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University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Mammalogy Papers: University of Nebraska State Museum, University of Nebraska State Museum

2004 Global Climate and the Evolution of Large Mammalian during the Later in Robert Hunt Jr. University of Nebraska-Lincoln, [email protected]

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Hunt, Robert Jr., "Global Climate and the Evolution of Large Mammalian Carnivores during the Later Cenozoic in North America" (2004). Mammalogy Papers: University of Nebraska State Museum. 286. http://digitalcommons.unl.edu/museummammalogy/286

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Global Climate and the Evolution of Large Mammalian Carnivores during the Later Cenozoic in North America

ROBERT M. HUNT, JR.

ABSTRACT Taxon ranges of larger mammalian carnivores can be grouped into seven temporal intervals during the later Cenozoic. These intervals are of varied duration and seem to correspond to periodic faunal reorganizations that accompanied the progressive climatic deterioration occur- ring from the late to the . Recent oxygen isotope records from deep-sea cores serve as proxy for the pattern of global climate during the Cenozoic and compare reasonably well with the large intervals. Intervals A, B, and the early part of C characterize a time of cooler global climate (␦18O: ϩ1.3 to ϩ3.0½) following the early Eocene climatic optimum. The later part of Interval C, following the mid- climatic optimum, and Intervals D through F record a gradual climatic deterioration (␦18O: ϩ2.0 to ϩ3.8½) from the mid-Miocene to early . Interval G (␦18O: ϩ3.8 to ϩ5.0½) corresponds to the extreme global cooling of the later Pliocene and Pleistocene. Glacioeustatic decline in sea level during these intervals probably made possible the entrance of migrant Eurasian carni- vores and other into the New World via the Bering route. The periodic emergence of this land bridge and the effect of the climatic oscillations of the later Cenozoic on the mammalian fauna appear responsible for the faunal shifts.

INTRODUCTION logical pyramid, in¯uence the dynamic of prey populations (e.g., Schaller, 1972; Sin- An improved record of and Qua- clair and Norton-Grif®ths, 1979). The nature ternary mammals has afforded a new of social organization of a , its body perspective on faunal succession in North size, territorial extent, style of , and America during the last 40 million years. mode of locomotion, among other such fac- Well-constrained taxonomic ranges have re- tors, determine its interaction with various sulted from (1) new discoveries of Neogene prey species. Thus, the identi®cation of in- mammals from more re®ned lithostratigraph- tervals characterized by stable, well-de®ned ic contexts, and (2) continuing study of his- carnivore assemblages contributes to an im- toric collections, most notably that of Childs proved understanding of mammalian faunal Frick at the American Museum of Natural dynamics overall. Here I provide a summary History. Taking advantage of these condi- of taxon ranges for North American large tions, this study focuses on the evolutionary carnivores, calibrated against the Magnetic history of North American carnivorans. In Polarity Time Scale (MPTS; Berggren et al., this case, range diagrams for large carnivores 1995). These taxa are arranged in faunal in- demonstrate distinct, episodic transforma- tervals from ϳ40 Ma to the Holocene (®g. tions at guild level: such reorganizations 11.1, table 11.1). Seven intervals are recog- seem to have occurred at much greater fre- nized, each with its characterizing carnivore quency during the late Cenozoic, seemingly taxa, and de®ned by ®rst appearances (FADs) in step with climatic deterioration as charted and last occurrences (LADs). The implica- by oxygen isotope curves. tions of these intervals and their temporal Large carnivores, at the apex of the eco- spacing are discussed below. The usage fol-

139 140 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 285

Fig. 11.1. Taxon range diagram of larger late Eocene to Pleistocene carnivores (carnivorans and creodonts) in relation to the North American Land ``Ages'' (NALMAs) and Lyellian epochs, calibrated against the Magnetic Polarity Time Scale of Berggren et al. (1995). North American Larger Carnivore Turnover Events (NALCTE) are indicated at ϳ37±36, 33.7, 23.8, 18.8, 17.5, 9.0, 6.0, 4.5, and 3.0 Ma (see text for discussion). Temporal intervals between the turnover events are designated Intervals A through G. Amphicyonine Group I, Ysengrinia, ; II, , Cynelos, Pliocyon; III, Pseudocyon, Ischyrocyon. Hemicyonine I, ; II, ; III, ; IV, new Clarendonian . lowed here employs the term ``carnivoran'' nivore fauna in the Chadronian North Amer- for members of the Order and ican Land Mammal Age (NALMA) (Hunt, ``creodont'' for members of the Hyaenodon- 1996: ®g. 2). Duchesnean and earlier asso- tidae and Oxyaenidae that currently comprise ciations of carnivores included only archaic the Order . The term ``carnivore'' early Tertiary groups placed in the miacoid refers to any ¯esh-consuming mammal. Carnivora and the hyaenodont and oxyaenid Creodonta. Viverravid and miacid species FAUNAL ASSOCIATIONS OF LARGER comprise the miacoids (Matthew, 1909; NORTH AMERICAN CARNIVORES Flynn, 1998), none larger than a modern coy- ote ( latrans). The largest of the mia- INTERVALS A TO G(LATE EOCENE coids was the viverravid Didymictis vanclev- TO PLEISTOCENE) ei from the early Eocene of the Huerfano ba- Large (Ͼ10±20 kg) carnivorans take the sin, Colorado, with a basilar length of skull stage with the advent of the White River car- of ϳ15±16 cm; the very small viverravid 2004 HUNT: CENOZOIC CARNIVORES AND GLOBAL CLIMATE 141

Fig. 11.1. Continued.

Viverravus minutus from the Bridger Basin, 40 million years in North America (Hunt Wyoming, had a basilar length of ϳ4 cm, and Tedford, 1993). Here I amplify our ini- and most miacids measured Ͻ12 cm. tial designations, extending and re®ning the With the arrival of the Chadronian car- de®nition of these associations, and describ- nivoran-creodont fauna at ϳ37 Ma, there is ing their characterizing taxa in greater de- an evident increase in the average body size tail. of carnivores. The largest carnivorans in North America were nimravid cats and coy- HYAENODONT-NIMRAVID ASSOCIATION (Inter- ote-sized daphoenine beardogs (Daphoenus val A: ϳ37±23.7 Ma) vetus), accompanied by particularly large hyaenodont creodonts (Hyaenodon megalo- The hyaenodont-nimravine nimravid-da- ides, H. horridus, grandis; phoenine amphicyonid association is typical Mellett, 1969, 1977). Hyaenodon attained of Chadronian, Orellan, Whitneyan, and ear- its maximum species diversity in the Chad- ly Arikareean faunas of North America. Of ronian (Mellett, 1977). This hyaenodont- the intervals recognized here, its 13.3 million nimravid association was previously recog- year extent is by far the longest. Hyaenodont nized as the ®rst of four sequential mid- to and nimravine nimravid diversity during this late Cenozoic associations of large carniv- interval is remarkable, with nine species of orous mammals that characterized the last hyaenodonts and ten of nimravids currently 142 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 285

