Myoconcha americana Stanton, 1895

Jenkins, Robert G., Kaim, Andrzej, Little, Crispin T. S., Iba, Yasuhiro, Tanabe, Kazushige & Campbell, Kathleen A., 2013, Worldwide distribution of the modiomorphid bivalve genus Caspiconcha in late Mesozoic hydrocarbon seeps, Acta Palaeontologica Polonica 58 (2), pp. 357-382 : 372

Myoconcha americana Stanton, 1895

Fig. 12 .

1895 Myoconcha americana sp. nov.; Stanton 1895: 48, pls. 2–11. 1930 Myoconcha americana ; Stewart 1930: 104.

2010 Myoconcha americana ; Kiel et al. 2010: 37.

Holotype: USNM 23042 , moderatly preserved almost complete right valve.

Type locality: Stephenson’s, Cold Fork of Cottonwood Creek, Tehama County, California, USA. <1 km northeast of Stevenson Peak, and ~ 1.5 km due west of Woodyard Flat ( Jones and Bailey 1973). Detailed map can be found in Campbell et al. (2002).

Type horizon: Lower Cretaceous Knoxville Beds, Albian according to Kiel et al. (2008a).

Dimensions. —The holotype is 40.4 mm long, 21.2 mm high, and 4.7 mm wide.

Description.—The specimen is a right valve only and is preserved in a dark grey mudstone with abundant skeletal detritus, including crinoid brachials, echinoid spines and some other unidentified shell fragments. The right valve is almost complete apart from the most anterior part that is partially broken. It is slender cuneiform in shape with fine, regularly increasing growth lines. The hinge is almost straight and extends over 28 mm. The umbonal part is ornamented by faint radial and commarginal ribs. The commarginal ribs are slightly angulated at the crossing with the radial ribs ( Fig. 12D ), a pattern typical for Myoconcha . Unfortunately the specimen surface is covered by some kind of glue that prevents further observations. The internal surface is covered by sediment apart from the most anterior part where a small and elongated lateral tooth is visible on the anterodorsal margin ( Fig. 12C ).

Remarks.—Observations of the holotype of Myoconcha americana confirms that it belongs to the genus Myoconcha rather than to Caspiconcha . Most diagnostic features are the umbonal ornament and presence of a probable tooth. We are not sure, however, whether the presence/absence of the tooth is a stable character during ontogeny, or a feature of juvenile individuals. Stewart (1930) and Kiel et al. (2010) interpreted Myoconcha americana as a juvenile of C. major and synonymized this species with the latter. Here we recommend a return to the original concept of Stanton (1895), retaining the species in Myoconcha .

Figures

Fig. 12. Modiomorphid bivalve Myoconcha americana Stanton, 1895 (USNM 23042), right valve. A. External view. B. Dorsal view. C. Detail of the hinge area showing a possible tooth. D. Detail of shell surface ornamentation. Growth lines show rectoangular shape of shell. Black arrowheads point to faint radiaxial ribs mostly obscured by the glue in this image. White arrowheads point to the anterior.