Loxochlamys corallina 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. 43, figs. 12.1-12.6]
2005 Loxochlamys corallina Waller in Waller & Stanley, 2005
T. R. Waller & G. D. Stanley Jr., 2005, figure 12.
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«Diagnosis.— Loxochlamys having 13-15 (commonly 14) prominent radial rounded-trigonal plicae that begin early in ontogeny and remain of moderate height throughout ontogeny.
Description.— Shell outline: Shell of small size, with heights ranging from about 9-25 mm and with height consistently exceeding length (Ht/L 1.02 to 1.41, x¯= 1.20, n = 16); acline to slightly opisthocline, with anterior half-diameter of disk commonly exceeding posterior half-diameter. Anterodorsal margin of disk straight to slightly outwardly concave; shorter posterodorsal margin straight to slightly convex. Umbonal angle about 908, with anterodorsal margin of disk forming a greater angle with the hinge line than does the posterodorsal margin. Left valve slightly more convex than right valve, the convexity of the left about 14% of valve height and convexity of right valve about 12% of valve height. Disk flanks low and with shallow slope toward plane of commissure. Disk gapes not observed directly but probably absent. Hinge line short and asymmetric, commonly about 50%–60% of shell length. Auricles unequal in length, the anterior commonly 1.5 to nearly twice the length of the posterior measured along the hinge line. Right anterior auricle with acute to rounded byssal notch of moderate depth; byssal fasciole clearly demarcated, moderately broad, slightly outwardly convex and lamellose; fine functional ctenolium present; anterior margin of this auricle with shallow outwardly convex curvature meeting hinge line at about 90°, dorsal margin straight or with a shallow scroll. Left anterior auricle with only a very shallow byssal sinus or none at all, the free margin of this auricle commonly with a central outwardly convex part bordered dorsally and ventrally by a shallow sinus, the overall trend of the margin being oblique to hinge line. Posterior margins of posterior auricles outwardly concave, with dorsal part of this margin intersecting hinge line at about 90° or less and overall trend of margin oblique to hinge line; posterior dorsal auricular margins straight, not scrolled. Ornament: Both valves with from 13-15, most commonly 14, simple interlocking radial plicae that originate less than 2 mm from beak and continue to ventral margin; plicae rounded to broadly trigonal in cross section, with shallowly sloping flanks, narrowly rounded crests, lacking carinate edges on shell interior, and interplical spaces rounded in cross section. Shell micro-ornament of disk surface complex, resulting from interplay of oblique microridges and commarginal lamellae. In proximal region, oblique microridges dominate. In midontogeny, oblique microridges and commarginal lamellae interact to produce a quincuncial pattern of tiny cusps or nodes, recalling shagreen microornament of some modern pectinids (Waller, 1972a, fig. 12), but adjacent cusps not merging to form typical shagreen pattern; beginning at shell height between 10 and 19 mm, commarginal lamellae become more lamellose and dominate the ornament pattern (Fig. 12.1, 12.2); commarginal lamellae shallowly tongued distally in plical interspaces and deeply tongued or ‘‘V’d’’ dorsally over tops of plicae. Disk flanks either smooth except for fine commarginal growth lines or crossed by continuations of oblique microridges of disk, these becoming antimarginal in trend across disk flanks and on adjacent surfaces of posterior auricles. Ornament on auricles consists of commarginal lamellae; radial costae absent except on right anterior auricle, where they are only very weakly developed and few in number. Shell interior and shell microstructure: Inner shell layers and accompanying muscle scars not preserved, but with a commarginal ridge near the shell margin indicating the distal edge of what was probably an extensive inner aragonitic shell layer that comprised the shell interior including the hinge structure; outer calcitic shell layer recrystallized but displaying distinct antimarginal fibrous microstructure; evidence of regularly foliated calcite absent; columnar prismatic outer shell layer in early ontogeny of right valve not preserved but if present probably less than 2 mm in height based on position of origin of radial plicae (see Waller, 1972b, p. 52, on the relationship between the termination of prismatic microstructure and the onset of ribbing). Etymology.