The Entelodont Brachyhyops (Mammalia, Artiodactyla) from the Upper Eocene of Flagstaff Rim, Wyoming

Home , Archaeotherium, Duchesnean, Entelodont, Hyaenodon

Lucas, S.G., Zeigler, K.E. and Kondrashov, P.E., eds., 2004, Paleogene Mammals, New Mexico Museum of Natural History and Science Bulletin No. 26. 97 THE ENTELODONT BRACHYHYOPS (MAMMALIA, ARTIODACTYLA) FROM THE UPPER EOCENE OF FLAGSTAFF RIM, WYOMING

SPENCER G. LUCAS1 AND ROBERT J. EMRY2

1New Mexico Museum of Natural History, 1801 Mountain Rd. NW, Albuquerque, NM 87104; 2Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, D. C. 20560

Abstract—Specimens of the entelodont Brachyhyops from the early Chadronian of Flagstaff Rim, Wyoming, document new morphological characteristics of the upper dentition and cranial anatomy of the genus. Brachyhyops fossils are present in Saskatchewan, Montana(?), Wyoming, Utah, New Mexico and Texas in strata of late Duchesnean-early Chadronian age. The genus first appeared in North America as an immigrant from Asia, where it occurs in Irdinmanhan-Ergilian (middle-late Eocene) strata in China, Kazakstan and Mongolia. Keywords: Brachyhyops, Artiodactyla, Entelodontidae, Wyoming, Flagstaff Rim, Eocene

INTRODUCTION Strata of the White River Formation exposed at Flagstaff Rim in central Wyoming (Fig. 1) yield an extensive record of Chadronian (late Eocene) fossil mammals directly tied to magnetostratigraphy and a series of radioisotopic ages between ~35 and 32 Ma (Emry, 1973, 1992; Emry et al., 1987; Prothero, 1996a). This section (Fig. 1) thus is criti- cal to understanding Chadronian mammal biostratigraphy, biochronology and correlation. Emry (1992) plotted the stratigraphic ranges of the mammalian taxa in the Flagstaff Rim section, and indicated the pres- ence of two entelodont taxa. Brachyhyops records are low in the section, 20-60 ft above its base, whereas Archaeotherium occurs higher in the section, 220-480 ft above its base (Fig. 1). Here, we document the Brachyhyops specimens from Flagstaff Rim and summarize the geographic and stratigraphic distribution of the genus. Institutional abbreviations: AMNH = American Museum of Natural History, New York; CM = Carnegie Museum of Natural His- tory, Pittsburgh; USNM = National Museum of Natural History, Smithsonian Institution, Washington, D. C. DESCRIPTION AND MORPHOLOGICAL SIGNIFICANCE Two specimens of Brachyhyops are known from the Flagstaff Rim section, and they provide new morphological data on this rare taxon. USNM 521245 is a left maxillary fragment with the posterior end of the canine alveolus, P1 alveolus, P2-3 and DP4 (Fig 2D-E). The specimen has no teeth in common, and thus little basis for direct comparison, with the holotype of B. wyomingensis Colbert, 1938, which is most of a skull (CM 12048) that includes P1, but which has no other premolars preserved except for part of one broken and very worn P4. However, the measurement from the posterior edge of the canine alveolus to the posterior edge of DP4 is 78 mm in USNM 521245, whereas the same measurement in B. wyomingensis (but to P4 rather than DP4) is 62 mm. Thus, based on this one comparable measurement, USNM 521245 is approximately 25% larger than the B. wyomingensis type. Further, USNM 521245 is of the appropriate size to occlude almost perfectly with a cast of the holotype lower jaw of B. viensis Russell, 1980, the species from the Cypress Hills Formation of Saskatchewan. Russell (1980a, p. 3) calculated that B. viensis is about 30% larger FIGURE 1. Location map and stratigraphic section at Flagstaff Rim, Wyoming, than B. wyomingensis, based on comparison of the type of B. viensis to showing stratigraphic distribution of entelodont taxa (after Emry, 1992). a mandible from Texas referred by Wilson (1971) to B. wyomingensis. We conclude that USNM 521245 is the right size to be B. viensis, and trenchant P2 follows. This tooth has not previously been described in we refer it to the species on that basis. Brachyhyops. It is antero-posteriorly long, transversely narrow, and two The maxilla is concave laterally and has an antero-posteriorly rooted. The single cusp is a tall cone with a distinct posterior ridge that oriented cavity (sinus) medially that extends back to over the P3. There runs onto a very low, narrow talon heel (with a posterior cingulum). is a large infraorbital foramen above the anterior edge of P4. The C The enamel is slightly rugose and lineated. Measurements (in mm) alveolus is larger than the double alveolus of P1, which is at an angle to are: length = 15.2, width = 5.7 the long axis of the tooth row. A short alveolus follows P1, and the The P3 is not complete but has two large, pointed labial cusps, 98

