A new skeleton of Theropithecus brumpti (Primates ...

Nina G. JablonskiCalifornia Academy ofSciences, Golden Gate Park,San Francisco, California94118-4599, U.S.A. E-mail:[email protected]

Meave G. LeakeyandChristopher KiarieDepartment of Palaeontology,National Museums of Kenya,P.O. Box 40658, Nairobi,Kenya. E-mail:[email protected];[email protected]

Mauricio AntonDepartamento dePaleobiologıa, MuseoNacional de CienciasNaturales, Jose GutierrezAbascal 2, 28006 Madrid,Spain. E-mail:[email protected]

Received 17 January 2002Revision received31 July 2002 andaccepted 4 August 2002

Keywords: Cercopithecidae,Cercopithecinae, Papionini,Papionina, baboons,locomotion, opposabilityindex, Pliocene.

A new skeleton of Theropithecus brumpti(Primates: Cercopithecidae) fromLomekwi, West Turkana, Kenya

A relatively complete skeleton of the fossil papionin, Theropithecusbrumpti, from the site of Lomekwi, west of Lake Turkana, Kenya, ishere described. The specimen, KNM-WT 39368, was recovered atthe site of LO 5 (3�51�N and 35�45�E), from sediments dated toapproximately 3·3 Ma. The skeleton is that of an old adult maleand preserves a number of articulated elements, including most ofthe forelimbs and tail. The cranial morphology is that of a large, earlyT. brumpti, exhibiting a deep mandible with a deeply excavatedmandibular corpus fossa, and mandibular alveoli and cheek teetharrayed in a reversed Curve of Spee. The forelimb skeleton exhibits aunique mixture of characteristics generally associated with a terres-trial locomotor habitus, such as a narrow scapula and a highly stableelbow joint, combined with those more representative of habitualarborealists, such as muscle attachments reflecting a large rotator cuffmusculature and a flexible shoulder joint. The forelimb of KNM-WT39368 also presents several features, unique to Theropithecus, whichrepresent adaptations for manual grasping and fine manipulation.These features include a large, retroflexed medial humeral epicondyle(to which large pronator, and carpal and digital flexor musclesattached) and proportions of the digital rays that denote capabilitiesfor precise opposition between the thumb and index finger. Takentogether, these features indicate that one of the earliest recognizedrepresentatives of Theropithecus exhibited the food harvesting andprocessing anatomy that distinguished the genus through timeand that contributed to its success throughout the later Pliocene andPleistocene. Based on the anatomy of KNM-WT 39368 and theknown habitat preference of T. brumpti, the species is reconstructed asbeing a generally terrestrial but highly dexterous, very large-bodied,sexually dimorphic, and possibly folivorous papionin. T. brumpti wasadapted for propulsive quadrupedal locomotion over generally evenground, and yet was highly adept at manual foraging. The estimateof 43·8 kg body mass for KNM-WT 39368 renders unlikely thepossibility that the species, or at least adult males of the species, werehighly arboreal. T. brumpti, as represented by KNM-WT 39368, isseen as a large, colorfully decorated, and basically terrestrial papioninthat was restricted to riverine forest habitats in the Lake TurkanaBasin from the middle to latest Pliocene.

� 2002 Elsevier Science Ltd. All rights reserved.

Journal of Human Evolution (2002) 43, 887–923doi:10.1006/jhev.2002.0607Available online at http://www.idealibrary.com on

Introduction

To a paleontologist, the value of an associ-ated skeleton is inestimable. In primatepaleontology alone, the study of specimens

0047–2484/02/120887+37 $35.00/0

such as ‘‘Lucy’’ (AL-288-1, Australopithecusafarensis), the ‘‘Turkana Boy’’ (KNM-WT15000, Homo ergaster), the 1958 skeletonof Oreopithecus bambolii, and the nearlycomplete skeleton of the giant East African

� 2002 Elsevier Science Ltd. All rights reserved.

