Parvamussium bayosense del Rio, Beu & Martínez, 2008
DEL RÍO, C. J., A. G. 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. 285, figs. 3.1-3.5]
2008 Parvamussium bayosense del Rio, Beu & Martínez, 2008
C. J. Del Río, A. Beu & S. A. Martínez, 2008, figure 4.
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«Etymology: After Cerros Bayos, where this species was collected.
Types: Holotype MACN-Pi 4879 (right valve) and paratypes MACN-Pi 4877-4878 (left valves); MACN-Pi 4880 (left valve).
Type locality and horizon: General Roca, Río Negro Province, Argentina; Danian, Roca Formation.
Other material examined: one left valve MACN-Pi 4876 from General Roca; one left valve GHUNLPam 22872 and numerous fragments from Cerros Bayos (La Pampa Province).
Occurrence: Danian exposures of the Roca Formation at Cerros Bayos (La Pampa Province) and General Roca (Río Negro Province).
Measurements (in mm): (H = height, L = length, AL = dorsal auricle length)
Diagnosis: Shell up to 4 mm high, weakly inflated, acline. Auricles large, deeply separated from disc. Right disc with numerous fine commarginal ridgelets and 2-3 more prominent ridges on umbonal area; left disc with commarginal ridges and up to 8 fine radial costae, with up to 4 fine radial costae in each interspace. Interior of valves with 7 thick internal ribs.
Description: Shell small, very thin, translucent, weakly inflated, nearly equidimensional, acline, up to 4 mm in height; umbonal angle 90-110º. Auricles of almost equal size, large, separated from disc by deep sulcus; dorsal auricular margin straight, dorsal length ca. 83 % of total disc length; free margin of left anterior auricle slightly concave, inclined anteriorly; free margin of posterior auricle almost straight, almost vertical to slightly inclined anteriorly (Fig. 3.5); right anterior auricle unknown; auricular crura prominent; auricles ornamented by prominent commarginal ridges. Right disc sculptured with numerous fine, commarginal ridgelets with evenly convex crests (18-20 ridgelets/mm) and 2 or 3 more prominent commarginal ridges on umbonal area; left disc sculptured with commarginal ridges and up to 8 fine radial costae, with up to 4 intercalated thinner radial striae in the interspace between each pair of costae. Because of poor preservation, neither height at which radial costae appear nor whether there are any nodes or tubercles on ribs can be determined, although Fig. 3.4 seems to indicate that small nodes are present. Interior of valve with 7 thick, internal radial costae with evenly rounded crests, reaching almost to ventral margin, where they are separated by interspaces 1 mm wide.
Comments: The material collected is very fragile, and in most cases it is partially fragmented and auricles are missing. Most of it consists only of internal views of translucent valves revealing external features. Comparison with other species is difficult because the exterior surface of most valves cannot be observed.
The type species of Parvamussium, P. duodecimlamellatum (BRONN, 1831) (DEPERET & ROMAN 1928: pl. 27, figs. 2-6; HERTLEIN 1969: N350, fig. C73.2a, b, Miocene-Pliocene, Europe) is slightly longer than high, with shorter auricles than P. bayosense n. sp., and its left valve disc is smooth; the right valve disc is commarginally lirate “as a result of alternating commarginal bands of small polygonal and radially elongate prisms” (WALLER 2006b: 342) (note that the illustration by HERTLEIN 1969: fig. C73.2c shows P. felsineum (FORESTI, 1895)). Parvamussium felsineum (DEPERET & ROMAN 1928: pl. 27, figs. 7-12, Miocene-Pliocene, Europe) resembles P. bayosense n. sp. in having the left valve disc sculptured with numerous radial riblets with many intercalated narrow riblets, but the two species can be separated by height exceeding length in P. felsineum, which also has shorter auricles than in P. bayosense n. sp. As noted above, “P.” pumilum (LAMARCK, 1819) (DAMBORENEA 2002: pl. 1, figs. 12-20, Jurassic, Europe and Argentina) was referred to a phylogenetic position between Parvamussium and Filamussium WALLER, 2006 by WALLER (2006b: 324) and need not be compared here. Parvamussium vafer (MARWICK, 1931: 64, pl. 4, figs. 55-57; Middle Miocene, New Zealand) reaches a slightly larger size than P. bayosense n. sp., and is higher than long. It also differs from P. bayosense n. sp. in having only 8 commarginal costae per mm, 9 to 10 rather than 7 internal ribs, and more numerous radial ribs on the left valve. P. aucklandicum (ZITTEL, 1864) (BEU & MAXWELL 1990: 374, pl. 51a, b, f; Early Miocene, New Zealand) has narrower internal ribs, strongly squamous radial ribs, only one narrow costa intercalated between primary ones, and 14 commarginal costae per mm. P. stanfordense (ARNOLD, 1906) (MOORE 1984: pl. 1, figs. 14, 15, 17; Eocene, California; probably also occurring in Japan, MOORE 1984) has larger shells with smaller auricles than P. bayosense n. sp., and 10-12 rather than 7 internal ribs.» CLAUDIA JULIA DEL RÍO, ALAN GLENN BEU & SERGIO AGUSTÍN MARTÍNEZ CHIAPPARA, 2008
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