Ambonychia alata

Phylum: Mollusca
Class: Bivalvia
Order: Pterioida
Family: Ambonychiidae
Genus: Ambonychia
Species: Ambonychia alata (Meek, 1872)

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Originally: Anomalodonta costata, Anomalodonta alata (Holland, UGA Strat Lab 2013)
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Stratigraphic Occurrences

A. alata_strat

Geographic Occurrences

Map point data provided by iDigBio.
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Sequences (Formations)

  • C5 Sequence (Waynesville)
  • C4 Sequence (Arnheim)

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Identification in Hand Sample

  • Straight hinge line
  • Strong, radiating costae
  • Strongly defined imbricating growth marks/li>
  • Very large posterior alation
  • Numerous fine, crowded lines crossing all costae and the furrows between

Ambonychia alata from the Arnheim Formation of Warren County, Ohio (MUGM 29488)

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Foerste (1916):

  • The type of Anomalodonta costata is numbered 790 in the James collection at Chicago University. It possesses 19 simple radiating plications but the original number may have equaled 24. Compared with Anomalodonta alata, the radiating plications are distinctly narrower and are separated by relatively broader interspaces, which on account of their considerable width appear comparatively flat. Anomaldonta costata is a smaller species, usually not exceeding 50 mm. in height. The concave curvature of the anterior margin and the sinuous curvature of the posterior margin are both less, and the latter can scarcely be said to be alate; both of these features are possessed also by Anomalodonta alata during its younger stages. The posterior margin of Anomalodonta costata rarely is well preserved but its direction is indicated frequently by the direction of the concentric striae upon such parts of the shell as remain. The type is labeled as coming from Cincinnati, Ohio, but the horizon of the species is known to extend from the Arnheim, where frequently it is common, into the Waynesville member of the Richmond. As in the case of Anomalodonta alata, one or two additional plications are intercalated occasionally along the hinge-line or on the anterior slope of the shell. The total number of plications may reach 27 or 28.

Meek (1872):

  • Shell attaining a moderately large size, subtrigonal in general outline, compressed posterior-dorsally, and more convex in the umbonal and anterio-central regions; umbonal slopes ranging at an angle of about fifty degrees below the hinge line, and broadly rounded; hinge line straight, very nearly or quite equaling the greatest anterio-posterior diameter of the valves, and ranging nearly at right angles to the anterior side of the same; posterior alation very large, not separated from the swell of the umbonal and central regions by any defined sulcus, slightly rounded at its immediate extremity above; posterior margin faintly sinuous a little below its intersection with the hinge margin above, thence sloping forward and downward, and finally rounding into the regularly rounded base; anterior side more or less concave and nearly vertical above, but rounding regularly into the base below; beaks terminal, rather pointed, rising little above the hinge line, and directed a little obliquely upward and forward, with more or less inward curvature.
  • Surface marked by 24 to 28 simple, strong, radiating costae to each valve, that are nearly equal in breadth to the furrows between; those on the central portions of the valves passing nearly straight from the beaks obliquely to the posterior basal margins, those on the anterior side curving more or less forward below, and those near the cardinal margin curving a little upward behind. Crossing all of these costae and the furrows between, are numerous fine crowded lines, and at regular distant intervals, a few strongly defined imbricating marks of growth that curve parallel to the basal and posterior margins.
  • Height, 2.30 inches; breadth, 2.20 inches; convexity, about 0.80 inch.

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