888 . . ET AL.

colobine monkey, Paracolobus chemeroni(KNM-BC 3), has demonstrated thesuperiority of complete over composite indi-viduals in the elucidation of functionalanatomical complexes, body size and pro-portions, and life history. Thus, even in thecase of the present skeleton, in which thespecimen does not represent a new taxon,we have a rare opportunity to investigate indetail the cranial and postcranial mor-phology of an undoubted individual extinctprimate. This opportunity allows us tobetter illuminate the feeding and locomotorbehaviors and ecological niche of one ofthe most distinctive fossil cercopithecids yetrecognized.

The Old World monkey genus Thero-pithecus is one of the best known of fossilprimate taxa, being recognized from thou-sands of specimens from sites in northern,eastern and southern Africa from themiddle Pliocene through most of the LatePleistocene. The most widespread and long-lived species of the genus, T. oswaldi, hasalso been identified from Pleistocene-agedsites outside of Africa, in India and Spain(Delson, 1993; Gibert et al., 1995). T.brumpti was more localized in its spatialdistribution, being recognized only fromPliocene-aged sites of the Lake TurkanaBasin, namely, the Nachukui Formationof West Turkana and the Koobi ForaFormation of East Turkana in Kenya, andthe Shungura Formation of the Omo inEthiopia. At these sites, there is a temporaloverlap between T. brumpti and T. oswaldi ofabout 1 Ma (Behrensmeyer et al., 1997), buttheir relative frequencies during this intervalare strikingly different. At the beginning ofits tenure, in the middle Pliocene, T. oswaldiis barely discernible in the fossil record ofthe Turkana Basin, being greatly out-numbered by fossils of T. brumpti. Near theend of the Pliocene, about 1·9 Ma, however,the situation reversed, with T. oswaldigreatly outnumbering T. brumpti and theformer on its way to being the common

baboon from the later part of the TurkanaBasin Pliocene and Pleistocene sequences.T. brumpti has been interpreted as occupy-ing a riverine gallery forest habitat (Eck& Jablonski, 1987), whereas T. oswaldi isassociated with more open habitats (Eck,1987). The turnover from the dominance ofone species to another appears to be a partof a more prevalent trend among the largemammals of the Turkana Basin thatcoincides with the transition from a river-dominated to a lake-dominated ecosystem(Feibel et al., 1991). This transitioninvolved a reduction in the number ofspecies associated with forest-edge environ-ments, and a corresponding increase ingrazing herbivores to exploit the marshygrasslands of the delta and lake margins andthe drier grasslands flanking them (Feibelet al., 1991; Behrensmeyer et al., 1997). Theco-occurrence of fossils of Theropithecus withthose of hominins at many African sites hasprovided a continuing stimulus for the studyof the adaptations and evolutionary historyof the genus, and the basis of models ofhominin differentiation (Jolly, 1970; Foley,1993).

Thanks to its generous fossil record andthe molecular evidence of its phylogeneticrelationships (see papers in Jablonski,1993b; Disotell, 2000), the evolution ofthe genus is now fairly well understood.Theropithecus appears to be most closelyrelated to baboons of the genus Papio, withthe common ancestor of both havingdiverged from the common ancestor ofMandrillus and Cercocebus, probably inthe early Pliocene (Disotell, 2000). Thesegenera, commonly grouped together in theSubtribe Papionina, are believed to haveoriginated in the early Pliocene from aspecies of Parapapio or other similarlygeneralized African cercopithecid. The fossilrecord has, unfortunately, shed little light onthe mode or tempo of differentiation ofthe large papionins, or on their individualancestries.

