Chokekenia nicolasi (Morra, 1985)
MORRA, G. A. 1985. Revisión de Zygochlamys (Mollusca: Bivalvia). Ameghiniana, 22: 300-308. [p. 304, pl. 2, figs. 2a, 2b]
1985 Zygochlamys nicolasi Morra, 1985
«Type specimens.— Holotype, an articulated shell, CPBA 12465 and CPBA 12466, right and left valves, respectively. Morra (1985) designated a right valve, CPBA 12465, as the holotype and the left valve of the same specimen, CPBA 12466, as a paratype. As they constitute a single articulated specimen, they are both part of the holotype; from Cañadón El Lobo (Santa Cruz Province), San Julián Formation (early Miocene).
Diagnosis.— Shell medium sized with one growth ledge. Left valve with nine simple plicae of two widths and right valve with eight paired plicae. Plicae covered with numerous ribs of equal width that bifurcate twice on right valves, covered with scales that appear at 20–40 mm height from beaks.
Occurrence.— Monte León Formation (early Miocene) at Punta Guanacos and Estancia Darwin (= Canadón Darwin) and San Julián Formation (early Miocene) at Estancia El Lobo (Punta Casamayor).
Description.— Shell medium sized, up to 75 mm high, thin, subcircular in outline, higher than long or almost as long as high, right valve slightly more convex, opisthocline and posteriorly elongate; dorsal margins of disc concave, strongly inclined ventrally. Hinge dorsal margin straight, 45%–53% of total disc length. Both valves with one growth ledge at 20–40 mm from beaks. Umbonal angle 86°–93°. Auricles asymmetrical, moderately large, and with free margins concave, sloping anteriorly; right anterior auricle with dorsal margin straight, not projected upward, free margin rounded; byssal notch rectangular, moderately deep; ctenolium functional throughout ontogeny, with four or five strong teeth; byssal fasciole wide with corrugations convex toward umbo; byssal sinus very shallow. Right auricles sculptured with 7–10 radial ribs, left auricles with 10–12 ribs; ribs on right anterior auricle thicker than those on remaining auricles. Disc sculptured with wide plicae of subrectangular section that flatten toward ventral margin, interspaces each narrower than one plica, both covered with ribs ornamented with low, ventrally concave scales that appear at 25–40 mm height from beaks, along with development of ledges; right interspaces with one central rib and two secondary riblets, left interspaces with one secondary and one or two tertiary riblets. Right valve sculptured with eight paired plicae, partially bifurcated in some specimens, ornamented with five to six ribs that bifurcate on tops of plicae at 4–5 mm and at 15–20 mm height from beaks, and two weakly defined single plicae on each side of each plica. Left valve with nine single plicae of two widths; wider plicae covered with six ribs, narrower plicae with three or four ribs. Entire surface of disc with antimarginal ridgelets that appear in radial stage; commarginal microsculpture absent; shagreen microsculpture restricted to a band situated between 2 and 40 mm from beaks on most specimens; commarginal lamellae restricted to initial radial stage to 2.5 mm height on left anterior auricle and on right posterior auricle of some specimens; coarse commarginal corrugations covering right anterior auricle; left auricles with shagreen on proximal area.
Materials.— Four articulated specimens, six right valves and four left valves MACN-Pi 6404; 6407; CPBA 16373–16374; 21585, PRI 66414.
Dimensions (in mm).—Holotype CPBA 12465, L = 59.8, H = 60.9; CPBA16373, L = 52.2, H = 56.0; CPBA16374 L = 49.3, H = 51.3.
Remarks.— This species was originally described from an articulated specimen from Estancia El Lobo and included in the genus Zygochlamys by Morra (1985), who designated only the right valve as the holotype.
Analysis of the right valve illustrated by Ortmann (1902, pl. 23, fig. 2d) from Estancia Darwin identified as Pecten geminatus quemadensis showed that it is a specimen of Chokekenia nicolasi and allows us to expand the range of C. nicolasi to the early Miocene Monte León Formation exposed in the Austral Basin.» SANTELLI, M. B. & C. J. DEL RÍO. 2019. Neogene Pectinidae (Bivalvia) of tribe Chlamydini Teppner, 1922 in Patagonia (Argentina): Zygochlamys Ihering, 1907 and three new genera. Journal of Paleontology, 93 (2): 312-336, figs. 1-11. [p. 325, 327]
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Chokekenia nicolasi (Morra, 1985) n. comb; M. B. Santelli & C. J. del Río, 2019,Neogene Pectinidae (Bivalvia) of tribe Chlamydini Teppner, 1922 in Patagonia (Argentina): Zygochlamys Ihering, 1907 and three new genera, figures 8.1-8.7.
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