Zygochlamys sebastiani Morra, 1985
MORRA, G. A. 1985. Revisión de Zygochlamys (Mollusca: Bivalvia). Ameghiniana, 22: 300-308. [p. 304]
«Type specimens.— Holotype CPBA 12457, a left valve, and paratype CPBA 12458, a right valve, both from Monte Entrada (Santa Cruz Province), Monte León Formation (early Miocene).
Diagnosis.— Shell equiconvex. Spaces between plicae sculptured with up to three riblets. Antimarginal microsculpture thickened on top of plicae; commarginal lamellae present on entire disc, but dominant from 40 or 50 mm height from beaks; developing a reticulate pattern when crossing antimarginal ridgelets.
Occurrence.— Monte León Formation (early Miocene) at Monte Entrada, Cañadón de los Misioneros, mouth of Santa Cruz River, and Punta Guanacos; Chenque Formation (earliest–middle Miocene) at Pico Salamanca and Cerro Chenque.
Description.— Shell of moderate size, up to 95 mm in height, triangular, higher than long, equiconvex, acline, subequilateral, with anterodorsal margin concave and posterodorsal margin straight. Hinge dorsal margin straight, 45%–50% of total shell length. Umbonal angle 90°–95°. Auricles large, asymmetrical, and with free margins sloping anteriorly, those on posterior auricles concave, dorsal margin of anterior auricle slightly projected upward, free margin rounded; byssal notch very deep, wide, and rounded, with functional ctenolium with 5 or 6 strong teeth; byssal fasciole with corrugations convex toward umbo; byssal sinus shallow. Right auricles sculptured with 5 or 6 radial ribs, left anterior auricle with 11–13 ribs, left posterior auricle with 5–7 ribs; ribs on anterior auricles thicker than on posterior auricles, dorsal rib of right anterior auricle thickened. Exterior surface with rounded plicae sculptured with one primary rib that appears between 9 and 11 mm height from beaks and two narrower secondary riblets that originate between 25 and 60 mm height; interspaces with secondary riblets; ribs and riblets covered with ventrally concave scales. Right valve with 10 or 11 paired plicae with a well-defined geminate pattern at
beginning of radial stage but vanishing toward ventral margin, where they are separated by narrower interspaces; left valve with 15–20 single plicae of two intercalated sizes and interspaces of equal width. Antimarginal microsculpture extending from 18 mm on plicae and interspaces, thickening on primary ribs over crest of plicae; commarginal sculpture of very narrow lamellae that arise at 40–50 mm height from beaks; when crossing, the two microsculptures form a reticulate pattern. Shagreen microsculpture uncommon, restricted to umbonal area of both valves up to 10–12 mm height; auricles of both valves covered with antimarginal microsculpture. Materials.— One articulated specimen, two right valves, one left valve, three fragments of right valves, and four fragments of left valves: CPBA 11010, 15961–15963, 21577; MACN-Pi 5819; PRI 72684 and PRI 72685
Dimensions (in mm).— Holotype CPBA 12457, L = 78.9, H = 90.9; CPBA 12458, L = 50.4 , H = 60.2; MACN-Pi 5819, L = 56.2, H = 61.4.
Remarks.— Morra (1985) described Z. sebastiani based on a left (holotype) valve and a right (paratype) valve collected at Monte Entrada, and new collections expand its presence northward to the Monte León Formation exposed at Punta Guanacos (northern sector of the Austral Basin, Santa Cruz Province) and to the upper beds of the Chenque Formation in the Chubut Province (Golfo San Jorge Basin). Zygochlamys sebastiani differs from Z. geminata in having smaller shells, and its auricles are covered with fewer ribs. It differs from both Z. geminata and Z. jorgensis in having equiconvex shells characterized by the dominance of very fine commarginal lirae still present in the advanced stages of ontogeny, where they give rise to a reticulate pattern when crossing the thick antimarginal striae on the tops of plicae. Moreover, Z. sebastiani can be distinguished from Z. jorgensis by having shells sculptured with more numerous plicae with a primary rib restricted to their crests and interspaces on the left valve with fewer riblets.»
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. 323, 324]
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Zygochlamys sebastiani Morra, 1985, 2019, Neogene Pectinidae (Bivalvia) of tribe Chlamydini Teppner, 1922 in Patagonia (Argentina), figure 7.
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