— The species name refers to the close association of the new species with colonial corals in the New Pass Range. Types and other material examined.— The holotype, UMIP 18048B, is a right valve with the concave inner surface of the outer shell layer facing upward from the rock matrix (Fig. 12.1), from the coral beds of Unit E, South Canyon, Locality 1. Nearly its entire outline is preserved, with some details of external ornament preserved as an impression in the matrix and with remnant antimarginal fibrous microstructure present (Fig. 12.2). Paratypes from Locality 1 that show certain morphological features particularly clearly are catalogued as USNM 526484–526496 (examples shown in Fig. 12.4–12.6). Other USNM paratypes from Locality 1 are uncatalogued but bear USGS locality numbers 34045, 34046, 34048, 34056, 34058, and 34067. In the Museum of Paleontology of the University of Montana, paratypes are present on rock pieces bearing the catalogue numbers UMIP 18014A–C, 18029, 18029A, 18035, 18035B, 18048B (which also contains the holotype), and 18048-C. In total, the material examined consists of 24 partial or nearly whole right valves, 20 partial or nearly whole left valves, and about 150 fragments. Measurements.— Holotype, height (restored estimate) 18.8 mm, length 14.9 mm. Occurrence.— The holotype and most of the paratypes are from the coral beds of Unit E in South Canyon of the New Pass Range. A few specimens were also found in South Canyon (Locality 1) in Unit E about 12 m above the coral beds, i.e., about midway between the coral beds and the overlying ammonoid bed. In ‘‘Brachiopod Canyon’’ (Locality 2), 3.9 km north of New Pass Mine on the western flank of the New Pass Range, poorly preserved fragments of the species were found in association with colonial corals and Pleuronectites newelli n. sp. This assemblage indicates a continuation of the South Canyon coral beds into this canyon. Discussion.— The three known species of Loxochlamys share similar shape and micro-ornament, including a tendency to become commarginally lamellose late in ontogeny. The new species differs from Loxochlamys chiwanae (McLearn, 1941a; Fig. 12.7-12.9) and L. sasuchan (McLearn, 1941a) (Fig. 12.10) in having a greater modal rib count (14 as compared to 12), with the ribs of the new species distinctly higher and beginning earlier in ontogeny. In the series L. corallina–L. chiwanae–L. sasuchan, there is a stepwise decrease in rib height and a lengthening of the time of appearance of the ribs during ontogeny, the plicae of L. sasuchan being so low in amplitude and beginning so late in ontogeny (at a distance from beak of about 12 mm) that they are difficult to count. In the same series, there is also a decline in the relative convexity of the right valve relative to the left valve. The two Canadian species are much closer to one another morphologically than either is to L. corallina. The morphological series Loxochlamys corallina–L. chiwanae–L. sasuchan is also a stratigraphic series. McLearn (1941a, p. 32) found that Loxochlamys chiwanae is restricted to the Mahaffy Cliffs fauna in the Peace River foothills of British Columbia, whereas L. sasuchan occurs in the ‘‘Lima? poyana fauna’’ in the same area. Later McLearn (1941b, p. 99; 1941c, p. 96) described the ‘‘Lima? poyana fauna’’ as stratigraphically succeeding the Mahaffy Cliffs fauna which, in turn, occurs well above the ‘‘Nathorstites fauna.’’ The Nathorstites Zone was later subdivided into the Meginae, Maclearni, and Sutherlandi Zones (Tozer, 1967, p. 64). Because Loxochlamys corallina in South Canyon occurs stratigraphically lower than the Unit E ammonoid bed containing ammonoids of these zones, it is clearly the oldest member of this series. Loxochlamys corallina is the most abundant bivalve in the coral beds. Although no articulated specimens were found, right and left valves occur in nearly equal numbers (27 right, 21 left), indicating a lack of extensive postmortem transport. Fragments of the pectinid are common bioclasts in the coral beds but are rare above the coral beds and unknown below them. The persistent byssal notch and ctenolium indicate that L. corallina was byssally attached throughout life, probably to dead coral or shells on bioherm surfaces.» THOMAS RICHARD WALLER, 2005
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