TABLE 11.1 Late Eocene to Recent Faunal Associations of Large Carnivores in North America

recognized (Mellett, 1977; Bryant, 1991, in the Whitneyan and early Arikareean. Skull 1996). If size alone is considered, the dom- lengths of the larger nimravine genera range inant carnivores of this association are the from ϳ13 to ϳ26 cm (Bryant, 1996; personal large creodonts, Hemipsalodon grandis (bas- observations): , ϳ13 to 18 cm; Po- ilar length, ϳ41 cm), Hyaenodon megaloides gonodon, ϳ20 cm; , ϳ15 to 23 cm; (skull length, ϳ40 cm), and H. horridus , ϳ15 to 26 cm. (skull lengths, 25±35 cm). The largest nim- Daphoenine amphicyonids commonly ac- ravids occur toward the end of the interval company these groups and are represented by 2004 HUNT: CENOZOIC CARNIVORES AND GLOBAL CLIMATE 143 four genera (Daphoenus, Daphoenictis, Bra- Clarendonian, the dominant large carnivor- chyrhynchocyon, Paradaphoenus) and at ans are large amphicyonines and - least six species (Hunt, 1996). The largest is ines, joined in the later part of the period by Daphoenus vetus, with males attaining basi- a few borophagine canids. However, this era lar skull lengths of ϳ20 cm in the Orellan can be subdivided into two intervals. The when the species is particularly well-repre- ®rst interval (Interval B), from ϳ23.7 to sented in the White River Group of the Great ϳ17.5 Ma (late Arikareean through early Plains. Daphoenus achieves a maximum bas- Hemingfordian), is characterized by temno- ilar length of ϳ23 cm by the time of its last cyonine and daphoenine amphicyonids that occurrence in the early Arikareean. seem to be primarily if not entirely North Hyaenodonts, nimravine nimravids, and American endemics, diluted by the arrival of Daphoenus became extinct within the early the earliest species of amphicyonines and Arikareean interval. The last records of hemicyonines. The second interval (Interval hyaenodonts and Daphoenus in late Oligo- C), from ϳ17.5 to 9 Ma (medial Hemingfor- cene rocks of the lower Arikaree Group in dian through Clarendonian), sees the extinc- the Great Plains are dated at ϳ28.0 Ma and tion of all temnocyonines and daphoenines; ϳ28.6 Ma, respectively; Daphoenus sur- the larger carnivorans are amphicyonine, vived to ϳ27 Ma in the Paci®c Northwest in hemicyonine, and felid immigrants, and the the John Day beds of Oregon. Nimravine endemic borophagine canids. cats persist only a short while longer to ϳ25± 24 Ma. Hence these long-dominant groups TEMNOCYONINE-DAPHOENINE- appear to be extinct by ϳ27±24 Ma. Nim- ASSOCIATION (Interval B: ϳ23.7±17.5 Ma) ravines are represented by at least three gen- era (Nimravus, Pogonodon, Eusmilus)in Beginning in the earliest Arikareean, the lower Arikaree Group sediments, contem- ®rst temnocyonine amphicyonids are record- poraneous with the last occurrences of ed in North America at Logan Butte in the hyaenodonts and Daphoenus. Although cre- John Day beds of Oregon (ϳ29±29.5 Ma), odonts became extinct in North America at in the Sharps Formation of the Wounded ϳ28 Ma, they survived in southern and Knee area, South Dakota (LACM Locality into the Miocene. 1872, estimated at ϳ28±29.5 Ma), and in In the richly fossiliferous sediments of the pumice-bearing levels of the Gering Forma- central Great Plains in Nebraska, South Da- tion at Wildcat Ridge, Nebraska (ϳ28.3 Ma). kota, Wyoming, and Colorado, UNSM pa- These early temnocyonines attained the size leontologists have found no of these of or small (ϳ15±30 kg). groups above the lower Arikaree Group. Ex- Identi®ed by a uniquely specialized denti- tensive explorations for fossil mammals in tion, temnocyonines are present throughout rocks of the upper Arikaree Group in Ne- the Arikareean NALMA, and become extinct braska, Wyoming, and South Dakota have in the latest Arikareean. The last documented failed to produce a hyaenodont, nimravid, or occurrences of temnocyonines are in Upper Daphoenus. Thus the hyaenodont-nimravid Harrison sediments in northwest Nebraska association is apparently extinct by late Ari- and southeastern Wyoming. Temnocyonines kareean time (estimated to begin at ϳ23 Ma) actually appear late in Interval A and extend in the North American midcontinent. into the early part of Interval B and are the only group of large carnivorans, other than DOMINANCE OF AMPHICYONINES-HEMICYON- the endemic hesperocyonine canids, span- INES (EXTENDING OVER INTERVALS B AND C) ning the Arikareean NALMA. Terminal spe- cies of large temnocyonines (ϳ80 kg) in the Immigrant Eurasian amphicyonine - latest Arikareean attained skull lengths of dogs and hemicyonine ursids characterize the ϳ30 cm. large carnivoran assemblage in North Amer- With the of the hyaenodonts, ica from the beginning of the Miocene to ϳ9 nimravids, and Daphoenus, the mid-Arika- Ma at the end of the Clarendonian. During reean interval (ϳ25±23 Ma) displays a dearth the late Hemingfordian, Barstovian, and of large carnivores. Remnocyonines diversity 144 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 285

Fig. 11.2. The Cenozoic oxygen isotope curve of Zachos et al. (2001) and global climatic events relative to the guilds of larger carnivores designated by Intervals A through G. Subevents e1±e3 within Intervals C and D are discussed in text (modi®ed from Zachos et al., 2001). 2004 HUNT: CENOZOIC CARNIVORES AND GLOBAL CLIMATE 145 at this time, seeming to ®ll this void.1 How- that survives to the end of Interval B and ever, the failure of other carnivorans to in- attains great size (basilar skull lengths, 28± crease in size and occupy the available meat- 30 cm). Borocyon last occurs in the early eating and carcass-consuming niches allows Hemingfordian of a group of artiodactyls, the Entelodontidae, Nebraska, and with its demise, daphoenine to successfully enter this domain. amphicyonids become extinct and are re- increase in size from the late into placed by immigrant amphicyonines. the . By the time of their ex- Endemic New World daphoenines (Da- tinction at ϳ17.5 Ma, they were of enormous phoenodon, Borocyon) and temnocyonines size, large males approaching ϳ800 kg. Doc- coexist with Old World amphicyonines umented evidence of scavenging by entelo- (Ysengrinia, Amphicyon, Cynelos) within In- donts is now known from a number of late terval B. These are the largest terrestrial car- Arikareean sites in North America (Joeckel, nivorans (ϳ50±100 kg) evolved on the con- 1990; Hunt, 1990); it is likely that they were tinent up to this time (Hunt, 1998b). The im- important carcass processors, able to crush migrant amphicyonines Ysengrinia, Cynelos, bones of large . Both entelodonts and Amphicyon appear at ϳ23.0, ϳ19.2, and and temnocyonines possess large, robust pre- ϳ18.8 Ma, respectively, and herald the be- molars suitable for durophagy. Their worn ginning of a Eurasian amphicyonine migra- premolars commonly exhibit ¯attened tips tion into North America that continues into with crimped enamel rims similar to the the mid-Miocene. blunted premolars of living durophagous hy- Accompanying the amphicyonines are im- enas. migrant hemicyonine ursids. The oldest In the mid- to late Arikareean, beardogs of North American records of hemicyonines are the genus Daphoenodon appear together with referable to Cephalogale, discovered in late temnocyonines at many localities in the Arikareean sediments of western Nebraska Great Plains and in Florida. At the carnivore and southeastern Wyoming (Hunt, 1998a). den site at Agate National Monument a large New World species of Cephalogale range temnocyonine was found in a burrow only a from ϳ23 to ϳ17.5 Ma. The youngest re- meter from dens with individuals of Da- cords of the genus are found in early Hem- phoenodon superbus (Hunt et al., 1983; ingfordian sites in western Nebraska and Hunt, 1990). The genus Daphoenodon in- Florida where they are as large as a small cludes several lineages, all probably de- americanus. scended from the earliest species, D. notion- An enormous predatory mustelid, Mega- astes, known only from Florida (Frailey, lictis ferox, also is con®ned to Interval B. 1979) and the Gulf Coast (Albright, 1996). Characterized by a robust, short-limbed post- Some species maintain a short-legged sub- cranial skeleton indicative of powerful mus- digitigrade stance, but one lineage evolves a culature, coupled with a short-faced cranium, long-legged digitigrade predator, Borocyon, shearing carnassials, strong canines, and well-developed crushing premolars, Megal- 1 Nonmarine rocks in North America dating from ϳ25 ictis apparently functioned as a giant wol- to 23 Ma have been dif®cult to con®dently identify. In verine-like predator (Hunt and Skolnick, the John Day Formation of north-central Oregon, recent dating of tuffs (Fremd et al., 1994) has established that 1996). the interval between the Tin Roof Tuff (ϳ25.1±25.3 Ma) Canids are endemics in Intervals A±C, re- and the ATR Tuff (22.6 Ϯ 0.13 Ma) falls within this stricted to North America. Throughout Inter- time interval. However, no large carnivores have been val B, canids are relatively small , the recently collected from this interval nor can fossils from hesperocyonines and borophagines reaching the older 19th-century collections be placed with cer- tainty in these beds. The Arikaree Group of the Great skull lengths of ϳ14±18 cm in only a few Plains and Rocky Mountain basins may include sedi- species (Wang, 1994; Wang et al., 1999). ments of this age, but reliable radiometric dates have not been obtained because of the scarcity of datable tuffs. AMPHICYONINE-HEMICYONINE-BOROPHAGINE In intervals within the Arikaree Group that may fall ASSOCIATION (Interval C: ϳ17.5±9 Ma) within the ϳ25 to 23 Ma window, temnocyonine bear- dogs are the only large carnivorans that have been With the disappearance of endemic da- found. phoenines at the end of Interval B (ϳ17.5 146 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 285

