Pronodens transmontanus Douglass, 1903

Calede, Jonathan J., Constenius, Kurt N., Famoso, Nicholas A. & Kehl, Winifred A., 2024, Discovery of Oligocene-aged mammals in Glacier National Park (Kishenehn Formation), Montana, Geodiversitas 46 (9), pp. 367-386 : 371-376

Pronodens transmontanus Douglass, 1903

( Fig. 2 ; Appendix 1 )

Leptomeryx transmontanus Douglass, 1903: 167 , fig. 11.

TYPE MATERIAL. — Holotype. United States of America • CM 726, partial skull with right and left P2-M3 lacking parts of skull anterior to P2 and posterior to the frontals.

TYPE LOCALITY. — CM 2377 (KU-MT-31, UMPC MV6203, UWBM C1709), United States of America, Montana, East of Drummond.

MATERIAL. — From GLAC-PALLOC-0001: GLAC 26988, partial left dentary bearing p2-m3 and partial right dentary bearing p3-m3.

DIAGNOSIS. — Much smaller species than Pronodens silberlingi ; dentary shallower than in P. silberlingi ; p2 less mesiodistally elongated than in P. silberlingi ; labial cusps of m3 (protoconid, hypoconid, and hypoconulid) more buccally expanded than in P. silberlingi ; protocone of P3 smaller and more buccolingually elongated than in P. silberlingi .

STRATIGRAPHIC AND GEOGRAPHIC RANGE. — Lower unit of the Cabbage Patch beds (Renova Formation, early Arikareean) and upper member of the Kishenehn Formation, western Montana, United States of America (early Arikareean).

DESCRIPTION

The Pronodens material from the Kishenehn Formation is composed of partial left and right dentaries. The left dentary preserves a little damaged or worn dentition including p2-m3, whereas the right dentary includes a sheared and worn dentition including p3-m3. The left jaw is only the second specimen including the entire cheektooth row ever discovered for the species after CM 8938, the most complete lower jaw known for the species. It is also one of the bestpreserved dentition, albeit worn, available for description for P. transmontanus ( Fig. 2 ).

The dentaries of GLAC 26988 are similar to those of Pronodens transmontanus in their robustness of the anterior portion of the jaw ( Fig. 3A ; Table 1 ). In the posterior portion of the molar row, the dentary of the Kishenehn Formation material is deeper than in the specimens from the Cabbage Patch beds, more closely resembling MOR 951. Overall, both the Kishenehn Formation specimen and Cabbage Patch material are dorsoventrally shallower than the dentary of P. silberlingi ( Fig. 3A ). Both the left and right jaws show evidence for the absence of p1 unlike in Leptomeryx , but like all known Pronodens material. The mental foramen can be observed on the left side ( Fig. 1C ); its position far anterior to the anterior edge of p2 is similar to that observed in three known specimens of P. transmontanus, UMPC 3030, UMPC 1782, and UWBM 97460. Two other specimens, an edentulous jaw assigned to P.transmontanus (KUVP 20724) as well as CM 8938, show a more posteriorly located foramen, immediately ventral to the anterior edge of p2, as in the type of P. silberlingi , a feature previously considered a characteristic of the genus ( Taylor & Webb 1976). The postsymphysis is posterior to the mental foramen in the Kishenehn Formation material ( Fig. 1C ), unlike in CM 8938 and the holotype of P. silberlingi, YPM 13952. In both GLAC 26988 and YPM 13952, the postsymphysis terminates posteriorly halfway through p2; in CM 8938, it is fully anterior to p2, terminating posteriorly at the anterior edge of the tooth ( Fig. 1C ).

