Placenticeras costatum, , Hyatt, 1903

LANDMAN, NEIL H., TSUJITA, CAMERON J., COBBAN, WILLIAM A., LARSON, NEAL L., TANABE, KAZUSHIGE & FLEMMING, ROBERTA L., 2006, Jaws of Late Cretaceous Placenticeratid Ammonites: How Preservation Affects the Interpretation of Morphology, American Museum Novitates 3500, pp. 1-48 : 29

Placenticeras costatum

Pierre Shale, Colorado

A lower jaw occurs inside the body chamber of an incomplete adult (USNM 529075), missing part of the outer whorl ( figs. 24 , 25 ). Based on the rate of whorl expansion, the shell would have attained a diameter of approximately 195 mm. The jaw lies in the back of the body chamber on the right side. It is convex and approximately 25 mm long; the left wing is approximately 15 mm wide (W/L of the left wing 5 0.6). The ratio of shell diameter (restored) to jaw length is 7.75. The jaw is folded along the midline, and more of the left wing is exposed than the right wing. Part of the jaw is covered with a black film but the jaw appears to extend beyond the edge of the black color.

The apex is slightly projected, and the rest of the anterior margin is fairly straight. The apical angle is approximately 1408. A slit appears on the midline and is filled with matrix. It is flanked by ridges, which broaden posteriorly, and which are themselves bordered by radial depressions. Starting approximately 10 mm from the tip, the two wings of the jaw are faulted with the right wing thrown upward. Part of the outer surface of the jaw bears an orange crystalline layer.

Figures

Fig. 24. Placenticeras costatum Hyatt, 1903, with lower jaw (right arrow) in body chamber, USNM 529075, U.S. Geological Survey loc. D1353, Baculites cuneatus Zone, Pierre Shale, Grand County, Colorado. The left arrow indicates the base of the body chamber, 31.

Fig. 25. Close­up of the lower jaw of Placenticeras costatum, Hyatt, 1903, USNM 529075 (see fig.