POSTCRANIAL ANATOMY OF VIVERRAVUS (MAMMALIA, CARNIVORA)AND IMPLICATIONS FOR SUBSTRATE USE IN BASAL CARNIVORA

RONALD E. HEINRICH1 and PETER HOUDE2

1Division of Biological Sciences, University of Kansas, Lawrence, KS 66045, U.S.A., heinrich@ku.edu;2Department of Biology, New Mexico State University, Las Cruces, NM 88003, U.S.A., phoude@nmsu.edu

ABSTRACT—A postcranial skeleton of the viverravid carnivoran, Viverravus acutus from the early Eocene of Wyoming,is described and compared to contemporary carnivorans (the viverravid Didymictis, and the miacids Miacis and Vulpa-vus), and to extant taxa belonging to the families Mustelidae, Procyonidae, Canidae, Viverridae, and Herpestidae. Basedon humeral and femoral midshaft diameters, body mass for this animal is estimated to have been about 250 g, less thanall but a few living species of Carnivora. Shoulder and hip morphology indicate a considerable range of motion and thestructure of the humeroulnar joint suggests habitually flexed postures, characteristics typical of extant carnivorans that areexcellent climbers. These similarities are also shared with miacids, supporting the hypothesis that basal members of theorder Carnivora were well adapted for exploiting arboreal habitats. An astragalus tentatively attributed to the middleEocene species Viverravus gracilis, however, is similar to that of Didymictis and suggests a greater emphasis on terrestriallocomotion than is found in miacids.

INTRODUCTION

Carnivora is a taxonomically, morphologically, and behav-iorally diverse mammalian order. With respect to locomotionand substrate use, the range of specializations exhibited by livingcarnivorans is exceptional: carnivorans show adaptations forspeed in open habitats, controlled climbing through tropical for-ests, and seemingly effortless movement through the world’soceans. This array of locomotor behaviors evolved from one orboth of the early Tertiary families Miacidae and Viverravidae, anassortment of carnivorans whose phylogenetic relationships toextant Carnivora remain incompletely understood. The work ofWang and Tedford (1994) is the strongest evidence yet that livingcaniforms (i.e., Canidae, Ursidae, Procyonidae, Mustelidae, andthe Pinnipedia) evolved from taxa included within Miacidae.Postcranial skeletons of all five early Eocene genera are rela-tively well known and generally similar in morphology althoughranging between 1 and 10 kg in body mass (Heinrich, 1995).Analyses of this material have concluded that the joints of theforelimb and hind limb were capable of a large range of powerfulmotion, suggesting habitual use of flexed and abducted limb pos-tures, as is found in modern carnivorans that regularly climbvertical supports (Matthew, 1909, 1915; Heinrich and Rose, 1995,1997). Some of these animals, such as Vulpavus, probably reliedmore heavily on arboreal habitats for foraging than others suchas Miacis, but all of the miacids were probably excellent climbersincluding the largest of the taxa, Vassacyon.

Less well understood is the origin of extant feliform carniv-orans (i.e., Felidae, Hyaenidae, Herpestidae, and Viverridae)collectively referred to as the Aeluroidea. Some workers haveargued that aeluroids were also derived from some, as of yet,unidentified member of Miacidae (Gingerich, 1983; Gingerichand Winkler, 1985, Wesley-Hunt and Flynn, 2005) whileothers, dating back to the late 1800s and early 1900s (Flower andLydekker, 1891; Wortman and Matthew, 1899; Matthew, 1909;Gregory and Hellman, 1939), concluded that aeluroids evolvedfrom viverravids, a hypothesis that was revived by the early cla-distic analysis of Flynn and Galiano (1982, see also Flynn, 1998).Hunt and Tedford (1993) have even elaborated on a specificrelationship between the viverravids and aeluroids notingdental similarities between the North American middle Eocenespecies Viverravus sicarius and the European Oligocene aeluroid

Anictis, leading them to conclude that “. . . the modern aeluroidpostcranial skeleton is probably derived from Eocene speciescurrently attributed to Viverravus. . .” (1993:65). To date, how-ever, the only description of Viverravus postcrania is that ofMatthew (1909) who, in describing a middle Eocene specimen ofViverravus gracilis (see below), stated that it, “. . .so far as maybe judged from the fragments preserved, was proportionatelysmall, with slender digitigrade pes, and grooved astragalar troch-lea . . .” (1909:356). The only viverravid for which considerablepostcranial remains are known is the larger late Paleocene–earlyEocene genus Didymictis. Heinrich and Rose (1997) concludedthat Didymictis was a primarily terrestrial and at least incipientlycursorial animal with locomotor behaviors comparable to thoseof the Oriental civet, Viverra. This dichotomy of locomotor ad-aptations and inferred substrate use between miacids on the onehand and Didymictis on the other, raises the question of whetherbasal members of Carnivora (defined here as the common an-cestor of Miacidae and Viverravidae plus all of their descen-dants) were habitually terrestrial, primarily arboreal, or equallyat home on the ground and in the trees. Alternatively, as somehave suggested, were the earliest carnivorans specialized for asemifossorial existence like small mustelids that chase rodentsinto their burrows (Gambaryan, 1974), or like the less agile andambulatory striped skunk, Mephitis (Van de Graaff et al., 1982)?A partial skeleton of Viverravus provides not only the first goodinformation about locomotor behavior in the genus but, moreimportantly, helps shed light on our understanding of the diver-sification of locomotor behaviors during early stages of carniv-oran evolution.

Institutional Abbreviations—AMNH, American Museum ofNatural History; USGS-USNM, United States GeologicalSurvey-National Museum of Natural History; USNM, NationalMuseum of Natural History.

MATERIALS AND METHODS

USNM 489122 and AMNH 11514

USNM 489122 was removed from a large calcareous nodulecollected at Houde site 8 within or near University of Michigansite SC-4, an early Wasatchian (Wa1) locality (Gingerich, 2001)in the Clark’s Fork Basin of northwestern Wyoming. This nod-ule, 8ABC, contains a vast array of vertebrate remains ranging

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from fully articulated to completely disarticulated skeletons, andrepresents the largest faunal assemblage yet recorded from acalcareous nodule in the Clark’s Fork Basin (see Houde [1988]and Bowen and Bloch [2002] for discussion of the depositionalenvironment and taphonomy of these fossiliferous limestones).Identified taxa are predominantly birds and arboreal mammals,including representatives of Sandcoleidae, Didelphidae, Leptic-tidae, Nyctitheriidae, Microsyopidae, Micromomyidae, Paramo-myidae, and Apatamyidae (Houde and Olsen, 1992; Bloch andBowen, 2001; Bloch and Boyer, 2001). USNM 489122 was foundarticulated except for those parts of the skeleton missing due topostdepositional bioturbation processes (Bowen and Bloch,2002). The skeleton was removed from the surrounding matrixusing a seven percent acetic acid preparation and some manualcleaning, although some parts of the skeleton remain in matrix.

The partial skeleton is attributed to Viverravus acutus on thebasis of its location within the stratigraphic section as well asdescriptions and dental measurements given for this species byPolly (1997). The skull is crushed and flattened, but most of thedentition is preserved as are parts of the basicranium (Pollyet al., in press) including both petrosals and a partial ossifiedauditory bulla, the first identified for any early Tertiary carniv-oran. Other preserved axial-skeleton elements include severalrib fragments, the atlas, five isolated thoracic vertebrae of vary-ing degrees of completeness, and a section of 12 nearly completevertebrae that includes the last thoracic, all seven lumbars, thesacrum, and the first coccygeal vertebra. Forelimb elements in-clude part of the left scapula, a complete left and distal righthumerus, nearly complete left radius and ulna, distal right radius,right second metacarpal, left pisiform, and a proximal phalanx.Hind-limb elements include right and left os coxae, nearly com-plete right femur, and the proximal right tibia. Although theentire permanent dentition had erupted, epiphyseal fusion wasincomplete at the distal ends of the radius, ulna, and femur andthe proximal end of the tibia, indicating that the animal had notyet reached skeletal maturity at the time of death.

