Agerchlamys boellingi Taylor & Guex, 2002
TAYLOR, D. G. & J. GUEX. 2002. The Triassic/Jurassic System boundary in the John Day Inlier, east-central Oregon. Oregon Geology, 64 (1): 3-28, pls. 1-4. [p. 14, pl. 1, figs. 1-4, 8-11; pl. 4, figs. 1-3]
2002 Agerchlamys boellingi Taylor & Guex, 2002
D. G. Taylor & J. Guex, 2002,
plates 1, 4. |
«Holotype: NWMNH No. 25510. Provenance: From lower part of Ferguson Hill Member, New York Canyon area (Fig. 1A, loc. d). Dimensions: H=29.5 mm, W=29.4 mm. Left and right valves.
Etymology: Named after Karen Boelling, for her contribution to our understanding of the geochemistry of the Volcano Peak Group. Diagnosis: Ribbing fine; left valve with umbonal angle greather than 90°; radial ribbing weak on posterior ear, shell in vicinity of byssal notch preserves only growth striae. Posterior auricle about ½ to ⅔ the length of the anterior auricle. Description: Shell of moderate size; valves gently inflated, left valve more inflated than right valve. Anterior right auricle with fairly deep byssal notch. Posterior auricles more than half the length of anterior auricles. Posterior margin of posterior auricles steeply sloping, anterior margin of anterior auricle on left valve subvertical to gently overhanging. Antero-dorsal margin on right valve moderately concave; dorsal margins otherwise gently concave to straight. On right valve umbonal angle may at first be <90°, but quickly widens with growth to >90°; umbonal angle on left valve >90° degrees. Radial ribs very fine, threadlike, with relatively wide interspaces; intercalatory ribbing common; normally >50 radial ribs on a valve, concentric ribbing weaker than radial ribbing, although on some specimens concentric ribbing may be strong enough to result in reticulate ornamentation. Auricle preserves a few to several radial ribs which are weak to moderate in strength on anterior ears and faint on posterior ears. Shell in vicinity of byssal notch sculptured only by growth striae. Discussion: This species is characterized by its fine and closely spaced radial ribbing which, in conjunction with comarginal ribbing, gives a delicate cancellate ornamentation. This species is similar in appearance to A. wunshanae Marwick from New Zealand and South America (Damborenea, 1993), which differs by having an anterior auricle shorter (<½ of length) than the posterior auricle. Specimens unequivocally assigned to the species occur in strata superjacent to the Crickmayi Zone. One specimen from the Crickmayi Zone (Figure 10, bed 16) has slightly less dense radial costation than is typical, while another shell fragment from the same level at stratigraphic section e-e’ (Figure 9, bed 17) may also belong to this species.» DAVID G. TAYLOR & JEAN GUEX, 2002
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«The bivalve Agerchlamys boellingi is known from several sections in North America at similar, or slightly higher, stratigraphic levels, but never from undisputed Triassic strata. McRoberts (2004) discussed well-preserved representatives of this species from several sections spanning the Triassic–Jurassic boundary interval in northeastern British Columbia. In these localities (e.g. Ne Parle Pas Point, Black Bear Ridge and Crying Girl Prairie), Agerchlamys first occurs within 1m above the youngest undisputed Triassic strata and approximately 0.2-3.0 m below psiloceratid ammonoids. Recent work in Austria (McRoberts field work, 2006) documented similar bivalves, often identified as ‘Chlamys textoria’ (see, for example, Golebiowski & Braunstein, 1988) as first occurring in greater abundance within 0.2–3.0 m above the CIE in many alpine sections, including Kendlebachgraben, Tiefengraben, Kuhjoch, Schlossgraben and Steinplatte (Kammerker and Möseralm sections).
As with the ammonoids of the Psiloceras tilmanni group discussed above, neither Agerchlamys boellingi nor ‘Chlamys textoria’ are part of a demonstrable evolutionary series in which potential ancestors are known, and therefore they are not suitable for defining a datum for the base Hettangian GSSP. The original definition of Agerchlamys by Damborenea (1993) only lists three included species, of which two (the type Agerchlamys wunchae and Agerchlamys proprius) are Jurassic. The one Triassic species listed by Damborenea (1993, 2002) as belonging to this genus was originally described as Chlamys (Camptochlamys) inspecta Kiparisova (in Kiparisova et al., 1966) from the Carnian and Norian of Siberia. This species lacks radial ornament on its auricles and is here considered to be distinct from, but closely related to, Agerchlamys.» MCROBERTS, C. A., P. D. WARD & S. HESSELBO. 2007. A proposal for the base Hettangian Stage (= base Jurassic System) GSSP at New York Canyon (Nevada, USA) using carbon isotopes. ISJS Newsletter, 34 (1): 43-49, figs. 1-4. [p. 45]
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