The dentition of GLAC 26988 is best preserved in the left jaw ( Appendix 1 ). The m1 and m2 of the right jaw are damaged and p2 is missing ( Fig. 1A ); only the roots of the tooth are preserved. The size of the p2 of GLAC 26988 falls within the range of measurements observed in Pronodens transmontanus from the Cabbage Patch beds (the type area) for both length and width ( Figs 3B ; 4A ; Table 2 ). The p2 of P. silberlingi is much more mesiodistally elongated. The shape of the p 2 in the Kishenehn Formation specimen is very similar to that observed in the Cabbage Patch fossils ( Fig. 1E ). The protoconid and entoconid are more robust than in UWBM 97460, but this character varies across specimens in the sample from Cabbage Patch (see UMPC 3030, CM 8938, and UWBM 97460 in particular; Fig. 4A ). The length of the p3 of P. transmontanus from Cabbage Patch overlaps the length of the p3 of the Kishenehn Formation material, but the buccolingual width of the latter is larger, more similar in fact to P. silberlingi ( Figs 3B ; 4B ; Table 2 ). In both the left and right jaws of GLAC 26988, the portion of the tooth anterior to the paraconid is heavily worn, much more so than the posterior portion of the tooth and cannot be described in detail ( Fig. 1A, C, E ). The paraconid itself is larger in GLAC 26988 than in KUVP 18452, KUVP 18668, and CM 8938. The same observation can be made of the protoconid, which is broader buccolingually in the Kishenehn Formation material than in the Cabbage Patch specimens. The ridge extending lingually from the protoconid is longer and narrower in the Cabbage Patch material than in the Kishenehn Formation specimen where it terminates more anteriorly ( Fig. 1E ). The anatomy of the posterior-most portion of the tooth, including the entoconid and hypoconid, varies greatly with wear across specimens within the Cabbage Patch sample; the Kishenehn Formation specimen is most similar to KUVP 18452 and CM 8938, although the ridge extending lingually from the entoconid is more lingually inflated in the Kishenehn Formation specimen than in the Cabbage Patch fossils. The length of p 4 in the Kishenehn Formation specimen overlaps with the range of sizes observed in P. transmontanus from the Cabbage Patch beds ( Fig. 3B ; Table 2 ). GLAC 26988 is comparable in width to the largest measured specimen of P. transmontanus from Cabbage Patch as well as the type specimen of P. silberlingi , but it is proportionately broader ( Fig. 4C ). The anterior-most portion of the p4 of the Kishenehn Formation specimen is similar to that of UWBM 97460, the only well preserved p4 of P. transmontanus from Cabbage Patch, when accounting for wear. The more worn KUVP 18668 shows a similar paraconid morphology as the Kishenehn Formation specimen as well. The shape of the metaconid is complex, but once again, when accounting for wear, is essentially identical to that of UWBM 97460 and CM 8938 ( Fig. 1E ). The advanced wear stage of the Kishenehn Formation specimen leads to an uninterrupted connection between the metaconid and entoconid ( Fig. 1E ). The shape of the entoconid is simple, once more as a consequence of wear. It is larger, rounder, and more lingually inflated than in UWBM 97460 or CM 8938. There is, however, little difference in the morphology of the hypoconid between GLAC 26988 and UWBM 97460 when accounting for wear.

The m1 of GLAC 26988 is more heavily worn than in UWBM 97460, UMPC 1482, UMPC 3030, and CM 8938 ( Fig. 1E ). The Kishenehn Formation specimen overlaps in size with the Cabbage Patch fossils ( Fig. 3C ; Table 3 ) although it is broader for a given length than the specimens from the type area ( Fig. 4D ). All these specimens are much smaller than the type of Pronodens silberlingi . GLAC 26988 is heavily worn making the morphologies of the metaconid and entoconid difficult to compare between the Kishenehn Formation specimen and the Cabbage Patch fossils, but the overall shapes of these lingual cusps are consistent across all specimens. The protoconid and hypoconid are more buccally extended in the Glacier National Park specimen ( Fig. 1E ), leading to the greater buccolingual width of the tooth in GLAC 26988 compared to the Cabbage Patch fossils. A similar pattern can be observed in the m2 with an overlap in size of the Kishenehn Formation specimen with the Cabbage Patch fossils ( Fig. 3C ; Table 3 ). The Kishenehn Formation specimen is once again slightly wider for a given length than the Cabbage Patch sample ( Fig. 4E ), but they are very similar, and even resemble a specimen of Pronodens from the Six Mile Creek Formation, MOR 951. The type of P. silberlingi is larger than any other specimen of the genus measured. The shape of the metaconid of GLAC 26988 is similar to the one in CM 8938; the shape of the entoconid is similar to that observed in UMPC 1482. The protoconid and hypoconid are buccally expanded in the Kishenehn Formation specimen compared to the Cabbage Patch sample. There are little to no differences in m3 size between the Glacier specimen and the sample of fossils from the Cabbage Patch beds ( Fig. 3C ; Table 3 ). Unlike in m1 and m2, the anterior lophid of the m3 of GLAC 26988 is buccolingually narrower than that observed in the sample of P. transmontanus from Cabbage Patch or MOR 951 ( Fig. 4F ). The buccal cusps (metaconid, entoconid, and entoconulid) of GLAC 26988 are most similar to those observed in CM 8938 and UMPC 1482 or MOR951; they are large and round with tapered mesial and distal ends. In the Kishenehn Formation specimen, the lingual surface of the entoconid is not as rounded as in CM 8938 and KUVP 18838. Instead, it resembles more closely the morphology observed in UWBM 97494 with a lingually displaced cusp point. The entoconulid is teardrop shaped with the narrow end at the mesial end of the lophid contacting the distal end of the entoconid as in KUVP 18838 and CM 8938; the shapes of the entoconid and entoconulid in UWBM 97494 differ from those found in the Kishenehn Formation fossil as well as other P. transmontanus specimens. The entoconulid of MOR 951 is more symmetrical than in P. transmontanus from Cabbage Patch or the Kishenehn Formation specimen. The protoconid is not as buccally expanded in the Kishenehn Formation specimen as they are in the Cabbage Patch sample at similar wear stages; this is also true of the other labial cusps, the hypoconid and the hypoconulid ( Fig. 1E ). However, these cusps are more buccally expanded than in P. silberlingi or MOR 951. Both the hypoconid and the hypoconulid are more mesiodistally expanded in GLAC 26988 than in the Cabbage Patch specimens, leading to more robust bulbous cusps.

