A survey of the Cretaceous ammonite Placenticeras Meek, 1876, in the United States Western Interior, with notes on the earliest species from Texas

William A. Cobban (Edited and with additions by W.J. Kennedy)


This contribution documents the record of the late Cretaceous ammonite Placenticeras Meek, 1876, from the late Cenomanian of Texas and the southern part of the U. S. Western Interior up to the late Middle Campanian zone of Baculites scotti, reconstructed and updated from an incomplete manuscript by the late W. A. Cobban based on the collections of the U. S. Geological Survey. The original manuscript dates from the late 1980’s, and there is now additional information on the occurrence of the genus that is incorporate here; much of this comes from Neal Larson of Hill City, South Dakota, to whom I am indebted for his help in preparing Bill’s manuscript for publication. It now provides an objective documentation of the distribution of Placenticeras in space and time on which any subsequent analysis of the evolution of the genus will depend.


Cretaceous; Ammonites; Placenticeras; U.S. Western Interior

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