Nevadapecten lynnae Waller in Waller & Stanley, 2005
WALLER, T. R. & G. D. STANLEY JR. 2005. Middle Triassic Pteriomorphian Bivalvia (Mollusca) from the New Pass Range, west-central Nevada: Systematics, biostratigraphy, paleoecology, and paleobiogeography. Journal of Paleontology, [Memoir Paleontological Society 61], 79 (1): 1-59, figs. 1-14. [p. 46, figs. 13.1-13.4]
2005 Nevadapecten lynnae Waller in Waller & Stanley, 2005
T. R. Waller & G. D. Stanley Jr., 2005, figure 13.
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«Diagnosis.— As for the genus.
Description.— Shell outline: Shell of medium size, with maximum shell length of about 60 mm, length exceeding height (Ht/L 0.8 for the holotype); slightly opisthocline and prosogyrous to nearly acline and orthogyrous, nearly equilateral; ventral margin of disk broadly circular, curving more sharply to meet anterior and posterior disk flanks. Anterodorsal and posterodorsal margins of disks of both valves straight to very slightly outwardly concave in lateral view. Umbonal angle very broad (125° on the holotype, a right valve, and 140° on the paratype, a nonmatching left valve), with the anterior umbonal angle greater than the posterior. Valves equally convex, with maximum convexity about 16% of valve height; maximum inflation of both valves anterior to midline; curvature of right disk flattening toward anterior and posterior margins, especially toward the posterior; curvature of left disk greatly flattened toward anterior and posterior margins, producing flat anterior and posterior sectors separated by a sulcus from central part of disk. Disk flanks of both valves very low but steep, clearly setting off auricles from disk. Disk gapes not observed. Hinge line symmetric, with hinge length about half shell length. Right anterior auricle not preserved except for a remnant indicating a persistent byssal notch of unknown depth; ctenolium not preserved or detected. Left anterior auricle shallow, with straight dorsal margin and obtuse, outwardly concave anterior margin lacking a byssal sinus. Right posterior auricle with proximal part of dorsal margin straight, possibly rising into a scroll distally, posterior margin outwardly concave with overall trend forming an obtuse angle with hinge, curving distally near dorsal margin to form an acute point. Left posterior auricle with straight dorsal margin, posterior margin poorly preserved, apparently forming an obtuse angle with dorsal margin. Ornament: Right disk with 16 low, rounded, simple plicae broader than interspaces and decreasing in height and width toward anterior and posterior margins. Left disk with 17 low, rounded, simple plicae narrower than interspaces and almost disappearing on the anterior and posterior flattened sectors. Radial plicae and interspaces of both valves crossed by regular, fine commarginal lirae with spacing of about 6/mm at a shell height of about 35 mm. These commarginal lirae broadly tongue dorsally across plicae and ventrally across interspaces (Fig. 13.3). Antimarginal microornament absent or not preserved. Right posterior auricle with commarginal growth lines spaced at 4/mm; radial costae apparently absent on left auricles and right posterior auricle, not determined on right anterior auricle. Shell interior and shell microstructure: Muscle scars and hinge apparatus not accessible. Thin outer shell layer with preserved but recrystallized antimarginal fibrous microstructure suggesting original calcitic composition; thicker inner shell layer coarsely recrystallized without preserved microstructure, suggesting original aragonitic composition. Columnar prismatic structure not preserved or detected. Etymology.— The new species is named in honor of my daughter, Lynn Waller Fegley, who assisted with field work in South Canyon and collected the specimen that is now the holotype. Types.— All from the coral beds of Unit E, South Canyon, Locality 1. Holotype: USNM 526497, a right valve exterior (Fig. 13.1–13.3). Paratypes: USNM 526498, a left valve exterior (Fig. 13.4); USNM 526499, fragment of disk interior of right valve, on same block as a paratype (USNM 526477) of Pleuronectites newelli n. sp.; USNM 526500, fragment of posterior disk of right valve; USNM 526501, external mold of fragment of right? valve. Measurements.— Holotype, height 46 mm, length 56.5 mm, length of posterior hinge line 14 mm. Occurrence.— Known only from the coral beds of Unit E, South Canyon, New Pass Range, Nevada, late Ladinian. Discussion.— The new species differs from all other members of the Tosapectininae in having a combination of uniform biconvexity and simple radial plicae of a single order throughout ontogeny. Compared to Pseudopecten of the Lower Jurassic, the new species has typical tosapectinine flattened sectors on the left valve, much lower disk flanks lacking cross-ridging, and a thinner outer calcitic shell layer that does not appear to be foliated.» THOMAS RICHARD WALLER, 2005
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