Notochlamys iheringii (del Río, 1995)
DEL RÍO, C. J. 1995. The genus Swiftopecten Hertlein, 1936, (Bivalvia: Pectinidae) in the Tertiary of Southern South America. Journal of Paleontology, 69 (6): 1054-1059 [p. 1057, figs. 2.1-2.8]
1995 Swiftopecten iheringii del Río, 1995
C. J. del Río, 1995, figure 2.
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«Diagnosis. — Shell nearly equiconvex, right valve slightly more convex than left. Exterior surface of the shell ornamented with four or five, commonly deeply grooved, wide, radial plicae on right valve and three or four plicae on left valve; ribs narrower than interspaces.
Description. — Shell ofmedium size, moderately inflated, subequilateral, higher than long, with right valve slightly more convex than left. Dorsal margins of disk straight, steeply sloping downwards. Umbonal angle 75-82 degrees. Auricles small, triangular, unequal in size; dorsal auricular margin attaining lengths of 40-53 percent of total length of shell. Right valve with anterior auricle much longer than the posterior, with rounded margin and sculptured with 7 radial riblets. Byssal notch deep; ctenolium formed by 4 or 5 teeth. Posterior auricle very small, ornamented with 3 or 4 riblets, with straight free margin, sloping backwards. Two pairs of hinge teeth. Surface of the disk sculptured with 4 or 5 rounded plicae, sometimes longitudinally divided with a radial deep groove and with narrower and concave interspaces; riblets usually of equal width, sometimes finer in interspaces; ledging usually fairly well developed. Anterior left auricle longer than posterior, byssal sinus shallow; free margins of both posterior and the anterior auricles steeply sloping backwards. Exterior surface of left valve with 3 central plicae ornamented with riblets; ledging distinctly developed, producing strong nodes on top of radial elements (Figure 2.1). Discussion. — The specimens described in this paper were initially identified as Myochlamys nodosoplicata Ihering (Ihering, 1907, p. 256), but now they are here assigned to Swiftopecten iheringii new species, the only known representative ofthe genus Swijtopecten in South America. The holotype of M. nodosoplicata (Ihering, 1897, p. 227, plate 5, figure 36) is a deformed and poorly preserved right valve, which was later placed in Zygochlamys Ihering, 1907 (Morra, 1985). However, the presence in M. nodosoplicata of disks ornamented with very fine radial threads, sometimes only developed on the ventral margin of the shells, indicates that this species cannot be related phylogenetically either to "Zygochlamys" or to Swijtopecten. Characteristic morphological features of Swijtopecten are the presence of few plicae ornamented with radial costae, the development of ledging often absent on right valves but always distinct on left valves, and shagreen microsculpture covering the shell surface. The development of these elements in Swiftopecten iheringii new species allows the placement of this new species in Swijtopecten. The exterior ornamentation of Swijtopecten iheringii new species varies greatly. Some specimens from Punta Casamayor lack bifurcated right plicae, whereas others have deeply longitudinally grooved plicae. The type species of the genus, Swiftopecten swiftii (Bernardi) (see Masuda, 1972, page 399, plate 48, figures 1-5, plate 49, figures 1-5, Recent, Japan) has a larger shell (height 11.0 cm, length 8.0 cm) with the right valve more convex. Adults have a more triangular outline, higher plicae, they do not have longitudinal grooves and left valves have four distinct plicae that are narrower than those in Swiftopecten iheringii new species. Pecten etchegoini Anderson (Grant and Gale, 1931, page 173, plate 10, figure 3a-3b, Middle Pliocene, California) differs from Swijtopecten iheringii new species in having a more convex right valve and shallower byssal notch. Swiftopecten donmilleri MacNeil (1973, page 122, plate 11, figures 1-6, early Middle Miocene, Alaska) may be easily separated from the Patagonian representative of Swiftopecten because the northern hemisphere species has more convex shells, a more distinctive ledging, and fine macrosculpture in which the secondary ribbing ofthe superimposed riblets is broader and usually split towards the ventral margin of the valves. Swiftopecten nutteri (Arnold, 1909, plate 27, figures 3-4, Late Miocene, California) may be distinguished from Swiftopecten iheringii new species because the former species has six strongly divided plicae on the right valve that become obsolete towards the ventral margin, fewer and wider radial riblets, and left valve sculptured with four or six plicae wider than the interspaces, which are longitudinally sculptured with three or four riblets. Dimensions. — Holotype: MACN 252: height 27.0 mm, length 22.4 mm; paratypes MACN 253: height 26.7 mm, length 21.7 mm; MACN 255: height 44.4 mm, length 48.4 mm; MACN 256: height 23.0 mm, length 19.0 mm; MACN 2656: height 25.0 mm, length 21.0 mm. Etymology. — Named in honor of H. von Ihering, who dedicated so many years to the study of the Tertiary fauna of Argentina. Material.— Holotype: one right valve, MACN 252, from Yegua Quemada, a locality situated 50 km south of the Mouth of the Santa Cruz River (Santa Cruz Province), Monte Lean Formation (Early Miocene-Late Oligocene). Paratypes: 3 right valves and two left valves, MACN 253, from Cerro Espejo (San Julian Formation, Late Eocene-Early Oligocene), MACN 256, MACN 2656, MACN 255, from Punta Nava and Punta Casamayor ("Juliense" levels, Late Eocene-Early Oligocene) (Santa Cruz Province). Repository. — Ihering Collection, Museo Argentino de Ciencias Naturales "Bernardino Rivadavia." Buenos Aires, Argentina.» CLAUDIA JULIA DEL RÍO, 1995
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«Del Rio (1995) referred a new evenly plicate, shagreen-bearing chlamydinine species from the Early Miocene Chenque Formation (as used by Barreda 1996) of the Gulf of San Jorge, Argentina, to Swiftopecten Hertlein, 1936. The new species, S. iheringii del Rio, has a narrower umbonal angle than most Notochlamys species (although similar to that of some small specimens of the Pliocene N. kendricki sp. nov.) but resembles Notochlamys species in its five evenly rounded plicae, its shagreen microsculpture and its long anterior auricles. In shape and in the presence of growth ledges, S. iheringii resembles North Pacific species of Swiftopecten closely related to S. swiftii (Bemardi) (eg., MacNeil 1967: pl. 3; Waller 1991: pl. 2, figs 9, 10) more closely than it does Notochlamys. Nevertheless, the overall similarity of S. iheringii to Notochlamys species and the intermediate geographic position of S. iheringii raise the possibility of a closer phylogenetic relationship
between Notochlamys and Swiftopecten than their present morphologies and distributions would seem to suggest. However, this relationship is unclear at present, and further similar, plicate, shagreenbearing chlamydinines occur in other parts of the world, such as Manupecten and Semipallium (Waller 1991: 24-26) and the phylogeny of all these taxa, as suggested by Waller (1991), needs reconsideration in light of the earlier Australian records of Notochlamys reported here. As Notochlamys Cotton, 1930 is the earlier of the two possible synonyms Notochlamys and Swiftopecten, this question does not affect the generic identity of the Australian taxa.» BEU, A. G. & T. A. DARRAGH. 2001. Revision of southern Australian Cenozoic fossil Pectinidae (Mollusca: Bivalvia). Proceedings of the Royal Society of Victoria, 113: 1–205. [p. 58]
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