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Theropithecus is represented in the fossilrecord by two distinct lineages, dominatedrespectively by the species T. oswaldi and T.brumpti. Delson (1993) has distinguishedthese taxonomically as the subgeneraT. (Theropithecus) and T. (Omopithecus),respectively. The single extant species of thegenus, T. gelada, has been accommodated inthe former subgenus, but the nature of itsphyletic relationship to other species in thegenus remains unclear because of theabsence of any relevant fossil evidence andits many distinctive autapomorphies. T.gelada lives on the high montane grass-lands of central Ethiopia, and appears torepresent a long-isolated sublineage withinTheropithecus. The naming of species withinthe genus and the allocation of species to thegenus have been controversial. Of relevancehere is the debate that has surrounded theaccommodation of species other than T.brumpti itself into the T. (Omopithecus) line-age. Eck & Jablonski (1984) argued forthe inclusion of two Pliocene species,Papio baringensis and P. quadratirostris, intoTheropithecus and specifically into the T.brumpti lineage. Delson and colleagues(Delson & Dean, 1993; Frost, personalcommunication) have argued that, whileP. baringensis may be a theropith, P. quad-ratirostris is clearly not, and is better accom-modated as a species of Papio (Dinopithecus).A detailed recitation of the arguments madeby these authors is outside the scope ofthe present work, and readers are referredto the papers by Eck & Jablonski (1984)and Delson & Dean (1993) for thisdiscussion.

The preservation of nearly complete fore-limb skeletons for KNM-WT 39368 isfortuitous insofar as the forelimbs of cerco-pithecoids are widely recognized as beingmore informative of modes of locomotionand manipulation than the hindlimbs, whichfunction mainly to provide propulsive thrustin locomotion. Thus, this account will focusprimarily on a description and interpretation

of the shoulder girdle and forelimb forT. brumpti.

Discovery and geological andpaleoenvironmental contexts

One of the most productive regions forTheropithecus fossils representing both the T.oswaldi and T. brumpti lineages has been theLake Turkana Basin. Located in the easternRift Valley of East Africa, the Lake TurkanaBasin has provided a remarkable record ofMiocene, Pliocene and Pleistocene fossilsthat has made a significant contribution toour current understanding of the tempo andmode of evolution of terrestrial Africanfaunas (Coppens & Howell, 1985, 1987a,b;Feibel et al., 1989; Harris 1981, 1983;Harris et al., 1988a,b; Leakey et al., 1996;Behrensmeyer et al., 1997). Whereas muchof this evidence has come from theShungura and Koobi Fora Formations tothe north and east of Lake Turkana respec-tively, the Nachukui Formation to the west,has made a significant contribution. TheNachukui Formation outcrops along a10 km wide strip on the western shores ofthe lake between the settlements at Kalakolto the south and Lowarengak to the north.These deposits were initially investigated inthe 1980s by the West Turkana ResearchProject as part of the National Museumsof Kenya (NMK) field research program(Harris et al., 1988b). Fieldwork over thesubsequent decade led to the recovery ofover 1000 specimens of 93 mammalianspecies. Geological studies revealed asequence of tuffs interbedded in the 730 mof section, 23 of which had been previouslyrecognized in the Koobi Fora and ShunguraFormations. Recently field research in theNachukui Formation was re-established byNMK teams led by Meave and LouiseLeakey in collaboration with Frank Brown(University of Utah), who is studyingdetailed aspects of the geology.

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Figure 1. Reconstruction of the skeleton of KNM-WT 39368, with preserved elements indicated inyellow. Skull restoration based on male crania of T. brumpti from the Omo and West Turkana. Postcranialelements not preserved with the original specimen were reconstructed on the basis of other fossilTheropithecus and living papionins.

