An Early Miocene Dome-Skulled Chalicothere

Home , Aletomeryx, Arikaree Formation, Chalicothere, Chalicotheriinae, Chalicotherioidea, Merycochoerus

PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024 Number 3486, 45 pp., 26 ®gures, 8 tables October 27, 2005

An Early Miocene Dome-Skulled Chalicothere from the ``Arikaree'' Conglomerates of Darton: Calibrating the Ages of High Plains Paleovalleys Against Rocky Mountain Tectonism

ROBERT M. HUNT, JR.1

CONTENTS Abstract ...... 2 Introduction ...... 2 Geologic Setting ...... 3 Systematic Paleontology ...... 12 Age of the Dome-Skulled Chalicothere ...... 28 The ``Arikaree'' Conglomerates of N.H. Darton ...... 32 Post-Laramide Evolution of the Rocky Mountains ...... 39 Acknowledgments ...... 42 References ...... 42

1 Department of Geosciences, W436 Nebraska Hall, University of Nebraska, Lincoln, NE 68588-0549 ([email protected]).

ABSTRACT Fragmentary skeletal remains discovered in 1979 in southeastern Wyoming, associated with a mammalian fauna of early Hemingfordian age (ϳ18.2 to 18.8 Ma), represent the oldest known occurrence of dome-skulled chalicotheres in North America. The chalicothere and accompanying fauna were found in silica-cemented ¯uvial sandstones and conglomerate capping a series of west-to-east trending buttes in Goshen County, Wyoming, extending eastward into Sioux County, Nebraska. These butte caprocks, herein named the Carpenter Ranch For- mation, are the reversed topographic remnants of a major paleovalley incised in the early Miocene and ®lled with coarse granitic/ma®c gravels and sands derived from Precambrian- cored upifts to the west. The Carpenter Ranch Formation is identi®ed as a coarser-grained, westward extension of the early Hemingfordian Runningwater paleovalley of Skinner, Skinner, and Gooris (1977) situated in northwest Nebraska. The age-diagnostic mammal faunas and coarse crystalline gravels of the Carpenter Ranch Formation of Wyoming and the coeval Martin Canyon beds of northern Colorado document the timing of tectonic rejuvenation of Rocky Mountain uplifts to the west in the early Hemingfordian. A second line of buttes situated 3 miles (ϳ 4.8 km) north of the Carpenter Ranch paleovalley includes the regional landmark recorded as ``Spoon Butte'' in 19th century military and topographic surveys of the region. Spoon Butte, its caprock in northwest-southeast alignment with adjacent satellite buttes, represents a second and younger mid-Miocene paleovalley trend, characterized by silica-cemented sandstones and basal granitic/chert gravel with Barstovian mammals. N.H. Darton and G.I. Adams of the U.S. Geological Survey ®rst described these caprocks as Arikaree conglomerates and mistakenly assumed they were of the same age as the caprock at Spoon Butte and its accompanying buttes to the north. However, this investigation shows that (a) mammal faunas from the two caprock trends are of markedly different ages; (b) the clast composition of the gravels is distinct; and (c) the paleovalleys align along two entirely different geographic trends. Furthermore, a loose gravel found on top of Spoon Butte, previously believed to date the caprock, is identi®ed as a lag deposit sharing the same distinctive clast composition as high Quaternary terrace gravels in the vicinity. These terrace gravels include acid volcanics (not present in the Carpenter Ranch and Spoon Butte basal gravels) and a composite assemblage of water-worn mammal bones of Barstovian, Clarendonian, and Pleistocene ages. Thus they demonstrate that late Cenozoic deposits once occupied the extensive tableland east of the Hartville Uplift, but were later removed by erosion and reworked into Pleistocene and Holo- cene drainages.

INTRODUCTION America. North American schizotheriines are Miocene chalicotheres are uncommon in of early and mid-Miocene age, and are rep- North American faunas, yet when present are resented by two genera (Coombs, 1998, often preserved in riverine environments and 2004), Moropus and Tylocephalonyx. waterholes. Chalicotheres apparently were Dome-skulled chalicotheres were ®rst de- water-dependent mammals, their teeth and scribed by Munthe and Coombs (1979), fol- skeleton indicating a browsing habit in well- lowing the discovery of two North American vegetated settings. Bones of chalicotheres skulls with dorsally in¯ated or ``domed'' cra- should resist damage and disintegration sim- nia. Coombs (1979) created the genus Tylo- ply because of their large size, and so their cephalonyx for these mammals, based on scarcity in most Miocene faunas contributes dental, cranial, and postcranial remains from to the belief that they were restricted in hab- the western United States. Fossils attributed itat and limited in numbers of individuals. to the genus have been recorded from the late The Chalicotheriidae comprise two sub- Hemingfordian of Nebraska, Wyoming, Col- families, the Chalicotheriinae and the Schi- orado, Utah, and Oregon, and from the late zotheriinae. The Chalicotheriinae are con- Hemingfordian and early Barstovian of Mon- ®ned to the Old World, whereas the Schi- tana. zotheriinae occur in both Eurasia and North During exploration of the Patrick Buttes in 2005 HUNT: DOME-SKULLED CHALICOTHERE 3 southeastern Wyoming by University of Ne- east by 9 mi north-south. More than 20 sand- braska paleontologists in 1979, chalicothere stone-capped buttes are aligned along two bones were discovered in indurated gravels trends, which represent the vestiges of two and sandstones capping several of the buttes. deeply incised Miocene paleovalleys in Although much of this material was frag- southeastern Wyoming. The buttes occur mentary, a mandible, a few postcranial ele- within the boundaries of the U.S. Geological ments, and a partial skull indicated the pres- Survey 30-minute Patrick Quadrangle (104Њ ence of a large chalicothere similar in size to to 104Њ30ЈW, 4 2 Њ to 42Њ30ЈN), ®rst topo- the early Miocene schizotheriine Moropus graphically mapped in 1895 and named for elatus, so abundantly represented in the Ag- the Patrick Post Of®ce on Rawhide Creek, ate waterhole bonebed in western Nebraska Goshen County, Wyoming (Adams, 1902: pl. (Hunt, 1990). Recently, removal of coarse- 3). Rising dramatically from the surrounding grained sediment enclosing the partial skull plains, the buttes were ®rst mentioned in ear- revealed the expanded cranial architecture of ly military surveys (Warren, 1857, 1875) and a dome-skulled chalicothere. Associated with also noted during the initial geological re- the skull at the site of collection were a man- connaissance studies of western Nebraska dible and partial femur, probably from the and southeastern Wyoming by the U.S. Geo- same individual, which are here attributed to logical Survey (Darton, 1899; Adams, 1902). the schizotheriine Tylocephalonyx. The Pat- They were visited brie¯y by paleontologists rick Buttes chalicothere displays the largest exploring in the region (Peterson 1907, 1909; cranial dome yet discovered, and it is pre- Osborn, 1909), but apparently they failed to sumed to belong to a male. yield noteworthy fossils. Fossil mammals associated with the chal- The buttes are capped by indurated Mio- icothere indicate an early Hemingfordian cene ¯uvial sandstone and granitic gravel, age; hence these remains represent the oldest commonly silica-cemented, with occasional record of a dome-skulled individual in North lenses of soft interbedded silty to clayey America. Additional fragmentary chalico- sandstones lacking intergranular cement. The there limb elements were found in the cap- ¯uvial deposits deeply incise tuffaceous rock of adjacent buttes in the same ¯uvial sandstones and siltstones of the Arikaree and sands and gravels as the dome and mandible White River Groups that form the lower and are provisionally attributed to Tyloce- slopes of the buttes. phalonyx. The species is considered indeter- The age of the capping sandstones re- minate pending discovery of more complete mained enigmatic until 1977±1980, when a specimens. geological survey of the buttes was under- taken by University of Nebraska paleontol- ABBREVIATIONS ogists. Prior to this, Adams (1902) had AMNH Division of Paleontology, American mapped the butte caprocks as basal Arikaree Museum of Natural History, New York conglomerate (implicitly late Oligocene); CM Division of Vertebrate Fossils, Carne- Love et al. (1980) assigned them to the upper gie Museum of Natural History, Pitts- Miocene; and others have considered their burgh age uncertain (Rapp et al., 1957; Denson, F:AM Frick Collection, American Museum of 1974). We were encouraged when an initial Natural History, New York geological reconnaissance in 1977±1978 pro- FMNH Field Museum of Natural History, Chi- duced a few fossil mammals indicating a cago Miocene age. Summer ®eld seasons in the UNSM University of Nebraska State Museum, buttes in 1979 and 1980 resulted in the dis- Lincoln covery of fossil mammals from nearly all butte caprocks and demonstrated that the GEOLOGIC SETTING capping sandstones were of two different The Patrick Buttes are situated in north- ages, early Miocene and medial Miocene. eastern Goshen County, Wyoming, extending Sedimentary structures, geometry of sand ϳ5 mi eastward into Sioux County, Nebraska and gravel bodies, and paleochannel cut-and- (®g. 1). They occupy an area ϳ14 mi west- ®ll features indicate that the capping sand- 4 AMERICAN MUSEUM NOVITATES NO. 3486

Fig. 1. Geographic map of the Patrick Buttes, Wyoming-Nebraska. The areal extent of each butte is indicated by solid and/or dashed lines marking the limits of the butte caprocks, formed by silica- cemented sandstone. Names for individual buttes are derived from historical usage, local practice, and UNSM ®eld terms. stones and gravels are ¯uvial in origin. Two dont, a rhinoceros, and the chalicothere Ty- major ¯uvial systems can be distinguished by locephalonyx. gravel content, linear channel trend, paleo- The linear courses of the northern and current data, and contained fossil mammals southern butte lines are evident in satellite (®g. 2). A northern butte line (including photo imagery as axial paleovalleys trending Spoon Butte, a regional landmark 3.5 mi in eastward from the Rocky Mountain uplifts length and up to 1 mi in width) of medial onto the plains. The northern butte line is Miocene age is dated by a Barstovian fauna aligned from northwest to southeast; the of mesodont equids (Merychippus s.l., Pro- southern butte line trends west to east, turn- tohippus, Desmatippus), tapir, rhinoceros, ing to the northeast at its eastern terminus. gomphothere proboscidean, dromomerycid Topographic mapping of the base of the ¯u- and merycodont artiodactyls, camels, pecca- vial deposits of the southern buttes indicates ry, and canid and amphicyonid carnivores. an eastward decrease in elevation of about The southern butte line of early Miocene age 17 ft/mi; the northern butte line is similar. At yielded an early Hemingfordian fauna that several localities, paleovalley geometry is included the dromomerycid Aletomeryx, observable in cross-section and/or plan view, moschid Pseudoblastomeryx, oreodonts Mer- revealing steep lateral margins and sinuous ycochoerus and Merychyus, canid Phlao- trends (®g. 3). Maximum observed depth of cyon, amphicyonid Daphoenodon, the cam- incision in the northern buttes occurs where els Protolabis and Michenia, a large entelo- a paleochannel cuts 140 ft into Arikaree 2005 HUNT: DOME-SKULLED CHALICOTHERE 5