The Bridgerian specimen of Viverravus gracilis, AMNH 11514,described by Matthew (1909), includes fragmentary right and leftdentaries (the right dentary is better preserved and includes p4and the talonid of m1 as well as alveoli from the canine to thesecond molar), an isolated premolar, the distal part of a meta-podial, and an astragalus. Assignment of the postcrania to thesame skeleton as the teeth is problematic, but the size of theastragalus and metapodial, as well as the bleached coloring ofthese two bones, is consistent with this interpretation. Additionalsupport for the association is found in the similarity between theastragalus of AMNH 11514 and astragali of Didymictis. Giventhis evidence and the importance of tarsals in reconstructinglocomotor adaptations of fossil mammals, a description andanalysis of AMNH 11514 is prudent, although it should be keptin mind that the Bridgerian deposits are on the order of 5–6million years younger than those from which USNM 489122 wascollected.

Taxa Used For Comparison

USNM 489122 and AMNH 11514 are compared to the earlyEocene viverravid Didymictis protenus and the miacids Miacispetilus and Vulpavus canavus, all three of which have been de-scribed in detail elsewhere (Heinrich and Rose, 1995, 1997). Av-erage body mass estimates for these taxa are 1.3 kg for M. petilus(one specimen), 4.5 kg for V. canavus (range of 3.5–6.0 for threespecimens), and 5.0 kg for D. protenus (range of 3.9–7.2 for eightspecimens).

Very few taxa among living carnivorans have adult bodymasses that approach, let alone overlap, the small size estimatedfor USNM 489122 (see below) and, of those that do, most belongto the genus Mustela. Comparisons here are made to 10 species

from five carnivoran families (Table 1). These taxa representmuch of the locomotor diversity exhibited by the order (aquaticadaptations excluded) including cursorial (Fennec fox) and ar-boreal (kinkajou and African palm civet) specializations, adap-tations for digging (spotted skunk), and more generalized behav-ior where animals are strictly ground-dwelling (mongooses) oradditionally utilize trees (martens and genets). Because of theircomparable size, however, the most detailed comparisons aremade to three taxa: the long-tailed weasel, Mustela frenata; thespotted skunk, Spilogale putorius; and the dwarf mongoose,Helogale parvula. Helogale is the most committed to the ground,climbing even sloping trunks poorly (Kingdon, 1977). Spilogalecan climb but is primarily terrestrial and ambulatory (i.e. gener-ally employs a walk rather than a trot as in Helogale or a half-bound as in Mustela), and uses its specialized forelimbs for dig-ging out subterranean prey (Kinlaw, 1995). The long-tailed wea-sel is the most likely to be found in trees, readily climbing inpursuit of prey and to avoid predators (Sheffield and Thomas,1997, and references therein) but it also hunts in rodent burrowsunderground and the relatively short limbs of all members of thegenus are generally thought to be a morphological adaptation forthis behavior (Gambaryan, 1974; King, 1989).

Of the larger carnivorans considered, the African palm civet,Nandinia binotata, is of particular interest for two reasons. First,unlike carnivorans that are described as good climbers, Nandiniaspends most of its time in the trees and possesses adaptations forthis arboreality that include powerful and highly flexible limbs,the capacity to descend a trunk in a controlled headfirst manner,and a muscular tail that can be used as a brace when its forepawsare holding prey (Taylor, 1970; Kingdon, 1977). Nandinia is alsoimportant because it has been argued to be the sister taxon to allextant aeluroids (Hunt, 1987, 1989; Flynn, 1996; Yoder et. al.,2003). If this latter contention is correct and if Viverravus is nearthe base of the aeluroid diversification, then comparisons be-tween Nandinia and Viverravus might be expected to reveal anumber of skeletal similarities indicative of arboreal adaptation.

Muscle and Body Mass Reconstructions

Since there are inherent difficulties in reconstructing musclesfor fossils (Bryant and Seymour, 1990), attachment sites for spe-cific muscles are inferred for several of the more distinct bonyrugosities following the Extant Phylogenetic Bracket (EPB)method for soft-tissue reconstruction (Witmer, 1995). The extantoutgroups ‘bracketing’ Viverravus in this analysis (Fig. 1) areCaniformia (including Mustelidae, Procyonidae, and Canidae)and Aeluroidea (including Viverridae and Herpestidae); only

TABLE 1. Extant taxa used in this study for comparative purposes.Body mass (kg) ranges from Nowak (1991).

Taxon Common nameBodymass Locomotor

MUSTELIDAEMustela frenata Long-tailed weasel 0.1–0.4 ScansorialSpilogale putorius Spotted skunk 0.2–1.0 SemifossorialMartes americana Pine marten 0.3–1.3 Scansorial

PROCYONIDAEBassariscus astutus Ringtail 0.8–1.3 ScansorialPotos flavus Kinkajou 1.4–4.6 Arboreal

CANIDAEFennecus zerda Fennec fox 1.0–1.5 Terrestrial/

CursorialHERPESTIDAE

Helogale parvula Dwarf mongoose 0.2–0.7 TerrestrialHerpestes ichneumon Gray mongoose 1.7–4.0 Terrestrial

VIVERRIDAEGenetta genetta Common genet 1.0–2.3 ScansorialNandinia binotata African palm civet 1.7–2.1 Arboreal

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muscles that can be reconstructed unambiguously are consid-ered. Of the taxa used for comparison in this study, musculo-skeletal descriptions are available for the spotted skunk (Hall,1926), pine marten (Leach, 1976), kinkajou (Beswick-Perrin,1871), African palm civet, common genet and the dwarf and graymongooses (Taylor, 1974, 1976), and the domestic dog (Evans,1993, considered for purposes here to be identical to the Fennecfox).

The importance of body mass as an ecological variable in gen-eral, and as a factor related to locomotor behavior and substrateuse specifically, has been discussed at length elsewhere (Damuthand McFadden, 1990, and references cited therein), and a num-ber of allometric regressions using dental and postcranial mea-surements have been derived for estimating body mass in fossilcarnivorans (Legendre and Roth, 1988; Anyonge, 1993; Egi,2001). Here body mass is estimated for USNM 489122 usingregressions derived (1) from the length of the lower first molar(Van Valkenburgh, 1990) and (2) from bone length and averagemidshaft diameters of the humerus and femur (Heinrich, 1995).Although these two sets of regressions include behaviorally andphylogenetically diverse carnivoran species, few taxa in theanalyses approach the small body size of USNM 489122 and,therefore, these regressions are supplemented by a third data setusing humeral and femoral length and diameters calculated foreight species of Mustela ranging in body size from 30 to 1500 g(Appendix I). In this latter data set, external measurements weretaken using digital calipers for between 10 and 23 specimens perspecies with averages for males and females calculated sepa-rately. As in the two other studies, least squares regres-sions, correlation coefficients, and the standard error of esti-mate (SEE) were calculated from log-transformed data usingSYSTAT version 5.1 (Wilkinson, 1989). The percent SEE, ameasure of the predictive strength of these regressions (smallervalues are indicative of greater predictive power), was also cal-culated for each regression following Smith (1984).

Body mass estimates for USNM 489122 are given in Table 2.Regressions based on lower first molar length (Data Set I) giveestimates of 170 g and 480 g, while humeral and femoral regres-sions provide estimate ranges of 243–364 g (Data Set II) and143–265 g (Data Set III). The relatively narrow ranges providedby the postcranial regressions, and particularly the midshaft di-ameters, are consistent with the argument that the most accurateestimates of body mass in terrestrial vertebrates will be thosethat are based on parameters directly involved in weight bearing

(Jungers, 1990). Given that the estimates derived for humeraland femoral midshaft diameters are all very similar (230, 243, 251and 265 g), it seems likely that the skeleton of USNM 489122belonged to an animal that weighed about 250 g, an estimate verysimilar to that provided by Polly (2001) for V. acutus based onthe area of m1. The mandible of AMNH 11514 is substantiallylarger than that of USNM 489122. Although neither of the lowerfirst molars is complete, a length of 6.0 mm—the distance fromthe caudal-most part of the fourth premolar to the end of the m1trigonid—was estimated for this animal, giving body mass esti-mates of 1.10 and 1.03 kg using the regression equations of VanValkenburgh (1990, Data Set I).

