other material examined: Three left valves and one right valve GHUNLPam22870; 22873-22875 from Cerros Bayos; two right valves and two left valves MACN-Pi 4872-4875 from General Roca; numerous fragments of disks and auricles.
Occurrences: Danian exposures of the Roca Formation at Cerros Bayos (La Pampa Province) and General Roca (Río Negro Province).
Measurements: (in mm; abbreviations as above):
Diagnosis: Shell up to 5 mm high, rather inflated. Auricles large; anterior auricles deeply demarcated. Byssal notch with rounded apex; no ctenolium. Left anterior auricle with 5-6 narrow radial riblets, right anterior auricle with commarginal lamellae and up to 5 radial riblets; disc with fine commarginal ridges, coarser on umbonal area than lower down.
Description: Shell small, subcircular, up to 5 mm high, thin, translucent, equiconvex, rather inflated, acline to only slightly prosocline and from equidimensional to slightly longer than high. Umbonal angle 100-112°. Auricles subequal, large, dorsal auricular length 70 % - 85 % of total disc length; free auricular margins straight, inclined posteriorly; dorsal auricular margin straight; posterior auricles not differentiated from disc, anterior auricles strongly demarcated from disc by a deep groove; right anterior auricle with deep byssal notch with a rounded apex. Left anterior auricle with 5 or 6 narrow radial riblets; right anterior auricle with moderately prominent commarginal lamellae and up to 5 radial riblets, the most ventral one much coarser than the others. Disc sculptured with fine commarginal ridges, more pronounced on left than on right valve; coarser in umbonal area than lower down, more numerous and closely spaced on ventral region than higher up, 4 ridges per mm on ventral region of disc; extremely fine antimarginal ridgelets on disc and auricles, up to 25/mm in central part of disc and 35/mm in lower centre of disc.
Comments: Specimens GHUNLPam 22868-22870 were partially described and illustrated by DEL RÍO et al. (2007, fig. 3V-X ) who classified this material as Pectinidae gen. and sp. indet., as diagnostic features such as antimarginal microsculpture or ornamentation on anterior auricles were not preserved. Recent collections from the Cerros Bayos and Cholino sections and SEM photographs of the new material allowed the recognition that these new specimens, and those considered by DEL RÍO et al. (2007) belong in Delectopecten neuquenensis n. sp.
Delectopecten neuquenensis n. sp differs from the type species, D. vancouverensis (GRAU, 1959: pl. 15, RAINES & POPPE 2006: 61-63, pl. 3, fig. 5-9), in lacking its many fine, scaly radial riblets. D. neuquenensis n. sp. resembles D. randolphi (DALL, 1897, illustrated by GRAU 1959: pl. 16, figs. 4-5; but considered to be a synonym of D. vancouverensis by RAINES & POPPE 2006: 15) (Recent, Bering Sea to Guaymas, Mexico) in having an almost acline, orbicular, smooth shell only sculptured with fine antimarginal striae and commarginal lamellae, and in having anterior auricles of both valves sculptured with radial costae. However, D. neuquenensis n. sp. differs from D. randolphi in having smaller, equidimensional shells, a byssal notch with a rounded apex, a longer auricular dorsal margin, and the commarginal costae are weaker than the strong undulation observed in D. randolphi. D. zacae (HERTLEIN 1935: pl 18, figs. 3-6; 9-10, GRAU 1959, pl. 19) has sculpture ranging from almost smooth valves, except for the commarginal undulations, with prominent radial sculpture on the anterior auricle, to having up to c. 16 prominent, narrow radial riblets and numerous intercalated secondary riblets, all more prominent on the left valve than the right (RAINES & POPPE 2006: 64-65, pl. 4, figs. 7-9, 11, 13); it differs from D. neuquenensis in having a larger and more strongly prosocline shell, 12-18 mm high. Delectopecten neuquenensis n. sp. particularly resembles the acline, almost smooth-surfaced Neogene and Recent species of the eastern North Pacific. Among these, the small, thin, almost acline, nearly equilateral shells of D. alternilineatus (CLARK, 1918: pl. 13, figs. 14, 15, MOORE 1984: pl. 2, figs. 8-9; Neogene, California) strongly resemble those of D. neuquenensis n. sp. and the two species can only be differentiated because the Argentinean species has almost flat valves, whereas D. alternilineatus is more inflated and has 10 weakly nodulose radial ribs on the left posterior auricle and about 13 radial ribs on the anterior dorsal margin of the right disc. D. peckhami (GABB, 1869) (as interpreted by ARNOLD 1906: pl. 3, fig. 8, MOORE, 1984: pl. 3, figs. 4-5), a species from the Eocene-Miocene of California, also has flattened, acline, equidimensional shells with commarginal undulations, but differs from D. neuquenensis n. sp in being larger, with the dorsal margin of the auricles of the left valve inclined towards the beak, and in having radial ribs on the discs of both valves.
Delectopecten neuquenensis closely resembles D. crassistriatus BEU & DARRAGH (2001: 35, fig. 7 A-I), the only known species recorded from Cenozoic rocks of the southwest Pacific (Pebble Point Formation, Lower Paleocene, Victoria, Australia), in its small size, in the almost smooth surface except for the extremely fine commarginal sculpture, and in the sculpture of the anterior auricles. However, D. neuquenensis n. sp. has a less convex shell (nearly flat), a deeper and more rounded byssal notch apex, commarginal lamellae on both valves, and more closely spaced antimarginal ridgelets; there are only 20 wavy ridgelets per mm on the central lower part of the disc of D. crassistriatus. Although the sculpture of the anterior auricles of D. neuquenensis is highly abraded, it consists of up to five narrower ridges than the 7 coarse ones in D. crassistriatus. D. musorstomi POUTIERS, 1981 (DIJKSTRA & MARSHALL, 1997: 88, pl. 8, figs. 1-6, RAINES & POPPE 2006: 58-60, pl. 2, fig. 1; Recent, western Pacific from the Philippines to New Caledonia) is similar to D. neuquenensis n. sp. in size and inflation, but differs from both it and D. crassistriatus in having a longer right anterior auricle, a more strongly prosocline disc, and markedly coarser and more complex disc sculpture than either fossil species. Finally, the Recent New Zealand species D. fosterianus (POWELL, 1979: 379, pl. 73, fig. 3) reaches a much larger size than all others discussed here (up to 20 mm high), and is weakly inflated and almost smooth, with sculpture only of antimarginal ridgelets and fine commarginal striae, apart from a few small scale-like nodules on the commarginal striae near the anterior and posterior ends of the disc, and 6 narrow radial costae on the right anterior auricle.» CLAUDIA JULIA DEL RÍO, ALAN GLENN BEU & SERGIO AGUSTÍN MARTÍNEZ CHIAPPARA, 2008
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