REMARKS

The Kishenehn Formation specimen is very similar in size to the sample of Pronodens transmontanus from the type area, the Cabbage Patch beds ( Tables 1-3 ). None of the measurements of the Kishenehn Formation specimen fall outside of the range of measurements observed in P. transmontanus ± 10%; only two measurements of the Kishenehn Formation specimen (Ddm2 and p3W) are more than 5% larger than the measurements observed in the Cabbage Patch sample; this despite the small size of our sample (two and five specimens respectively) that is unlikely to capture the range of size variation in P. transmontanus . In contrast, P. silberlingi is much larger, with the type specimen (YPM 13952) more than 5% larger than P. transmontanus from Cabbage Patch in 83% of measurements taken (10 out of 12). YPM 13952 is also more than 10% larger than the Kishenehn Formation specimen in nine measurements (out of 12) available for both specimens. On average, the teeth of YPM 13952 are 14.96% larger than those of the Kishenehn Formation specimen. Differences between the Kishenehn Formation specimen and MOR 951 are reduced; however, the two specimens differ by 11.9 and 20.7% for two dental measurements (m2L, m3Wpost). The morphology of the teeth in the Kishenehn Formation specimen more closely resembles P. transmontanus than P. silberlingi or Pronodens sp. However, the dentition of the type specimen of P. silberlingi is worn beyond the possibility of tooth crown morphological comparisons. We conservatively assign the material from Kishenehn Formation to P. transmontanus .

Figures

FIG. 2. — Morphology of GLAC 26988,Pronodens transmontanus (Douglass,1903),from the Kishenehn Formation: A, lateral view of right dentary;B, medial view of right dentary; C, lateral view of left dentary; D, medial view of left dentary; E, line drawing of the occlusal view showing tooth morphology of the left dentary. A photo of the occlusal view of the teeth is showed in Appendix 1. Scale bars: A-D, 2 cm; E, 4 mm.

APPENDIX 1. — Dental morphology in occlusal view of GLAC 26988, Pronodens transmontanus (Douglass, 1903), from the Kishenehn Formation.

FIG. 3. — Size analysis of GLAC 26988 and other leptomerycids from the Arikareean and Hemingfordian: A, comparison of the depth of the dentary at different tooth positions; B, comparison of the size of the premolars; C, comparison of the size of the molars. GNP denotes the specimen from Glacier National Park (Kishenehn Formation); data from the left jaw. Note that the data for Pseudoparablastomeryx Frick, 1937 are in fact a species mean. Sample size is indicated for each comparison. See Tables 1-3 for data. Abbreviations: ant, anterior lophid; post, posterior lophid; Ppf, Pseudoparablastomeryx francescita (Frick, 1937); Pps, P. Pseudoparablastomeryx scotti (Frick, 1937); Ps, Pronodens silberlingi Koerner, 1940; Psp, Pronodens sp.; Pt, P. transmontanus (Douglass, 1903).

FIG. 1. — Geographic context of the Oligocene-aged fossils from Glacier National Park: A, detailed map of the Kishenehn basin showing the North Fork and Middle Fork areas, faults in the area (continuous black lines), continental divide (red), national park boundaries (dashed blue line), and water courses (gray). Localities of GLAC 26988 (1), GLAC 26987 (2), and CM 87801 (3) are also showed (modified from Dawson & Constenius 2018). Detailed geographic information is available to qualified researchers from repositories; B, map of western Montana showing the location of five coeval Arikareean-aged deposits bearing leptomerycid and/ or equid fossils: the Kishenehn, Renova (Cabbage Patch beds), and Fort Logan formations as well as the Canyon Ferry Reservoir and the Peterson Creek Local Fauna (in Idaho). Select water bodies, cities, and the continental divide are also displayed. Grey areas represent basins with strata coeval with the Kishenehn Formation. Black box shows location of A; C, location of the state of Montana within the United States with black box showing location of B.

FIG. 4. — Shape analysis of GLAC 26988 and other leptomerycids from the Arikareean and Hemingfordian: A-C, comparison of the shape of the premolars; D-F, comparison of the shape of the molars. GNP denotes the specimen from the Glacier National Park (Kishenehn Formation); data from the left jaw. Note that the data for Pseudoparablastomeryx Frick, 1937 are in fact a species mean. Each point otherwise represents a specimen. Disparity is showed for Pronodens transmontanus (Douglass, 1903) from the Cabbage Patch beds when four or more specimens were measured. See Tables 1-3 for data. Abbreviations: ant, anterior lophid; Ppf, Pseudoparablastomeryx francescita (Frick, 1937); Pps, P. Pseudoparablastomeryx scotti (Frick,1937);Ps, Pronodens silberlingi Koerner, 1940; Psp, Pronodens sp.; Pt, P. transmontanus (Douglass, 1903).