Ma), the Old World amphicyonines (Amphi- the later Barstovian. With the extinction of cyon, Cynelos) became the largest carnivor- the great amphicyonids near the end of the ans of the Hemingfordian NALMA. By the Clarendonian, the late Clarendonian and late Hemingfordian Sheep Creek fauna of Hemphillian are typi®ed by large species of western Nebraska, these two genera reached the borophagine canids and Boro- skull lengths of 33±34 cm (Cynelos idoneus) phagus. and 37±39 cm (Amphicyon frendens). These In the later Hemingfordian, near the be- amphicyonine lineages eventually attained ginning of Interval C, the carnivorous Eur- maximum size in the early Barstovian (Cy- asian hemicyonine Phoberocyon appears in nelos sinapius, 39±44 cm; Amphicyon in- eastern North America, represented by dental gens, 42±52 cm; Pliocyon, 29±30 cm), im- and postcranial remains at Thomas Farm in mediately prior to their apparent extinction Florida (Tedford and Frailey, 1976) and pos- by ϳ14 Ma. With the disappearance of the sibly in Delaware (Emry and Eshelman, large scavenging entelodonts, it is likely that 1998:158). During the early Barstovian, these amphicyonines processed carcasses in Phoberocyon is succeeded by the Old World addition to active hunting. The appearance of hemicyonine Plithocyon, known from a large hyaenoid borophagines at this time also sug- sample from the Barstow syncline, Califor- gests a response to the availability of the du- nia, and from rare remains from rophagous niche, climaxing with the huge and Nebraska. These hemicyonine ursids dif- bone-crushing canid Epicyon haydeni (basi- fered from the living ursine in their less lar skull lengths, late Clarendonian±early specialized, hypercarnivorous dentitions (rel- Hemphillian, 28±32 cm) in the Ogallala For- ative to ancestral amphicynodont ursids; mation of the Great Plains. Hunt, 1998a) and their long-footed, digiti- There is a major shift in the large amphi- grade stance, suggesting that they were ac- cyonines at ϳ14 Ma, the boundary between tive pursuit predators. early Barstovian and early late Barstovian Early in Interval C, at ϳ16.5±17 Ma, the faunas in the Great Plains. Amphicyon, Cy- ®rst felids enter North America from nelos, and Pliocyon are replaced by new lin- and persist as relatively small lynx- to leop- eages of large Pseudocyon and mid-sized Is- ard-sized cats until the appearance of the chyrocyon, which then dominate the later large lion-sized felid Nimravides in the Clar- Barstovian and Clarendonian of North Amer- endonian. Also, true ursine bears (), ica. The only known skull of Pseudocyon although never very large at this time (Ͻ50 from the late Barstovian of Nebraska has a kg), become evident in Hemingfordian and basilar length of 37 cm, and several mandi- Barstovian (but not Clarendonian) faunas, bles also indicate its great size. Ischyrocyon, probably inhabiting well-vegetated environ- known from a number of skulls from 36 to ments as isolated individuals. 47 cm, attains enormous size in the Claren- donian. The earliest populations of Ischyro- BOROPHAGINE CANID-URSID -FELID cyon from the Barstow syncline, , ASSOCIATION (Interval D: ϳ9±6 Ma) are somewhat smaller (basilar skull lengths, 28±34 cm, N ϭ 7). During the early Hemphillian (ϳ9±7 Ma), Early Barstovian canids contemporaneous the large canids Epicyon, , and with the Amphicyon-Cynelos-Pliocyon group the smaller Carpocyon are joined by the remain of modest size: the largest are the great immigrant ursavine bear Indarctos. Ap- borophagines Tomarctus (basilar lengths, parently Indarctos oregonensis, the large ϳ19±19.5 cm), Paracynarctus (ϳ14±16 cm), borophagine dogs (Epicyon haydeni), the (ϳ20 cm), and the hesperocyo- large long-limbed felid Nimravides, and the nine Osbornodon (ϳ20 cm). Coincident with barbourofeline nimravid have the appearance of the Pseudocyon-Ischyro- replaced the great amphicyonines and hemi- cyon group at ϳ14 Ma, borophagine canids cyonines, marking a radical alteration in the begin a steady and conspicuous size increase, large carnivore assemblage. The ``cats'' and documented by larger species of Aelurodon, borophagines are species that have continued Carpocyon, Protepicyon, and cynarctines in to increase in size from the Clarendonian 2004 HUNT: CENOZOIC CARNIVORES AND GLOBAL CLIMATE 147 faunas into the Hemphillian. At ϳ7 Ma, later EARLY CANID-FELID-URSID ASSOCIATION in the Hemphillian interval, appear addition- WITH MACHAIRODONT AND HYAENOID CANID al Eurasian immigrant species: the machai- (Interval F: ϳ4.5±3 Ma) rodont felid , the arctoid ``dog'' At ϳ4.5 Ma the earlier faunas in- Simocyon, and the mustelid Eomellivora clude species of felids, canids, and ursids that (Tedford et al., 1987). The earliest tremarc- re¯ect at least to some degree the modern tine ursid also appears at this representatives of these families in North time (ϳ7 Ma; Tedford and Martin, 2001; America. Ursus appears near the beginning Hunt, 1998a). Intervals D and E have been of the interval (ϳ4.3±4.5 Ma), as does Puma commented on by others (Tedford et al., (Martin, 1998). Ursus is accompanied by the 1987; Webb and Opdyke, 1995) as a time of tremarctine bear Plionarctos, marking the renewed immigration from the Eurasian initiation of a shared occupation of North mainland across the Bering route into North America by Ursus and tremarctines that will America, and a number of species of smaller extend to the Holocene. The canine canids carnivores and mammals also participate in that arise in the Hemphillian begin to radiate this migration. into the lines leading toward modern foxes, coyotes, and wolves, but most Blancan ca- nines (the ancestral Canis lepopha- BOROPHAGINE CANID-URSID - gus, the small wol¯ike Canis edwardii, and FELID ASSOCIATION (Interval E: ϳ6±4.5 Ma) foxes) are small. The late Hemphillian is populated by new Archaic holdovers that remain in the in- terval include the tremarctine ancestor Plion- faunal elements, recording a continuing di- arctos, the machairodont , and lution of the North American carnivoran fau- the hyaenoid canid Borophagus. na by Old World species. At ϳ6 Ma the mi- grant ursid Agriotherium and mustelid Ple- MODERN CANID-FELID-URSID ASSOCIATION siogulo enter, soon followed by the machai- WITH MACHAIRODONT AND HYAENID (Interval rodont felid Megantereon. The nimravid cats G: ϳ3 Ma±Holocene) and simocyonine arctoids are absent and pre- sumably extinct. A diverse group of smaller The presence of modern groups ancestral felids (Lynx, Adelphailurus, Pratifelis) and or closely related to the living North Amer- the large Nimravides are joined for the ®rst ican canids (Canis, , ), felids time in North America by several machai- (Felis, Lynx, Puma, Panthera), and ursids rodont felids (Machairodus, Megantereon), (Ursus sensu lato) characterize the interval. and this coexistence of machairodonts and The tremarctine ursid ®rst occurs the less specialized felines continues into the at ϳ2.5 Ma in California and , and its sister taxon the arctodont bear Arctodus, the late Pleistocene. Modern procyonids (Pro- largest land carnivoran to have ever existed, cyon) also make their ®rst appearance. The appears late in the interval at ϳ1 Ma, accom- canids Epicyon, Borophagus, and Carpocyon panying the much smaller species of New continue through this interval, suggesting World Ursus. that the endemic canid assemblage is largely The late Cenozoic radiation of the canine unperturbed, simply receiving occasional canids (subfamily ), stemming from pulses of immigrant stocks from Eurasia. the small early and mid-Miocene , However, a number of of char- was initiated in the Hemphillian. Only small acteristic taxa occur at the end of the interval foxes (Urocyon, Vulpes) coexisted with coy- at ϳ4.5±4.7 Ma (Hemphillian-Blancan ote-sized ``Canis'' davisi (Berta, 1987) in the boundary). The felids Machairodus and Nim- Hemphillian, and so canines did not yet rival ravides, the ursid Agriotherium, and the ca- in size the large carnivorans of the latest nids Epicyon and Carpocyon are missing Miocene in North America. Canine diversi- from Interval F, representing a signi®cant ®cation took place in North and possibly shift in the carnivoran guild in the early Pli- Central America from ϳ6 to 3 Ma (Berta, ocene. 1987). By ϳ3.5 Ma in the Blancan a coyote- 148 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 285 sized canid, Canis lepophagus, was widely hyaenids at ϳ1.5 Ma and the felid distributed and is a possible ancestor of Ca- at ϳ2±2.3 Ma. nis latrans (KurteÂn and Anderson, 1980). No -sized canids were present in the New OXYGEN ISOTOPES,CENOZOIC GLOBAL World at this time. Early wolves, Canis CLIMATE, AND INTERVALS A±G etruscus, evolved in the Old World in the early Pleistocene from the C. arnensis±C. le- Although the fossil history of larger car- pophagus group of Holarctic coyotelike ca- nivorans and creodonts is punctuated by nu- nines (KurteÂn and Anderson, 1980). A small merous temporal gaps in the Cenozoic non- wolf-like canid does not occur in North marine sedimentary record of North Ameri- America until the appearance of Canis ed- ca, the richer sampling of lineages made pos- wardii in the late Blancan and Protocyon tex- sible by renewed ®eld efforts and the anus (known only from the American South- opening of the Frick Collection in New York, west) in the early Irvingtonian. These small together with a progressively more re®ned wolves are followed by the larger C. arm- lithostratigraphy and biochronology of North brusteri in the late Irvingtonian, and the ar- America mammals, make possible an im- rival of Canis lupus, the gray wolf, in the proved assessment of faunal turnover in latest Irvingtonian and Rancholabrean. The these groups. Most striking in the compila- large (Aenocyon dirus) of North tion of table 11.1 is the relative duration of America is contemporary with the gray wolf these intervals through time. Intervals A in the Rancholabrean. Thus, large wolf-sized through C are signi®cantly longer than those canids are a relatively recent phenomenon in to follow (13.3, 6.2, 8.5 m.y. versus 3.0, 1.5, 1.5, and 3.0 m.y.). Although the boundary North America, appearing only in the early between intervals is not always sharply de- Pleistocene (Irvingtonian) and continuing to ®ned, with some taxon ranges overlapping the present. These wolves do not become im- and others beginning or ending diachronous- portant in the large carnivoran assemblage of ly, the composition of the guilds of larger North America until the extinction of the carnivores re¯ects an evident measure of fau- hunting Chasmaporthetes at ϳ1.5 Ma nal stability over these temporal intervals. in the early Irvingtonian and the disappear- Recent advances in our understanding of ance of the canid Borophagus at or shortly global climate and tectonism