Posidonotis semiplicata (Hyatt, 1894)
HYATT, A. 1894. Trias and Jura in the Western States. Bulletin of the Geological Society of America, 5: 395-434. [p. 414]
1894 Monotis semiplicata Hyatt, 1894
1894 Monotis symmetrica Hyatt, 1894
1928 Entolium balteatum Crickmay, 1928
1894 Monotis symmetrica Hyatt, 1894
1928 Entolium balteatum Crickmay, 1928
«Loc., American canyon.
This shell has a more elongated form than Monotis subcircluaris, being narrow anteriorly and then widening out posteriorly. There are concentric lines over the whole valve, but only the umbonal ridges and posterior parts of the valves are adorned by radiating ridges. These consist of coarse, linear, straight ridges or costse widely separated and with finer lines between them. The extreme border of the posterior part of the shell in the vicinity of the hinge is also destitute of radial lines. There is a distinct but very small posterior wing, and the anterior edge has also a very slight extension, just enough to make it appear straight. The anterior or oral region is short, the umbo varying from nearly terminal to about one-fifth of the antero-posterior diameter from the oral end. The posterior end is much broader, the shape being an extended oval, narrower anteriorly than posteriorly. The umbo is situated nearer the center in young specimens, but always well toward the anterior end, and the shape of the valve does not change materially with growth. The species is gregarious, thickly crowded on the slabs, having had similar habits in this respect with Monotis salinaria and subcircularis. It is obviously either a Triassic or Rhetic species.»
ALPHEUS HYATT, 1894
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«Description
The shell of P. semiplicata is small to medium sized, suborbicular in shape, very thin, and of low convexity. Growth line patterns of the right valve anterior auricle reveal a small byssal notch early in ontogeny and its subsequent allometric reduction in size (Pl. 1, fig. 1) (see also Damborenea 1987, text-figs. 28e-28g). Interpretation The presence of a small byssal notch at an early age indicates that the juvenile obtained stability by means of byssal fixation. Ammonites and dead shells of Posidonotis on the sea floor may have served as attachment sites. Subsequent negative allometric growth of the byssal notch implies gradual abandonment of byssal attachment in favour of a reclining mode of life. An adult free-lying, reclining life habit on soft sediments in a low-energy environment is consistent with both a relatively small adult size and a thin, low-convexity shell of suborbicular shape (see also Stanley 1970; Johnson 1984) as well as with the paleoenvironmental interpretation given below. Shells of low convexity imply a relatively high surface-to-volume ratio. This is interpreted as an adaptatlon of an organism to increase oxygen uptake by absorption and diffusion of oxygen via surface tissues (e.g., Oschmann 1994). Posidonotis therefore appears to be well adapted to reduced oxygen conditions on the sea floor. A byssal juvenile phase precludes a holopelagic, that is, swimming or drifting, life style. There remains the possibility of an early pseudoplanktic mode of life, in which the bivalve is attached to a floating substrate, and subsequently a benthic phase, where it is reclining on the sea floor. Alternatively, an early epibyssate phase on the sea bottom, followed by a later reclining phase is conceivable. Further evidence is provided by the relative abundance and facies distribution of P. semiplicata.» ABERHAM, M. & J. PÁLFY. 1996. A low oxygen tolerant East Pacific flat clam (Posidonotis semiplicata) from the Lower Jurassic of the Canadian Cordillera. Canadian Journal of Earth Sciences, 33: 993-1006, pl. 1, text-figs. 1-5. [p. 998]
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Posidonotis semiplicata (Hyatt, 1894). M. Aberhan & J. Pálfy, 1996, A low oxygen tolerant East Pacific flat clam (Posidonotis semiplicata) from the Lower Jurassic of the Canadian Cordillera, plate 1, figures 1-10.
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