DESCRIPTIONS AND COMPARISONS

Axial Skeleton

The dentition of Viverravus acutus has been described else-where (Polly, 1997) as has the base of the skull of USNM 489122(Polly et al., in press). Descriptions and comparisons are limitedhere to the postcranial elements of USNM 489122 and AMNH11514.

Vertebrae—Eighteen vertebrae (fifteen nearly complete and4 partial) of USNM 489122 were recovered; the atlas, five sepa-rate thoracic specimens, and an intact section of the vertebralcolumn including the last thoracic, seven lumbar, three sacral,and the first coccygeal vertebrae. The atlas (Fig. 2A) has sufferedsome crushing and distortion and the alar wings lateral to thetransverse foramina are not preserved. At its widest pointsthe atlas measures 9.9 mm mediolaterally and 5.5 mm dorso-ventrally. Incomplete preservation makes it impossible to articu-late the five isolated thoracic vertebrae but what is preserved ofthese specimens indicates that they are all from the caudal partof the thoracic region and are likely successive vertebrae. Thelast thoracic is in association with the first lumbar and can beclearly identified by the presence of articular facets on the cen-trum for articulation with the head of the last rib. The spinousand transverse processes of the thoracic, lumbar, and sacral ver-tebrae are relatively less robust than comparable vertebrae ofMustela, Spilogale, and Helogale. The sacrum of USNM 489122

FIGURE 1. Cladogram after Flynn (1998) showing a hypothetical phy-logenetic relationship between Viverravus and the extant taxa (Table 1)considered in this study. The alternative phylogenetic hypothesis (Gin-gerich and Winkler, 1985; Wesley-Hunt and Flynn, 2005) is that viver-ravids form the sister group to extant Carnivora.

TABLE 2. Body mass estimates for USNM 489122.

Skeletal variable Slope Intercept r SE %SEE Mass

DATA SET ILength of m1-A 2.97 −2.27 0.83 0.377 138 170 gLength of m1-B 1.21 −0.93 0.42 0.299 99 480 g

DATA SET IIHumeral Length 2.152 −3.630 0.86 0.213 63.3 298 gAve. Humeral

Diameter 2.389 −1.544 0.95 0.113 30.4 243 gFemoral Length 2.156 −3.741 0.86 0.209 61.8 364 gAve. Femoral

Diameter 2.646 −1.721 0.95 0.119 31.5 251 gDATA SET III

Humeral Length 3.821 −3.355 0.99 0.094 24.2 143 gAve. Humeral

Diameter 3.363 1.053 0.99 0.082 20.7 230 gFemoral Length 3.475 −2.925 0.98 0.096 24.7 249 gAve. Femoral

Diameter 3.390 0.989 0.99 0.096 24.8 265 g

Data Set I consists of allometric regressions from Van Valkenburgh(1990) where regression A is based on data for 72 species ranging in sizefrom 140 to 210 kg and B is data for the 30 species weighing less than 6kg. Data Set II consists of allometric regressions from Heinrich (1995)calculated for 24 carnivoran species with mean body masses between 500g and 15 kg. Data Set III consists of regression equations derived foreight species of Mustela (see Appendix I).Abbreviations: r, correlation coefficient; SE, standard error of estimate;%SEE, percentage standard error of estimate; Mass, estimated bodymass.

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(Fig. 2B) is slightly longer than that of Mustela, but is similar inincluding three vertebrae (the sacra of Spilogale and Helogaleeach consist of two vertebrae). It differs from all three genera,however, in that the auricular surface for articulation with theilium is not expanded dorsoventrally, thus reducing the sur-face area of contact between the two bones (see also descriptionof os coxae below). Conversely, the first coccygeal vertebra ofUSNM 489122 is absolutely as large as that of Mustela frenata,which has 19–23 caudal vertebrae and a tail length 44–70% ofbody length (Sheffield and Thomas, 1997), strongly suggestingthat Viverravus acutus had a strong, well-developed tail. Amongmodern carnivorans, vertebral numbers range from four to eightfor lumbars and two to four for sacrals (Ewer, 1973); the sevenlumbar and three sacral vertebrae of USNM 489122 may wellrepresent the plesiomorphic condition for Carnivora. Finally, us-ing the vertebral proportions found in Mustela and the length ofthe vertebral column from last thoracic to the end of the sacrumin USNM 489122 (63 mm), the length of the vertebral columnminus the tail is estimated to have been about 16 cm for USNM489122.

Forelimb

Scapula—A fragment of the left scapula is preserved that in-cludes the glenoid cavity, coracoid process, and portions of thesupra- and infraspinous fossae (Fig. 3A). The glenoid cavity is 3.9mm long and has a maximum width of 2.4 mm. The supraglenoidtubercle projects well beyond the margin of the cavity, giving theglenoid a distinctly concave appearance in lateral view. The cora-coid process for attachment of the shoulder adductor m. pecto-ralis minor and the adductor, rotator, and protractor m. coraco-brachialis (and in some taxa the forearm flexor m. bicepsbrachii), extends medial to the glenoid cavity and is extremelywell developed. No acromion process is preserved, and the rootof the scapula, separating a larger supra- from smaller infra-scapular fossa, is set well back from the margin of the glenoid.

The well-developed supraglenoid tubercle and resulting con-cave glenoid cavity of USNM 489122 is very similar to that ofDidymictis and Mustela (Fig. 3B) and unlike the shallow glenoidof miacids and Spilogale (Fig. 3C). No intact coracoid process isknown for Miacis or Vulpavus, but that of Didymictis is not as

relatively well developed as in USNM 489122. In modern car-nivorans, the largest coracoid processes are found in scansorialand arboreal taxa but in none of these is it as relatively large asin Viverravus. Coracoids are exceptionally well developed in treeand flying squirrels, and it has been hypothesized that this mayenhance the adductive force generated by m. coracobrachialis todrive claws into the tree during climbing (Essner and Scheibe,2000). In sciurids (Thorington et al., 1997) and several car-nivoran taxa studied (Martes, Potos, and some specimens ofGenetta), m. coracobrachialis has two separate muscle bellies; abrevis that inserts into the proximal humerus (found in nearly allcarnivorans), and a longus that inserts just proximal to the ent-epicondylar foramen. The large coracoid process of Viverravus

FIGURE 2. Selected vertebrae of Viverravus acutus USNM 489122.A, the atlas in cranial (bottom) and caudal (top) views. B, the sacrumin cranial (bottom) and dorsal (top) views. C, the first coccygeal ver-tebra in cranial (bottom) and dorsal (top) views. Abbreviations: as,auricular surface; tf, transverse foramen; tp, transverse process. Scale barequals 5 mm.

FIGURE 3. Left scapula of (A) Viverravus acutus USNM 489122, (B)Mustela, and (C) Spilogale in ventral (left) and lateral (right) views.Abbreviations: cp, coracoid process; gc, glenoid cavity; sg, supraglenoidcavity. Scale bars equal 5 mm.

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may indicate that a two-headed m. coracobrachialis condition isplesiomorphic for Carnivora and retained in lineages wherestrong adduction, protraction and rotation of the humerus, allmotions associated with climbing, are maintained. In Spilogale, ataxon that does not possess a m. coracobrachialis (Hall, 1926),the coracoid is nearly absent (Fig. 3C).

