Formerly: Leptaena planumbona
Map point data provided by iDigBio.
- C5 Sequence (Liberty, Waynesville)
- C4 Sequence (Arnheim)
Identification in Hand Sample
- Delicate striations
- Small to medium size
- Semicircular form
- No fold or sulcus present
- General lack of rugae
- Triangular plate (symphytium) in flat ventral interarea
Jin & Zhan (2001):
- Remarks: Only six specimens from the southern Manitoba collection are assigned to Strophomena planumbona on the basis of their medium-sized shell, relatively gentle geniculation, general lack of rugae except for the presence of short, weak, oblique wrinkles in the postero-lateral part of some shells (pl. 7, figs. 1, 2). Such inconspicuous rugae are present also in some shells of the same species described from Anticosti Island (Dewing, 1999) and the Hudson Bay Lowlands (Jin et al. 1997).
- In the Cincinnati area, Strophomena plaumbona is largely confined to the Richmondian strata of the Arhneim, Waynesville and Liberty Formations (Davis, 1985).
- Articulate brachiopod. Differs from similar species by having delicate striations, a relatively small size, and a rectangular form, and by lacking a fold and sulcus. Arnheim through Liberty.
Jin et al. (1997):
- Remarks: S. planumbona has a medium-sized, subquadrate shell (length rarely exceeding 20 mm), with relatively low convexity, subrhomboidal ventral muscle field, and relatively large adductor scars. The exterior generally lacks rugae. In external shape, the Hudson Bay specimens are most similar to those illustrated by Caster et al. (1961) from the Richmondian rocks of Ohio in their roundly subquadrate to semicircular outline, whereas the shells described by Wang (1949) and Howe (1988) are more typically subquadrate. The type specimens of S. planumbona as illustrated by Rong and Cocks (1994), vary from subquadrate to semicircular. Surface sculpturing consists of clearly unequal to nearly equal multicostellae (very fine, with 5 to 7 per 1 mm), crossed by even finer growth lines (about 15 per 1 mm). The Hudson Bay specimens of S. planumbona show very fine, dense, and irregularly spaced pseudopunctae, as in most species of Strophomena (Dewing, 1995). Naturally exposed internal structures are rare but a few specimens expose a subrhomboidal ventral muscle field occupying the posterior one-third of the valve length, with a thin, prominent median ridge, relatively high lateral bounding ridges, and large adductor scars extending to the anterior margin of the muscle field.
Leighton & Savarese(1996):
- Strophomena planumbona which is small, oblate, and never geniculate, but varies in valve convexity.
- Biomechanical experiments uphold the hypothesis that concavo-convex strophomenid brachiopods lived in the convex-down position. Moreover, the convex hypothesis is not supported. These results hold for both sandy and muddy substrates.
- Shell finely striated, quadrate. Brachial (dorsal) valve flattened 2/3 of the way anteriorly from the hinge. Muscular area of pedicle (ventral) valve circular, deeply impressed, bordered by curved, sharply elevated ridges, which are deflected forward producing an anterior gap. Shell distinctly thickened along the anterior and lateral margins, crossed by vascular grooves. Striae normally subequal or alternating in size, less frequently an alternation of one coarse with 2 or 3 fine ones. WIdth, 30mm, length, 0.75 width, striage 5 per 2 mm.
- In the Richmond areas of southern Ohio S. planumbona is introduced near the base of the Clarksville Member, more commonly as the variety elongata, the typical forms common in the upper Blanchester and Liberty. Southward in Kentucky on both east, and west flank it is limited to the uppermost Waynesville and Liberty.
- Shell rather small, with subquadrangular outline, anterior broadly rounded; pedicle (ventral) valve usually curved downward only moderately toward its anterior margin; surface striae fine; muscle impression in interior of pedicle (ventral) valve having high border along its entire margin, except anteriorly where the border is open, its anterior parts on each side of median line curving slightly forward; callous thickening along anterior margin of valve narrow, and crossed radially by vascular markings. Some shells are more prolonged along hinge-line and some have oblique wrinkles along this line.
- Locality and Horizon. Type locality probably from Waynesville, Oxford, Ohio. It ranges at this horizon through Ohio, Indiana, and Kentucky. Common in lower half of Liberty in all three states. Rare in Ontario and Quebec, though closely similar specimens are widely distributed , but scarce. On Manitoulin island specimens of this type occur in the Waynesville member at Gore Bay, Kagawong, on Bass Lake road south of Little Current, and at Clay cliffs; southeast of Meaford; Oakville and Streetsville. In the province of Quebec, in Huron River section; in erratic blocks at St. Hugues, and in situ at St. Hilaire and in the Nicolet River section.