Fig. 2. Geology of the capping sandstones and gravels of the Patrick Buttes. Cross-hatching indicates buttes of the northern trend capped by the Barstovian Spoon Butte Beds; an open pattern indicates buttes of the southern trend capped by the early Hemingfordian Carpenter Ranch Formation. Gravels of the Carpenter Ranch Formation (table 1) were sampled at sites marked I through VI; gravels from localities marked by black diamonds were combined to form the composite Spoon Butte Beds sample. Site OBQ marks the acid volcanic-bearing terrace gravel of the Oberg Quarries. A stippled pattern shows the reconstructed areal extent of the Lay Ranch Beds, an early Miocene paleovalley ®ll containing latest Arikareean mammals. rocks. Maximum observed incision of the are devoid of acid volcanics (rhyolite, daci- southern paleovalley in outcrop is ϳ100 ft, te). The northern buttes generally lack pebble but true topographic relief could have been as (or coarser) gravel; their caprocks are chie¯y much as 350 ft if paleovalley incision is mea- an indurated cross-bedded sandstone, but sured from the highest topographic elevation within the base of the caprock unit are rare of Arikaree rocks in the vicinity. Maximum pebbles of pink orthoclase granite and a dis- thickness of the capping sandstones ranges tinctive yellow-brown chert, either ¯oating in from 100 to 140 ft; however, average thick- sandstone matrix, or as clasts within a ma- ness for both butte lines is ϳ30 to 40 ft. trix-supported basal conglomerate of round- Pebble gravel from the northern and south- ed Tertiary sandstone blocks up to several ern buttes was sampled with Ϫ3ù and Ϫ4ù feet in diameter. Pink orthoclase granite and sieves (U.S. Standard, W.S. Tyler Co.). Lith- yellow-brown chert can be traced westward ologically distinctive gravel fractions char- to the granites and Paleozoic limestones of acterized each ¯uvial system (table 1). The the Hartville Uplift 30 mi to the northwest, southern buttes are distinguished composi- indicating the provenance of the basal gravel tionally by a granitic/ma®c gravel with rare of the northern buttes. The source of the anorthosite and chert clastsÐthese gravels gravels of the southern butte line are likely 6 AMERICAN MUSEUM NOVITATES NO. 3486

derived from the Laramie Mountains, either as primary clasts or reworked from older ¯u- vial gravels. Over 15 geographically isolated buttes comprise the southern butte line, extending from Tea Kettle Rock on the west, through Sturdivant Gap, to East Sturdivant Butte on the east (®gs. 1, 2), a distance of ϳ15 miles. The lithologic composition of the capping sandstones and gravels and the manner of occurrence of the fossils are alike on all buttes of the southern trend. The caprocks are ®ne- to coarse-, but chie¯y medium-grained, silica-cemented feldspathic sandstone (table 2: a subarkose to arkose, with 24±27% feldspar content) and pebble gravel, the gravel scat- tered throughout in lenses and stringers. Al- though silica-cemented sandstone and gravel is the dominant lithology, occasional lenses of ®ne-grained, beige- or cream-colored, un- cemented silty sand occur at various levels within these caprocks and have yielded age- diagnostic fossils like those of the more indurated beds. Fossils occur as isolated specimens on all southern buttes, and they are usually more complete than those from the northern butte line. Bones are often well preserved with little evidence of reworking: a large femur at one locality had been broken during scavenging, yet the angular pieces remained un- abraded and in close association in the cap-

← Fig. 3. Geologic cross-section from Spoon Butte to East Sturdivant Butte and Carpenter Butte (north-south pro®le, vertical exaggeration,

X10). Barstovian silica-cemented sandstone (TSB) caps Spoon Butte and incises latest Arikareean

sediments (TLR) of the Lay Ranch paleovalley. The caprocks of East Sturdivant Butte and Car- penter Butte preserve a transverse pro®le of the Carpenter Ranch paleovalley where sandstones and gravel of the early Hemingfordian Carpenter

Ranch Formation (TCR) represent the stratotype at Cow Trail Notch. Acid volcanic-bearing gravels distinguish Holocene terraces of the modern drainage system that have been reworked from high terrace gravels (ϳ4700 ft, Oberg Quarries, east of cross-section) yielding mid- and late Mio-

cene mammals and rare Pleistocene fossils. TA, Arikaree Group undifferentiated; TWR, White Riv- er Group. 2005 HUNT: DOME-SKULLED CHALICOTHERE 7

TABLE 1 Pebble Composition (in %) of Crystalline Gravels of Carpenter Ranch and Runningwater Formations, Spoon Butte Beds of O.A. Peterson, and Oberg Quaternary Terrace Gravel, Goshen County, Wyoming, and Sioux County, Nebraska

rock, indicating a synchronous burial undis- topographically low capping sandstones of turbed by later cut-and-®ll. Yet only three lo- the southern buttes supposedly represented calities in the southern buttes have provided the initial ®lling of the valley, and the higher most of the age-diagnostic mammals (®g. 4). caprock of the northern buttes marked the No record of fossils from these buttes had terminal phase of aggradation (®g. 5). Fossil been published prior to the UNSM study. mammals discovered by us, however, prove However, in September 1968, M.F. Skinner that this cannot be the correct interpretation: of the American Museum (New York) found deposition of the northern and southern cap- a castorid mandible (F:AM 42983) in a talus rock sandstones was separated by at least 3 block, presumably from the caprock at Mer- million years. The markedly different clast ycochoerus Butte, the ®rst fossil mammal re- content of the gravels, the temporally distinct corded from the southern buttes. mammal faunas, and the divergent linear It was believed at ®rst (Adams, 1902; Dar- trends of the northern and southern paleo- ton, 1899) that the sandstones capping both valleys demonstrate the existence of two dis- the northern and southern buttes might be crete Miocene ¯uvial systems. In the eastern part of a single ¯uvial system that simply part of the study area, the northern and south- migrated laterally as it ®lled its valley. The ern butte lines converge, and here where they 8 AMERICAN MUSEUM NOVITATES NO. 3486

TABLE 2 Petrography (in %) of the Indurated Sandstone Caprocks of the Southern and Northern Patrick Buttes, Goshen County, Wyoming, and Sioux County, Nebraska

are in closest proximity, the incised bases of T27N, R58W, extending southwest into the the two paleovalleys are ϳ200 ft apart in el- extreme southeast corner of sec. 34, T27N, evation. R60W, Spoon Butte 7.5-minute quadrangle, The capping sandstones and gravels of the 1990); the measured section at Cow Trail southern butte line have not been formally Notch (®gs. 6A, 7) is designated the strato- named. An informal term, Carpenter Ranch type. The gap is situated astride the Wyo- beds, was applied to these rocks at the con- ming-Nebraska state boundary where it is clusion of our initial survey (Hunt, 1985a). traversed by Sheep Creek, a small spring-fed Because of their mappability, distinguished stream. Outcrops in and adjacent to the gap by a uniform and distinctive lithology (with are readily accessible by a maintained gravel emphasis on the characteristic gravel com- road. Early Hemingfordian mammals have position), and the geographically well-cir- been collected from the stratotype section at cumscribed outcrops, identi®ed and mapped the Cow Trail Notch promontory (®g. 7). in their entirety in the region (®g. 2), the Car- The early Hemingfordian mammal fauna penter Ranch beds merit formation-rank des- of the Carpenter Ranch Formation is most ignation as a formal lithostratigraphic unit. similar to fossil mammals from the lower This rock unit is herein designated the Car- part of the Runningwater Formation (North- penter Ranch Formation, from the location east of Agate local fauna, MacFadden and of important and accessible fossiliferous ex- Hunt, 1998). The Northeast of Agate local posures in the Carpenter Ranch 7.5-minute fauna comes from Runningwater strata ϳ20 quadrangle (secs. 7±10, T26N, R60W) in mi northeast of the eastern terminus of the proximity to the headquarters of the Carpen- Carpenter Ranch paleovalley. The Carpenter ter Ranch (NE1/4, SE1/4, NW1/4, sec. 12, Ranch mammals are also similar to species T26N, R58W) at the southern entrance to from Runningwater Quarry, which is strati- historic Sturdivant Gap. The exposures in graphically low in the Runningwater For- Sturdivant Gap (®g. 6A,B) can be considered mation and contains one of the oldest of the the type area of the formation (W1/2, sec. 36, Runningwater fossil assemblages. The prox- 2005 HUNT: DOME-SKULLED CHALICOTHERE 9

Fig. 4. Principal fossil mammal localities (indicated by arrows) in the capping sandstones of the Patrick Buttes. Cow Trail Notch at East Sturdivant Butte and sites at Merycochoerus Butte have yielded most of the age-diagnostic specimens in the early Hemingfordian Carpenter Ranch Formation. Deahl Butte produced the partial dome and mandible of the chalicothere (UNSM 44800, 44801). Barstovian sites in the Spoon Butte Beds include South Rim, GF Butte, North Point and Box Canyon. The Oberg Quarries are gravel pits containing waterworn mid- and late Miocene mammals and Pleistocene Equus.

Fig. 5. Redrafted version of N.H. Darton's cross-section (1899: ®g. 213) from Spoon Butte to Sturdivant's Ranch, showing the silica-cemented sandstone caprocks descending in elevation to the south. Darton regarded all the caprocks as ``Arikaree'' conglomerates, ``consisting in large part of crystalline rocks and apparently lying on a steeply sloping surface''. The letters A-E were not explained in his caption. Note correspondence to ®gure 3. 10 AMERICAN MUSEUM NOVITATES NO. 3486

Fig. 6. Type area of the Carpenter Ranch Formation, early Miocene, Wyoming-Nebraska: (A) Cow Trail Notch stratotype section (at arrow) in NW1/4, SW1/4, NE1/4, SW1/4, sec. 36, T27N, R58W, Sioux County, Nebraska, looking northeast toward the west escarpment of East Sturdivant Butte within Stur- divant Gap. The Cow Trail Notch fauna was collected entirely from the promontory where the section was measured. The high surface of East Sturdivant Butte is mantled by a thin lag deposit of Oberg gravel not to be confused with the indurated Carpenter Ranch Formation gravel in the type section and contiguous outcrops in the gap. Sheep Creek runs in the foreground. (B) East escarpment of West Sturdivant Butte in Sturdivant Gap, looking west, immediately opposite ®gure 6A, showing the cliff- forming Carpenter Ranch Formation caprock overlying ®ne-grained Arikaree and White River strata. Truck on access road through the gap is situated at the Wyoming-Nebraska state boundary. 2005 HUNT: DOME-SKULLED CHALICOTHERE 11