Humerus—Only the distal third of the right element is pre-served while the left humerus is complete (Fig. 4A) and mea-sures 27.7 mm from the proximal-most aspect of the head to thedistal articular surface between capitulum and trochlea. There issome slight crushing along the length of the bone but this doesnot appear to have significantly altered its shape or proportions.The humeral head is nearly as wide (3.8 mm) as it is long (4.0mm), and a distinct epiphyseal line is present between head anddiaphysis. The greater tubercle extends above the humeral head,and a well-defined intertubercular groove lies between this andthe robust, lesser tubercle which is the insertion site of an im-portant medial rotator of the shoulder, m. subscapularis. Distinctattachment sites are also present on the greater tubercle for thelateral rotators of the shoulder, m. teres minor and m. infraspi-natus. The diaphysis is strongly compressed mediolaterally (cra-niocaudal and mediolateral diameters are 3.2 and 1.7 mm, re-spectively) and the deltopectoral crest extends along the cranialmargin of the humerus to about midshaft, tapering to a narrowridge distally. An entepicondylar foramen and well-developedmedial epicondyle for forearm flexor musculature are present,and a strong lateral supracondylar crest merges with the diaph-ysis proximally about one-fourth the length of the bone. Thecapitulum for articulation with the radial head is twice as wide asthe more distally projecting trochlea, and the articular surface ofthe capitulum extends proximolaterally toward the base of thelateral supracondylar crest. A weakly developed coronoid fossacan be distinguished from the relatively wide and deep radialfossa just proximal to the capitulum. Caudally, the olecranonfossa is shallow and proximodistally short, and between the me-

dial epicondyle and trochlea is a well-delineated pit that prob-ably served as an attachment site for the olecranon ligament,which binds the distal humerus to the olecranon process of theulna.

The humeral heads of extant carnivorans tend to be moreelliptical (long axis is craniocaudal) than that of Viverravus andmiacids. In primarily terrestrial taxa (Fig. 4C) and animals thatdig such as Spilogale, the humeral head is considerably larger inproportion to the rest of the bone. Like Viverravus, the lowgreater tubercle of miacids and mustelids (Fig. 4B) merges withthe diaphysis, whereas in Didymictis, Fennecus, and the aeluroidsstudied (Fig. 4C), the greater tubercle extends well above andcranial to the humeral head. The marked mediolateral flatteningof the diaphysis of Viverravus is similar to that of Didymictis andunlike that of extant carnivorans, in which the diaphysis is ellip-tical proximally but nearly round in cross section at midshaft.The miacid diaphysis superficially resembles that of Viverravusbut the relatively greater craniocaudal diameter in miacids is afunction of a very different deltopectoral crest morphology, onewhere the crest extends as a narrow raised ridge of bone from thegreater tubercle to an abrupt end at or just distal to the midshaft,rather than gradually tapering into the diaphysis. The medialepicondyle and lateral supracondylar line are well developed inmost carnivorans (Fig. 4B, C), the exception being the cursoriallyadapted taxon studied, Fennecus. Terrestrial/cursorial carniv-orans, including Didymictis, herpestids (Fig. 4C), and Fennecusare also characterized by having a deep or perforate olecranonfossa (i.e., supratrochlear foramen) indicative of a very stablehumeroulnar joint in 180 degrees of extension, quite unlike theextremely shallow olecranon fossa of USNM 489122, miacids,Potos, and Nandinia. The attachment site of the olecranon liga-ment is not as distinct in extant carnivorans as in viverravids andmiacids, the exception being Potos.

Ulna—The left ulna of USNM 489122 (Fig. 5A) is nearly com-plete, missing only small fragments of the trochlear notch and

FIGURE 4. Left humeri of (A) Viverravus acutus USNM 489122, (B) Mustela, and (C) Helogale in cranial (left), medial (center), and caudal (right)views. Abbreviations: dp, deltopectoral crest; gt, greater tubercle; lt, lesser tubercle; me, medial epicondyle; of, olecranon fossa; rf, radial fossa;stf, supratrochlear foramen. Scale bars equal 10 mm.

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diaphysis. A poorly preserved distal epiphysis, probably the left,is abraded along its surface such that no significant contact canbe made between epiphysis and diaphysis. With the epiphysis,the ulna measures just over 29.0 mm in total length, 22 mm ofwhich is from the proximal radial notch to what is preserved ofthe radial facet distally. The olecranon process has a distinctcranial orientation with respect to the long axis of the diaphysissuch that the olecranon projects beyond the margin of the troch-lear notch. A shallow but not quite flat tendinal groove for thelong head of the triceps musculature separates the lateral andmore proximally projecting medial margins of the olecranon pro-cess. The anconeal process, for articulation with the olecranonfossa, is blunt and more proximally than cranially oriented, re-sulting in an ’open’ trochlear notch. The radial notch is relativelyflat and triangular, being wider caudally than cranially. The di-aphysis is mediolaterally compressed and craniocaudally deepalong nearly its entire length; the lateral surface is shallowlygrooved and the medial surface is slightly convex. Distally, thecranial surface is flanked by sharp bony ridges, between which isthe attachment site of the main forearm pronator, m. pronatorquadratus. The epiphysis has an incomplete radial facet for ar-ticulation with the ulnar notch of the radius, and the tip of thestyloid process for articulation with the pisiform and cuneiformcomes to a blunt point.

A cranially oriented olecranon process similar to USNM489122 is found in Nandinia, Potos, and miacids, whereas terres-trial taxa such as Helogale (Fig. 5C), Fennecus, and Didymictishave a straight or even caudally deflected olecranon. The ole-cranon of Mustela (Fig. 5B) has a slight cranial orientation butdiffers from Viverravus and the extant arboreal taxa in having adeep tendinal groove, a character that is also commonly found interrestrial/cursorial taxa. The anconeal process of Didymictis andall extant taxa studied is better defined than in USNM 489122(miacids share the Viverravus morphology). In taxa possessingdeep (e.g., Mustela) or perforate (e.g., Helogale) olecranon fos-

sae, the anconeal process tends to project farther cranially (Fig.5B, C). The shape and orientation of the radial notch of Nan-dinia and Didymictis are very similar to those of USNM 489122and unlike the concave/angled and more cranially oriented notchof Fennecus and the herpestids. The ulnar diaphysis of extantcarnivorans, while mediolaterally flattened, tends not to be asrelatively deep craniocaudally as in Viverravus, Potos, and to alesser extent Martes. The craniocaudal diameter of the ulna interrestrial taxa such as Helogale tapers along its length, and thedistal-most aspect is reflected caudally (Fig. 5C). A sharp lateralmargin for attachment of m. pronator quadratus, as in USNM489122, is characteristic of the scansorial and arboreal taxa ex-amined, while in terrestrial taxa the margin is much less welldefined.

Radius—Parts of both radii are preserved. The left element isrepresented by two sections, a 6.1-mm proximal fragment thatincludes the badly abraded radial head and bicipital tuberosity,and a 12.2-mm section of the diaphysis. The right radius is rep-resented by a better preserved and longer section (16.0 mm) ofthe diaphysis. The distal epiphysis is not known for either ele-ment. Based on the ulnar measurements given above, the radiusis estimated to have been 23 mm in length (distance betweenradial head and carpal articulating surface) suggesting that, ex-cept for the epiphysis, nearly the entire length of the bone isrepresented (Fig. 5A). The ulnar margin of the radial head forarticulation with the radial notch is preserved, as is a shallowdepression for articulation with the capitulum of the humerus.The oval-shaped radial tuberosity for insertion of the ante-brachial flexor m. biceps brachii is well developed, with a raisedridge along its lateral margin. The diaphysis is distinctly narrowmediolaterally and the distal third of the bone is triangular incross section with the ulnar surface being as wide, if not wider,than the cranial and caudal surfaces. A ridge at the junction ofthe caudal and ulnar surfaces of the bone is the attachment sitefor m. pronator quadratus.