during the Ce- after ϳ2 Ma. nozoic provide a template against which we An archaic and rather exotic component can evaluate Intervals A through G. Earth's remains early in the interval: in the later climate has been profoundly affected by the Blancan faunas the ailurid Parailurus in the orbital behavior of the planet, and by crustal Paci®c Northwest and a number of aeluroid movements and deformation generated by carnivores appear from ϳ3.5 to 2.8 Ma, in- plate tectonics. Insight into Cenozoic climate cluding the Old World hyaenid Chasma- change has been considerably enhanced by porthetes (the only hyaenid to ever reach the the development of deep-ocean oxygen and New World), the machairodont Homother- carbon isotopic records that supply detailed ium, and the felines Dinofelis and Miraci- information on long-term as well as abrupt, nonyx. The machairodont Megantereon per- or ``transient'', shifts in global climate (Mill- sists from the early Blancan and is believed er et al., 1987, 1991; Prentice and Matthews, to evolve into the late Blancan and Irving- 1988; Denton, 1999; Zachos et al., 2001). tonian at ϳ2.4 Ma. Earlier discussions of Cenozoic climate tend- The advent of the mass extinctions at the ed to emphasize a nearly continuous climatic Pleistocene-Holocene boundary sees the de- cooling from the early Paleogene into the mise of many of the great carnivorans of In- Pleistocene. Newer high-resolution isotopic tervals F and G: gone are in data have revealed a more complex and vary- the Wisconsinan; Miracinonyx at ϳ19,000± ing climatic signal characterized by intervals 20,000 years; Arctodus at ϳ12,000±13,000 of broad climatic stability of variable dura- years; Smilodon at ϳ8,000 years. Earlier last tion that record cooling and warming events. occurrences in the interval included the These intervals are themselves made up of 2004 HUNT: CENOZOIC CARNIVORES AND GLOBAL CLIMATE 149 shorter oscillating periods of cooling and at ϳ33.7 Ma (extinction of brontotheres, the warming, some exceedingly brief (104±105 creodont Hemipsalodon, several species of years). hyaenodonts, the archaic amphicyonids Da- Here I compare the North American Inter- phoenictis and Brachyrhynchocyon, oromer- vals A±G (®g. 11.1) with the recently pub- ycid artiodactyls, and cylindrodont rodents), lished deep-sea stable oxygen isotope record here designated as the Chadronian-Orellan for benthic foraminifera compiled from over boundary between subintervals A1 and A2 40 ocean drilling sites that sampled the Ce- (®g. 11.2). In the Old World the Eocene-Ol- nozoic record of sedimentation (®g. 11.2, igocene mammalian faunal shift was a more from Zachos et al., 2001). These intervals pronounced event (Grand Coupure, ϳ33.7 compare reasonably well with the ␦18O Ma, Montanari et al., 1988; LeÂveÃque, 1993). curve; several large carnivore faunal turn- From the EECO (early Eocene climatic over events occur at evident shifts in oxygen optimum, 50±52 Ma) a 17-million-year trend isotope values of the deep-sea curve. More- toward cooler climate is indicated by a 3.0½ over, subevents (e1±e3) within Intervals C and enrichment in ␦18O from 50 to 34 Ma (early D appear to correspond to abrupt, or ``tran- Eocene to early Oligocene). Prior to the late sient'', oxygen isotope enrichment events Eocene this enrichment is attributed to a 7ЊC. suggestive of marked climatic cooling. Next decline in deep ocean temperature (Zachos et I discuss the more detailed correspondence al., 2001). All later ␦18O change is regarded of Intervals A±G with the Zachos et al. as the combined effects of ice-volume and (2001) curve. temperature, ``particularly for the rapid en- Oxygen isotope curves for the Cenozoic richment event at 34 Ma'' (Zachos et al., (Miller et al., 1987; Prentice and Matthews, 2001). 1988; Zachos et al., 2001) record an evident The Oligocene oxygen isotope curve from shift toward greater enrichment in 18O ratios ␦ ϳ34 to 26 Ma shows ␦18O values of 2.4 to (e.g., 0.0 to 1.8½, Zachos et al., 2001) from 3.0½, indicating a marked enrichment in the end of the early Eocene (ϳ49 Ma) to the ␦18O over the Eocene interval where values beginning of the late Eocene (ϳ37 Ma). The were between 0.0 and 2.0½ (Zachos et al., shift from mid- to late Eocene corresponds 2001). Permanent ice sheets with mass as to the advent of the Chadronian carnivore great as 50% of the present-day ice sheet are fauna in North America characterized by the inferred during this early to mid-Oligocene large carnivore guild of Interval A (Hunt, 1996: ®g. 2). Prior to Interval A, the Paleo- interval (Zachos et al., 1994, 2001). From cene-Eocene carnivores of North America, ϳ33.7 Ma to ϳ26 Ma, the mammals of the the miacoids and creodonts, were not large Orellan, Whitneyan, and early Arikareean mammals, most much less than 50 kg and NALMAs represent a uniform faunal aggre- many much smaller (ϳ1±20 kg).2 The shift gate of species evolving without major inter- from a miacoid-creodont dominated fauna in ruption in faunal content; this is likewise an 18 the Paleocene and earlier Eocene to the fauna interval of uniform ␦ O values until the late of Interval A corresponds to the global cli- Oligocene warming eventÐthe Zachos et al. matic changes now documented at the end of (2001) curve places this event at ϳ26±24 the middle Eocene (ϳ37 Ma, Berggren et al., Ma, immediately prior to the Miocene Mi-1 1995; Prothero and Emry, 1996: 679). glaciation. A more moderate shift occurred in North Within Interval A2, a warming spike at America at the Eocene-Oligocene boundary ϳ28.3 Ma coincides with an episode of ma- jor erosional incision in the central Great Plains in which much of the upper White 2 Mesonychians, long considered creodonts but now removed from that order (McKenna and Bell, 1997), ex- River Group is removed at a number of lo- isted in North America from the early Paleocene to mid- calities in the late Oligocene, followed by de- Eocene and undoubtedly ®lled a scavenging, omnivo- position of the lower Arikaree Group. This rous, if not strictly carnivorous role for most species. event is coincident with a prominent fall in Placed in three families (Mesonychidae, Hapalodectidae, Triisodontidae), they were nearly all larger than contem- sea level recorded during the initiation of su- porary miacoids and many creodonts. percycle TB1 in the stratigraphic record (Haq 150 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 285 et al., 1988: ®g. 14; Woodburne and Swisher, momerycids and moschids) entered the New 1995: ®g. 3). World. From ϳ26 to 24 Ma, a marked warming Thereafter, a warming climate persists trend is recorded in the ␦18O values, which from the early Miocene until the mid-Mio- fall to between 1.4½ and 2.0½. From ϳ26 cene climatic optimum (®g. 11.2, MMCO; Ma until the mid-Miocene (ϳ14±15 Ma), ox- Zachos et al., 2001), which ends at ϳ14 Ma. ygen isotope values are believed to indicate Within this interval additional warming is in- reduced global ice volume and cooler bot- dicated from ϳ17.5 to ϳ16.2±16.0 Ma (Box tom-water temperatures, punctuated by sev- Butte±Sheep Creek interval of the Heming- eral brief glaciations (Mi events). These gla- fordian NALMA). An abrupt ␦18O enrich- ciations and concomitant lowering of sea lev- ment event occurs at ϳ16 Ma, followed el may have allowed brief migrations at these again by climatic warming from ϳ16 to ϳ15 times via the Bering land bridge. Ma. From ϳ15 Ma a steady climatic deteri- At approximately the Oligocene-Miocene oration begins that continues to the present, boundary (23.7±24.0 Ma), a marked cooling during which ␦18O values increase from event occurs (Mi-1 glaciation, ®g. 11.2), ev- 1.6½ in the mid-Miocene to 3.0±5.0½ in the idenced by an enrichment spike in the ␦18O Pleistocene, accompanied by the develop- ratio to 2.6½. The turnover of large carni- ment of the Antarctic and Arctic ice sheets. vorans between Intervals A and B is closely At ϳ15±14 Ma the steady and pervasive 18 tied to the Oligocene-Miocene boundary, and enrichment in ␦ O values marks the end of to the transformation of the early to late Ari- the mid-Miocene climatic optimum (Zachos kareean mammal fauna within North Amer- et al., 2001). A dramatic shift in the large ica, an event that coincides with the strong amphicyonids (e1 event, ®g. 11.2) and the global temperature decline recorded in the progressive rise of the borophagine canids at oxygen-isotope curves. This also coincides this time suggests that the great beardogs of with a major erosional interval in the Great the late Hemingfordian and early Barstovian were somehow tied to the stable climatic in- Plains between lower Arikaree Group sedi- terval of the early and mid-Miocene. Within ments and the upper Arikaree Group. 40Ar/ Interval C, the e event marks a pronounced 39Ar dating of a tuff in the Harrison Forma- 1 transition within the large carnivoran com- tion in northwestern Nebraska indicates an munity. At ϳ14 Ma the Amphicyon-Cynelos- age of 23 Ma (sanidine, 22.9 0.08; Izett ϳ Ϯ Pliocyon group was replaced by equally and Obradovich, 2001) above a major un- large Pseudocyon and Ischyrocyon species, conformity with lower Arikaree sediments. preceded somewhat earlier by extinction of The composition of the Harrison mammal the last hesperocyonine canids. The ϳ14 Ma fauna demonstrates a profound alteration of event marks the beginning of an ␦18O enrich- taxa from the early Arikareean assemblage ment phase that persists until ϳ9 Ma, with that preceded it. Thus, the agreement of the ␦18O values steadily increasing from ϩ2.0 to faunal shift, the regional erosional event ϩ3.0 ½. The extinction of amphicyonids and 18 within the Arikaree Group, and the ␦ O en- hemicyonine ursids coincides with the 9 Ma richment spike at ϳ24 Ma are particularly date. compelling in suggesting a major climatic From ϳ9toϳ6 Ma (Interval D), there is shift at the Oligocene-Miocene boundary that a period of more uniform ␦18O values in the may be coupled with a marked global low- Zachos curve corresponding to the Hemphil- ering of sea level that takes place from ϳ23 lian NALMA. The preceding period of glob- to 20 Ma (Prentice and Matthews, 1988). Sea al cooling may have contributed to aridi®- level fall associated with the Mi-1 glaciation cation of the North American continental in- probably made possible a migration event via terior at this time, and development of the the Bering route in which late Arikareean interior grassland faunas of the late Miocene. carnivores (Ysengrinia americana, Cephalo- Two subevents during Interval D appear to gale, Zodiolestes, Cynelos) and other mam- coincide with abrupt oxygen isotope enrich- mals ( , entelodont Di- ment spikes. They occur within a brief nohyus, , small dro- warming trend from ϳ9toϳ7 Ma. At ϳ8 2004 HUNT: CENOZOIC CARNIVORES AND GLOBAL CLIMATE 151

