Multituberculata

The Campanian multituberculates from the Judith River Formation afford clear evidence for ancestry of later Maestrichtian forms. Evidently well advanced from the plagiaulacids reported from the Trinity Sandstone (Albian) of Texas, they are much closer to the multituberculates of the Lance Formation. Most of the genera recognized by Clemens (1936 b) in the Lance Formation of Wyoming are present in the Judith River Formation. Additional genera have been described by Sloan and Van Valen (1965) from a series of Upper Cretaceous localities in eastern Montana; of these, only Cimexomys has so far been identified in the present collection. Allotherian teeth greatly outnumber those of the Metatheria and Eutheria. The occurrence of isolated teeth makes generic and specific identification in some instances quite tentative. Most assignments are based on the criteria used by Clemens (1963 b) in associating various multituberculate dentitions, and consist of comparative morphology, frequency of occurrence, and size. Unfortunately, size is not so useful a criterion in the Judith River multituberculates as it is in those from the Lance Formation. Cimexomys judithae, the smallest multituberculate, is the easiest to differentiate. Except for the largest species, Meniscoessus major, the others tend to approach each other in size. The Campanian Mesodma primaevus is significantly larger than M. formosa and M. thompsoni, but Cimolomys clarki and Meniscoessus major are smaller than their later descendants, Cimolomys gracilis and Meniscoessus robustus, and much closer to each other in size. The only specimen that can be referred to Cimolodon is a single left P 4 fragment. As is usually the case with isolated multituberculate teeth, the fourth lower premolar is most useful for generic and specific identification. Sloan (personal commun.) has pointed out the merits of using the labial rather than the lingual side of the lower blade as suggested by Jepsen (1940) for diagnostic purposes. In addition to the relative height of the anterior serration above the top of the depression for P 3, and the number and pattern of the ridges discernible on either side of the blade, the labial side shows more sculpture, particularly on the posterolabial corner of the crown. The shape and wear pattern of the posterolabial cuspule and the fine surface sculpture dorsal to that cuspule in this region are of diagnostic value. The shape of the labial enamel on the anterior root is also taxonomically useful. P 4 s have not been used as much for taxonomic differentiation. Complete P 4 s are more common than P 4 s in the collection and present morphological details that can be used for generic and specific identification. The following features are useful for diagnostic purposes: the number and shape of the cusps on the medial row; the number of cusps on the labial row; the height to length ratio (height taken as the vertical distance to enamel base from the penultimate cusp of the medial row); the inclination of the ridge connecting the posterior basal cusp with the penultimate cusp of the medial row; and the wear pattern and the number of cusps in the triangular area on the posterior medial slope of the crown.