FIGURE 5. Left ulnae and radii of (A) Viverravus acutus USNM 489122 (distal radius reversed), (B) Mustela, and (C) Helogale. Ulnae in lateral(left) and cranial (center) views and radii in caudal (right) view. Abbreviations: ap, anconeal process; bt, bicipital tuberosity; op, olecranon process;pq, attachment site of m. pronator quadratus; rn, radial notch; tg, tendinal groove for m. triceps; tn, trochlear notch. Scale bars equal 10 mm.

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The incomplete nature of the radial head of USNM 489122makes comparisons to modern taxa uninformative; in addition,the bicipital tuberosity is generally similar among all extant car-nivorans. The shape of the radial diaphysis of Viverravus is un-like that of the craniocaudally flattened diaphysis of Mustela(Fig. 5B) and much more like that of Helogale (Fig. 5C), Genetta,and Didymictis where the triangular distal diaphysis has a wideulnar border and craniocaudally deep articular surface for con-tact with the scapholunar carpal bone.

Metacarpal and Pisiform—The only parts of the hand knownfor USNM 489122 are a partial phalanx and two complete bones,the left pisiform and right second metacarpal. The pisiform(Fig. 6A) has a maximum length of 2.9 mm and is at least asrobust as that of Mustela (Fig. 6B). The articular facets for thetriquetrum and ulna are relatively larger in Mustela, but theattachment site for the important wrist flexor and abductor, m.carpi ulnaris (as well as several intrinsic hand muscles), is betterdeveloped in the fossil. Pisiforms have not been described formiacids or Didymictis. The metacarpal (Fig. 6C) is 7.9 mm inlength and very similar in proportion to that of Mustela(Fig. 6D).

Hind Limb

Os Coxae—The right hip bone of USNM 489122 is more com-plete than the left, with only a section of the ischiopubic ramusmissing (Fig. 7A). The bone measures 29.6 mm from iliac crest toischial tuberosity, 60% of which is preacetabular. The iliac bladeis relatively narrow, flaring to a maximum width just caudal tothe crest. The auricular surface for contact with the sacrum isslightly concave and makes up about 50% of the preacetabularlength. The nearly round acetabulum has a craniocaudal diam-eter of 3.7 mm. Just cranial to the acetabulum is a well-definedmuscle scar, the cranial inferior tubercle, for attachment of the

hip flexor m. rectus femoris. The moderately well-developed is-chial spine, origin of the lateral rotator and hip abductor m.inferior gemellus, is dorsal and a little caudal to the acetabulum.

The iliac blade of all extant carnivorans studied is relativelywider than that of USNM 489122, but the difference is mostnotable in terrestrial taxa such as Fennecus and Helogale (Fig.7C). Complete iliac blades are unknown for miacids and Didy-mictis, but in both what is known of the iliac blade is wide andmore similar to that of modern carnivorans than Viverravus. Asin Viverravus, the cranial inferior tubercle of Didymictis andmiacids is particularly well developed. Among extant carniv-orans, scansorial and arboreal taxa such as Nandinia, Bassaris-cus, and Genetta have better-developed tubercles than terrestrialtaxa. The os coxae of terrestrial and tree-climbing carnivoransalso tend to differ in the degree of buttressing dorsal to theacetabulum and in the size of the ischial spine, with considerablymore buttressing and smaller spines found in terrestrial taxa. Inboth these conditions, the hip bone of USNM 489122 is more

FIGURE 6. Left pisiform and right second metacarpal of Viverravusacutus USNM 489122 (A and C) and Mustela (C and D). Pisiforms inlateral (top) and medial (bottom) views, metacarpals in dorsal (left) andventral (right) views. Abbreviations: fcu, attachment site of m. flexorcarpi ulnaris; uln, articular facet for ulna; trq, articular facet for trique-trum. Scale bars equal 1 mm.

FIGURE 7. Right os coxae of (A) Viverravus acutus USNM 489122(top in medial view), (B) Mustela, and (C) Helogale in lateral view.Abbreviations: as, auricular surface; cit, cranial inferior tubercle; ic, iliaccrest; is, ischial spine; it, ischial tuberosity. Scale bars equal 5 mm.

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similar to that of Mustela than Helogale (Fig. 7B, C), ischialspines also being better developed in miacids than Didymictis.

Femur—Only the right femur is preserved and it is completeexcept for the distal epiphysis (Fig. 8A). The specimen measures31.3 mm and the total length of the bone is estimated to havebeen 34 mm based on comparisons with complete femora ofsmall carnivorans possessing distinct epiphyseal lines. The femo-ral head is nearly round (maximum diameter 3.2 mm) exceptwhere interrupted by the large proximocaudally oriented foveacapitis, and the articular surface extends onto the proximal andcaudal surface of the short and craniocaudally narrow femoralneck. The greater trochanter has a deep intertrochanteric fossa(attachment site of several lateral hip rotators), and it projectsslightly above the femoral head. The extremely well-developedlesser trochanter, for insertion of the hip flexor and lateral ro-tator, m. iliopsoas, projects medially. A prominent muscle scar,the third trochanter, is on the lateral side of the bone just distalto the lesser trochanter. At midshaft the diaphysis is a little widermediolaterally (2.9 mm) than craniocaudally (2.4 mm) and thereis a definite craniocaudal curvature along its length particularlyevident in the distal half of the bone.

The relative size of the femoral head, like that of the humeralhead, is smaller in Viverravus than in modern taxa (particularlySpilogale) although the fovea, attachment site of the lig. teres, isrelatively larger (Fig. 8). Viverravus also shows expansion of thearticular surface onto the femoral neck as is common in carniv-orans that employ abducted limb postures. In all of these re-spects, as well as the overall shape of the head, USNM 489122 ismost similar to miacids. Didymictis differs from Viverravus inthat the femoral head is hemispherical and there is little or no

expansion of the articular surface onto the neck. The highlyterrestrial Fennecus possesses a femoral head morphology com-parable to that of Didymictis and, like the larger viverravid, hasa relatively long femoral neck (found to a lesser extent in Helo-gale [Fig. 8B]) that likely enhances parasagittal motion of thehindlimbs during running. Greater trochanter morphology isvariable in extant carnivorans with arboreal taxa (e.g., Potos)tending to have lower trochanters than terrestrial taxa (Fenne-cus), but Nandinia has one of the highest greater trochantersand, in both miacids and Didymictis, the greater trochanter ex-tends above the head as in USNM 489122. The orientation of thelesser trochanter on the other hand is more clearly associatedwith specific locomotor behaviors; taxa that climb have mediallyoriented trochanters like Viverravus, and terrestrial taxa, havesmaller caudally oriented trochanters (Fig. 8B). Miacids areagain similar to USNM 489122 in this respect, while the well-developed lesser trochanter of Didymictis has an orientation in-termediate between those of Viverravus and Helogale.

Tibia—A proximal fragment of the diaphysis and the unfusedepiphysis of the right tibia are preserved for USNM 489122 (Fig.9). Some erosion of the bone has occurred along the cranial andlateral margins of the diaphysis at the epiphyseal plate, but thereis good congruence between epiphysis and diaphysis elsewhere.The condyles are separated by a low intercondylar eminence(higher laterally than medially), a poorly defined cranial inter-condylar fossa, and a better-developed but shallow caudal inter-condylar fossa. The medial condyle is distinctly concave next tothe eminence, but flattens out cranially and caudally. The lateralcondyle is slightly higher than the medial, extends farther cau-dally and is concave cranially and convex caudally. Just cranial

FIGURE 8. Right femora of (A) Viverravus acutus USNM 489122, (B) Helogale, and (C) Spilogale in cranial (left) medial (for USNM 489122 only),caudal (right), and proximal (top) views. Abbreviations: fv, fovea; gtr, greater trochanter; ltr, lesser trochanter; ttr, third trochanter. Scale bars equal10 mm.