Ma the appearance of the migrant Old World Cenozoic of North America identi®ed by ursid Indarctos corresponds to subevent e2. Webb and Opdyke (1995). Timing of these At ϳ7 Ma, an abrupt enrichment event ac- migration events was compared to the Pren- companies a signi®cant sea level fall that co- tice-Matthews (1988) ␦18O curve for the Ce- incides with the immigration of several Eur- nozoic, based on equatorial planktonic fora- asian carnivorans (Machairodus, Simocyon, minifera. Correspondences were also noted ,?Plionarctos) via the Bering with the sequence stratigraphic onlap-of¯ap route into North America. Similar abrupt en- chronology of Haq et al. (1988). Boundaries richment events also occur in the Zachos of Intervals D through G generally corre- ␦18O curve at ϳ6 Ma and ϳ4.5 Ma, which spond to the migration episodes of Webb and mark the boundaries between Intervals D±E Opdyke for the Hemphillian and Blancan and E±F. Zachos et al. (2001) recognized a land mammal ages, including the Clarendon- ``subtle warming trend'' from 6 to ϳ3.2 Ma, ian-Hemphillian boundary. More recent ``when oxygen isotope values increased re- work on faunas of the late Oligocene±early ¯ecting the onset of Northern Hemisphere Miocene Arikaree and Hemingford Groups Glaciation''. Sea level fall in Beringia may of the central Great Plains has improved the have contributed to the migration events re- resolution of taxon ranges of large carnivores corded at these times. and other mammals for Intervals A±C. 18 It becomes evident that as ␦ O values con- Webb and Opdyke (1995) noted important tinue to increase on average through the Pli- mammalian immigration events in the early ocene and Pleistocene, many instances of sea Miocene, two ®rst-order and one second-or- level lowering provide avenues across Ber- der, placed at 21, 20, and 18 Ma. These oc- ingia for mammal migration. Episodic ap- curred in the latest Arikreean (six genera), pearances in the New World of large carni- early Hemingfordian (14 genera), and medial vores with Eurasian af®nities are likely tied Hemingfordian (10 genera). They remarked to such climatic events. on the close temporal spacing of the two From 4.5 to ϳ3.2 Ma the climate in the events at ϳ21 and ϳ20 Ma. The placement early Pliocene remains somewhat stable and of the three events can be revised here so that warm, but after ϳ3 Ma a cooling trend be- they occur at 23.0, 19.2, and 18.8±18.5 Ma. gins. From ϳ3.2 to ϳ2 Ma the effect of glob- 23 Ma date is based on 40Ar/39Ar dat- al cooling and the full development of both The ϳ Antarctic and northern hemisphere ice mas- ing of the Agate Ash (Izett and Obradovich, ses undoubtedly in¯uenced the mammalian 2001) in the Harrison Formation, marking fauna of the northern continents, and resulted the initiation of upper Arikaree Group de- in the shift to a modern assemblage of large position in the Great Plains. The ϳ19.2 Ma carnivores in which large canids, ursids, and date (FT, zircon) establishes the earliest Up- felids became the dominant species. The per Harrison deposition in the same region marked shift to oxygen isotope values Ͼϩ3.0 (Hunt et al., 1983); and ϳ18.8±18.5 Ma de- occurs from ϳ3.2 to 2 Ma and corresponds ®nes the earliest Runningwater Formation to the transition from Interval F to G. sediments based on paleomagnetic calibra- At the beginning of the Pleistocene at tion (MacFadden and Hunt, 1998). The ϳ1.8 Ma the oscillations in the ␦18O values mammalian taxa associated with these inter- increase noticeably, and a succession of gla- vals are listed in table 11.2. cial-interglacial events continues from ϳ2.4 Webb and Opdyke's (1995) ``latest Ari- Ma to the present. Such climatic oscillations kareean'' event is better placed at or near the are likely related to (and perhaps largely re- Oligocene-Miocene boundary at ϳ23±23.7 sponsible for) shifts in carnivoran ranges, Ma (Interval A±B boundary). This event is and also for certain speciation events and ex- coincident with a major sea level fall (Haq tinctions, on the northern continents. et al., 1988) that could have made possible the introduction of Eurasian large carnivores COMPARISON OF INTERVALS A±G WITH CENO- and other mammals (table 11.2) into North ZOIC MAMMAL MIGRATION EVENTS America via the Bering land bridge. In the Intervals A to G are broadly comparable central Great Plains, Interval B includes 3 with land mammal migration episodes in the principal lithostratigraphic units: the two for- 152 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 285