Fig. 7. Stratotype section of the Carpenter Ranch Formation (®g. 6A), west escarpment of East Sturdivant Butte, Sioux County, Nebraska, which includes the Cow Trail Notch local fauna. Early Hemingfordian mammals of the Cow Trail Notch local fauna include the dromomerycid Aletomeryx, moschid Pseudoblastomeryx, and canid Phlaocyon. Lithic symbols as in ®g. 9. 12 AMERICAN MUSEUM NOVITATES NO. 3486 imity of the Runningwater and Carpenter considered indeterminate due to the fragmen- Ranch paleovalleys and their similarly tary nature of much of the skeleton. aligned west-southwest to east-northeast REFERRED MATERIAL: Included in this in- trend suggest that the Runningwater paleo- determinate species from the Carpenter valley is the northeastward continuation of Ranch Formation are a right mandible with the Carpenter Ranch trend. This conclusion m2±3, a partial cranial dome, and the diaph- is supported by the gravel composition of the ysis of the left femur, all found together at two formations, characterized by a granitic/ the same stratigraphic level within a 2-m in- ma®c gravel with rare anorthosite clasts and terval (®g. 9); also a proximal right ulna; a an absence of acid volcanic rock fragments distal metapodial; a proximal left metacarpal (table 1). 4 and the dorsal process of an ungual pha- Massive pink tuffaceous mudstones of the lanx; a distal right femur (the ®rst evidence White River Group exposed at the base of of this chalicothere found in the caprock the buttes of the southern line yield Oligo- sandstones of the southern buttes); a distal cene mammals typical of the Brule Forma- left femur in two parts (scavenged by an en- tion (®g. 8). The Carpenter Ranch ¯uvial sys- telodont) found within5mofaproximal left tem deeply incised these White River depos- humerus at the same stratigraphic level; a its, in the process removing almost all strata distal right tibia; a glenoid-bearing fragment of the Arikaree Group, but at some buttes of the scapula; and a fragment of the ecto- remnants of gray Arikaree sandstones inter- loph of a right M3. vene (®g. 7). At some localities the surface Localities and dates of collection of these of the White River Group is a deeply weath- elements are listed in table 3. ered carbonate-impregnated paleosol. All HORIZON AND AGE: All referred material is White River fossils found by our ®eld party from the early Hemingfordian Carpenter come from outcrops at Dog Jaw Butte and Ranch Formation, Goshen County, Wyo- Merycochoerus Butte in sections 8±9, T26N, ming. The associated mandible (UNSM R60W. Arikaree outcrops exposed along the 44800), cranial dome (UNSM 44801), and southern butte line are nearly barren of fossil partial femur (UNSM 44802) were found in mammals. place in a ¯uvial lens of sand and gravel exposed in a south-facing cliff at the western SYSTEMATIC PALEONTOLOGY end of Deahl Butte (®g. 9). Here a channel ORDER PERISSODACTYLA of the Carpenter Ranch paleovalley had incised, and mostly removed, the gray Arikaree FAMILY CHALICOTHERIIDAE GILL, 1872 sands, cutting to the level of the local paleo- SUBFAMILY SCHIZOTHERIINAE HOLLAND AND surface of the White River beds at the chal- PETERSON, 1914 icothere locality. The dome, mandible, and femur were found within a single gravel lens Tylocephalonyx Coombs, 1979 at the same stratigraphic level, situated ϳ25 TYPE SPECIES: Tylocephalonyx skinneri ft (7.6 m) above the base of the channel. Coombs, 1979: 10. INCLUDED SPECIES: The type species only. DESCRIPTIVE OSTEOLOGY KNOWN DISTRIBUTION: Early Hemingfor- dian to early Barstovian of western North MANDIBLE AND DENTITION: The mandible America. (UNSM 44800) preserves ϳ24 cm of the DIAGNOSIS: As given in Coombs (1979: 7). horizontal ramus from the angle forward to REMARKS: This report extends the temporal below m1 (®gs. 10, 11). Height of the man- range of the genus reported by Coombs dible below the center of m3 is 67.6 mm, and (1979) to include the early Hemingfordian below the center of m2 is 60.6 mm. Greatest chalicothere hypodigm from the Carpenter mandibular width below m3 is 37.4 mm. The Ranch Formation, Goshen County, Wyo- horizontal ramus appears to rapidly diminish ming. Material listed below generally con- in height from m3 forward, suggesting that forms in anatomical detail to the descriptions the length of the premolar row was some- given by Coombs (1979), but the species is what reduced. 2005 HUNT: DOME-SKULLED CHALICOTHERE 13

Fig. 8. Measured reference section of the Carpenter Ranch Formation at Merycochoerus Butte, Goshen County, Wyoming. The large oreodont Merycochoerus magnus was found at both low and high stratigraphic levels in Carpenter Ranch sediments ®lling a deeply incised early Hemingfordian paleovalley cut into the Brule Formation. Lithic symbols as in ®g. 9. 14 AMERICAN MUSEUM NOVITATES NO. 3486

TABLE 3 Referred Material of the Dome-Skulled Chalicothere Tylocephalonyx from the Carpenter Ranch Formation, Goshen County, Wyoming

The m2±3 are larger and more robust than erupted and essentially unworn. The crown these molars in Moropus elatus, including of m3 is intact and shows an anterior lophid those of large males. Maximum antero-pos- with a low yet distinct small paraconid, a terior length of m3 is 55.5 mm and of m2 is higher protoconid, and an even taller meta- 52.1 mm. Greatest width of the m3 trigonid conid, which is the tallest cusp. The meta- is 29.1 mm and of the m3 talonid, 27.9 mm. conid and metastylid are distinctly separated The combined length of m2±3 is 108.8 mm. cusps. The metastylid is the tallest cusp of The m2 is moderately worn, the m3 fully the posterior lophid, whereas the somewhat 2005 HUNT: DOME-SKULLED CHALICOTHERE 15

Fig. 9. Measured section of the Carpenter Ranch Formation at Deahl Butte at the locality yielding the partial cranial dome (UNSM 44801) and mandible (UNSM 44800) of the chalicothere Tylocephal- onyx (table 3). The dome, mandible, and diaphysis of a femur (UNSM 44802) were found in proximity within a crystalline gravel lens. Here Arikaree gray sands intervene between the incised Carpenter Ranch paleovalley and Brule Formation. Lithic symbols apply to ®gures 7±9. 16 AMERICAN MUSEUM NOVITATES NO. 3486

Fig. 10. Partial right mandible of the chalicothere Tylocephalonyx (UNSM 44800) with m3 and partial m2, labial view, Carpenter Ranch Formation, from the south escarpment of Deahl Butte, Goshen County, Wyoming (locality data provided in table 3). lower hypoconid and entoconid are of about ϳ2 m from the mandible, the cranial remnant equal height. appears to represent only half of the dome, Although only part of an upper molar broken to reveal an apparent sagittal section (UNSM 44808, presumably M3) represents that exposes a massive boxwork of compart- the upper dentition, several characters indi- mentalized sinuses reinforcing the interior of cate its referral to the family and to Schi- the dome (®gs. 12, 13). At the time of its zotheriinae. Coombs (1989: 438) noted that discovery, the boxwork was packed with chalicotheres are distinguished by upper mo- coarse sand and gravel. It likely survived due lars with a complete metaloph with no meta- to rapid burial, presumably after the carcass conule and an incomplete protoloph in which was scavenged, because the bone shows no the paraconule is retained. UNSM 44808 pre- evidence of prolonged exposure to subaerial serves part of the ectoloph from the meso- weathering. It is larger in its dimensions than style forward to the paracone. If the ectoloph any of the previously reported Tylocephalo- is restored, based on the M3 of Moropus, its nyx specimens: height of the dome from base length is estimated at ϳ52 mm. A small pro- to summit is ϳ247 mm; anteroposterior toloph 12 mm in length, typical of schizoth- length at its base is ϳ278 mm, gradually ta- eriine chalicotheres, extends linguad from pering upward so that 10 cm above the base the paracone, and includes the paraconule. this length is 178 mm, and 4.5 cm below the The ectoloph height measured at the unworn apex it is 114 mm. paracone is ϳ34.7 mm, indicating a relative- The orientation of the partial dome is ly tall-crowned molar typical of schizotheri- somewhat uncertain: the presumed posterior ines, and similar to molar crown heights face tapers downward more gradually than (e.g., M3, 38.2 mm) known in Moropus ela- the anterior surface, which descends more tus. The enamel rib found on the labial face abruptly (®g. 12). The external surface is of the ectoloph in schizotheriines such as moderately rugose, more so toward the pos- Moropus elatus is absent in UNSM 44808, terior margin. Small nutrient vascular foram- and thus is similar to Tylocephalonyx, which ina are clustered toward the posterior base, also lacks these ribs. similar to parietal-squamosal vascular foram- CRANIAL DOME: Found at the same level ina found in some mammalian skulls. Com- 2005 HUNT: DOME-SKULLED CHALICOTHERE 17

Fig. 11. Partial right mandible of the chalicothere Tylocephalonyx (UNSM 44800) with m3 and partial m2, lingual view, Carpenter Ranch Formation, from the south escarpment of Deahl Butte, Goshen County, Wyoming. Note distinct m3 metastylid. pact bone 4 cm thick occurs at the posterior that the damaged humerus and femur came base of the dome, interpreted as a remnant from the same scavenged carcass. The form of the skull roof. Thick bone may have con- of the proximal humerus does not differ in tinued forward under the dome but this part any respect from that of M. elatus. The an- of the skull has been lost. At the anterior tero-posterior length of the humeral head in base of the dome the bone transforms from this case is 16.2 cm whereas the same dian open network of thick-walled sinuses to a mension in a large M. elatus from University more normal, ®nely cancellous structure that Quarry is 19.2 cm. parallels the skull surface, suggesting that Coombs (1979: 23) observed that none of one is observing the transition between the the humeri of Tylocephalonyx skinneri re- interior of the dome and the surface of the tains the proximal end, so a comparison with skull in the fronto-parietal region. UNSM 44812 is not possible. Scavengers often damage the proximal humerus when fo- FORELIMB cusing on the concentration of muscle avail- SCAPULA: A scapular fragment (UNSM able in the shoulder region. The proximal hu- 44811) about 14 cm in length that includes merus was also damaged or destroyed in in- the glenoid fossa appears to belong to a chal- dividuals of M. elatus from University icothere. It compares with the glenoid of Mo- Quarry, where evidence of scavenging is ropus elatus and is similar in size to some commonplace in the Agate waterhole bone- individuals from the Agate bonebed. Only bed (Hunt, 1990). ϳ7±8 cm of the glenoid rim are preserved, ULNA: A proximal ulna (UNSM 44810) heavily reinforced by thick bone with abun- preserves the olecranon process, semilunar dant nutrient vascular foramina. notch, and coronoid process, demonstrating HUMERUS: A proximal left humerus the nature of the articulation with the radius. (UNSM 44812), damaged by preburial scav- The proximal ulna was similar in size and enging, was found within 5 m of a distal fe- form to the ulna of M. elatus. Features of M. mur at Childers Butte. The distal femur dis- elatus mentioned by Coombs (1978) are pre- plays a percussion fracture produced by a sent: a prominent anconeal process, deep large entelodont (®g. 14), and it seems likely semilunar notch with strong medial expan- 18 AMERICAN MUSEUM NOVITATES NO. 3486

Fig. 12. Cranial dome of the chalicothere Tylocephalonyx (UNSM 44801), external surface of the dome, Carpenter Ranch Formation, Deahl Butte, Goshen County, Wyoming (presumed anterior face to left, dorsal upward). sion, radius facet distal and at an acute angle Coombs (1979: 23) regarded the radius/ to the semilunar notch and forming a deep ulna of Tylocephalonyx skinneri as little dif- articular fossa. Ulnar width at the level of the ferent from that of M. elatus. She did note coronoid process is ϳ11 cm. The shape of that the lateral extent of the proximal facet the semilunar notch for articulation with the on the ulna for the radial head is slightly humerus and the reduced coronoid process smaller in T. skinneri; despite the damaged demonstrate that the radial head, which ®ts state of UNSM 44810, it seems to show this tightly against the radial notch, was immo- more restricted condition of the proximal bile and did not rotate. Only extension and facet. In both species the radius and ulna ¯exion occurred at the elbow; there was no were incapable of independent movementÐ rotation of the radius around the ulna, an as- locked by fusion of the distal shafts, the form pect of the anatomy of M. elatus previously of the proximal articulation, and by strong noted by Coombs (1978: 25). The ulnar frag- interosseous ligaments. ment exhibits considerable preburial ¯aking METACARPAL: A massive, robust proximal and cracking of the cortical boneÐit must metacarpal 4 (®g. 15, UNSM 44804) was have been scavenged and exposed to weath- found in talus of the Carpenter Ranch For- ering for some time prior to burial. mation at a small satellite butte ϳ0.6 km 2005 HUNT: DOME-SKULLED CHALICOTHERE 19

Fig. 13. Cranial dome of the chalicothere Tylocephalonyx (UNSM 44801), internal surface of the dome showing the well-developed compartmentalized architecture, Carpenter Ranch Formation, Deahl Butte, Goshen County, Wyoming (presumed anterior face to right, dorsal upward). southeast of the site at Deahl Butte where the pears reduced in area relative to the dorsal dome and mandible occurred. It was found facet, and is separated from it by a ϳ1cm above the base of the formation and can only gap. In addition, the volar facet does not ex- have been derived from that rock unit. tend proximad above the dorsal facet as it The articular surfaces and facets for the does in M. elatus but rather is situated di- unciform and for metacarpals 3 and 5 are directly posterior to it. agnostic for various chalicotheres, and it is The greater part of the dorsal surface of worthwhile to describe these in some detail metacarpal 4 (®g. 15B) is for articulation for UNSM 44804. with the unciform. This surface is much wid- The radial side of metacarpal 4 (UNSM er and appears to be ¯atter than those of ei- 44804) bears two articular facets for meta- ther males or females of M. elatus from Uni- carpal 3: a dorsal facet and a volar facet (®g. versity Quarry at Agate National Monument. 15A). In M. elatus the two facets are in con- However, this con®guration may be in¯u- tact, the volar facet placed somewhat more enced by the robust size of UNSM 44804, proximally on the side of the metacarpal, and probably a large male. In larger individuals only slightly smaller in area than the dorsal of M. elatus the articular surface for the un- facet. In UNSM 44804 the volar facet ap- ciform becomes broader and more planar 20 AMERICAN MUSEUM NOVITATES NO. 3486

