Mesodma primaevus ( Lambe, 1902 )

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 : 367-370

Mesodma primaevus ( Lambe, 1902)

Figures10A-J;11K

Ptilodus primaevus LAMBE, 1902 , p. 79 .

Cimolomys primaevus : SIMPSON, 1929, p. 114.

Mesodma primaevus : CLEMENS, 1963b, p. 53.

HOLOTYPE: NMC 1890, left mandible with P4 and M1.

TYPE LOCALITY: Near Steveville, Red Deer River Valley, Alberta.

KNOWN DISTRIBUTION: Oldman Formation, Alberta, and Judith River Formation, Montana.

REVISED DIAGNOSIS: Mesodma primaevus is larger than M. formosa but smaller than M. ambigua ; it is about the size of M. thompsoni . P4s have 11 serrations and an average length of 3.0 mm., and closely resemble the type specimen, NMC 1890, from the Oldman Formation of Alberta. The profile of the serrate crest is generally higher than in M. thompsoni and M. ambigua ; it is more like that of M. formosa .

DESCRIPTION: Lower incisor; AMNH 77273 and similar teeth are tentatively assigned to Mesodma primaevus on the basis of size and morphological similarity to lower incisors figured and described by Clemens (1963b, p. 33).

P4; similar to the type specimen of Mesodma primaevus , a P4, in size, morphology, and in having 11 serrations, are specimens AMNH 77121, 77122, and 77123. First and second external ridges of the P4 of M. primaevus are short and run ventrally at an acute angle to the ridges of the posterior serrations. The profile of the serrate crest of both P4s, AMNH 77121 and 77122, is a high, near symmetrical arc with its apogee at the fourth or fifth serration. The posterolabial side of the tooth is worn, but external ridges from the posterior three serrations in AMNH 77121 (fig. 1 OH, I) indicate that the ridges are not directed anteriorly. Ventral to the posterior two serrations a short, worn ledge represents what, in the primitive condition, was a row of cusps.

Height of the slightly convex anterior edge of the crown, measured from the top of the anterobasal cavity to the first serration, is approximately one-third the length of the crown. The anterobasal cavity is covered on all except the posterior side by bulging enamel; at the dorsal roof of the concavity the enamel may be cleft.

The first internal ridge on P4 is usually longer than, or equal to, the second internal ridge. The longest ridge seems to originate from the fifth serration, or from the serrations immediately adjacent to it. The anterior root is larger than the posterior, and a prominent interradicular crest is present.

Ml; discrimination of isolated M1s of Mesodma primaevus and Cimolomys clarki is not possible as their size and cusp formulae are either identical, or overlap. These M1s have an average length of 3.4 mm., and an observed range of 3.15 mm. to 3.6 mm., and a cusp formula of 5 or 6/4.

Molars referred to Mesodma in this work are tentatively identified on cusp formulas and similarity to the M1 figured by Lambe (1902). Cusp formula of AMNH 77125 is 6/4 (fig. lOF). The first external cusp is the smallest and is succeeded by cusps that are crescentic with their apexes deflected posteriorly. Internal cusps are higher than external cusps and stand more up- right; the posterointernal cusp is the largest. The teeth are rectangular in outline in occlusal view. Roots of the M1s are transversely rectangular in cross section and a prominent interradicular crest is present. There are no accessory roots.

M2; allocation of M2 to this species is also doubtful. Cusp formula of AMNH 77126 (fig. lOG) is 4/2, as compared with 3/2 of the later species of Mesodma . The anteroexternal cusp of M2 is conical, the other external cusps are crescentic with posteriorly directed apexes. The two internal cusps are much larger than the external ones. The anterointernal cusp is linked to the second external cusp by a ridge cutting off the median valley. This feature seems to be fairly consistent in similar M2s in the collection. The average length of M2 is 2.4 mm.

The anterior root has a transversely rectangular cross section, and an interradicular crest is absent.

Maxilla containing P2 and P3; a fragmentary maxilla (AMNH 77149) was obtained from the productive clam/bone bed about 40 feet east of Clambank Hollow Quarry and on the opposite side of the gully. The maxilla (fig. 1 OA) is the only multituberculate jaw fragment found during the three years of collecting. Anterior premolars bear close resemblance to the maxillary fragment, UCMP 46600, from the Lance Formation (Clemens, 1 963 b). Clemens tentatively identified the premolars as belonging to Mesodma "on the basis of the small size of the upper premolars and the circumstantial evidence of the joint occurrence " (of a broken P4 of M. thompsoni ). AMNH 77149, from the Campanian, probably represents Mesodma , although the possibility of its belonging to Cimexomys judithae cannot be ruled out.