TABLE 11.2 First and Last Occurrences of Early Miocene Mammals in the Harrison, Upper Harrison, and Runningwater Lithostratigraphic Units of Western Nebraska and Southeastern Wyoming

mations of the upper Arikaree Group (Har- mentation is observed in the Great Plains at rison Formation, ``Upper Harrison'' beds) the boundary between Intervals B1 and B2, and the oldest unit of the Hemingford Group in¯uencing faunal composition of the upper (Runningwater Formation). Mammals from Arikaree units relative to the fauna of the these rock units have collectively been Runningwater Formation. Arikaree Group named the ``Runningwater Chronofauna'' sedimentation is primarily ®ne-grained eo- (Webb and Opdyke, 1995) because of an ev- lian volcaniclastic sands and silts deposited ident faunal continuity through the interval, over a level aggrading landscape east of the particularly among the lineages Rocky Mountain uplifts (Hunt, 1990). Flu- (e.g., camels, oreodonts, rhinos, , pro- vial deposits are largely limited to broad toceratids, dromomerycids). shallow streams that reworked the ®ne- Within this chronofauna, Webb and Op- grained eolian materials but remained con- dyke (1995) identi®ed one of the most sig- ®ned to localized linear valley tracts. In con- ni®cant (®rst-order) migration events in the trast, Runningwater sediments include a later Cenozoic, involving the appearance of much greater proportion of granitic, epiclas- 14 genera of mammals in the Runningwater tic coarse sediment derived from the uplifts Formation of western Nebraska. This event to the west (Cook, 1965; Yatkola, 1978), de- occurs within Interval B, herein dated at posited within an enormous and deep paleo- ϳ18.8±18.5 Ma based on the paleomagnetic valley complex that can be traced from calibration of the initiation of Runningwater southeastern Wyoming through northwest deposition in western Nebraska by Mac- Nebraska to southwestern South Dakota. Fadden and Hunt (1998). A faunal shift This ``sudden'' appearance of riparian ¯uvial among large amphicyonine carnivores (from environments at ϳ18.8 Ma is the signature subgroup I to II, ®g. 11.1), the appearance of of Runningwater sedimentation, and these new lineages of daphoenines (Borocyon), the beds are replete with ®sh, alligator, salaman- ®rst appearance of the Eurasian ursine bear der, aquatic turtles, frogs, and mammals in Ursavus, and the extinction of temnocyo- numerous local channel deposits that remain nines and the giant mustelid Megalictis richly fossiliferous. Such depositional envi- (Hunt and Skolnick, 1996) mark this event. ronments are very rare in Arikaree rocks. It

Thus, Interval B can be subdivided into B1 is uncertain how many of the ``migrant'' and B2 segments (®g. 11.1: B1, ϳ23 to mammals of the Runningwater are the result ϳ18.8±18.5 Ma; B2, ϳ18.8±18.5 to 17.5 of the new wetter environments or whether Ma). they truly represent relatively synchronous A profound change in the style of sedi- introduction of Old World taxa into North 2004 HUNT: CENOZOIC CARNIVORES AND GLOBAL CLIMATE 153