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and lateral to this condyle is a distinct rounded tubercle possiblyfor the proximal-most attachment of m. tibialis anterior, an ex-tensor of the crurotalar joint. The tibial plateau measures 3.4 mmacross and 4.9 mm craniocaudally. Extending distally from therounded tibial tuberosity is the cnemial crest, which appearssharp and well defined owing to a concave surface laterally for m.tibialis anterior. A relatively flat fibular facet exists on the lateraland inferior margin of the epiphysis.

The general shape of the medial and lateral tibial condyles issimilar among extant carnivorans, miacids, Didymictis, andUSNM 489122. In terrestrial taxa (most notably Fennecus) andDidymictis the tibial tuberosity has a relatively greater and more-defined cranial projection, whereas in scansorial and arborealtaxa, miacids, and Viverravus the tibial tuberosity terminatesmore bluntly. A sharp cnemial crest characterizes many scanso-

rial and terrestrial taxa as well as USNM 489122, whereas in themiacids, Nandinia, and Potos the cnemial crest, like the tibialtuberosity, is more rounded and less well defined. In none of theextant carnivorans, miacids or Didymictis is the tubercle identi-fied at the cranial and lateral margin of the tibial plateau quite aswell defined as in USNM 489122, Genetta and Potos being themost similar.

Astragalus—The astragalus of AMNH 11514 is complete (Fig.10A). The trochlea is well grooved with a sharp lateral marginthat is higher, has a greater radius of curvature, and terminatesmore cranially than the medial margin, which extends well be-yond the small astragalar foramen. The trochlea ends cranially ata bony shelf that would have come into contact with the tibia tolimit dorsiflexion at the crurotalar joint. The surface on the lat-eral side of the astragalus for articulation with the fibula is nearlyflat and a slight depression on the medial side of the bone indi-cates contact with the medial malleolus of the tibia. The groovefor the tendon of the plantar flexors m. flexor hallucis longus(and m. flexor digitorum longus) is deep, oriented obliquely tothe long axis of the astragalus, and extends well on to the ventralsurface, where it is bordered laterally by the sharp margin of theectal facet. The ectal facet, for articulation with the caudal facetof the calcaneum, is relatively shallow, smoothly concave andspiral shaped—the more cranial part being oriented ventrallyand the more caudal part oriented laterally. The sustentacularfacet, for articulation with the sustentaculum of the calcaneum, isconvex, round, and medially oriented, and is clearly separatedfrom the ectal facet and astragalar head by deep and shallowdepressions respectively. The articular surface of the convexhead extends well onto the medial surface, and the long axis ofthe head is rotated about 25 degrees from the mediolateral plane.

The shape of the trochlear groove of AMNH 11514 is compa-rable to that of Mustela and Didymictis (Fig. 10B, C) being in-termediate between the almost flat surface of miacids (Fig. 10D),Martes, and Potos, and the very deep groove of Fennecus wherethe medial trochlear margin is higher and has a radius of curva-ture nearly equal to that of the lateral margin. In none of theextant taxa studied does the trochlea terminate cranially as asmall shelf, although miacids and Didymictis both share thischaracter with AMNH 11514. Extant carnivorans also differfrom AMNH 11514, miacids, and Didymictis in that the trochlearsurface is not interrupted by an astragalar foramen, so that thearticular surface extends much farther caudally, creating a hemi-spherical or nearly 180 degree articular surface. In miacids (Fig.10D), the lateral trochlear margin and articular surface both endin front of the large astragalar foramen, and the articular surfaceof the medial margin extends only slightly beyond the foramen.

FIGURE 10. Left astragali of (A) Viverravus gracilis AMNH 11514 (reversed), (B) Mustela, (C) Didymictis, and (D) Vulpavus, USNM 362847,USGS 27585, in dorsal (left), ventral (right) and cranial (bottom) views. Abbreviations: ast, astragalar foramen; ef, ectal facet; fhl, groove for tendonof m. flexor hallucis longus; hd, head; sf, sustentacular facet; tr, trochlea. Scale bars equal 2 mm.

FIGURE 9. Right tibia of Viverravus acutus USNM 489122 in proximal(top), caudal (center) and cranial (bottom) views. Abbreviations:cc, cnemial crest; lc, lateral condyle; tt, tibial tuberosity. Scale bar equals5 mm.

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In Didymictis (Fig. 10C) the foramen is also relatively large butmore caudally positioned with both sides of the trochlear surfaceextending beyond the foramen as in AMNH 11514. The ectalfacet in modern carnivorans and miacids is wider cranially thanin Viverravus, giving the articular surface a more rectangularthan triangular shape. The concave, spiral-shaped ectal facet inAMNH 11514 is similar to arboreal and scansorial taxa as well asmiacids, while in terrestrial taxa the ectal facet forms a distinctnotch. The result is that in arboreal/scansorial taxa the astragalussmoothly translates on the caudal calcaneal facet, while in ter-restrial taxa the astragalus pivots on the calcaneal facet. Thesustentacular facet of terrestrial taxa differs from that of climbersin being long, narrow, and relatively flat rather than being round,convex, and extending across the neck of the astragalus as inMustela and miacids (Fig. 10B, D). Orientation of the astragalarhead also differs markedly between arboreal and terrestrial taxa.The long axis of the head in the most arboreal forms is almostmediolaterally oriented as in miacids (Fig.10D) compared to asmuch as 45 degrees of rotation in Fennecus and about 35 degreesin Didymictis (Fig. 10C). In this respect AMNH 11514 moreclosely resembles terrestrial taxa.

FUNCTIONAL INTERPRETATIONS

Glenohumeral Joint

The scapula, the proximal attachment site of many musclesacting on the glenohumeral joint, is incomplete, but the intactglenoid cavity and coracoid process along with the proximal hu-merus provide for some reasonably clear interpretations of thisjoint. The exceptionally well-developed coracoid process of thescapula, the round humeral head, and the well-developed inser-tion sites on the greater and lesser tubercles and deltopectoralcrest indicate strong shoulder abduction, adduction, and medialrotation in Viverravus, characters common to extant carnivoransthat exhibit a high degree of joint mobility. Viverravus differsfrom terrestrially adapted carnivorans, which rely on morestrictly parasagittal movements at the shoulder, in that the hu-meral head is small relative to the rest of the bone, the head isnot craniocaudally elongate, and the greater tubercle does notextend well above and cranial to the head. Characters of theglenoid cavity of Viverravus, specifically the well-developed su-praglenoid tubercle and its distinctly concave appearance, havebeen considered indicative of a primarily terrestrial adaptation inherpestids and viverrids (Taylor, 1974), but the occurrence ofthese glenoid morphologies in Mustela demonstrates that theyare not restricted to terrestrial carnivorans. The glenohumeraljoint of USNM 489122 clearly indicates climbing adaptations.

Elbow and Wrist Joints

The humeroulnar articulation of Viverravus, specifically theshallow olecranon fossa of the humerus and the cranially inclinedolecranon process of the ulna, are indicative of a joint that washabitually maintained in a flexed position. This is quite differentfrom terrestrial taxa, which maximize the range of extension andhumeroulnar stability by possessing a strongly beaked anconealprocess that “locks” into a deep or perforate olecranon fossa. Inaddition, the enlarged capitulum for articulation with the radialhead and the proximal radioulnar joint, characterized by a flatand laterally oriented radial notch, demonstrate that the range ofsupination between radius and ulna was maximized in Viverravusand in these respects is similar to the morphology of Nandinia(Taylor, 1974). In terrestrially adapted animals, the radial notchlies on the cranial surface of the ulna and the forearm is generallymaintained in a pronated position with little capacity for supi-nation. Viverravus was also capable of strong pronation at theelbow as indicated by the distinct ridges for m. pronator quadra-tus attachment along the distal diaphysis of the radius and ulna.