P2; the tooth is three cusped, and it is more elevated than the adjoining P3. Two of the three conical cusps are more lingual in position than the third, and the anterior of these is higher and more slender. The posterolabial cusp is larger than the other two, and the area posterior to it is expanded and slopes gently. P2 iS two rooted, with a length of 1.0 mm. and a width of 0.7 mm.

P3; the crown of P3 has four cusps and the anterior edge of the tooth appears to be displaced lingually relative to P2. Although P3 iS worn, it is possible to tell that the posterior pair of cusps is larger and better separated from each other than the anterior pair. The crown is 1.0 mm. long and 0.65 mm. wide. P3 has two roots.

P4; the upper blade, AMNH 77150, is tentatively assigned to Mesodma primaevus (fig. lOB, C). It has a cusp formula of 2/6/2. The posteroexternal cusp is much larger than the anterior, and the medial row consists of six, well-separated conical cusps. The penultimate cusp is the highest, and the included angle between the anterior and the posterior slopes of the shearing face is around 120 to 125 degrees. The same angle in the Maestrichtian species M. formosa and M. thompsoni is less, and varies between 115 and 120 degrees. The crown of AMNH 77150 is unworn and the two to three basal cusps are distinct. The length of the tooth is 4.0 mm.

M1; cusp formula of AMNH 77154 is 6/7/5 or 6 (fig. 1 E). Of the external cusps, the anterior tend to be conical and the posterior subpyramidal, rather than crescentic, and their apexes are only slightly deflected anteriorly. The anterior cusp of the medial row is separated from the anterior cusp of the external row by a valley. The lingual side of the external row cusps is grooved. The internal row extends slightly more than one-half the length of the crown, although these five to six cusps on the internal row are not well separated from each other and are more like cuspules. Mls of Mesodma primaevus are distinct from those of other species from the Judith River Formation in size, cusp formula, and the ratio of length of the internal row to crown length. They can be distinguished from M1s of later Maestrichtian species of Mesodma by two, apparently primitive, features. The first is the tendency to have fewer cusps in the external and medial rows than in later species, and the second is the presence of five to six very small cuspules in the internal row in comparison to five to six distinct and well-separated cusps in the Maestrichtian species of Mesodma . The cross section of the anterior root is elongated, whereas that of the posterior is transversely lengthened, and an interradicular crest is present. The average length of M1 is 4.0 mm.; the width is 2.0 mm.

M2; cusp formula of AMNH 77159 is 1/3/4 (fig. 1 OD). The internal row consists of four wellseparated cusps, with the anterointernal cusp linked to the anteromedial cusp by a short ridge. Three medial cusps are large and crescentic. On the anteroexternal side of the crown, the single external cusp, or cuspule, is situated on a ridge. The tooth is 2.1 mm. long and 1.6 mm. wide.

DISCUSSION: Mesodma primaevus ( Lambe, 1902) was one of the earliest Campanian mammals to be described from the "Belly River Series" (Oldman Formation) of Alberta. Simpson (1929), in his review of the Mesozoic Mammalia, included Lambe's holotype in Cimolomys (sensu lato) as had been suggested previously by Matthew (1916). Both of these authors, and later Russell (1952) noted the similarity of the type specimen, NMC 1890, to some of the later Lance allotherians; a situation resulting partly, but not entirely, from the initial inclusion of most Lance multituberculates in the genus Cimolomys used in a "wastebasket sense." Additional evidence from the Judith River Formation tends to confirm the tentative assignment by Clemens (1 963b) of Lambe's type to Mesodma . There is a close resemblance between the Judith River specimens and the type specimen of Mesodma primaevus , NMC 1890, which is a broken right mandible containing P4 and Ml (not from the left side as suggested by Russell, 1952).

Stanton and Hatcher (1905) reported a specimen of this species from the Judit'h River Formation. The new material greatly adds to our knowledge of this species. Mesodma primaevus is a satisfactory ancestor for Mesodma formosa and Mesodma thompsoni of the Maestrichtian Lance and Hell Creek formations, and through them to the later ectypodontids.