America at this time. Many of these ®rst ap- of the Clarendonian NALMA, where a shift pearances might be predicted in such riparian to the Hemphillian faunas has long been rec- settings (e.g., the aquatic arctoid Potamoth- ognized (Tedford et al., 1987). Among the erium, otter Mionictis, mustelid Leptarctus, most striking extinctions at this time are the the ursine bear Ursavus, weasels and other loss of the great amphicyonines and hemi- small arctoid carnivorans, the procyonid cyonines that characterize much of the Mio- Edaphocyon, small merycodont antiloca- cene of North America. Similarly, the aelu- prids, and several insectivores). rodontine and cynarctine canids also fail to Nonetheless, there is an integrity to the survive the Clarendonian (Wang et al., faunas of the upper Arikaree Group and Run- 1999). ningwater beds that is terminated by the hi- Within Interval D, Webb and Opdyke rec- atus between this latter formation and the ognized as a second-order event the migra- Box Butte Formation of the medial Heming- tion of the great bear Indarctos at ϳ8 Ma, fordian, which rests disconformably on the and also events at ϳ7 and ϳ6 Ma. The ϳ8 terminal paleosol (``Platy Bench'') of the Ma event (e2 event, ®g. 11.2) corresponds to Runningwater at many localities in western Mi-7 (Miller et al., 1991) and the sea level Nebraska (Galusha, 1975). Box Butte mam- fall TB3.2 (Haq et al., 1988). The events at mals show a greater af®nity with the super- ϳ7Ma(e3 event, ®g. 11.2) and ϳ6 Ma (In- jacent fauna of the Sheep Creek Formation terval D±E boundary) also now appear to (Tedford et al., 1987; Galusha, 1975). The correspond to oxygen isotope excursions of 18 break between the Runningwater and Box the Zachos et al. (2001) ␦ O curve. Butte faunas was recognized by Webb and Also identi®ed by Webb and Opdyke is a Opdyke (1995) as a ®rst-order migration ®rst-order migration episode at ϳ5 Ma that event (with 10 migrant genera), correspond- appears to correspond to Interval E (from ϳ6 ing to the Interval B±C boundary of this to ϳ4.5 Ma). Moreover, their event at ϳ2.5 Ma, during the development of major North- study. In the Great Plains this boundary is ern Hemisphere ice sheets, correlates approx- marked by the appearance of hypsodont imately with the start of Interval G at ϳ3± horses with cement-covered cheek teeth, the 3.2 Ma. Despite slight differences in the tim- arrival of the ®rst true felids, a new large ing of these events, there is an evident cor- aceratherine rhinoceros and new mustelids, respondence between the migration events and the extinction of many taxa belonging to for Cenozoic mammals identi®ed by Webb Interval B (table 11.2). and Opdyke (1995) and the large carnivore Interval C represents a period of relative intervals proposed here. stability in North American Miocene faunas, The correlation of mammal migration culminating in the mid-Miocene climatic op- events to the oxygen isotope record from ma- timum (MMCO), after which the steady in- rine deposits of the global ocean was dis- 18 crease in ␦ O values heralds the initiation of cussed by Webb and Opdyke (1995) as the global cooling and the development of large- probable result of development of land bridg- volume Antarctic ice. Near the onset of cli- es (in the Neogene via the Bering Route) co- matic deterioration, the e1 subevent records incident with lowered sea level, accompanied the replacement of the large amphicyonine by climate shifts that in and of themselves fauna (Amphicyon, Cynelos, Pliocyon) of the transformed the North American ecosystem. early Barstovian by the large amphicyonines The continuing re®nement of mammalian (Pseudocyon, Ischyrocyon) of the medial-late biochrons in North America, based on new Barstovian (Hunt, 1998b). Webb and Opdyke collections and intensive study of older ma- (1995) remarked on the notable decline in terials, and accompanied by more detailed diversity of browsing and mixed feeding un- ®eld geological efforts, will provide a steadi- gulates following the mid-Miocene cooling, ly improving database for such investiga- and their replacement by grazing forms. The tions. cooling event seems to favor the radiation of borophagine canids in North America as ap- ACKNOWLEDGMENTS propriate predators of the grazing ungulates. For making available the Frick Collection The Interval C±D boundary marks the end of carnivores at the American Museum over 154 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 285 several decades, I am especially grateful to T.G. Bromage and F. Schrenk (editors), African M. C. McKenna and R. H. Tedford. Field and biogeography, climate change, and human evo- laboratory research was funded in part by the lution: 94±114. New York: Oxford University H. B. Meek Fund of the University of Ne- Press. braska and by grants from the National Geo- Emry, R.J., and R.E. Eshelman. 1998. The early Hemingfordian (early Miocene) Pollack Farm graphic Society. The two ®gures were pre- local fauna: ®rst Tertiary land mammals de- pared by UNSM illustrator Angie Fox, scribed from Delaware. In R.N. Benson (edi- whose skills and helpful suggestions were es- tor), and paleontology of the lower sential to the project. My colleague M. R. Miocene Pollack Farm fossil site, Delaware. Voorhies shared his expertise on Barstovian Delaware Geological Survey Special Publica- and Clarendonian faunas in several discus- tion 21: 153±173. sions that improved my understanding of Flynn, J.J. 1998. Early Cenozoic Carnivora these intervals. (``Miacoidea''). In C. Janis, K. Scott, and L. Malcolm McKenna served as mentor, ad- Jacobs (editors), Evolution of Tertiary mam- visor, and friend during my graduate study at mals of North America: 110±123. New York: Cambridge University Press. the American Museum from 1965±70; his Frailey, D. 1979. The large mammals of the Buda scholarship so ably evidenced in his studies local fauna (Arikareean: Alachua County, Flor- in mammalian paleontology has served as an ida). Bulletin of the Florida State Museum, Bi- example many of us have attempted to fol- ological Sciences 24(2): 123±173. low, yet not equaled in originality and in- Fremd, T., E.A. Bestland, and G.J. Retallack. sight. I appreciate the invitation of Gina 1994. John Day basin paleontology ®eld trip Gould and Susan Bell to contribute this pa- guide and road log. 1994 Society of Vertebrate per. Paleontology Annual Meeting, Seattle, WA. Kimberly, OR: John Day Fossil Beds National Monument Publication 94-1. REFERENCES Galusha, T. 1975. Stratigraphy of the Box Butte Albright, L.B. 1996. Insectivores, rodents, and Formation, Nebraska. Bulletin of the American carnivores of the Toledo Bend local fauna: an Museum of Natural History 156: 1±68. Arikareean (earliest Miocene) assemblage from Haq, B.U., J. Hardenbol, and P.R. Vail. 1988. Me- the Coastal Plain. Journal of Vertebrate sozoic and Cenozoic chronostratigraphy and Paleontology 16(3): 458±473. eustatic cycles. Society of Economic Paleon- Berggren, W.A., D.V. Kent, C.C. Swisher, and tologists and Mineralogists Special Publication M.P. Aubry. 1995. A revised Cenozoic geo- 42: 71±108. chronology and chronostratigraphy. In W.A. Hunt, R.M., Jr. 1990. Taphonomy and sedimen- Berggren, D.V. Kent, M.P. Aubry, and J. Har- tology of Arikaree (lower Miocene) ¯uvial, eo- denbol (editors), Geochronology, time scales lian, and lacustrine paleoenvironments, Nebras- and global stratigraphic correlation: 129±212. ka and Wyoming. In M.J. Lockley and A. Rice Tulsa: SEPM (Society for Sedimentary Geolo- (editors), Volcanism and fossil biotas. Geolog- gy) Special Publication 54. ical Society of America Special Paper 244: 69± Berta, A. 1987. Origin, diversi®cation, and zoo- 111. geography of the South American . In Hunt, R.M., Jr. 1996. Amphicyonidae. In D.R. B.D. Patterson and R.M. Timm (editors), Stud- Prothero and R.J. Emry (editors), The terrestrial ies in neotropical mammalogy: essays in honor Eocene-Oligocene transition in North America: of Philip Hershkovitz. Fieldiana: Zoology (n.s.) 476±485. New York: Cambridge University 39: 455±471. Press. Bryant, H.N. 1991. Phylogenetic relationships and Hunt, R.M., Jr. 1998a. Ursidae. In C. Janis, K. systematics of the (Carnivora). Scott, and L. Jacobs (editors), Evolution of Ter- Journal of Mammalogy 72: 56±78. tiary mammals of North America: 174±195. Bryant, H.N. 1996. Nimravidae. In D.R. Prothero New York: Cambridge University Press. and R.J. Emry (editors), The terrestrial Eocene- Hunt, R.M., Jr. 1998b. Amphicyonidae. In C. Jan- Oligocene transition in North America: 453± is, K. Scott, and L. Jacobs (editors), Evolution 475. New York: Cambridge University Press. of Tertiary mammals of North America: 196± Cook, H.J. 1965. Runningwater Formation, mid- 227. New York: Cambridge University Press. dle Miocene of Nebraska. American Museum Hunt, R.M., Jr., and R.I. Skolnick. 1996. The gi- Novitates 2227: 1±8. ant mustelid Megalictis from the early Miocene Denton, G.H. 1999. Cenozoic climate change. In carnivore dens at Agate National Monument, 2004 HUNT: CENOZOIC CARNIVORES AND GLOBAL CLIMATE 155

