Cimexomys judithae, 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 : 365-366

Cimexomys judithae , new species

Figure 10L-R ETYMOLOGY: Named for Julia "Judith" Hancock Clark, for whom the Judith River was named by Lewis and Clark.

HOLOTYPE: AMNH 77100, a right P4.

TYPE LOCALITY: Clambank Hollow, Chouteau County, Montana.

KNOWN DISTRIBUTION: Judith River Formation.

DIAGNOSIS: Teeth smaller than those of Cimexomys priscus and larger than those of Cimexomys minor . The lower molars are small, the average length of M1 is 1.8 mm.

DESCRIPTION: P4; there is only one complete P4, the holotype (AMNH 77100), that can be referred to the genus Cimexomys (fig. 1OL, M). It has nine serrations and a length of 3.0 mm. The first external ridge is weakly developed and converges on the second external ridge at an acute angle; the other ridges are wide-spaced and curve anteroventrally except at the posterior two serrations where they are feebly developed. The apogee of the lateral profile is at the third or fourth serration, dorsal to the ventrolabial expansion of the enamel on the anterior root. The anterobasal cavity for P3 is covered with enamel except for the posterior wall. The posterolabial cusp is represented in Cimexomys by a faint ledge. The anterior root is much larger than the posterior and an interradicular crest is present. Wear appears to be the greatest on the posterolabial side of the premolar. Ml; M1s have been referred to Cimexomys judithae on the basis of size rather than on any noticeable morphological difference from other Mls in the collection. Cusp formula of AMNH 77103 is 6/4 (fig. lON). The conical first external cusp is linked to the first internal cusp by a low ridge, notched by the median valley. External cusps are smaller and more crescentic than the lingual ones. Roots have transversely rectangular cross sections with a feebly developed interradicular crest. Length of the crown is 1.8 mm. M2; cusp formula of AMNH 77105 is 4/2 (fig. 100). This small tooth (length 1.4 mm.) has four external cusps of which the most posterior may be considered divided, as the valley separating the last and next to last cusp is very narrow and shallow. External cusps are approximately equal in height to the two larger internal cusps. All the cusps are crescentic with their apexes deflected posteriorly. The cross section of the anterior root is transversely rectangular, whereas that of the posterior is triangular; no interradicular crest is present. P4; there are two P4s in the collection that are quite small in comparison to the others, and are referred to Cimexomys judithae . One of these is thoroughly worn, but the other, AMNH 77109, is complete and is illustrated in figure lOP, Q. Although the uncertainty of referring this tooth to the present species is even more apparent because of the slightly larger than expected length of the tooth (2.4 mm.), its distinct morphology separates it from the premolars of Mesodma with which it can be most easily confused. Sloan and Van Valen (1965), in their description of Cimexomys minor from the Hell Creek Formation of Montana, did not distinguish P4. The cusp formula of AMNH 77109 is 3/5/2. The external row consists of three wellseparated cusps that are parallel or converge only slightly with the medial row. Cusps are as well spaced on the external row as they are on the medial row; on the latter, they are nearly equal in size but the posterior cusps tend to be higher because the height of the crown increases posteriorly. The crown in the case of Cimexomys , however, is much lower than in Mesodma species, not only of the Campanian, but also of the Maestrichtian. Height to length ratio for P4 of Cimexomysjudithae is 0.4, whereas that for Mesodma formosa is 0.6. Its roots are rectangular in cross section and a prominent interradicular crest is present.

Ml; there are two Cimexomys upper first molars that are morphologically similar but differ from each other in size. Both have the identical cusp formula of 5/5/1. The distinctive feature of these molars is that the internal row is extremely short and in all cases is less than one-half the length of the molar. Cimexomys has the shortest internal row of all multituberculates except the M1s of the plagiaulacids, which have only two rows. Of the two molars, the smaller (AMNH 77110) is of the size expected for C. judithae . The larger M1 is discussed later.

Specimen AMNH 77110 is a worn tooth with five conical external cusps (fig. 1 OR). The anteroexternal cusp is connected to the anterior cusp of the medial row by a short, low ridge. Conical to pyramidal medial row cusps are parallel to the cusps of the external row except for the posteromedial cusp, which is more labial in position. The tendency for the posterolabial cusp to be situated more labially is seen also in the illustration of Ml by Clemens (1963b, p. 46) of? Mesodma sp., later referred by Sloan and Van Valen (1965) to C. minor . The internal row is more like an elevated ridge than a separate cusp and its anterior termination is usually at the next to last cusp on the medial row. The length ofAMNH 77110 is 2.4 mm.

M2; an M2, AMNH 771 1 1, resembles M2 of C. minor and is of a size that one would expect for C. judithae . Its cusp formula is 1/3/3. The most anterior cusp of the internal row is conical in shape and it is preceded by two cusps, the apexes of which are deflected anteriorly. The anterior cusp of the medial row is more a part of the anterior ridge than a separate cusp. A worn external ridge bearing a cuspule joins the posteromedial cusp. Its length is 1.5 mm. The cross section of the anterior root is transversely rectangular, whereas that of the posterior is rounded; no interradicular crest is present.