Fig. 14. Distal chalicothere femur (UNSM 44806) in posterior view, referred to Tylocephalonyx, Carpenter Ranch Formation, Childers Butte, Goshen County, Wyoming. A circular percussion fracture (at the arrow) produced by the premolar of a large entelodont suggests postmortem scavenging of the chalicothere carcass. No other mammal identi®ed in early Hemingfordian faunas can have made these circular bite marks. than seen in small individuals, but not to the from it by a deep channel ϳ7 mm in width. extent evident in UNSM 44804. This could represent a secondary area of con- UNSM 44804 demonstrates that the Car- tact for metacarpal 5. penter Ranch chalicothere had retained a re- Coombs (1979: 11) listed three metacarpal duced metacarpal 5 in the manus as does M. 4s belonging to Tylocephalonyx skinneri, one elatus. On the ulno-volar side of proximal of which was complete (F:AM 54887b). Of metacarpal 4 in M. elatus, Coombs (1978: the two facets (dorsal and volar) for meta- 31) described a facet for the articulation of carpal 3 present on the proximal metacarpal metacarpal 5, located on the ulno-volar angle 4ofM. elatus, T. skinneri entirely lacked the of metacarpal 4 and separated from the prox- volar facet, which Coombs considered an im- imal edge by paired tubercles. This facet was portant distinction. However, UNSM 44804 ``on the distal volar surface of the ulnar tu- displays both a volar and dorsal facet, which bercle''. On two metacarpal 4s of M. elatus are of a different shape and placement than in the UNSM collection from University those seen in M. elatus and T. skinneri. Other Quarry, this facet is present, as well as a sec- subtle differences from the metacarpal 4 of ond articular facet behind it, the two facets UNSM 44804 can be seen in photographs of together creating a modest concavity for the the T. skinneri metacarpal 4 (Coombs, 1979: head of metacarpal 5, which clearly was a 33, ®gs. 16B, C, D), and these suggest that reduced element. UNSM 44804 also shows the manus of the early Hemingfordian Car- a facet for a reduced metacarpal 5, slightly penter Ranch chalicothere was somehow dif- concave, and similarly situated on the distal ferently con®gured relative to the manus of volar surface of the ulnar tubercle. However, the late Hemingfordian T. skinneri. Because the second facet found in M. elatus is ques- the presence of both a dorsal and volar facet tionable in UNSM 44804, in which a very for metacarpal 3 represents a plesiomorphic small, smooth articular(?) surface does occur trait, it is possible that early Hemingfordian directly behind the principal facet, separated Tylocephalonyx possessed both facets, with 2005 HUNT: DOME-SKULLED CHALICOTHERE 21

Fig. 15. Proximal metacarpal 4 (UNSM 44804) referred to (?)Tylocephalonyx, Carpenter Ranch Formation, Goshen County, Wyoming: A, radial face showing the dorsal (d) and volar (v) facets for metacarpal 3; B, proximal articular surface for the unciform (u). Stereopairs. Finest divisions of scale in mm. only the dorsal facet retained in late Hem- latter a much larger individual, probably ingfordian T. skinneri. male. Coombs (1978: 50), on good evidence, Coombs (1978: 53, ®gs. 25C,D) also il- believed P13000 to be a female. One can en- lustrated the metacarpal 4 of Moropus hol- tertain the possibility that the Carpenter landi (FMNH P13000) from near Jay-Em, Ranch metacarpal 4 might represent the ear- Wyoming, presumably from the Upper Har- lier M. hollandi and not the dome-skulled Ty- rison beds. The placement of the facets for locephalonyx. However, the only known articulation of metacarpal 3 shows a closer skull of M. hollandi (FMNH P12094) lacks correspondence to that of UNSM 44804 than any indication of doming, but whether a to M. elatus or T. skinneri; the dorsovolar dome was an exclusively male trait is not length measured across these facets on the known because of the scarcity of intact skulls radial side of metacarpal 4 is ϳ48 mm in M. (Munthe and Coombs, 1979: 85). hollandi and 63.5 mm in UNSM 44804, the UNGUAL PHALANX: A dorsal process of the 22 AMERICAN MUSEUM NOVITATES NO. 3486

Fig. 16. Diaphysis of chalicothere limb bone (UNSM 44825) showing green-bone fracturing produced by scavengers, with broken margins overprinted by preburial abrasion, East Sturdivant Butte, Cow Trail Notch local fauna, Carpenter Ranch Formation, Sioux County, Nebraska. Note marrow cavity packed with granitic gravel. ungual phalanx of a chalicothere (UNSM Another distal left femur and accompanying 44805) was found at the same locality as diaphysis (UNSM 44806) were found in out- UNSM 44804 (proximal metacarpal 4). The crop in 1979, together with a proximal hu- remnant of the process is 4.6 cm in length merus (UNSM 44812), over a distance of ϳ5 and ϳ2.5 cm in width; it includes the apical m at the same stratigraphic level on Childers termination of the process and much of the Butte. Despite damage due to preburial scav- articulation with the intermediate phalanx. Its enging, the distal femur and diaphysis (UNSM robust size and rugosity suggest it is the dor- 44806) can be identi®ed con®dently as chali- sal process of the ungual phalanx of digit II cothere. These pieces show green-bone frac- of the manus. tures, and the distal femur carries a depressed circular percussion fracture produced by the HINDLIMB premolar of a large entelodont (®g. 14). This ϳ FEMUR: Remains of several femora, in- femur had an estimated length of 60 cm, thus cluding a diaphysis of a left femur (UNSM a size similar to the femur of M. elatus. 44802) found in proximity to the dome and Lengths of the femur in male and female M. mandible described above, come from sites elatus have been reported as 65 and 43 cm, at Deahl Butte and adjacent Childers Butte. respectively (Holland and Peterson, 1914:362), A distal right femur (UNSM 44807) found and two intact femora from University Quarry at Childers Butte in November 1977 provid- in the University of Nebraska collections mea- ed the ®rst evidence of the presence of chal- sure ϳ60 and 64 cm in length. icotheres in the Carpenter Ranch Formation. TIBIA: A distal right tibia (UNSM 44809) These femora differ in no evident features from the Duncan Buttes is similar to that of from those of M. elatus, showing the same M. elatus. Coombs (1978: 55) noted that placement and development of the lesser tro- both the tibia and femur of M. hollandi could chanter and conspicuous third trochanter. not be distinguished from these same bones Such bones often display green bone frac- of M. elatus. The tibia of Tylocephalonyx tures and appear to be scavenged (®g. 16). skinneri was also considered by Coombs 2005 HUNT: DOME-SKULLED CHALICOTHERE 23

(1979: 34) to be ``similar in proportions and there. Until discovery of more complete ma- morphology to tibiae of Moropus''. The few terial proves otherwise, I have assumed that differences she cited do not involve the distal chalicothere elements from the Carpenter part of the bone. A ®bula articulated with the Ranch beds belong to a single species. It is distal tibia in these chalicotheres: the distal improbable that any of the chalicothere fos- facet for attachment of the ®bula is present sils from the Carpenter Ranch Formation be- in UNSM 44809 and suggests a ®bula much long to a taxon such as Moropus elatus, since like that of M. elatus (Holland and Peterson, the presence of two such similar chalico- 1914: pl. 71). theres in the same environment seems un- Of some importance is that the distal tibia likely, and because none of the remains preserves the articular surface for the astrag- shows any particular af®nity to the Agate alus and demonstrates rather shallow grooves species. Speci®cally, the length and width of for the astragalar trochlea relative to those of the m2±3 exceed almost all known M. elatus M. elatus (particularly with regard to the me- molars in size, the proportions of the astrag- dial trochlear depression). The width of the alar trochlea differ from Moropus, and astragalar trochlea in M. elatus, based on domed skulls are unknown in M. elatus specimens in the UNSM collection from the males and females. Agate waterhole bonebed, ranges from 58.6 Among the postcranial elements referred to 79.6 mm (N ϭ 14; mean, 69.3 mm). Mea- here to Tylocephalonyx, it is the metacarpal surement across the trochlear depressions in 4 that is the most dif®cult element to include the distal tibia of UNSM 44809, which in that genus; it does not appear to corre- serves as a proxy for the width of the astrag- spond to that bone in T. skinneri but better alar trochlea, indicates that the Carpenter matches metacarpal 4 of Moropus hollandi. Ranch chalicothere had a broad astragalus Nonetheless, because the size and form of with a trochlear width of ϳ92 mm, larger chalicothere postcranial elements from the than that of M. elatus. However, in addition southern buttes suggest the presence of only to this difference in size, it is the relative a single species, and the postcranial elements proportions of these astragalar depressions occur with dental and cranial material attrib- that suggest referral of the Carpenter Ranch utable to a dome-skulled animal, all chali- chalicothere to Tylocephalonyx. Coombs cothere elements from the Carpenter Ranch (1979: 34, ®g. 19A) observed that the tibial beds are tentatively attributed here to Tylo- and ®bular parts of the astragalar trochlea in cephalonyx, while recognizing the possibility T. skinneri are of about equal width, whereas of the presence of a second species similar in M. elatus the tibial part of the trochlea is to or the same as M. hollandi. narrower, occupying less than half the troch- At the time of the initial discovery of lear width (Coombs, 1978: 33, ®gs. 14C,D). dome-skulled chalicotheres, Munthe and In UNSM 44809 the trochlear width of the Coombs (1979) did not describe the internal tibial side is 45 mm, and the ®bular side 47 architecture of the dome in detail due to the mm, which is more similar to Tylocephalo- existence of only three skulls. Although ra- nyx. These same measurements taken from a diographic examination of the two complete tibia of M. elatus from University Quarry are skulls provided no additional information, 31 mm and 40 mm, respectively. cranial fractures did allow construction of transverse sections of one of the skulls, in- DISCUSSION cluding a schematic pro®le through the dome. They reported ``bony partitions con- The tall-crowned molar fragment lacking necting the dome to the roof of the braincase an enamel rib on the ectoloph, the large and which divided this cavity into a series of ir- robust m2±3, a distal tibia, and the cranial regularly shaped sinuses'' (Munthe and dome all suggest identi®cation of the mate- Coombs, 1979: 82). rial as the schizotheriine Tylocephalonyx. The partial dome of the Carpenter Ranch The partial femora, humerus, ulna, and other chalicothere (UNSM 44801) preserves a fragments do not in themselves provide a re- boxwork of compartmentalized sinuses ex- liable generic identi®cation for this chalico- tending from the summit to the base of the 24 AMERICAN MUSEUM NOVITATES NO. 3486 dome (®g. 13), which conforms to the pre- sagittal plane through the hollow upper por- liminary observations of Munthe and tion. One can conclude that the dome prob- Coombs (1979: 82, ®g. 6). In domed skulls ably had a hollow dorsal cavity, bordered on from the Split Rock Formation (Wyoming) its margins by strong sinusoidal bone rein- and from Greenside Quarry (Nebraska), they forcing the walls throughout, and with a net- found evidence of a similar internal osseous work of sinus compartments continuous boxwork, a remnant of which can be seen in across the basal part of the dome above the their photograph of the Greenside Quarry braincase. skull (Munthe and Coombs, 1979: ®g. 3; In their evaluation of the functional role of note bony septa immediately above the roof the dome, Munthe and Coombs (1979) con- of the braincase). However, they were not sidered several plausible hypotheses. Al- able to determine the exact location and ori- though recognizing that information at their entation of the cranial sinuses from the avail- disposal was limited, they suggested that the able skulls. Thus, UNSM 44801 makes pos- dome may have been involved in acoustic sible for the ®rst time an examination of the signaling, in visual displays, or for low-im- internal architecture of the dome. pact butting (or some combination of these UNSM 44801 shows that the interior of behaviors). UNSM 44801 fortunately sup- the dome was entirely supported by the si- plies evidence, lacking in the Munthe and nusoidal boxwork from the base to the sum- Coombs study, that establishes the thickness, mit (®gs. 13, 17). The sinuses are nearly in- orientation, and distribution of the dome's intact in the upper half of the dome, an area of ternal bony partitions. The pronounced thick- ϳ17 by 14 cm seen in parasagittal section: ness of the walls of the sinus compartments about 40 sinus compartments occupy the in- and their distribution within the dome indi- terior of the dome as preserved, with indicate a well-reinforced architecture. Robust vidual diameters ranging from ϳ9mm to 3 bony struts bridge the area above the brain- cm (®g. 17A). The diameters of 20 of these case, supporting the walls of the dome from compartments measured at random in the up- base to apex. Only the upper part of the per half of the dome averaged 16 mm. Sim- dome appears to have been hollow. One can ilarly, the thickness of the bony partitions conclude that the dome was reinforced, du- (septa) separating the compartments aver- rable, and could have withstood fairly strong aged 3.3 mm with a range from 1.6 to 6.0 external impacts. mm. In the upper part of the dome the thick- After carefully considering a variety of ness of the bone forming the roof ranges possible hypotheses, Munthe and Coombs from thin (3.2 mm) to very thick (24 mm). (1979: 88) focused their attention on low-im- Of considerable interest is that the inner mar- pact ¯ank-butting behavior in the living gi- gins of almost all septa in the upper half of raffe (Giraffa camelopardalis). The skull of the dome are smooth, in life apparently cov- a male giraffe, which they examined in sag- ered by membrane, indicating that one is ob- ittal section, contained a cranial sinus dorsal serving the intact, unbroken inner edges of to the braincase, partitioned by bony septa, individual compartments. However, compart- that they judged similar in form and size to ments in the lower part of the dome exhibit that of the dome-skulled chalicothere. Con- broken margins, suggesting that they were sidering that the internal septa of the Car- continuous across the base of the dome, and penter Ranch dome are thicker and more ro- were broken apart at the time the skull was bust than those of the giraffe, low-impact damaged, presumably by scavengers. butting cannot be ruled out. However, other Thus, the interior of the dome, at least in functions attributed to the dome seem equal- its upper part, must have been an open air- ly plausible: as a male visual display or (if ®lled space, yet strongly reinforced along the both sexes possessed the domed skull) in sides by bone. This morphology provides a species recognition. In addition, the air si- plausible explanation for the fact that only nuses within the dome also might have half of the dome was found, for if scavenged served an acoustic role similar to that attrib- or otherwise damaged, the dome might tend utable to the expanded cranial sinuses in the to split apart into two halves along a mid- living African and Asian elephants (Loxo- 2005 HUNT: DOME-SKULLED CHALICOTHERE 25

