Meniscoessus major ( Russell, 1937 ), 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 : 373-376

Meniscoessus major ( Russell, 1937) ,

new combination

Figure 12

Cimolomys major RUSSELL, 1937 , p. 75.

Cimolomys major : RUSSELL, 1952, p. 111.

HOLOTYPE: NMC 8743, a lower jaw with an M1 and M2, and incisor root, from the Oldman Formation of Alberta.

TYPE LOCALITY: Legal subdivision 10, sect. 29, T. 21, R. 12, W. 4th meridian; Red Deer Valley, near Steveville, Alberta.

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

REVISED DIAGNOSIS: Meniscoessus major is the smallest species of Meniscoessus yet described. P4 with 11 serrations, lateral profile of the blade is a symmetrical arc, anterobasal cavity enamel covered except for a part of the posterior wall, posterior two serrations with prominent grooves leading into a well-developed posterolabial ledge. M1 cusp formula is 8/6 with more cusps than in any contemporary allotherian. M1 with a high width/length ratio, M2 with an accessory root near the posterior root.

DESCRIPTION: Incisor; one partial incisor, AMNH 77254 (fig. 12F), can tentatively be referred to Meniscoessus on the basis of its large dimensions and the presence of deep longi- tudinal labial grooves that appear to be characteristic of the genus. The cross section at the junction of enamel and the root is ovoid.

P4; no complete P4s have been recovered from the Judith River Formation. Fragments of the lower premolar present in the collection include the anterior half of the tooth, AMNH 77247 (fig. 1 2A, B). A similar number of serrations are present on P4, NMC 8820, referred to Cimolomys major by Russell (1952) from the Oldman Formation. P4 is from the left side, as stated by Russell (1937), and not from the right side as stated by Russell (1952). The lateral profile of the serrate crest is a high symmetrical arc, with its apogee at the fourth or fifth serration. The anterobasal cavity is enamel covered except for the ventral portion of the posterior wall. The last two or three serrations are characteristic and resemble the condition in Cimolomys to a certain degree. These posterior serrations are well separated from those anterior and their apexes may be deflected posteriorly. Grooves are present on the serrations which terminate ventrally at the prominent posterolabial ledge.

The roots of all three specimens from the Judith River Formation referred to Meniscoessus have been broken, although a striated interradicular crest is present in most cases.

Ml; the only M1 that can be assigned confidently to the genus is AMNH 77253 (fig. 12D, E). It is very similar to specimen NMC 8743 in most respects. Cusp formula in both the holotype and referred specimens is 8/6, and is thus much higher than that of M. robustus . This formula is within the range of variation of C.

gracilis Clemens (1963b) , but M. major differs from this species in other features.

The conical anteroexternal cusp is the smallest cusp on M1 and is succeeded by seven other cusps that are pyramidal in shape and ornamented by grooves on both the labial and lingual side. The posteroexternal cusp is not fully separated from the adjoining cusp. There are six conical internal cusps that are much taller than the external cusps and are grooved only on the labial side. The anterior root is only slightly larger than the posterior, and an interradicular crest links them together. A subsidiary root is present labial to the crest.

M2; not present in the collection.

Upper incisor; AMNH 77265 is a left incisor (fig. 1 2M). The upper incisors of cimolomyids are typically bifid in contrast to ectypodontid incisors.

P4; there are two P4s that can confidently be assigned to the genus. AMNH 77257 (fig. 12I, J) is complete, except for breakage at the anterolabial border and the absence of roots. Its cusp formula is 2/4, 5/2, or 3. The posterior cusp is the larger of the two external cusps. External cusps are closely appressed to those of the medial row, and a deep valley separates the two rows. The anterolabial side of the crown is damaged but was probably expanded. There are four or five cusps on the medial row, and they only slightly increase in height posteriorly. Vertical striations are present on the cusps of the two rows. The posterior slope of the posteromedial cusp is quite steep and terminates ventrally in a basal cusp. Two other cusps are situated in this region lingual to the basal cusp. A ridge connects the anterior of these lingual cusps to the base of the posterolabial cusp.

Shape of P4, in occlusal view, is characteristically constricted in the middle. Although the roots are missing in both P4s, their cross sections, which are subrounded, indicate that the two roots were equal in size. P4 differs from P4 of Meniscoessus borealis in having one additional cusp in the medial row.

A deciduous upper posterior premolar, AMNH 77255 (fig. 12K), is tentatively referred to Meniscoessus . Its cusp formula is 2/4, and its length is 2.5 mm. The two external cusps are equally large, well separated, and closely appressed to the medial row. The medial row consists of four small cusps ornamented with vertical striations. There is a distinct constriction in the middle ofthe tooth, in occlusalview, as in P4.

