Cimolodon, Marsh, 1889

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 : 376-378

Cimolodon sp.

Figure11M,N

There is only one tooth, a fragment of P4, that can be referred with confidence to Cimolodon .

P4; the anterior half of a P4 from Clambank Hollow, AMNH 77267 (fig. 1 1M, N), has been referred to Cimolodon . It is morphologically similar to P4 of C. nitidus Marsh (1 889b), but slightly smaller. The tooth, in labial view, shows a high arched profile of the serrate crest. The first external ridge is very short and meets the longer second external ridge at an acute angle. Twelve anteroventrally curving ridges are visible on P4, but evidently more than 12 serrations were present. The anterobasal cavity is deep and covered by enamel on all sides except the posterior wall.

MULTITUBERCULATE POSTCRANIAL ELEMENTS

Recovery of limb and girdle elements associated with dentitions of small Cretaceous mammals is impossible by screening techniques. It is therefore practically impossible to make any generic identification of isolated postcranial material. Gidley (1909), Broom (1914), Simpson (1928), Simpson and Elftman (1928), Granger and Simpson (1929), McKenna (1961), Clemens (1963b), and Deischl (MS) have been the principal contributors to our knowledge of the postcranial osteology of allotherians.

Deischl (MS) tried to differentiate various postcranial elements present in the large Hell Creek collections from the Bug Creek Anthills locality, McCone County , Montana, on the basis of size, frequency of occurrence, and morphological differences. Although the three criteria listed above are not conclusive in themselves, they are the principal means of identifying and associating allotherian teeth, limb, and girdle elements.

A few multituberculate limb bone elements have been found in the Judith River Formation. These include the distal portion of a humerus, three calcanea, and three fragmentary shafts of femora. These limb elements, in general, bear close resemblance to the specimens described by Deischl.

HUMERUS

The distal portion of a humerus, AMNH 77175, is referable to Mesodma (fig. 1OK). Although much smaller than a fragmentary humerus of M. thompsoni , UMVP 1406, described by Deischl (MS, fig. 9), AMNH 77175 is morphologically similar to it in the shape of the ulnar condyle and the position of the ulnar pit. These two characteristics were used by Deischl in the differentiation of the humeri of Cimexomys minor , Mesodma formosa , M. thompsoni , and Stygimys kuszmauli .

Width or length of AMNH 77175 cannot be estimated as the extremities are broken, but measurements taken from the entepicondylar foramen to the ectepicondyle indicate that the limb bone may belong to a juvenile of Mesodma primaevus , or, less likely, to an adult of Cimexomys judithae . Measurements of AMNH 77175 are nearly identical with those of the distal fragment of a Cimexomys minor humerus, UMVP 1404 (Deischl, MS, fig. 7), and would tend to support the less likely alternative, that it belongs to an adult of C. judithae , but a number of morphological differences exist. The ulnar pit in AMNH 77175, and in UMVP 1406 ( Mesodma thompsoni ), is situated at the proximal termination of the ulnar condyle, whereas in specimen UMVP 1404, of Cimexomys minor , it is situated on the anterolateral surface of the ulnar condyle. Also, in UMVP 1404, the entepicondylar foramen is situated more proximal to the ulnar condyle and not lateral to it as in AMNH 77175 and UMVP 1406. The shape of the ulnar condyle is also different in the two genera.

AMNH 77175 is incomplete; the entepicondylar region and most of the hollow shaft is missing. The shape of the ulnar condyle in this specimen is similar to that in specimen UMVP 1406, of Mesodma thompsoni , and the proximal termination of the condyle does not taper as in M. formosa .

ALLOTHERIAN FEMORA

Proximal portions of three eroded and damaged allotherian femora have been found. A right multituberculate femur (AMNH 77178) recovered by surface picking at the Ankylosaur Point locality (fig. 11K) has a relatively long and narrow neck, but lacks its head. The proximal digital fossa is deep. Most of the greater trochanter is not preserved and distally the femur is broken and the gluteal crest is weak. The lesser trochanter is relatively large. Width of the femur shaft taken below the lesser trochanter is 1.76 mm., which indicates that the femur is smaller than the multituberculate femora described by Deischl (MS, figs. 26-28).

The femur AMNH 77178, however, reveals some morphological differences from UMVP 1421, a left femur fragment of Cimexomys minor . Its neck is relatively longer, and the distal digital fossa is less well developed. Some resemblances to Mesodma femora are apparent, especially in the shape and development of the lesser trochanter. Perhaps AMNH 77178 is the femur of a young individual of Mesodma primaevus .

CALCANEA

Only a few papers have been published discussing calcanea of Cretaceous and early Tertiary allotherians, notably those by Marsh (1 889b), and Granger and Simpson (1929). The work of Deischl (MS) also needs mention. The distinctive morphology of multituberculate calcanea makes them easy to distinguish from calcanea of therians. Proximally, the groove for the tendon of Achilles is feebly developed or absent from the calcanea. The posterior astragalar facet is large and highly convex; a deep groove separates it from the sustentaculum which is situated farther distally and medially. The trochlear process also is characteristically hook-shaped and situated medioventrally. The cuboid facet is usually oval to subrounded but invariably slightly concave and distomedially positioned.