Fig. 17. Detail of the internal compartmentalized architecture of the cranial dome of Tylocephalonyx (UNSM 44801), Deahl Butte, Carpenter Ranch Formation, Goshen County, Wyoming. A, Enlarged view of the sinus compartments showing the smooth unbroken edges of septa in the central part of the dome; B, Stereopair of the dome interior shown in A. Finest divisions of scale in B in mm. 26 AMERICAN MUSEUM NOVITATES NO. 3486 donta africana, Elephas maximus). Elephants frontal sinus but that these spaces were non- are known to employ low-frequency sound olfactory. However, the horse, tapir, and rhi- in communication over long distances (Payne noceros all have large sphenoidal sinuses in- et al., 1986; Poole et al., 1988). Low-fre- vaded by olfactory tissue. In his survey of quency sound perception is enhanced when paranasal sinuses, Negus (1958) remarked on middle ear volume is expanded so that the the presence in many ungulates of ``supra- ossicular vibrations within the middle ear are cranial cavities'' in the frontal, parietal, and not damped by an air space of small volume. occipital areas of the skull that did not con- The living African elephant possesses a large tain ethmoturbinals but that communicated foramen in the exoccipital bone forming a with the nasal region. Based on the limited passageway between its middle ear and the anatomical evidence, invasion of the chali- voluminous cranial sinuses in its skull. This cothere dome by olfactory ethmoturbinals communication between middle ear and cra- seems unlikely, but it is possible that a pas- nial air sinuses would prevent ossicular sage from the dome into the nasal cavity ex- damping and favor the perception of low fre- isted. Such a connection between the frontal quency sound. Possibly a similar passageway sinus and nasal cavity occurs in many un- existed in dome-skulled chalicotheres be- gulates (Sisson and Grossman, 1953; Negus, tween their middle ear and the sinuses in the 1958: ®g. 156). The broad enlarged snout of dome, but if so, domes would be expected in chalicotheres suggests that the ethmoturbi- both sexes. Only dissection or noninvasive nals may have been suf®ciently well devel- CT (computerized tomography) scanning of oped within the nasal fossa such that no need a complete skull can demonstrate the pres- existed for ethmoturbinal invasion of the cra- ence of such an anatomical arrangement in nial dome. these large perissodactyls. Evidence reported above demonstrates that Another rationale for the dome lies in the large entelodonts scavenged the remains of enhancement of olfaction. Negus (1958) not- these chalicotheres (only entelodont teeth ed that to obtain greater room for ethmotur- match bite marks in these bones). It is sig- binals, hence improved olfaction, extension ni®cant that no complete chalicothere limb into surrounding bones and the development bones, despite their large size and durability, of spaces dorsal to the braincase were viable survived intact in the sands and gravels of strategies. Extension of the ethmoturbinals this ¯uvial system. All limb bones have been covered with olfactory mucosa often occurs broken, and several show green-bone frac- in mammals by invasion of the frontal and tures that would have required considerable sphenoid paranasal sinuses. In some Carniv- force to produce (®g. 16). The record of en- ora, such as felids, the ethmoturbinals invade telodont tooth marks on chalicothere limb newly created spaces within the frontal and bones has now been documented at three lo- nasal bones, which Negus (1958: 300) con- calities of early Miocene age in the Great sidered as ``the construction of a superim- Plains: (1) the late Arikareean waterhole posed upper story'' added to the dorsal sur- bone bed at Carnegie Hill, Agate Fossil Beds face of the skull. The extension of ethmotur- National Monument, Sioux County, Nebras- binals into paranasal sinuses clearly serves ka (Hunt, 1990: ®g. 25); (2) the latest Ari- an olfactory role because olfactory nerves kareean Lay Ranch paleovalley, Anderson supply the invading turbinals. Ranch Formation at Spoon Butte, Goshen Most such olfactory extensions occur in County, Wyoming (®g. 18); and (3) early living Carnivora but it is noteworthy that Hemingfordian ¯uvial sediments of the Car- among living perissodactyls the tapir has penter Ranch Formation, Goshen County, evolved olfactory penetration into both the Wyoming (®g. 14). nasal and frontal areas of the frontal sinus, Coombs (1979: 5) visualized Tylocephal- and the horse has a very small olfactory ex- onyx as a browsing ungulate of the temperate tension into the nasal (not frontal) part. Ne- forests of northwestern North America with gus (1958: ®g. 147) reported that a rhinoc- ``a signi®cant distribution onto the Great eros (species not indicated) retained very Plains, perhaps associated with riparian com- large nasal and frontal parts of an enlarged munities''. The presence of a dome-skulled 2005 HUNT: DOME-SKULLED CHALICOTHERE 27

Fig. 18. Chalicothere left distal humerus (UNSM 44826) scavenged by a large entelodont, showing bite marks on both (A) anterior and (B) posterior surfaces of the bone, Lay Ranch beds (a paleovalley ®ll within the Anderson Ranch Formation, latest Arikareean), east side of Spoon Butte, 500 ft west of the Wyoming-Nebraska state boundary, Goshen County, Wyoming. Arrows mark the principal percussion fractures. 28 AMERICAN MUSEUM NOVITATES NO. 3486 chalicothere apparently referable to Tyloce- Agate post of®ce in central Sioux County phalonyx in association with abundant silic- where these pale orange-brown sandstones i®ed wood in ¯uvial sediments of a major yield a fauna of earliest Hemingfordian as- early Miocene paleovalley in the central pect. A key index taxon in this assemblage Great Plains, supports her hypothesis. Chal- is the large oreodont Merycochoerus mag- icotheres in the Miocene of the Great Plains nus, which occurs both in the Northeast of are commonly found in ¯uvial and waterhole Agate local fauna and in the Carpenter Ranch settings, where they have been regarded as mammal assemblage. water-dependent mammals in recent ®eld These earliest Hemingfordian faunas in studies (Hunt, 1990). western North America are considered to fall within the 18.2±18.8 Ma interval, hence old- AGE OF THE DOME-SKULLED er than any of the previously reported occur- CHALICOTHERE rences of dome-skulled chalicotheres. Here I summarize mammals contributing to Coombs (1979) and Munthe and Coombs the age assessment of the Carpenter Ranch (1979) attributed all known fossils of the beds: the oreodont Merycochoerus, a dromo- dome-skulled Tylocephalonyx to the mid- merycid Aletomeryx, the moschid Pseudob- Miocene, that is, to the late Hemingfordian lastomeryx, the small canid Phlaocyon, and and early Barstovian land mammal ``ages''. the amphicyonid carnivore Daphoenodon. They believed that none of these remains was ϳ older than 17 Ma. The fauna associated ORDER ARTIODACTYLA with the dome-skulled chalicothere in the Carpenter Ranch Formation is of early Hem- FAMILY OREODONTIDAE ingfordian age, and is representative of the Merycochoerus magnus earliest assemblage of Hemingfordian mam- Figure 19 mals yet identi®ed in the Great Plains. Mam- malian faunas of similar age are now known Remains of this large oreodont were found from western Nebraska (Northeast of Agate both near the base and at the uppermost lev- local fauna, MacFadden and Hunt, 1998; els of the Carpenter Ranch beds at Mery- Tedford et al., 1987) and northeastern Colo- cochoerus Butte (®g. 8), and they demon- rado (Martin Canyon local fauna, Tedford, strate that nearly the entire paleovalley ®ll at 1999), and from less well-known assemblag- that locality is within the temporal range of es from near Wheatland in southeastern Wy- this species. A right mandible (UNSM oming (Hunt, unpublished observations), and 44813) was found within a meter of the basal from the Troublesome Formation in north- contact of the Carpenter Ranch beds with the central Colorado (Izett, 1968). These strata underlying White River sediments, establish- record a regional tectonic/climatic event ing the age of the earliest ¯uvial deposits in identi®ed by the ®rst appearance of Heming- the paleovalley at this location. An edentu- fordian faunas in the Great Plains and adja- lous maxilla and associated fragmentary cent Rocky Mountain basins. At this time an postcranial bones (UNSM 44814) occurred in¯ux of coarse sands and granitic gravels 67 ft higher in the section, 14 ft below the indicates increased competence of streams top of the butte. This was the only locale in bringing Rocky Mountain detritus from the the southern buttes in which Merycochoerus uplifts eastward onto the Great Plains. occurred. The Northeast of Agate local fauna has The stage of evolution of the three species been paleomagnetically calibrated and is of Merycochoerus can be estimated by the considered to fall in Chron C5E of the Mag- degree of fusion of the premaxillae, which is netic Polarity Time Scale (Berggren et al., an index of the degree of nasal retraction de- 1995), dated at ϳ18.2 to 18.8 Ma. This local veloped in this oreodont lineage over time, fauna comes from the stratigraphically low- plotted against a measure of the size of M3 est exposures of the Runningwater Forma- (Hunt and Stepleton, 2004: ®g. 18). The ear- tion in western Nebraska (MacFadden and liest species, M. matthewi, precedes the larg- Hunt, 1998), best developed northeast of the er M. magnus, which is followed by the ter- 2005 HUNT: DOME-SKULLED CHALICOTHERE 29

