Merycochoerus magnus, Loomis, 1924

HUNT, ROBERT M., 2005, An Early Miocene Dome-Skulled Chalicothere from the ‘‘ Arikaree’ ’ Conglomerates of Darton: Calibrating the Ages of High Plains Paleovalleys Against Rocky Mountain Tectonism, American Museum Novitates 3486, pp. 1-46 : 28-32

Merycochoerus magnus

Figure 19

Remains of this large oreodont were found both near the base and at the uppermost levels of the Carpenter Ranch beds at Merycochoerus Butte (fig. 8), and they demonstrate that nearly the entire paleovalley fill at that locality is within the temporal range of this species. A right mandible (UNSM 44813) was found within a meter of the basal contact of the Carpenter Ranch beds with the underlying White River sediments, establishing the age of the earliest fluvial deposits in the paleovalley at this location. An edentulous maxilla and associated fragmentary postcranial bones (UNSM 44814) occurred 67 ft higher in the section, 14 ft below the top of the butte. This was the only locale in the southern buttes in which Merycochoerus occurred.

The stage of evolution of the three species of Merycochoerus can be estimated by the degree of fusion of the premaxillae, which is an index of the degree of nasal retraction developed in this oreodont lineage over time, plotted against a measure of the size of M3 ( Hunt and Stepleton, 2004: fig. 18). The earliest species, M. matthewi , precedes the larg­ er M. magnus , which is followed by the terminal species, M. proprius . This species progression underwent a shortening of the skull, nasal retraction likely indicating the development of a fleshy proboscis, and an increase in molar dimensions.

When compared with representative samples of these three species, the mandible from Merycochoerus Butte compares most closely with those of M. magnus from the lowermost Runningwater Formation northeast of Agate post office in Sioux County, Nebraska. The maxilla from the higher level on the butte is also similar to maxillae of M. magnus , yet appears somewhat more gracile. Because it lacks teeth, and its degree of nasal retraction can only be estimated, it is less certainly referred to that species. This oreodont suggests an earliest Hemingfordian age, equivalent to the fauna from the lowermost Runningwater Formation northeast of Agate, Nebraska.

ORDER ARTIODACTYLA

FAMILY DROMOMERYCIDAE Aletomeryx gracilis Figure 20 A right mandible with dp4 and m1 (UNSM 44820) was found in sands and gravels of the Carpenter Ranch beds at East Sturdivant Butte in close proximity to the maxilla of the small canid Phlaocyon (fig. 7). The dp4 and m1 are most similar to these teeth in Aletomeryx specimens from the Runningwater Formation in western Nebraska.

From their first occurrence in the early Hemingfordian of North America, species of Aletomeryx gradually increase in size, eventually evolving into the larger descendant genus Sinclairomeryx in the late Hemingfordi­ an. This size increase is documented by mandibular teeth such as dp4 and m3. Also, over this interval the premolar row (p2–4) shortens relative to the length of the molars (m1– 3).

Table 4 shows that the dp4 dimensions of the Carpenter Ranch specimen fall within the lower part of the range of a population of juvenile Aletomeryx found in Runningwater Quarry (UNSM locality Bx­58). Similarly, m1 measurements fall almost exactly at the mean values for this same population sample (table 4). This quarry occurs in the lowermost part of the Runningwater Formation in the type section, located in the northwest corner of Box Butte County, Nebraska. UNSM localities south of the Niobrara River in Box Butte County, collectively termed the Hemingford Quarries, sample the upper part of the Runningwater Formation. From one of

TABLE 4 Dimensions of dp4 and m1 (in mm) of Juvenile Aletomeryx [from Runningwater Quarry (UNSM Locality Bx­58), Box Butte Co., Nebraska, and from the Northeast of Agate local fauna, Sioux Co., Nebraska, compared to the juvenile Aletomeryx from East Sturdivant Butte]

these quarries, UNSM Hemingford Quarry 7B, over 30 juvenile Aletomeryx provide an adequate sample of dp4 for comparison with the Runningwater Quarry population (table 5). The Quarry 7B sample shows a significantly larger dp4 than the population from Runningwater Quarry, and it is evident that the dp4 from the Carpenter Ranch beds falls entirely outside the range of the Quarry 7B sample. Considering this evidence, an age coeval with the upper part of the Runningwater Formation is unlikely.