Nebraska: earliest evidence of dimorphism in level history, and continental margin erosion. New World (Carnivora, Mamma- Paleooceanography 2(1): 1±19. lia). Contributions to Geology, University of Miller, K.G., J.D. Wright, and R.G. Fairbanks. Wyoming 31(1): 35±48. 1991. Unlocking the icehouse: Oligocene-Mio- Hunt, R.M., Jr., and R.H. Tedford. 1993. Phylo- cene oxygen isotopes, eustasy, and margin ero- genetic relationships within the aeluroid Car- sion. Journal of Geophysical Research 96(B4): nivora and implications of their temporal and 6829±6848. geographic distribution. In F. Szalay, M.C. Mc- Montanari, A., A. Deino, R. Drake, B. Turrin, D. Kenna, and M.J. Novacek (editors), Mammal DePaolo, G. Odin, G. Curtis, W. Alvarez, and phylogeny: placentals: 53±73. New York: D. Bice. 1988. Radioisotopic dating of the Eo- Springer-Verlag. cene-Oligocene boundary in the pelagic se- Hunt, R.M., Jr., X.-X. Xue, and J. Kaufman. 1983. quence of the northeastern Apennines. In I. Pre- Miocene burrows of extinct beardogs: indica- moli-Silva, R. Coccioni, and A. Montanari (ed- tion of early denning behavior of large mam- itors), The Eocene-Oligocene boundary in the malian carnivores. Science 221: 364±366. Marche-Umbria basin (Italy), Ancona, Italy: Izett, G.A., and J.D. Obradovich. 2001. 40Ar/39Ar 195±208. International Union of Geological ages of Miocene tuffs in basin-®ll deposits Sciences, Commission on Stratigraphy. (, New Mexico, and Trouble- Prentice, M.L., and R.K. Matthews. 1988. Ceno- some Formation, Colorado) of the Rio Grande zoic ice-volume history: development of a system. The Mountain Geologist 38(2): 77±86. composite oxygen isotope record. Geology 16: Joeckel, R.M. 1990. A functional interpretation of 963±966. the masticatory system and paleoecology of en- Prothero, D.R., and R.J. Emry. 1996. Summary. telodonts. Paleobiology 16: 459±482. In D.R. Prothero and R.J. Emry (editors), The terrestrial Eocene-Oligocene transition in North KurteÂn, B., and E. Anderson. 1980. Pleistocene America: 664±683. New York: Cambridge Uni- Mammals of North America. New York: Co- versity Press. lumbia University Press. Schaller, G.B. 1972. The Serengeti lion: a study LeÂveÃque, F. 1993. Correlating the Eocene-Oligo- of predator-prey relations. Chicago: University cene mammalian biochronological scale from of Chicago Press. SW with the marine magnetic anomaly Sinclair, A.R.E., and M. Norton-Grif®ths (edi- sequence. Journal of the Geological Society of tors). 1979. Serengeti: dynamics of an ecosys- London 150: 661±664. tem. Chicago: University of Chicago Press. MacFadden, B.J., and R.M. Hunt, Jr. 1998. Mag- Tedford, R.H., and D. Frailey. 1976. Review of netic polarity stratigraphy and correlation of the some Carnivora (Mammalia) from the Thomas Arikaree Group, Arikareean (late Oligocene- Farm local fauna (Hemingfordian: Gilchrist early Miocene) of northwestern Nebraska. Geo- County, Florida). American Museum Novitates logical Society of America Special Paper 325: 2610: 1±9. 143±165. Tedford, R.H., and J. Martin. 2001. Plionarctos, Martin, L.D. 1998. . In C. Janis, K. Scott, a tremarctine bear (Ursidae: Carnivora) from and L. Jacobs (editors), Evolution of Tertiary western North America. Journal of Vertebrate mammals of North America: 236±242. New Paleontology 21(2): 311±321. York: Cambridge University Press. Tedford, R.H., M.F. Skinner, R.W. Fields, J.M. Matthew, W.D. 1909. The Carnivora and Insectiv- Rensberger, D.P. Whistler, T. Galusha, B.E. ora of the Bridger Basin, middle Eocene. Mem- Taylor, J.R. Macdonald, and S.D. Webb. 1987. oirs of the American Museum of Natural His- Faunal succession and biochronology of the tory 9: 291±567. Arikareean through Hemphillian interval in McKenna, M.C., and S.K. Bell. 1997. Classi®ca- North America. In M.O. Woodburne (editor), tion of mammals above the species level. New Cenozoic mammals of North America: 153± York: Columbia University Press. 210. Berkeley: University of California Press. Mellett, J.S. 1969. A skull of Hemipsalodon Wang, X.-M. 1994. Phylogenetic systematics of (Mammalia, Deltatheridia) from the Clarno the (Carnivora: Canidae). Formation of Oregon. American Museum Nov- Bulletin of the American Museum of Natural itates 2387: 1±19. History 221: 1±207. Mellett, J.S. 1977. Paleobiology of North Ameri- Wang, X.-M., R.H. Tedford, and B.E. Taylor. can Hyaenodon (Mammalia, Creodonta). Con- 1999. Phylogenetic systematics of the Boro- tributions to Vertebrate Evolution 1: 1±134. phaginae (Carnivora: Canidae). Bulletin of the Miller, K.G., R.G. Fairbanks, and G.S. Mountain. American Museum of Natural History 243: 1± 1987. Tertiary oxygen isotope synthesis, sea 391. 156 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 285

Webb, S.D., and N.D. Opdyke. 1995. Global cli- SEPM (Society for Sedimentary Geology) Spe- matic in¯uence on Cenozoic land mammal fau- cial Publication 54. nas. In S.D. Webb, and N.D. Opdyke (editors), Yatkola, D.A. 1978. Tertiary stratigraphy of the Effects of past global change on life: 184±208. Niobrara River valley, Marsland Quadrangle, , DC: National Academy Press. western Nebraska. Nebraska Geological Survey Woodburne, M.O., and C.C. Swisher, III. 1995. Special Paper 19: 1±66. Zachos, J., L.D. Stott, and K.C. Lohmann. 1994. Land mammal high-resolution geochronology, Evolution of early Cenozoic marine tempera- intercontinental overland dispersals, sea level, tures. Paleooceanography 9(2): 353±387. climate, and vicariance. In W.A. Berggren, D.V. Zachos, J., M. Pagani, L. Sloan, E. Thomas, and Kent, M.P. Aubry, and J. Hardenbol (editors), K. Billups. 2001. Trends, rhythms, and aberra- Geochronology, time scales and global strati- tions in global climate 65 Ma to Present. Sci- graphic correlation: 335±364. Tulsa, OK: ence 292: 686±693.