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We describe upper and lower jaws of PlacenticerasMeek, 1876, from the Upper Cretaceous (upper Campanian) Bearpaw Shale and Pierre Shale of the Western Interior of North America and lower jaws of the related ammonite MetaplacenticerasSpath, 1926, from the Campanian Yasukawa Formation of Hokkaido, Japan. One lower jaw is preserved inside the body chamber of Placenticeras costatumHyatt, 1903. The other jaws are isolated but are generally associated with fragments of placenticeratid shells. The jaws from North America are attributed to Placenticeras meekiBöhm, 1898, and P. costatum, while those from Japan are attributed to Metaplacenticeras subtilistriatum (Jimbo, 1894). All of the jaws are presumed to be from adults.
The jaws of Placenticeras attain lengths of up to 95 mm. They are preserved as steinkerns with a thin film of black material, representing diagenetically altered chitin. X-ray diffraction analysis of samples of this material indicates that it consists of magnesium-rich calcite, pyrite, and amorphous material (organic compounds). The upper jaw is approximately the same length as the lower jaw and is U-shaped, with narrow wings that converge anteriorly to a dome-shaped hood. The lower jaw is composed of two lamellae. The outer lamella is broad and consists of two wings terminating in a bilobate posterior margin. The inner lamella is one-half the length of the outer lamella. The two lamellae are separated except in the apical region and along the sides. The junction between the lamellae appears as a U or V-shaped outline on the anterior portion on the ventral surface of the jaw. This junction is especially conspicuous in specimens in which part of the inner lamella has eroded away.
In crushed specimens, the lower jaw is subquadrate in shape. In specimens that retain some or all of their original curvature, the central portion is gently convex and the sides bend steeply dorsally. The rostrum projects slightly anteriorly and dorsally and there is a thickened rim of chitin along the anterior margin where the two lamellae are doubled over. A small indentation appears at the apical end and, in most specimens, develops into a midline slit that extends posteriorly 10–15 mm. However, as shown in well-preserved specimens and based on comparisons with the jaws of closely related ammonites, this slit represents the remnants of a narrow ridge on the ventral side of the inner lamella. This ridge is surrounded by an elongate boss of thickened chitin, which corresponds to a depression on the dorsal side. The ventral surface of the outer lamella bears a midline ridge with a central groove, which essentially forms a continuation of the ridge on the inner lamella. The ventral surface of the outer lamella is ornamented with thin, radial striations and irregular broad undulations paralleling the posterior margin. The posterior end is generally incomplete, probably as a result of predation or postmortem degradation, and the lateral margins are commonly creased, indicating postmortem plastic deformation.
The lower jaws of Metaplacenticeras subtilistriatum are much smaller than those of Placenticeras but are otherwise similar in morphology. However, they retain pieces of a very thin, fibrous outer layer comprising two plates. X-ray diffr