Other than at Runningwater Quarry, fossiliferous strata of the lower Runningwater Formation containing Aletomeryx occur northeast of Agate post office in Sioux County, where they have yielded two juvenile individuals that retain dp4 and m1 (Northeast of Agate local fauna, MacFadden and Hunt, 1998). The dimensions of these teeth from the two individuals fall within the documented range of dp4 and m1 measurements from Runningwater Quarry (table 4). Specimens found northeast of Agate were identified as Aletomeryx marshi by Frick (1937: 160); however, they are not distinguishable from the Runningwater Quarry sample on present criteria—additional specimens from the lower Runningwater Formation, northeast of Agate, would be necessary to determine whether they are in fact a different population from the Runningwater Quarry species. Here I employ the name Aletomeryx gracilis Lull 1920 for the species from the lower Runningwater Formation, and Aletomeryx marslandensis Frick 1937 for the specimens from upper Runningwater strata.

TABLE 5

Comparison of dp4 and m1 (in mm) of Aletomeryx from Runningwater Quarry (lower Runningwater Fm., A. gracilis ) and Hemingford Quarry 7B (upper Runningwater Fm., A. marslandensis )

There is some question whether Aletomeryx might be present in strata of late Arikareean age; no confirmed Arikareean occurrences are known at this time. The association of Aletomeryx with early Hemingfordian species such as Merycochoerus magnus in the Carpenter Ranch fauna indicates that the East Sturdivant Butte occurrence of the small dromomerycid is not likely to be of late Arikareean age. An earliest Hemingfordian age is supported here for the Carpenter Ranch Aletomeryx .

ORDER ARTIODACTYLA

FAMILY MOSCHIDAE

Pseudoblastomeryx sp. , cf. P. schultzi

Figure 21

A partial maxilla with M1–3 of a small moschid (UNSM 44821) was found in the Carpenter Ranch beds at East Sturdivant Butte (fig. 7); 18 ft above the specimens of Aletomeryx and Phlaocyon . Although the labial aspect of these molars was damaged, the molar row (length; 23.4 mm) corresponds in its dimensions and form of the teeth to several of the small late Arikareean and early Hemingfordian blastomerycine moschids. These small moschids require taxonomic revision and so a specific identification is uncertain; however, the molars show close correspondence to those of Pseudoblastomeryx schultzi from the Bridgeport Quarries, Morrill County, Nebraska, of early Hemingfordian age. UNSM 44821 is tentatively referred to that species pending discovery of more complete material.

ORDER CARNIVORA

FAMILY AMPHICYONIDAE

Daphoenodon , advanced species

Figure 22

A partial left mandible with m1–3 and a damaged p4 (UNSM 44815) can be referred to an advanced species of the beardog Daphoenodon . It was discovered in an indurated fluvial sandstone at Merycochoerus Butte , 95 ft above the base of the Carpenter Ranch beds (fig. 8). Coarse, granitic gravel lenses occurred above and below the mandible, which was found 18 ft below the maxilla of Merycochoerus (UNSM 44814).

Although the molars are heavily worn, their form, size, and the squared posterior border of p4 establish its identity as Daphoenodon . Daphoenodon evolved from the smaller late Arikareean D. superbus into a much larger early Hemingfordian species that occurs in the Runningwater Formation. This increase in size is reflected in larger, more robust skulls, teeth, and postcrania. The molars and p4 establish that UNSM 44815 was a carnivore larger than D. superbus from the late Arikareean of western Nebraska, yet smaller in molar dimensions than the large terminal species of Daphoenodon from the upper Runningwater beds of Box Butte County and the Bridgeport Quarries in Morrill County (table 6). Although Daphoenodon is represented by only a few individuals in the lower Runningwater Formation, including one from the exposures northeast of Agate, they compare most closely with the Carpenter Ranch mandible, and suggest an early Hemingfordian age corresponding to the age of the Northeast of Agate local fauna.

ORDER CARNIVORA

FAMILY CANIDAE

Figures

Fig. 19. Right mandible (UNSM 44813) and maxilla (UNSM 44814) of the oreodont Merycochoerus magnus from the Carpenter Ranch Formation, Merycochoerus Butte, Goshen County, Wyoming.

Fig. 20. Right mandible of a juvenile dromomerycid Aletomeryx gracilis (UNSM 44820) with dp4 and m1, Cow Trail Notch local fauna, Carpenter Ranch Formation, East Sturdivant Butte, Sioux County, Nebraska. Stereopair. Scale bar, 1 cm.

Fig. 21. Right maxilla of the moschid Pseudoblastomeryx (UNSM 44821) with M1–3, Cow Trail Notch local fauna, Carpenter Ranch Formation, East Sturdivant Butte, Sioux County, Nebraska. Stereopair. Scale bar, 1 cm.

Fig. 22. Left mandible of the amphicyonid Daphoenodon (UNSM 44815) with m1–3, partial p4, Merycochoerus Butte, Carpenter Ranch Formation, Goshen County, Wyoming. Scale bar, 1 cm.