Well-developed flexor musculature crossing both the elbow andwrist is indicated by the large muscle attachment sites: the medialepicondyle and lateral supracondylar lines of the humerus, raisedbicipital tuberosity, and large pisiform. The flexed limb posturesand decreased emphasis on extension reconstructed for USNM489122 are typical of climbers and quite unlike the joint mor-phology characteristic of runners and fossorial taxa.

Hip Joint

The morphology of the os coxae and proximal femur ofUSNM 489122 suggest that the hip joint of Viverravus was regu-larly employed through a wide range of positions. Characteristicsthat support this interpretation include the round femoral headand acetabulum, articular expansion of the femoral head ontothe proximal aspect of the femoral neck, and the well-developedattachment sites for muscles associated with hip flexion (i.e.,cranial inferior tubercle), abduction (i.e., ischial spine), andlateral rotation (i.e., ischial spine and lesser trochanter). Thisis in contrast to habitually terrestrial carnivorans, where thehemispherical femoral head, long femoral neck, small and cau-dally oriented lesser trochanter, reduced or absent third tro-chanter, and dorsally buttressed acetabulum are related to anemphasis on locomotor behaviors employing hip flexion and ex-tension.

Crurotalar and Subtalar Joints

The uneven heights and unequal radii of curvature of the me-dial and lateral trochlear surfaces of the astragalus of AMNH11514 imply that during plantar flexion between astragalus anddistal tibia, some passive inversion of the foot occurred. Pedalinversion seems to have been enhanced at the subtalar joints, thespiral-shaped ectal, and round, convex sustentacular facets, al-lowing for some conjunct rotation of the astragalus relative tothe calcaneum during cranial-caudal translation. Jenkins andMcClearn (1984) showed that an increased range of pedal inver-sion at the crurotalar, subtalar, and transverse tarsal joints ischaracteristic of arboreal quadrupeds that employ hindfoot re-versal, a mechanism that enables these animals to rotate the hindfoot 180 degrees and thereby hang from their hind limbs anddescend vertical supports in a headfirst manner. The extent towhich each of these joints contributes to hind-foot reversal canvary, but in Potos most of the inversion is produced at the sub-talar and transverse tarsal joints (Jenkins and McClearn, 1984).Hind-foot reversal was not possible in the foot of AMNH 11514for two reasons. First and foremost, the oblique orientation ofthe astragalar head would have limited the range of motion pos-sible between the astragalus and the navicular, such that therange of inversion at the transverse tarsal joint would have beenconsiderably less than the 90 degrees found in Potos. Amongextant carnivorans, the most extreme astragalar head rotation isfound in cursorially adapted taxa such as Fennecus, in which thecrurotalar (characterized by a deeply grooved astragalus flankedby trochlear rims of nearly equal height and similar radii ofcurvature), subtalar (characterized by a notched ectal facet thatallows the astragalus to simply rock or pivot on the calcaneum),and transverse tarsal joints serve to enhance, as well as limitmotion to, flexion and extension. The second character that in-dicates a more limited pedal inversion in the foot of AMNH11514 is the presence of an astragalar foramen. The soft tissuestructures associated with the foramen (i.e., blood vessels andnerves) probably limited the extent of plantar flexion comparedto extant taxa where the articular surface of the astragalus isgenerally hemispherical. Without the distal tibia it is difficult toassess the range of plantar flexion possible, but in miacids andDidymictis, maximum plantar flexion (angle formed betweenlong axes of tibia and astragalus) has been estimated to be 115

HEINRICH AND HOUDE—POSTCRANIAL ANATOMY OF VIVERRAVUS 431

and 140 degrees respectively (Heinrich and Rose, 1997). Giventhat the astragalar foramen of AMNH 11514 is less well devel-oped and more dorsally positioned than in these other miacidsand in that the medial articular surface extends farther caudallythan in Didymictis, it seems likely that the crurotalar joint ofAMNH 11514 was capable of greater than 140 degrees of plantarflexion but nowhere near the 180 degrees possible in Potos.

Locomotor Adaptations of Viverravus

The skeleton of Viverravus acutus clearly indicates that, as faras substrate use goes, this early Eocene species was at least ascapable a climber as Mustela frenata and probably an excellentclimber comparable to Martes and Genetta (Clark et al., 1987;Kingdon, 1977). It shows none of the morphologies characteristicof taxa such as Helogale (e.g., supratrochlear foramen, caudallyoriented lesser trochanter) that are essentially limited to terres-trial habitats, nor does the skeleton of USNM 489122 share anyof the semifossorial attributes of Spilogale, such as robust limbswith enlarged articular surfaces. Furthermore, the combinationof gracile skeleton and interpreted use of habitually flexed joints(i.e., crouched postures) makes it unlikely that Viverravus em-ployed ambulatory gaits as found in skunks and raccoons amongothers, or that it regularly utilized a trot as do terrestrial mam-mals with leanings toward cursoriality. With respect to gait andlocomotor behavior, the picture one is left with for Viverravusacutus is of a small, agile, and scampering or bounding animal(probably moving similar to Mustela) that was as comfortablemoving through the trees as on the ground. Limb proportionsthat can be discerned for V. acutus, however, are very differentfrom those of Mustela (which has a very short radius relative tothe humerus, and humeri and femora of comparable length) andare more comparable to those of Nandinia (Table 3).

The astragalus of AMNH 11514, assigned to the middleEocene Viverravus gracilis, provides a more equivocal picture.Some characteristics of this bone (e.g., shape of subtalar facets)support an interpretation of enhanced mobility in the foot aswould be expected of an animal climbing vertical supports, butother features (e.g., oblique head orientation) are indicative ofan increased reliance on parasagittal limb motions emphasizingflexion and extension, adaptations characteristic of terrestriallyadapted carnivorans. This composite of astragalar characteristicsis not particularly similar to that of any extant carnivoran studiedto date, but is comparable to the astragalus of Didymictis. Ifindeed this bone belongs to the genus Viverravus, it almost rulesout the possibility that Viverravus was arboreal in the sense of

Potos or Nandinia, carnivorans that spend nearly all of theirforaging time in trees (Eisenberg, 1989; Kingdon, 1977). It ispossible, however, that V. acutus and V. gracilis, separatedby 3–4 million years in the fossil record, were adapted for some-what different habitats, the former more arboreal and the lattermore terrestrial.

LOCOMOTOR BEHAVIOR IN BASAL CARNIVORAAND EARLY AELUROIDEA

The diversity of locomotor behaviors exhibited by extant car-nivorans has evolved from the postcranial morphologies of earlymiacid and possibly viverravid carnivorans. While it is clear thatextant caniforms trace their evolutionary origins to a miacid an-cestor that was very well adapted to life in the trees, if not trulyarboreal, it has not been at all clear what locomotor behavior andsubstrate preferences characterized basal members of the orderor the earliest members of Aeluroidea. How does the skeleton ofViverravus described in this study bear on these questions?

Ideally, the identification of a primitive character trait, such assubstrate use in basal Carnivora, should employ outgroup com-parisons. The taxon most commonly cited as the sister group toCarnivora is Creodonta (Flynn et al., 1988; Wyss and Flynn,1993, Flynn, 1996), an extinct order of mammals that has in-cluded a variety of taxa since it was proposed by Cope in 1875,but which is generally regarded today as including only Hyaen-odontidae and Oxyaenidae. Support for a close relationship be-tween creodonts and carnivorans has been questioned by severalauthors, including Fox and Youzwyshyn (1994), who providedpersuasive arguments for why Creodonta should be removedfrom consideration as the sister group to Carnivora. Polly (1994,1996) has convincingly demonstrated that the taxon Creodontaitself is unlikely to be monophyletic. While other taxa (Lipo-typhla, palaeoryctoids, etc) have also been suggested to beclosely related to, if not the sister group of, Carnivora, no soundphylogenetic hypothesis is currently available to support thesecontentions. At present, then, there is no taxon that can be usedconfidently in an outgroup analysis to help assess the primitivelocomotor adaptations of basal Carnivora; inferences here relyinstead on the postcranial morphology of miacids and viverrav-ids.