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. minal species, M. proprius. This species pro- gravels of the Carpenter Ranch beds at East gression underwent a shortening of the skull, Sturdivant Butte in close proximity to the nasal retraction likely indicating the devel- maxilla of the small canid Phlaocyon (®g. 7). opment of a ¯eshy proboscis, and an increase The dp4 and m1 are most similar to these in molar dimensions. teeth in Aletomeryx specimens from the Run- When compared with representative sam- ningwater Formation in western Nebraska. ples of these three species, the mandible from From their ®rst occurrence in the early Merycochoerus Butte compares most closely Hemingfordian of North America, species of with those of M. magnus from the lowermost Aletomeryx gradually increase in size, even- Runningwater Formation northeast of Agate tually evolving into the larger descendant ge- post of®ce in Sioux County, Nebraska. The nus Sinclairomeryx in the late Hemingfordi- maxilla from the higher level on the butte is an. This size increase is documented by man- also similar to maxillae of M. magnus, yet dibular teeth such as dp4 and m3. Also, over appears somewhat more gracile. Because it lacks teeth, and its degree of nasal retraction this interval the premolar row (p2±4) short- can only be estimated, it is less certainly re- ens relative to the length of the molars (m1± ferred to that species. This oreodont suggests 3). an earliest Hemingfordian age, equivalent to Table 4 shows that the dp4 dimensions of the fauna from the lowermost Runningwater the Carpenter Ranch specimen fall within the Formation northeast of Agate, Nebraska. lower part of the range of a population of juvenile Aletomeryx found in Runningwater ORDER ARTIODACTYLA Quarry (UNSM locality Bx-58). Similarly, FAMILY DROMOMERYCIDAE m1 measurements fall almost exactly at the Aletomeryx gracilis mean values for this same population sample Figure 20 (table 4). This quarry occurs in the lower- A right mandible with dp4 and m1 most part of the Runningwater Formation in (UNSM 44820) was found in sands and the type section, located in the northwest cor- 30 AMERICAN MUSEUM NOVITATES NO. 3486

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. ner of Box Butte County, Nebraska. UNSM these quarries, UNSM Hemingford Quarry localities south of the Niobrara River in Box 7B, over 30 juvenile Aletomeryx provide an Butte County, collectively termed the Hem- adequate sample of dp4 for comparison with ingford Quarries, sample the upper part of the Runningwater Quarry population (table the Runningwater Formation. From one of 5). The Quarry 7B sample shows a signi®- cantly larger dp4 than the population from Runningwater Quarry, and it is evident that TABLE 4 the dp4 from the Carpenter Ranch beds falls Dimensions of dp4 and m1 (in mm) of Juvenile Aletomeryx entirely outside the range of the Quarry 7B [from Runningwater Quarry (UNSM Locality Bx-58), Box sample. Considering this evidence, an age Butte Co., Nebraska, and from the Northeast of Agate local coeval with the upper part of the Running- fauna, Sioux Co., Nebraska, compared to the juvenile Ale- water Formation is unlikely. tomeryx from East Sturdivant Butte] Other than at Runningwater Quarry, fossiliferous strata of the lower Runningwater Formation containing Aletomeryx occur northeast of Agate post of®ce in Sioux Coun- ty, 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). Speci- mens found northeast of Agate were identi- ®ed as Aletomeryx marshi by Frick (1937: 160); however, they are not distinguishable from the Runningwater Quarry sample on present criteriaÐadditional 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 Aleto- meryx marslandensis Frick 1937 for the specimens from upper Runningwater strata. 2005 HUNT: DOME-SKULLED CHALICOTHERE 31

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 Aletome- ORDER ARTIODACTYLA ryx might be present in strata of late Arika- FAMILY MOSCHIDAE reean age; no con®rmed Arikareean occurrences are known at this time. The associa- Pseudoblastomeryx sp., cf. P. schultzi tion of Aletomeryx with early Hemingfordian Figure 21 species such as Merycochoerus magnus in A partial maxilla with M1±3 of a small the Carpenter Ranch fauna indicates that the moschid (UNSM 44821) was found in the East Sturdivant Butte occurrence of the small Carpenter Ranch beds at East Sturdivant dromomerycid is not likely to be of late Ari- Butte (®g. 7) ϳ18 ft above the specimens of kareean age. An earliest Hemingfordian age Aletomeryx and Phlaocyon. Although the la- is supported here for the Carpenter Ranch Al- bial aspect of these molars was damaged, the etomeryx. molar row (length ϳ23.4 mm) corresponds

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. Ster- eopair. Scale bar, 1 cm. 32 AMERICAN MUSEUM NOVITATES NO. 3486

in its dimensions and form of the teeth to ORDER CARNIVORA several of the small late Arikareean and early Hemingfordian blastomerycine moschids. FAMILY CANIDAE These small moschids require taxonomic re- Phlaocyon leucosteus vision and so a speci®c identi®cation is un- Figure 23 certain; however, the molars show close cor- A left maxilla with P3-M2 (UNSM 44822) respondence to those of Pseudoblastomeryx belonging to a small canid is attributable to schultzi from the Bridgeport Quarries, Mor- Phlaocyon, a hypocarnivorous borophagine rill County, Nebraska, of early Hemingfor- lineage noted for the addition of accessory dian age. UNSM 44821 is tentatively re- cusps to its P4 and molars. It was found in ferred to that species pending discovery of a sand-and-gravel lens at East Sturdivant more complete material. Butte at the same stratigraphic level as Ale- tomeryx (UNSM 44820, ®g. 7). The Carpen- ORDER CARNIVORA ter Ranch maxilla belongs to a continuum of FAMILY AMPHICYONIDAE species whose members progressively increase in size, beginning in the late Arika- Daphoenodon, advanced species reean with Phlaocyon annectens (Agate Na- Figure 22 tional Monument, Quarry 3 carnivore dens, A partial left mandible with m1±3 and a Peterson, 1907: 53); continuing through damaged p4 (UNSM 44815) can be referred Phlaocyon leucosteus from the lower Run- to an advanced species of the beardog Da- ningwater Formation; and terminating in the phoenodon. It was discovered in an indurated large, robust species, P. marslandensis from ¯uvial sandstone at Merycochoerus Butte, 95 the Hemingford Quarries of the upper Run- ft above the base of the Carpenter Ranch ningwater Formation, Nebraska. Table 7 pre- beds (®g. 8). Coarse, granitic gravel lenses sents measurements of the upper carnassial occurred above and below the mandible, and molars of these species taken from in- which was found 18 ft below the maxilla of dividual specimens, supplemented by mea- Merycochoerus (UNSM 44814). surements recently provided by Wang et al. Although the molars are heavily worn, (1999). Inspection of these data indicates that their form, size, and the squared posterior UNSM 44822 is best referred to Phlaocyon border of p4 establish its identity as Da- leucosteus of early Hemingfordian age, coe- phoenodon. Daphoenodon evolved from the val with faunas from the lower part of the smaller late Arikareean D. superbus into a Runningwater Formation of Nebraska. much larger early Hemingfordian species BIOSTRATIGRAPHIC SUMMARY: Taken to- that occurs in the Runningwater Formation. gether, the age-diagnostic genera from the This increase in size is re¯ected in larger, Carpenter Ranch sands and gravels indicate more robust skulls, teeth, and postcrania. The an early Miocene age (table 8). The stage of molars and p4 establish that UNSM 44815 evolution of the species, however, is earliest was a carnivore larger than D. superbus from Hemingfordian, age-equivalent or possibly the late Arikareean of western Nebraska, yet slightly older than the mammalian fauna smaller in molar dimensions than the large from the lower part of the Runningwater For- terminal species of Daphoenodon from the mation northeast of Agate post of®ce (North- upper Runningwater beds of Box Butte east of Agate local fauna, MacFadden and County and the Bridgeport Quarries in Mor- Hunt, 1998). rill County (table 6). Although Daphoenodon is represented by only a few individuals in THE ``ARIKAREE'' CONGLOMERATES the lower Runningwater Formation, includ- OF N. H. DARTON (1899) ing one from the exposures northeast of Ag- The ¯uvial gravels and sands here referred ate, they compare most closely with the Car- to the Carpenter Ranch Formation and the penter Ranch mandible, and suggest an early previously established Spoon Butte beds Hemingfordian age corresponding to the age were recognized in the earliest geologic re- of the Northeast of Agate local fauna. connaissance studies in southeastern Wyo- 2005 HUNT: DOME-SKULLED CHALICOTHERE 33

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. ming and western Nebraska conducted by the Ridge are composed primarily of stream- U.S. Geological Survey (Darton, 1899; Ad- rounded sandstone concretions cemented by ams, 1902). A summary of this work on the calcite, mixed with granitic sand and gravel. butte caprocks will supply historical back- Based on faunal content, they were thought ground necessary to understanding UNSM to represent the channel deposits of a late ®eld efforts in the region. Arikareean ¯uvial system later mapped by In N.H. Darton's (1899) classic study of Vondra (Vondra et al., 1969), who referred the geology of western Nebraska, the Ogal- them to the Harrison Formation. Darton lala and Arikaree Formations were identi®ed, (1899: 746, ®g. 212) illustrated the strati- named, and mapped for the ®rst time, and graphic relationships of these Arikaree con- conspicuous conglomerate beds were recog- glomerates in suf®cient detail that they can nized in both formations. Darton noted that be readily identi®ed today south of Gering, ``the basal Ogallala beds are mainly con- Nebraska, along the Wildcat Ridge, where glomeratic'' and illustrated this with mea- they contain the UNSM late Arikareean sured sections in the southern Nebraska pan- mammal locality Sf-105. handle (Darton, 1899: 744, pls. 87±88). Darton (1899: 747) reported that the ``Ari- Ogallala conglomerates were recognized as karee'' conglomerates in the vicinity of Spoon crystalline gravels derived from the Rocky Butte differed from those along the Wildcat Mountains to the west. In contrast, the Ari- Ridge in their chie¯y crystalline gravel com- karee conglomerates were lithically hetero- position. We can be certain of the location and geneous, and were restricted to the Wildcat identity of these conglomerates because he Ridge, south of Scottsbluff, Nebraska, and published a stratigraphic pro®le from Spoon the vicinity of Spoon Butte in Wyoming Butte south to Sturdivant Gap (redrafted as (Darton, 1899: 735). ®g. 5), showing the conglomerates overlying At Wildcat Ridge, Darton did not consider Arikaree beds on the north and White River the conglomerates as the earliest Arikaree de- Brule clay to the south (Darton, 1899: ®g. posits, reporting that they occurred above the 213, pl. 85[1]). His pro®le shows the con- base of the Arikaree within the formation. glomerates as descending in elevation south- The Arikaree conglomerates at Wildcat ward, deposited on a ``steeply sloping sur- 34 AMERICAN MUSEUM NOVITATES NO. 3486

TABLE 6 Dental Dimensions (in mm) of the Early Miocene Amphicyonid Daphoenodon from Western Nebraska

Fig. 23. Left maxilla of the canid Phlaocyon leucosteus (UNSM 44822) with P3-M2, Cow Trail Notch local fauna, Carpenter Ranch Formation, East Sturdivant Butte, Sioux County, Nebraska. Ster- eopair. Scale bar, 1 cm. 2005 HUNT: DOME-SKULLED CHALICOTHERE 35

