Cimolomys clarki, 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 : 371-373

Cimolomys clarki , new species

Figure11A-J

ETYMOLOGY: For Captain William Clark, pioneer explorer of the Missouri River.

HOLOTYPE: AMNH 77179, right P4.

TYPE LOCALITY: Clayball Hill, Judith River Formation, Montana.

KNOWN DISTRIBUTION: Chouteau County, Montana.

DIAGNOSIS: Cimolomys clarki is smaller than, but similar to, its Maestrichtian descendant, C. gracilis .

DESCRIPTION: P4; no complete P4 of C. clarki has yet been found. A reconstruction of the premolar was made from AMNH 77179 and 77180 (fig. I1 A-D). P4 probably had eight or nine serrations arranged in a nearly symmetric arc. Ridges arising from the anterior two or three serrations are extremely short; those from the middle are well developed and extend anteroventrally to the level of the posterolabial ledge. The posterior two or three serrations are distinct, and are grooved labially. The posterolabial ledge is well developed and indicates the presence of three worn cuspules.

The anterolabial lobe of the crown at the anterior root is poorly formed, as in C. gracilis ; the anterobasal cavity is small and enamel covered except for the posterior wall. The anterior root is much larger than the posterior, and a low interradicular crest is present.

Ml; this tooth is apparently identical in size and cusp formula with that of Mesodma primaevus , suggesting a P4/M1 length ratio of about 1.0±0.1, which is similar to that in the other species of this family ( Clemens, 1963b, p. 80).

M2; morphologically, these teeth bear close resemblance to M2s of C. gracilis except that they are slightly smaller. Cusp formula of AMNH 77212 is 4/2 (fig. 1 1E). External cusps are four in number, subcrescentic with deep intervening valleys. The posteroexternal cusp occupies the posterolabial portion of the crown and is linked to the internal row by a low ridge, in some instances bearing small cuspules. The internal row consists of two large cusps which are much higher than the external ones and separated from them by broad valleys. An anterior ridge usually joins the anterior cusps of the external and internal rows.

Upper incisor; a right upper incisor, AMNH 77266, is referred to Cimolomys clarki on the basis of size and the presence of two distinct cusps (fig. 1 IJ).

P4; two specimens of P4, AMNH 77217 and 77218, both from Clambank Hollow, are referred to C. clarki . Cusp formula of AMNH 77217 is 2/5 or 6/2 (fig. 1 IH, I). Medial cusps gradually increase in height posteriorly and have prominent vertical striations on both their lingual and labial sides. The crown is low, and at its base enamel thickens and bulges on both external and internal sides. As is the case with C. gracilis , to which the teeth are similar, a distinct ridge connects the highest medial cusp to the basal cusps situated on the posterolabial corner of the crown. Lingual to the basal cusp, there is a depressed triangular area with two to three deep pits and grooves. Two external cusps in AMNH 77217 are situated on the anterolabial expansion of the crown; the posterior cusp is usually niuch larger than the anterior.

The coronal shape of P4 in occlusal view is roughly rectangular, with a constriction on both the lingual and the labial sides of the crown. Roots of AMNH 77217 are broken but their sockets indicate a subrounded cross section, and a small interradicular crest is present.

Ml; there are at least a dozen Mls in the collection from Clambank Hollow. They are similar to M1s of C. gracilis except that their dimensions are much smaller and they possess fewer medial cusps. Cusp formula of AMNH 77219 is 5/6/6 (fig. I1 G). External cusps are conical or pyramidal in shape. The anterior cusps are joined by a ridge to the anteromedial cusp. Other external cusps are well separated from each other by distinct valleys, especially the posterolabial cusp, in which the valley is wide and projects labially. Cusps ofthe medial row are crescentic with their apexes directed forward. Those on the internal row diminish in size anteriorly and are six in number. Anterolingual cusps are conical in shape, with circular bases, and extend to the secondmost anterior cusp of the medial row.

The cross section of the anterior root is elongated, whereas that of the posterior is transversely rectangular. Two small accessory roots are present near the posterior root and on either side of the interradicular crest.

M2; cusp formula of AMNH 77241 is 2/3/4 (fig. 11F). The internal row consists of four pyramidal cusps, which are well separated from each other by deep valleys. The anterointernal cusp is linked by a low ridge to the anteromedial cusp and also to the anteroexternal cusp, forming the anterior ridge which closely fits against the posterior side of M1. Medial cusps are well separated, and the most posterior one is crescentic and joined to the external cusps by a cuspidate ridge. Roots are equal in size, and an interradicular crest is feebly developed.

DISCUSSION: Although five species have been assigned to the genus Cimolomys , only three, C. clarki , from the Campanian and C. gracilis and C. trochuus , from the Maestrichtian, are validly included. Ptilodus primaevus Lambe (1902) from the Oldman Formation was referred to Cimolomys by Simpson (1929), and then later transferred to Mesodma by Clemens (1963b). Cimolomys major Russell (1937) is herein transferred to Meniscoessus . All teeth of Cimolomys clarki , except P4, are approximately 70 per cent of the size of their counterparts in C. gracilis . P4, on the other hand, is larger; the average length of the two specimens of C. clarki is 4.0 mm. in comparison with the average length of the specimens of C. gracilis ( Clemens, 1963b, p. 81) of 3.0 mm. This observation can be explained as the result of the progressive tendency toward reduction of size of the shearing premolars relative to the other teeth in species of the family. Morphologically, however, the teeth in the two species are similar and it seems reasonable to derive Cimolomys gracilis from C. clarki .

Absence of associated teeth in mandibles and maxillae from the Lance or Judith River formations has resulted in casual reference of teeth to specific taxa. All jaws previously described as belonging to Cimolomys have been removed from the genus; thus, in the Judith River and Oldman formations, mandibles referred to Cimolomys primaevus ( Lambe, 1902) and Cimolomys major Russell (1937) are placed in different genera. Also, with the removal of Mesodma and Cimolodon by Clemens (1963b) from the group of multituberculates that had been included by Simpson (1929) in Cimolomys , sensu lato, there are now no jaws that can be referred to Cimolomys , sensu stricto. The method used by Clemens (1 963b, p. 80) for distinguishing Cimolomys , sensu stricto was to eliminate teeth referable to Mesodma, Cimolodon , and Meniscoessus and to assign the few remaining teeth to Cimolomys . He corroborated his findings by computing ratios for various upper and lower teeth. Material from the Judith River Formation tends to confirm Clemens's identification.

Cimolomys major Russell (1937 , 1952) from the Oldman Formation is in reality closer to Meniscoessus than to Cimolomys and is discussed later. Only the reasons for excluding it from Cimolomys will be mentioned here. The type specimen is NMC 8743, a broken mandible with M1-M2, and the root of the incisor. Later a P4 was obtained at another locality. The root of this P4 is the same size as that expected for the alveolus of the type specimen. The teeth are large and the combined length of P4-M2 is 17.2 mm., nearly twice that of C. clarki . They are almost identical in size and morphology to teeth of the smallest species of Meniscoessus , found in the Judith River Formation. All species of Meniscoessus have characteristic grooves on the cusps, especially on the molars. In some cases similar grooves are present on the incisor and P4. Such grooves are present on M1 and M2 of Russell's type specimen NMC 8743. The structure of P 4 of Cimolomys and Meniscoessus is also different; in the Campanian Meniscoessus and NMC 8820 (P4 referred by Russell, 1952), 1 1 serrations are present instead of the usual eight or nine found in P4 of Cimilomys clarki and C. gracilis .