Ml; although the anterior portion of AMNH 77259 is broken, it confidently can be referred to Meniscoessus . Cusp formula, however, cannot be determined because of the fragmentary nature of the tooth (fig. 1 2H). Its cusps are conical, and those of the medial row are deflected strongly anteriorly. External cusps are more conical and less deflected anteriorly than those of the medial row. Vertical grooves and striations are present, not only on the labial and lingual side of the crown but also on the sides of the valleys separating the two rows.

The crown of AMNH 77259 is relatively unworn; thus the apexes of the cusps are sharp and pointed. Roots are absent in the specimen.

M2; although worn, M2s can be assigned to Meniscoessus on the basis of size and morphological similarity to M2s of the Maestrichtian species of the genus. The cusp formula of AMNH 77261 is difficult to decipher because of the worn and eroded nature of the tooth, but the external row appears to have two or three cusps (fig. 12G). There are four medial cusps, with some vertical striations still present on their lingual and labial sides. The internal row is so worn that the number of cusps cannot be discerned. The anterior root is slightly larger than the posterior one, and both have rectangular cross sections; as in Meniscoessus robustus , an interradicular crest is absent.

DISCUSSION: Meniscoessus major , the largest multituberculate from the Judith River Formation, is nevertheless the smallest species of that genus. The reconstructed dentition is fairly complete except for anterior premolars. M2 has not been found in the present collection but this tooth is present in association with M1 in the type mandible (NMC 8743) from the Oldman Formation ( Russell, 1952). Several characteristics, including large size, enlarged molars, and vertical grooves and striations on the face of the cusps, make the genus easily distinguishable from all other contemporary allotherians.

Russell's type specimen had originally been referred to Cimolomys, sensu Clemens (1 963 b). Although the obvious similarity between the two genera cannot be denied, it is unlikely that NMC 8743, the holotype of Meniscoessus major , is a large species of Cimolomys . The presence of a small Meniscoessus in the Judith River Formation, with teeth comparable in size to the teeth described by Russell (1937, 1952), is indicated by the distinctive P4, M1, and M2, which are about 70 per cent of the size of comparable teeth of M. robustus . M2 of M. major is smaller by about 30 per cent than M2 of M. conquistus , the type species (AMNH 301 1) and the smallest and perhaps the earliest of the Maestrichtian species. Coincidentally, as in Cimolomys clarki , P4 of M. major is the same size as the average size of 34 P4s of M. robustus measured by Clemens (1963b, p. 93). This is another indication that the shearing premolars of this family, especially P4s, were undergoing reduction relative to the molars during the Maestrichtian.

Cimolomys and Meniscoessus are similar in gross morphology. The lateral profile of the serrate crest of P4 in both is a high symmetrical arc with relatively few serrations (eight to 1 1). Posterior serrations, usually two to three in number, are demarcated with well-defined ridges leading into a prominent posterolabial ledge that may bear up to three cuspules. Similarity of the two Mls has already been discussed. P4, on the other hand, is apparently dissimilar. P4 in Cimolomys is lower and longer relative to the molars than in Meniscoessus . M1s have similar internal row/ length and width/length ratios.

Four species of Meniscoessus have been proposed, M. major in the Campanian, and M. conquistus Cope (1882) , M. robustus ( Marsh, 1889 b) , and M. borealis Simpson (1927a) confined to the Maestrichtian. Clemens (1 963 b), in his discussion of Lance mammals, described only M. robustus in detail; he confined M. conquistus to the type specimen, and regarded M. borealis as a species of "questionable validity" within the range of variation of M. robustus . Additional work by Sloan (personal commun.) in Montana, however, and Wilson (personal commun. to Sloan) in South Dakota indicates that the three Maestrichtian species are distinct.

Figures

FIG. 12. Meniscoessus major (Russell), new combination. A. AMNH 77247, labial view of anterior fragment of left P4. B. Lingual view. C. AMNH 77248, labial view of ventral fragment of left P4. D. AMNH 77253, labial view of right M1. E. Occlusal view. F. AMNH 77254, lingual view of right lower incisor. G. AMNH 77261, occlusal view of left M2. H. AMNH 77259, occlusal view of right M'. I. AMNH 77257, lingual view of left P4. J. Occlusal view. K. AMNH 77255, labial view of left upper posterior premolar. L. AMNH 77264, medial view of left calcaneum. M. AMNH 77265, labial view of left upper incisor. All x 10.