TABLE 7 Dental Dimensions (in mm) of the Borophagine Canid Phlaocyon

face'', which led Darton to regard them as ers and streams. The ability to drill deep wells strata within a single paleovalley of consid- far from these streams, permitting the use of erable width. He was aware that nowhere in windmills and irrigation, led to increasing hu- the Spoon Butte area were these capping con- man settlement and a growing interest in a glomerates overlain by Arikaree sediments (as geologic survey of water resources. they were at Wildcat Ridge), but this did not Although Darton (1899) mentioned Spoon seem to in¯uence his belief that they belonged Butte and the adjacent buttes to the south, to the Arikaree Formation. Adams (1902: pl. 3) was the ®rst to map and George Adams (1902) extended Darton's clearly illustrate the topographic trends of the geological mapping into southeastern Wyo- northern and southern buttes. He published ming, summarized in U.S. Geological Survey the ®rst and, to date, the only photograph of Water-Supply Paper No.70 on the geology the Carpenter Ranch beds, shown capping and water resources of the Patrick and Goshen Deahl Butte (Adams, 1902: pl. VIa, Point of Hole quadrangles. That region at the time of Rocks). Adams con®rmed the ``Arikaree'' Adams' work was undergoing accelerated de- conglomerates as ¯uvial deposits, and he ex- velopment for the raising of livestock, and plained their occurrence at various topo- ranching operations were rapidly invading graphic levels as explicit evidence of stream semiarid grasslands distant from the local riv- aggradation, in which ``the sediments en- 36 AMERICAN MUSEUM NOVITATES NO. 3486

TABLE 8 Mammalian Fauna of the Carpenter Ranch Formation, Goshen County, Wyoming, and Sioux County, Nebraska

croached upon the higher levels''. His chosen O.A. Peterson (1907: 23) of the Carnegie example was taken from Sturdivant Gap Museum of Natural History (Pittsburgh) was north to Spoon Butte (Adams, 1902: 17): the ®rst paleontologist to report an attempt The relation of these [conglomeratic] lenses is best to ®nd fossils in the caprock of Spoon Butte. seen in the locality of Spoon Butte, in the Patrick He described the caprock as ``a hard pinkish- quadrangle, and for a short distance to the south. In gray sandstone, 35±50 ft in thickness. In this the escarpment and lone hills north of Sturdivant's hard cap of sandstone, which is regarded as ranch, conglomerates [Carpenter Ranch Formation] are seen resting on Brule clay, or separated from it the top of the Upper Harrison beds, our party by a very thin bed of Arikaree sand. In Spoon Butte found no fossils''. Peterson failed to ®nd fos- the lenses are coarse sandstones exhibiting a cross- sil mammals because the caprock itself is al- bedded structure, and their occurrence is 400 ft above most entirely barren of mammalian remains; the lowest exposed portion of the Arikaree. In the intermediate localities the lenses occur at somewhat only in the basal sands and gravels of the lower horizons and seem to justify perfectly the in- Spoon Butte beds at a few select localities terpretation [of progressively aggrading ¯uvial de- do any mammalian fossils occur, and Peter- posits] here placed upon them. son must have bypassed these sites. His al- If Adams had known of the fossil content of location of the caprock to the Upper Harrison these conglomerates capping the buttes, he beds, today considered a unit of the Arikaree might have realized that separate ¯uvial sys- Group, suggests that he was aware of Ad- tems were indicated, but paleontologists who ams' description of the Spoon Butte caprock visited the buttes at about the same time as an Arikaree conglomerate. failed to note any fossil occurrences of sig- Later, Peterson (1909: 74±75, ®g. 27) re- ni®cance. considered this opinion, suggesting that the 2005 HUNT: DOME-SKULLED CHALICOTHERE 37 age of the caprock might be as young as Pli- that late Miocene beds were once present in ocene, and again remarked on the absence of the region and were subsequently removed fossils. It was at this time that he named the by erosion. These acid volcanic±bearing caprock at Spoon Butte the ``Spoon Butte gravels derive from mid- and late Miocene beds''. At about the same time, the paleon- deposits once present on the Hartville table- tologist H.F. Osborn (1909: 73, ®g. 14), in a land that were reworked during the Pleisto- survey of Cenozoic mammal horizons of cene and Holocene into stream terraces and western North America, included the caprock low hills that correspond to the present local at Spoon Butte in the broadly construed Ari- topography. karee Formation of Darton, but apparently he The Spoon Butte caprock continued to be was unaware of Peterson's suggestion of a mistakenly regarded as Pliocene or late Mio- much younger age. cene in age for the next two decades. Denson The only published claim of anyone hav- (1974) mapped the caprock of Spoon Butte ing found fossils at Spoon Butte occurred in as Ogallala Formation, at that time consid- 1956 when Paul O. McGrew of the Univer- ered to be Pliocene or upper Miocene ``poor- sity of Wyoming (in L. McGrew, 1963; also ly cemented calcareous claystone, siltstone, Rapp et al., 1957) collected ``waterworn sandstone, and conglomerate of ¯uviatile or- fragments of Pliocene(?) horse teeth . . . igin''. Whereas this quote adequately de- found in gravels on top of Spoon Butte and scribes the lithologic character of typical other hills in T.27N., R.60W''. This state- Ogallala strata to the south, the Spoon Butte ment appears to be the basis for later assign- beds are characterized by silica-cemented ment of the Spoon Butte caprock to the Pli- sandstones and basal granitic/chert gravel ocene or late Miocene. A loose lag gravel and are thus lithically distinct from the Ogal- does occur at a few localities on top of Spoon lala strata; evidently the Spoon Butte beds Butte, and it contains stream-rounded and were not actually visited during the mapping. abrasion-resistant Miocene mammal fossils, Crist (1977), in a hydrologic study of the including fragments of equid teeth. However, area, followed Denson in showing the Spoon this lag gravel is a much younger deposit Butte caprock as Ogallala Formation of late than the basal Spoon Butte gravel, and it con- Miocene age. tains more diverse and exotic clasts, partic- Love et al. (1980), in their compilation of ularly acid volcanics (rhyolite) never found the geology of the Torrington Quadrangle in in the Spoon Butte sandstone caprock or in southeastern Wyoming, indicated that the the basal Spoon Butte gravel. capping sandstone of Spoon Butte and the In fact, the lag gravel on Spoon Butte has adjacent buttes of the northern butte line, the a complex history and is similar in compo- caprocks of the southern butte line (Carpen- sition to loose ¯uvial lag gravels mantling ter Ranch beds), as well as similar sandstone low hills, ridges, and stream terraces at much butte caprocks east of and within the Hart- lower topographic elevations in the valleys ville Uplift (Flattop, Sheep Mountain) were adjacent to Spoon Butte (®g. 3, Quaternary all of late Miocene age. They noted Denson's terraces). These granitic gravels contain con- mapping of some of these as Ogallala For- spicuous amounts of both acid volcanics and mation. No mention was made of the uni- chert in a matrix of loose quartz sand (table form, silica-cemented sandstone lithologic 1, Oberg gravels). Angular blocks of the composition typical of these butte caprocks. Spoon Butte caprock contained in some of It was only when mammalian fossils from these gravels indicate that they postdate the Spoon Butte and the adjacent buttes to the Spoon Butte beds, a fact con®rmed by teeth south became available to UNSM paleontol- and bone fragments of late Miocene mam- ogists from 1977±1980 that their divergent mals in the gravel. On the George Oberg ages became apparent. ranch, immediately east of the butte, a high Today we recognize that the northern cap- terrace gravel deposit of this type (®gs. 2±4, rocks on Spoon Butte and adjacent buttes of Oberg Quarries) yielded a single tooth of the northern trend are Barstovian (mid-Mio- Pleistocene Equus, found with water-worn cene) in age, and the caprocks of the south- bones of Clarendonian mammals, indicating ern buttes are early Hemingfordian (early 38 AMERICAN MUSEUM NOVITATES NO. 3486

Miocene). These areas represent two unre- land carries a granitic/amphibolite/anortho- lated paleovalley systems separated by a sig- site clast content most similar to that of the ni®cant interval of time. Late Miocene and Carpenter Ranch Formation. younger sediments were deposited in the re- Mapping of Precambrian rocks of the gion east of the Hartville Uplift, and then southern Hartville Uplift (Millgate, 1965; nearly entirely removed by later erosion, so Snyder, 1980; Sims et al., 1997; Sims and that the geographic distribution of these stra- Day, 1999) identi®ed a granite dome (Hay- ta is dif®cult to reconstruct. stack Range) in contact with quartzofeld- We now realize that silica-cemented cap- spathic schists, metadiabase/amphibolite ping sandstones of the numerous buttes east dikes, and pegmatite east of the north-trend- of, and within, the Hartville Uplift are of di- ing Hartville Fault. Metadolomite, metacon- verse ages. Nevertheless, insofar as is glomerate, schist, phyllite, maroon quartzite, known, all fall within the Miocene. These and banded ironstone of the Precambrian caprocks record multiple episodic intervals Whalen Group occur west of the fault. Gran- of ¯uvial deposition, during which crystal- ite, gneiss and amphibolite in the Haystack line sands and gravels derived from the Lar- Range are similar to Carpenter Ranch gravel, amie Mountains and Hartville Uplift were which lacks metasediments, despite proxim- transported eastward to the bordering plains. ity of the Whalen Group only 20 miles (ϳ32 The provenance of the Carpenter Ranch km) west of the Carpenter Ranch Formation. gravels is of particular interest because, to- Arikaree sediments buried much of the Hart- gether with the gravels of the Runningwater ville Uplift in the early Miocene and it re- Formation, the clast types indicate derivation mains an open question as to what extent the from either the Precambrian core of the southern part of the uplift was accessible to north-central Laramie Mountains or possibly these streams. from the southern Hartville Uplift. Archean Carpenter Ranch and Runningwater grav- granitic and gneissic rocks cut by numerous els are granitic in composition, with amphib- amphibolite dikes form the greater part of the olite (metabasalt, metagabbro) common, northern and central Laramie Mountains gneiss, chert and quartzite present, and an- (Condie, 1969; Sims and Day, 1999; Sims orthosite rare. Based on clast content, the and Stein, 2003). To the south, these Archean provenance for these early Miocene gravels rocks are bordered by the Proterozoic Lara- would seem to be the north-central Laramie mie anorthosite body. Mountains. The Haystack Range possibly Archean granites of the northern Laramie contributed during the Carpenter Ranch ¯u- Mountains are separated from an orthogneiss vial cycle, but lack of metasediments in the terrain to the south by the Laramie Peak gravels favors the Laramie Mountains as the shear zone (Condie, 1969; Chamberlain et al, principal source. Scarcity of anorthosite 1993; Resor 1996; Sims and Day, 1999), also clasts suggests that the streams originated in termed the Garret-Fletcher Park-Cottonwood granitic terrain north of the Laramie anortho- Park shear zone (Snyder et al., 1995), which site body. includes deformed granitic and gneissic The west-to-east stream-¯ow trend of the rocks, quartzite, and amphibolite dikes. Ma- Carpenter Ranch Formation caprock belt is ®c amphibolite dikes of the north-central evident from topographic maps and satellite Laramie Mountains often exhibit a relict ig- photoimages. Our ®eld inspection of Carpen- neous texture that characterizes Carpenter ter Ranch outcrops from Duncan Buttes in Ranch amphibolite clasts. Streams accessing Wyoming eastward to Carpenter Butte in Ne- the Precambrian core of the north-central braska revealed a major paleochannel incised Laramie Mountains are capable of transport- into the White River beds (®g. 24). It rep- ing granitic/gneissic gravel with high am- resents the deepest incision of the paleoval- phibolite content. Sampling of sur®cial ley and con®rmed the west-to-east alignment stream and pediment gravels east of the Lar- of the paleotrend. The paleovalley then turns amie Mountains from the vicinity of Wheat- to the northeast upon entering Nebraska. land north to Esterbrook suggests that terrace The easternmost exposures of the Carpenter gravel of the Laramie River north of Wheat- Ranch Formation that we have identi®ed oc- 2005 HUNT: DOME-SKULLED CHALICOTHERE 39