FIGURE 2. Brachyhyops from Flagstaff Rim, Wyoming. A, USNM 521246, incomplete skull roof of B. viensis in lateral (A), dorsal (B) and ventral (C) views. D-E, USNM 521245, left maxillary fragment with C and P1 alveoli and P2-3, DP4, lateral (E) and occlusal (F) views. the paracone and metacone, and only a small, low, very narrow labial with glenoid fossae, and other fragments of basicranial elements. It cingulum, better developed labial to the metacone than the paracone. A represents an individual substantially larger than CM 12048, the holo- parastyle is present but no better developed than the cingulum. Low type of B. wyomingensis: for example, the width across the frontals ridges run between the paracone and metacone, creating an incomplete above the dorsal edges of the orbits (79 mm in CM 12048; 101 mm in ectoloph-like crest. The crown lingually is only a small lobe between USNM 521246) is about 28% larger, and the distance from the poste- the paracone-metacome upon which presumably a low, bulbous proto- rior edge of the orbit to the posterior end of the nuchal crest (108 mm in cone was located. Lingual to the metacone, the posterior cingulum is CM 12049; 152 mm in USNM 521246) is about 41% larger (the greater taller and better developed than the labial cingulum. Length (in mm) = degree of difference in this latter measurement is due in part to the 18.6. Note that Colbert (1938, fig. 3) reconstructed P3 incorrectly as a exaggerated posterolateral wings of the nuchal crest in USNM 521246). single-coned, trenchant tooth. The size of USNM 521246 is in the range expected for B. viensis, The DP4 is a bunodont tooth with a nearly triangular outline. It based on the size of the lower jaw, and we refer the specimen to that has a large paracone and metacone labially and prominent cingula an- species. teriorly and posteriorly that essentially surround the crown. The proto- In dorsal view, the roof is broadest across the orbits and tapers cone is the largest lingual cusp, and is followed by a somewhat smaller slowly back to the postorbital constriction, flares again slightly to taper hypocone on the postero-lingual cingulum. The metaconule and more sharply back to the posterior tip of the braincase, then flares out paraconule are slightly larger than the hypocone. Measurements (in again slightly with parasagittal crests that are sinusoidal (cf. Colbert, mm) are: length = 18.8, width = 18.2. 1938, fig. 2). The top of the skull has three sets of long ridges: (1) a USNM 521246 (Fig. 2A-C) is a skull roof from the anterodorsal medial ridge that runs with slight irregularity from frontal to occiput; margin of the orbits (left side) to the occipital (nuchal) crest, the poste- (2) two ridges laterally that are lower, broader and converge to meet rior parts of both zygomatic arches (not connected to the brain case) over the braincase constriction; and (3) the parasagittal ridges that de- 99 fine the outer margin of the dorsal surface of the skull (except for the orbit, which projects laterally from them). Between the ridges the bone is textured into irregular rugose lines and pits, similar to the texture of crocodilian dermal bone. Parts of the glenoid portion of the zygomata are preserved on each side. These have a wide, shallowly concave glenoid fossa with a short, blunt post-glenoid process medially. The anterior wall of the glenoid roof is a broad plate that is slightly concave anteriorly. There is a prominent process on the dorso-lateral margin. Ventrally, the frontal is invested with irregular, concave pockets and grooves (sinuses). The orbital wall is smooth, with a complete postorbital bar; the braincase dorsal roof is a broad, roughened cup for the cerebral hemispheres with a distinct, fairly digitate frontal-parietal suture that cuts across it transversely; also, there is an irregular pattern of foramina on this surface. A convex ridge cuts off the anterior from the posterior concavity for the occipital lobe, which is antero-posteriorly short and bulbous. Laterally, the skull roof rises behind the orbits, and there is a concave depression between the braincase wall and overhanging parasagittal crests. The bone is generally smooth on the lateral aspect of the skull. The frontal-parietal suture continues down the lateral side of the skull. In occipital view, the occiput is high and narrow, and deeply concave between backward projecting nuchal crests. FIGURE 3. Geographic and temporal distribution of Brachyhyops in North America. Archaeotherium coarctatum has a broad skull roof across the Localities are: B = Baca Formation, west-central New Mexico; BD = Beaver Divide, orbits that tapers rapidly posterior to the orbits to form very narrowly Wyoming; CC = Canyon Creek, Wyoming; F = Flagstaff Rim, Wyoming; G = spaced parasagittal crests above the braincase (e.g., Peterson, 1909). Galisteo Formation, north-central New Mexico; L = Lapoint fauna, Utah; P = Porvenir Brachyhyops, in contrast, has much broader parasagittal crests above local fauna, Trans-Pecos, Texas; Pr = Prickly Pear Creek local fauna, Montana; S = Southfork local fauna, Saskatchewan. the braincase. The textured, lineated braincase roof of Brachyhyops is most similar to the condition in Archaeotherium