While miacid postcrania are generally similar, the skeletonsand locomotor interpretations of Viverravus and Didymictis arevery different; Viverravus was a crouched, scampering animalmaking regular use of arboreal and terrestrial habitats, whileDidymictis is argued to have been predominantly terrestrialand incipiently cursorial (Heinrich and Rose, 1997). Which ofthese locomotor adaptations is more likely to have been primi-tive for Viverravidae? To answer this question it is useful toconsider the only other postcrania described for a viverravid: afragmentary hip bone, proximal femur, and “dubiously associ-ated calcaneum and astragalus fragments” of the early Paleocenespecies Protictis haydenianus (MacIntyre, 1966:162), an animalof comparable size to Didymictis. The relative size and shape ofthe ischial spine on the hip bone and the relatively short femoralneck and medial orientation of the lesser trochanter on the fe-mur are all more similar to what is found in Viverravus acutusand miacids than to those of Didymictis. As far as the astragalusis concerned, the shapes of the trochlea and ectal facets, the largesustentacular facet, and the orientation of the astragalar head aremore similar to corresponding features of miacids than to thoseof either AMNH 11514 or Didymictis. While the data are admit-tedly weak, what can be discerned from the Protictis postcranialfragments indicates greater affinities with miacids and Viverra-vus acutus than Didymictis.

If substrate utilization is mapped onto a phylogenetic tree forthe miacid and viverravid taxa considered in this study (Fig. 11),

TABLE 3. Limb proportions for USNM 489122 and extant taxa ana-lyzed in this study.

Taxon Sample R/H H/F

MUSTELIDAEMustela frenata 8 0.62 0.94Spilogale putorius 10 0.78 0.89Martes americana 19 0.78 0.88

PROCYONIDAEBassariscus astutus 10 0.78 0.91Potos flavus 13 0.79 0.91

CANIDAEFennecus zerda 5 0.93 0.94

HERPESTIDAEHelogale parvula 1 0.77 0.89Herpestes ichneumon 1 0.76 0.85

VIVERRIDAEGenetta genetta 8 0.86 0.88Nandinia binotata 6 0.79 0.84

FOSSILViverravus acutus 0.83 0.81

Abbreviations: R, radius; H, humerus; F, femur.

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it is clear that the most-parsimonious interpretation for sub-strate use in the basal members of the order is something akin tothat described for Viverravus acutus and gracile miacids such asMiacis petilus. These earliest carnivorans were equally at homein the trees and on the ground, with no particular adaptations forsemifossorial activity or for following prey into burrows. Fur-thermore, it suggests that a similar pattern of habitat utilizationexisted in the immediate outgroup to Carnivora, a hypothesisthat will be tested with additional fossils and more work on LateCretaceous—Early Tertiary mammal diversification. The mor-phology of Didymictis, on the other hand, is best considered asecondary adaptation for increased use of open terrestrial habi-tats much as are found in present-day viverrine civets Viverra,Civettictis, and Viverricula. What about substrate use in basalAeluroidea? If, as Hunt and Tedford (1993) suggested, Viverra-vus is ancestral to this group, then the earliest aeluroids werewell adapted for climbing but were not truly arboreal like Nan-dinia, a genus that has been argued to be the sister taxon to allother extant aeluroids (Hunt, 1989; Flynn, 1996; Yoder et al.,2003). Rather, substrate use in basal Aeluroidea is most likely tohave been similar to that found in Genetta, a genus that climbsexceptionally well but which is found more frequently on theground. If, on the other hand, viverravids are the sister taxon toall other carnivorans (Gingerich and Winkler, 1985; Wesley-Hunt and Flynn, 2005), then substrate use in basal Aeluroideahas yet to be determined.

CONCLUSION

The first partial skeleton of Viverravus was found and re-moved from a large and extremely fossiliferous limestone nod-ule, specimen 8ABC, collected from Houde site 8 within or nearUniversity of Michigan site SC-4, an early Wasatchian locality ofthe Clarks Fork Basin, northwestern Wyoming. The diversemammalian taxa represented in this calcareous nodule, likeViverravus acutus, are overwhelmingly scansorial and arboreal intheir locomotor behaviors, and include didelphids, leptictids,nyctitheriids, microsyopids, micromomyids, paramomyids, andapatamyids among others. The postcranial skeleton of Viverra-vus acutus is similar to that of small miacids such as Miacis peti-lus, and the former species is reconstructed as a crouched, agileanimal that probably employed a bounding gait as it made itsway along the forest floor and into the trees. The results of this

study further suggest that the diverse habitats and locomotorbehaviors employed by living carnivorans are derived from anancestral condition that regularly used arboreal habitats. Whilethese basal members of Carnivora were not arboreal to the de-gree found in the living kinkajou (Potos) and palm civet (Nan-dinia), they clearly spent considerable time in the trees probablyto forage for small vertebrates and invertebrates and as a meansof avoiding predators.

ACKNOWLEDGMENTS

We thank the following for access to collections in their careand their unending patience with the extended length of loans:R. Thorington and H. Kafka, Division of Mammals, Departmentof Vertebrate Zoology, National Museum of Natural History; M.McKenna and Jin Meng, Department of Vertebrate Paleontol-ogy, American Museum of Natural History; P. Gingerich and G.Gunnell, Museum of Paleontology, University of Michigan; G.Musser, Department of Mammalogy at the American Museumof Natural History, and K. Rose, Department of FunctionalAnatomy and Evolution, The Johns Hopkins School of Medi-cine. We thank two anonymous reviewers for their comments.All figures were executed by Chris Akers except Figures 10c and10d, which were completed by Elaine Kasmer and originallypublished in Palaeontology (Heinrich and Rose, 1997); we thankBlackwell Publishing Ltd for permission to republish those. Thisresearch was funded in part by an Ohio University ResearchCouncil grant to REH.

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Submitted November 17, 2003; accepted 26 October 2005.

APPENDIX I. Mustela species, sample sizes by sex, and humeral andfemoral measurements on which allometric regressions of Data Set III inTable 2 are based. Abbreviations: N, sample sizes for females (f) andmales (m); BM, body mass in grams; H, humerus; F, femur; L, length inmm; CC, craniocaudal midshaft diameter in mm; ML, mediolateral mid-shaft diameter in mm.

Species N BM HL HCC HML FL FCC FML

Mustela nivalis f-3 30 16.6 1.37 1.30 16.2 1.30 1.27m-11 36 19.7 1.60 1.45 19.6 1.54 1.58

Mustela erminea f-8 73 24.9 1.90 1.63 26.23 2.08 2.04m-15 153 29.5 2.27 1.93 31.6 2.21 2.32

Mustela frenata f-12 142 28.3 2.46 1.96 30.0 2.32 2.29m-11 269 33.6 2.95 2.37 36.3 2.73 2.81

Mustela altaica f-2 171 30.3 2.35 2.05 31.4 2.10 2.30m-13 284 35.0 2.98 2.42 36.6 2.63 2.78

Mustela sibrica f-4 395 34.6 2.88 2.53 39.2 2.73 3.03m-6 735 42.1 3.77 2.95 47.4 3.47 3.67

Mustela putorius f-9 560 38.5 3.07 2.83 42.9 2.87 3.20m-5 1058 44.7 4.08 3.46 50.3 3.56 3.76

Mustela nigripes f-6 809 45.5 3.93 3.52 47.3 3.47 4.20m-11 1021 49.0 4.44 3.90 51.9 3.85 4.40

Mustela vison f-10 940 41.8 4.26 3.27 45.1 3.72 4.12m-9 1496 47.8 4.71 3.92 51.5 4.14 4.84

HEINRICH AND HOUDE—POSTCRANIAL ANATOMY OF VIVERRAVUS 435