Fig. 24. Interpretive restoration of sinuous west-to-east trend of the Carpenter Ranch paleochannel axis incised into rocks of the White River Group, Goshen County, Wyoming, and Sioux County, Ne- braska. Inset illustrates a typical northwest-to-southeast cross-section of the paleochannel from Duncan Buttes to Merycochoerus Butte. cur at the northeastern limit of East Sturdivant Runningwater granitic gravels and sands that Butte in the NE1/4, NE1/4, sec. 27, T27N, yielded most of the classic Runningwater R57W (Agate SW 7.5-minute quadrangle). fauna to the east in Dawes and Box Butte These outcrops indicate that the paleovalley counties, where most productive quarries has reoriented to a northeasterly course. At have been located. However, the stratigraph- this locality, Carpenter Ranch outcrops dis- ically lowest sites in that area, such as Run- appear beneath Quaternary dune sands, which ningwater Quarry, show an af®nity to the then blanket the landscape for ϳ10 miles to Carpenter Ranch fauna and to the fauna from the northeast. When Miocene sediments even- lower Runningwater strata northeast of Ag- tually reappear beyond the dune ®eld in the ate. The evident similarity in age of the eastern half of T28N, R56W, granitic gravel Northeast of Agate fauna to the Carpenter and tuffaceous sandstones have been assigned Ranch mammals suggests that the pale or- to the Runningwater Formation and represent ange-brown tuffaceous sandstones northeast the westernmost outcrops of that rock unit of Agate are a ®ne-grained sedimentary currently recognized (Skinner et al., 1977). equivalent of the Carpenter Ranch ¯uvial The mammals from tuffaceous sandstones of sands and gravels, the two lithofacies togeth- the lower Runningwater beds northeast of Ag- er representing the earliest deposits of the ate are close in age to the Carpenter Ranch Runningwater/Carpenter Ranch paleovalley fauna and, together with the paleovalley trend, system established in the earliest Hemingfor- suggest that the Runningwater paleovalley dian of the central Great Plains. identi®ed by Skinner et al. (1977) is a direct continuation of the Carpenter Ranch trend POST-LARAMIDE EVOLUTION OF THE (®g. 25). ROCKY MOUNTAINS The Carpenter Ranch mammals appear Coarse epiclastic debris dispersed by post- less advanced than those from the upper Laramide ¯uvial systems into the mid-con- 40 AMERICAN MUSEUM NOVITATES NO. 3486

Fig. 25. Alignment of the Carpenter Ranch and Runningwater paleovalley systems in southeastern Wyoming and western Nebraska. Fossil mammals in these early Miocene ¯uvial sediments indicate an early Hemingfordian age. tinent documents a mid- to late Cenozoic re- widespread sands and gravel of the White cord of episodic uplift of the adjacent Rocky River Group (Stanley, 1976). Following this Mountain ranges. Mammalian fossils pre- initial pulse of coarse granitic debris from the served in the sedimentary ®ll of major pa- Laramie Mountains, transported eastward by leovalleys emerging from the uplifts in basal White River channels of Chadronian southeastern Wyoming, northeastern Colo- age, no signi®cant evidence of regional uplift rado, and western Nebraska provide the basis recurs until the initiation of late Oligocene for a highly resolved tectonic chronology. stream incision, evidenced by basal Arikaree These paleovalleys are often identi®ed by a paleovalleys ¯owing eastward from the Lar- reversed topography of geographically amie Mountains and Hartville Uplift (Vondra aligned butte caprocks, forming conspicuous et al., 1969; Swinehart et al., 1985). Early linear trends within and east of the uplifts. Arikareean mammals and 40Ar/39Ar radiom- Recovery of mammalian faunas from the etry (ϳ28.1±28.3 Ma) date this tectonic Carpenter Ranch and Spoon Butte paleoval- event (Tedford et al., 1996). Removal of a leys in southeastern Wyoming demonstrated considerable volume of lower Arikaree unexpected ages for two such ¯uvial sys- Group strata by late Arikareean incision is tems. Similar ¯uvial trends east of the Lar- documented by east-northeast trending pa- amie Mountains and Hartville Uplift could leovalleys in western Nebraska at Wildcat also represent more diverse ages than previ- Ridge (Vondra et al., 1969) and along the ously supposed. Many remain to be exam- Niobrara River at Agate National Monument ined for biochronologically useful faunas. (Hunt, 1985b). At the latter locality both The oldest widely recognized post-Lar- mammals and an 40Ar/39Ar date of 22.9 Ϯ amide ¯uvial activity east of the Laramie 0.08 Ma (Izett and Obradovich, 2001) estab- Mountains begins in the late Eocene with lish the approximate time of regional inci- 2005 HUNT: DOME-SKULLED CHALICOTHERE 41 sion, and deposition of the Harrison Forma- ic event involving regional uplift of the Lar- tion. amie Mountains and Rocky Mountain Front Fine-grained sheetlike sediments of the Range is documented by the appearance of Arikaree Group continued to accumulate east paleovalley systems of early Hemingfordian of the Laramie and Hartville uplifts, repre- age (®g. 26) carrying coarse granitic debris sented by the Anderson Ranch Formation into the central Great Plains (Skinner et al., (Hunt, 2002). These sediments incorporate a 1977; Hunt, 1985a; Flanagan and Montagne, signi®cant eolian tuffaceous component with 1993; Tedford, 1999). Trending east-north- latest Arikareean mammals. Only a single east from the mountains, a northern paleo- major paleovalley (®g. 3, Lay Ranch beds) valley transporting sediments of the Carpen- within the Anderson Ranch Formation has ter Ranch and Runningwater Formations tra- been recognized and mapped; it deeply in- verses southeastern Wyoming and continues cises undifferentiated Arikaree rocks at into western Nebraska. The gravel composi- Spoon Butte in southeastern Wyoming tion of this system is chie¯y granitic, lacking (Hunt, 1985a, 1990). This valley, emerging any evidence of rhyolitic or other acid-vol- from the vicinity of the southern Hartville canic clasts (Yatkola, 1978; Hunt, 1985a). To Uplift, carried ®ne-grained tuffaceous ¯uvial the south in northern Colorado, a second ma- sands and silts with only minor amounts of jor system represented by sediments of the epiclastic granitic sand and granule gravel. Martin Canyon Formation (Scott, 1982; Ted- The scarcity of latest Arikareean paleoval- ford, 1999) follows a similar direction across leys on the Hartville tableland suggests that northeastern Colorado. Here the lithic com- this was a tectonically quiescent interval. position includes not only granitic debris, but The granitic cores of the mountains to the also volcanic clasts (rhyolite, andesite), in- west were likely nearly buried by tuffaceous dicating a source in the Rocky Mountains in- Arikaree strata, evidenced today by the onlap volving the Never Summer Range±Specimen of the Anderson Ranch Formation on the Mountain volcanics (Wahlstrom, 1944; Cor- granites of the Hartville Uplift (Hunt, 2002: bett, 1968). ®g. 1) and by residual Arikaree deposits bor- These paleovalleys contain earliest Hem- dering the Laramie Mountains. ingfordian mammal faunas that ®rmly estab- The conclusion of Arikaree deposition east lish their contemporaneity and allow the sed- of the Hartville Uplift at ϳ19 Ma is evi- iments to be correlated with considerable denced by the development of multiple sili- precision. The occurrence of the large oreo- ceous paleosols found throughout the upper dont Merycochoerus magnus matches them part of the Anderson Ranch Formation (pre- with a third locale in the Rocky Mountains viously Upper Harrison beds of Hunt, 1990: at Middle Park, Colorado, where early Mio- ®gs. 7, 8, 10). Densely packed, ®ne siliceous cene sediments of the Troublesome Forma- rhizoliths of uniform diameter (0.5 mm) form tion overlie a weathered Cretaceous paleo- highly anastomosed root networks in these surface (Izett, 1968). Evidently, regional up- paleosols typical of grasslands. The rhizolith lift of the Front Range±Laramie Mountain networks are restricted to level geomorphic axis initiated simultaneous deposition on surfaces that can be traced over 1200 km2 both the western and eastern ¯anks of the east of the Hartville Uplift, demonstrating the range at ϳ18.2±18.8 Ma. It has not escaped regional extent of these early Miocene grass- our notice that the early Hemingfordian pa- lands. However, Arikaree deposition contin- leovalleys of the High Plains trend northeast, ued into Hemingfordian time in central Wy- indicating a topographic gradient in that di- oming (Split Rock Formation, Love, 1970; rection. The sediments of these early Mio- Munthe, 1988) and eastward in the vicinity cene paleovalleys, primarily con®ned to the of the Laramie Mountains (McGrew, 1963). linear trend of the stream courses, differ in An 40Ar/39Ar date (17.4 Ϯ 0.08 Ma, Izett and the lithologic composition of their axial grav- Obradovich, 2001) and mammal fauna from els, which clearly re¯ect local source terrain the Split Rock Arikaree outcrops of Munthe in the mountains to the west. The early Hem- (1988) indicate a medial Hemingfordian age. ingfordian valley ®lls are followed in time Evidence of a major early Miocene tecton- by mid- and late Miocene Ogallala sands and 42 AMERICAN MUSEUM NOVITATES NO. 3486

Fig. 26. Trend of early Miocene paleovalleys of early Hemingfordian age east of the Rocky Moun- tain Front Range and Laramie Mountains in Colorado, Wyoming, and Nebraska. Oldest mammal faunas in the valley ®lls document the initiation of regional uplift at ϳ18.2±18.8 Ma. 1, Carpenter Ranch- Runningwater paleovalley; 2, Martin Canyon paleovalley; 3, Troublesome Formation basin-®ll, Middle Park, Colorado. gravels within paleovalleys and alluvial Geographic Society and more recently by the aprons derived from continuing regional up- Harriet Meek Fund of the University of Ne- lift of the Rocky Mountain plateau. braska. Preparation of particularly recalci- trant fossils from silica-cemented caprock ACKNOWLEDGMENTS was accomplished by Rob Skolnick, Ellen Stepleton, and Greg Brown. Angie Fox pro- I am particularly grateful to the landown- vided much appreciated expertise in the pro- ers of Goshen County, Wyoming, and Sioux duction of the various maps and cross-sec- County, Nebraska for access to their property tions. I bene®ted from numerous fruitful dis- during the course of this ®eld study: I espe- cussions of chalicothere systematics with cially wish to thank George and Dorothy Ob- Margery Coombs (University of Massachu- erg, Chuck Oberg, Gene Lay, the Carpenter, setts, Amherst), and pro®ted from thorough Bixler, and Desenfants families, and Larry reviews of the manuscript by Jason Lille- Rice. Jim Phipps provided much useful in- graven (University of Wyoming, Laramie), formation on the history of the area and Margery Coombs, and Ellen Stepleton (Uni- shared an interest in our work over many versity of Nebraska, Lincoln). years. Many students were involved in ®eld parties during the mapping of the Patrick Buttes: James Vanderhill, Rob Skolnick, REFERENCES Kevin Seevers, Katherine Wolfram, Mary Adams, G.I. 1902. Geology and water resources Rebone, Dan Miller, and Paul Chilbert. of the Patrick and Goshen Hole Quadrangles in James Vanderhill and Rob Skolnick both eastern Wyoming and western Nebraska. U.S. completed Master's thesis studies during the Geological Survey Water Supply Paper 70: 1± early 1980s. Funding for the early years of 50. this research was provided by the National Berggren, W.A., D.V. Kent, C.C. Swisher, and 2005 HUNT: DOME-SKULLED CHALICOTHERE 43

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a This paper meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper).