A third known specimen of the ammonite Rhaeboceras burkholderi Cobban, 1987, has been discovered in the upper part of the Pierre Shale of early Maastrichtian age (Late Cretaceous) Baculites eliasi zone of Dawson County, Montana. This finding extends both the geographical and stratigraphic ranges of the species. In addition, R. cedarense new species is described from the Baculites baculus/grandis zone of the same region, which extends the stratigraphic range of the genus as currently recognized. The new species is a small form of Rhaeboceras that appears to be closely related to and probably descended from R. burkholderi.

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