coarctatum, and it may be a derived feature uniting Brachyhyops to an Archaeotherium 5. Lower Chadronian strata of the White River Formation at Can- coarctatum clade. This pattern here differs from the holotype skull of yon Creek, Wyoming, about 50 km east of Beaver Divide. These are skull B. wyomingensis, so it may be unique to B. viensis or it may just be fragments in the USNM collection (including USNM 455679) that match variable among Brachyhyops. well the cranial material of B. viensis from Flagstaff Rim described here. DISTRIBUTION OF BRACHYHYOPS 6. The Lapoint Member of the Duchesne River Formation, in northeastern Utah, which is the “type” fauna of the Duchesnean land- Brachyhyops, the oldest North American entelodont, is an im- mammal “age” (Lucas, 1992). Brachyhyops wyomingensis from Lapoint migrant from Asia (e.g., Brunet, 1979; Lucas and Emry, 1996; Foss, (Gazin, 1956; Wilson, 1971; Emry, 1981) is stratigraphically above an 2001). In Asia, Brachyhyops (= Eoentelodon) has a distribution in China, ash with an Ar/Ar age of 39.74 + 0.07 Ma and probably correlates to Kazakstan and Mongolia in strata of Irdinmanhan-Ergilian (middle- part of Chron 18n (Prothero and Emry, 1996). The Lapoint mammals late Eocene) age (Russell and Zhai, 1987; Wang and Qiu, 2002). suggest, however, a younger, late Duchesnean age, perhaps equivalent Brachyhyops fossils have a broad distribution in western North America, to Chrons 17r or 17n (Prothero and Emry, 1996). from Sakatchewan to the Big Bend of Texas in strata of late Duchesnean- 7. Duchesnean interval of the Galisteo Formation in north-cen- early Chadronian age (Fig. 3). These records are: tral New Mexico (Lucas and Estep, 1999). The Brachyhyops speci- 1. Southfork local fauna in the Cypress Hills Plateau of men, a m1 assigned to B. viensis, occurs at the Duchesneodus quarry Saskatchewan (Russell, 1980, a, b; Storer, 1984, 1996). This is the (Stearns’ quarry) near the top of the Galisteo Formation. This strati- type locality of Brachyhyops viensis Russell. Storer (1984, 1996) re- graphic level is in an interval of reversed magnetic polarity that Prothero garded the Southfork local fauna as late Duchesnean, but the taxa present and Lucas (1996) correlated to Chron 17r. The base of the Espinaso include species of Hyaenodon, Mesohippus, Hyracodon and Formation, about 10 m above the Duchesneodus quarry, is in a normal “Leptomeryx” that suggest an early Chadronian age. polarity interval correlated to Chron C17n3 and yields radioisotopic 2. Ostrander (1985, p. 213) lists the genus Brachyhyops without ages of ~38 Ma. Thus, the Brachyhyops specimen from the Galisteo documentation from the early Chadronian Prickly Pear Creek local fauna Formation is of late Duchesnean age. in Montana. A. Tabrum (written commun., 2004) informs us that Prickly 8. Duchesnean interval of the Baca Formation at Mariano Mesa Pear Creek is a drainage east of Helena and west of Canyon Ferry in west-central New Mexico. Originally reported by Schrodt (1980) where the “White River” deposits yielded a small collection of mam- and Schiebout and Schrodt (1981), the specimens are an edentulous mal fossils made by Earl Douglass in 1902 and housed at the Carnegie mandible fragment (Louisiana State University Museum of Geosciences Museum of Natural History. Tabrum says the Carnegie collection is [LSUMG] 2313-1) and a right p4-m3 (LSUMG 2313-2) of Brachyhyops mostly fragmentary but includes material of Colodon, Menodus and wyomingensis from the upper part of the Baca Formation. The latter, Oreonetes, but no specimens of Brachyhyops. The basis for Ostrander’s illustrated by Schrodt (1980, apppendix 3, fig. 1) is the correct mor- (1985) report of Brachyhyops may be in another collection and thus phology and size (measurements in mm are: m1l = 16.2, w = 11.6, m2l remains undocumented. = 17.7, w = 13.9, m3l = 18.6, w = 12.0) to be assigned to B. 3. The Flagstaff Rim records documented here, which are early wyomingensis. Recent paleomagnetic stratigraphy indicates that the Chadronian (Fig. 1; Emry, 1992). Brachyhyops locality is in an interval of reversed polarity that is imme- 4. The type locality of Brachyhyops wyomingensis, the early diately below the base of the Spears Formation, which yields radioiso- Chadronian (see Lucas et al., this volume) Big Sand Draw Sandstone topic ages of ~ 38 Ma. Thus, the Brachyhyops locality is best corre- Lentil of the White River Formation at Beaver Divide, Wyoming lated to Chron 17r (Ludtke et al., 2003), and it is approximately coeval (Colbert, 1938; Emry, 1975). with the Galisteo Formation record of Brachyhyops. 100 9. Late Duchesnean Porvenir local fauna in Trans-Pecos, Texas, ACKNOWLEDGMENTS where Brachyhyops wyomingensis is well documented (Wilson, 1971). The Porvenir local fauna is late Duchesnean; it is above an ash with an J. Schiebout and M. Dawson provided access to specimens in Ar/Ar age of 37.8 + 0.15 Ma and in an interval of normal polarity their care. A. Tabrum provided helpful information on the Prickly Pear equivalent to Chron 17n2 (Prothero, 1996b; Prothero and Emry, 1996). Creek local fauna We thank P. Kondrashov and S. Foss for comments on the manuscript.

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