Jaws of Late Cretaceous Placenticeratid Ammonites: How Preservation Affects the Interpretation of Morphology Author LANDMAN, NEIL H. Author TSUJITA, CAMERON J. Author COBBAN, WILLIAM A. Author LARSON, NEAL L. Author TANABE, KAZUSHIGE Author FLEMMING, ROBERTA L. text American Museum Novitates 2006 2006-01-12 3500 1 48 http://www.bioone.org/doi/abs/10.1206/0003-0082%282006%29500%5B0001%3AJOLCPA%5D2.0.CO%3B2 journal article 10.1206/0003-0082(2006)500[0001:JOLCPA]2.0.CO;2 0003-0082 5383501 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.