Cimexomys magnus, Sahni, 1972

Sahni, Ashok, 1972, The vertebrate Fauna of the Judith River formation, Montana, Bulletin of the American Museum of Natural History 147 (6), pp. 319-416 : 366-367

Cimexomys magnus , new species

Figures 10S-U; 11L ETYMOLOGY: Latin magnus , large.

HOLOTYPE: AMNH 77120, a right M1.

TYPE LOCALITY: Clambank Hollow, Chouteau County, Montana.

KNOWN DISTRIBUTION: Judith River Formation.

DIAGNOSIS: Teeth larger than Cimexomys minor or C. priscus .

DESCRIPTION: P4; two large P4s in the collection are tentatively referred to Cimexomys magnus . AMNH 77112 is illustrated in figure IlL. A study of the posterior part of the P4s reveals that the ridges arising from the posterior serrations are bifurcate, a feature also characteristic of? Mesodma sp. Clemens (1963b, p. 43) referred by Sloan (McKenna, Clemens, and Sloan, MS.) to Kimbetohia campi Simpson (1936) . The two branches of the ridges are not equal in length. On the middle serrations the anteroventral branch is much longer than the posteroventral, whereas on the anterior serrations the anteroventral branch is much shorter. The present species could be ancestral to Kimbetohia campi from the Lancian and Puercan, and, through Kimbetohia , to the various species of Ptilodus .

P4; the upper blade (AMNH 77114, fig. 10S, T) is morphologically similar to, but larger than, P4 of Cimexomys judithae . The premolar appears to be a relatively primitive tooth, similar to P4s of mid-Cretaceous Djadochta multituberculates except in the possession of a few more cusps (personal commun. to Sloan from

Kielan-Jaworowska) and, in structure, intermediate between them and the Maestrichtian upper blades of Cimolodon , Cimolomys , and Mesodma . Its cusp formula is 3 or 4/6 or 7/1 and its length is 3.2 mm. Ml; cusp formula of AMNH 77120 (fig. lOU) is 5/5/1. It is more than one-third longer than M1 of Cimexomysjudithae, and it is also larger than Cimexomys priscus Lillegraven (1969) and an undescribed species of Cimexomys from the Puercan of Montana (Sloan and Van Valen, 1965). The average length of the Puercan species is 3.2 mm. in comparison to 3.91 of AMNH 77120. DIscussION: Prior to the current study, two species of Cimexomys had been described. One, C. minor , ranged in age from the Maestrichtian to the Puercan, and the other, C. priscus , restricted to the Maestrichtian of Alberta. An unnamed, larger species also occurred in the Puercan (Sloan and Van Valen, 1965). The discovery of two more species from the Judith River Formation in Montana has extended the range of the genus down to the Campanian. The lower dentition has been reconstructed with greater certainty by reference to the type of C. minor (SPSM 62. 2115, a left mandible with P3 and P4 and alveoli for the lower incisor, M1 and M2), and other comparative material from the Bug Creek Anthills Quarry (Maestrichtian) of eastern Montana. The upper dentition, however, is more difficult to restore as no upper and lower jaws have been found in association. Since Cimexomys judithae is the smallest multituberculate in the collection, size has been used as a main criterion for identification. Size is important from another aspect; it is a probable cause for the under-representation of the species in the collection, as small teeth are suspected of being selectively eliminated in the screening process. The similarity of M1 of Cimexomys to that of the plagiaulacids has been discussed by Clemens (1963b, p. 48), who has pointed out that "the small size of the internal row of these molars (Ml,s) could be interpreted as a primitive character indicating little modification from the double-rowed Ml's of the plagiaulacids."