Within the Nachukui Formation (Harriset al., 1988a,b), several sites documentingthe earlier sediments in the Omo Groupdeposits are richly fossiliferous. One ofthese, LO 5, situated on the southernbranch of the Laga Lomekwi, in the foothillsof the Murua Rith range, was re-surveyed in1998. Preliminary observations of the clastsin the fossiliferous conglomerates at LO 5suggest that the site represents a paleo-stream channel that may have held drinkingwater; if this observation is borne out byfurther study, this may account for the con-centration of bones at the site (FrankBrown, personal communication). Althoughmany of the specimens found at LO 5 werefragmentary, at one locality, 3�51�N and35�45�E, a fairly complete skeleton ofT. brumpti was discovered by WambuaMangoa. The specimen, which preserves anumber of articulated elements (Figure 1),had begun to erode from the slope of a smallhill, and was initially detected by a few

fragments of the cranial and postcranialskeleton lying on the surface. Subsequentscreening of the site revealed further skeletalelements in situ and an excavation followedwhich led to the recovery of an unusuallycomplete monkey skeleton, catalogued asKNM-WT 39368. After retrieval of the sur-face fragments and commencement of theexcavation, the disposition of the skeletalelements was revealed. The animal was dis-covered lying on its back, curled up withits head and hindlimbs nearer the surface,covering the more deeply buried forelimbs,thorax and tail. The absence of puncture orgnaw marks denotes that the animal was notthe object of carnivore predation or scaveng-ing. Death due to senescence or disease isfar more likely, given the advanced state ofwear of the dentition. The extraordinarycompleteness and articulation of the bonesof KNM-WT 39368 raise questions over thedeposition and fossilization of the specimen.Site LO 5 has not yielded other partial or

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whole skeletons, and so one must invokerelatively uncommon taphonomic processesto account for the articulation of the fore-limbs and preservation of fragile elementssuch as the ribs and scapula. The twomost likely possibilities are that the carcassmay have been partially mummified beforedeposition, or that the animal fell intothe water and was rapidly covered withsediment thereafter.

The elements of the skeleton are mostlyoff-white, but are covered with a brownmatrix veneer derived from the clayey sedi-ments in which it was interred. The skeletonis not strongly mineralized, rendering thebones highly friable. Because of this, mostof the skeletal elements suffered crushingduring or after fossilization. The preparationof the skeleton, undertaken by one ofus (CK), was extremely exacting andtime-consuming, and required repeatedsequences of gluing, re-positioning ofelements, and stabilization.

The distribution of sedimentary facieswithin the Nachukui Formation suggeststhat, as today, the Labur and Murua Rithranges formed the western margins of thebasin and were drained by eastern-flowingrivers that fed into short-lived lakes, forerun-ners of the present lake, or into a major riversystem, the ancestral Omo River, flowingthrough the basin (Harris et al., 1988;Leakey et al., 2001). The site of LO 5 issituated close to the Marua Rith hills, andincludes alluvial fan deposits brought in byrivers draining from the west and character-ized by volcanic clasts derived from theTertiary volcanics in the hills. These sedi-ments include upward fining cycles of fine-grained clays, sandy silts and conglomerateswhich interfinger with sediments brought inby the ancestral Omo River drainage fromthe north. The section at LO 5 includes theLomekwi and Kataboi Members whichoccur above and below the Tulu Bortuff, respectively. The type section of theLomekwi Member is 158·5 m thick. The

upper boundary is defined as the LokalaleiTuff (Tuff D) which has been dated at2·52+0·05 Ma (Brown et al., 1985). TheTheropithecus skeleton was recovered fromsilty sands of the alluvial fan deposits20–22 m above the Tulu Bor Tuff which hasbeen correlated with the Sidi Hakoma Tuff

of the Hadar Formation in Ethiopia (Brown,1982) and is dated at 3·40+0·03 Ma(Walter & Aronson, 1993; Kimbel et al.,1994). Assuming linear sedimentation ratesbetween the two dated tuffs, the geologicage of the specimen can be estimated (byextrapolation based on the distance abovethe Tulu Bor Tuff), to be approximately3·3 Ma.

The faunal assemblages from LO 5, aswell as the geology, indicate a well-wateredand well-vegetated paleoenvironment, con-sistent with other contexts in which T.brumpti has been found. A mosaic ofhabitats is indicated by the bovids, butwith woodland- and forest-edge habitatsdominating.

Description of KNM-WT 39368

KNM-WT 39368 represents the skeleton ofan old adult male T. brumpti. The specimencomprises fragments of the cranium, a com-plete mandible, a partial sternum, frag-mentary thoracic, lumbar and a completesequence of caudal vertebrae, most ribs, themore or less complete forelimb skeletonsfrom both left and right, including completesets of carpals and digital rays preservedin situ, portions of the left os coxa, and bonesof the right leg. No femora, tarsal bones orpedal digits were recovered. A complete listof the elements of the skeleton preserved inKNM-WT 39368 is provided in Table 1.

CraniumMost of the cranium of KNM-WT 39368appears to have disintegrated as a result ofsurface exposure and erosion. Fortunately,some fragments of diagnostic and functional

892 . . ET AL.

The elements of the skeleton present in KNM-WT 39368

Element designation Element identification

A Rt digit II distal phalanxB Rt digit II middle phalanxC Rt digit II proximal phalanxD Rt digit III proximal phalanxE Rt digit III middle phalanxF Rt digit III distal phalanxG Rt digit IV proximal phalanxH Rt digit V metacarpalI Rt digit IV middle phalanxJ Rt digit IV distal phalanxK Rt digit IV metacarpalL Rt digit III metacarpalM Rt digit II metacarpalN Rt digit I distal phalanxP–Q Rt sesamoidsR Rt sesamoid with digit V metacarpalS Rt digit V proximal phalanxT Rt carpalU Rt lunateV Rt scaphoidW Rt ulnaX Rt radiusY Lt digit III distal phalanxZ Lt digit II distal phalanxAA Lt digit II middle phalanxAB Lt sesamoidAC Lt digit III middle phalanxAD Lt digit III proximal phalanxAE Lt digit IV proximal phalanxAF Lt digit IV middle phalanxAG Lt digit IV distal phalanxAH Lt digit II proximal phalanxAI Lt digit IV metacarpalAJ Lt digit V metacarpalAK Lt digit V proximal phalanxAL Lt digit V middle phalanxAM Lt digit V distal phalanxAN Lt pisiformAO Lt triquetralAP Lt ulnaAQ Lt hamateAR Lt sesamoidAS Rt rib fragmentAT Lt tenth or eleventh rib fragmentAU Lt eleventh or twelfth rib headAV Rt third or fourth fragmentAW Rt rib headAX Lt lower rib fragmentAY Lt eighth or ninth rib costal endAZ Rt seventh or eighth rib costal endBA Vertebra?BB Lt scapulaBC Sternal body segmentBD Rt second or third rib fragmentBE Lt first or second rib

Table 1

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Continued


BF Lt eighth/ninth ribs (fused)BG Rt lower rib fragmentBH Lt humerusBI Rt third or fourth ribBJ Rt rib fragmentBK Rt eighth/ninth/tenth ribs (fused)BL Lt lower rib fragmentBM Lt fourth or fifth rib fragmentBN Rt first or second ribBO Bone fragmentBP Lt capitateBQ Lt carpalBR–BS Rib headsBT Rt lower rib fragmentBU Rib headBV–BW Caudal vertebraeBX First two caudal vertebra fusedBY–BZ Sternal body segmentsCA–CO Caudal vertebraeCP–CQ Caudal vertebra fragmentsCR Sacrum dorsal fragmentCS Rib fragments in bagCT Rt lower rib fragmentCU Rt fourth or fifth rib fragmentCV Lt second or third rib costal endCW Lt sixth or seventh ribCX MandibleCY Lt upper canine tipCZ Rt upper canine fragmentDA Rt upper central incisorDB Rt upper lateral incisorDC Lt upper lateral incisorDD Upper premolar root?DE Upper molar crown and root brokenDF Upper molar root in alveolusDG Upper molar root fragmentsDH Upper molar root fragmentsDI Upper molar root fragmentsDJ Upper molar root fragmentsDK Upper molar root fragmentsDL Lt glenoid fossa partialDM Lt supraorbital marginDN Occipital bone frag. near the inionDO Nasal bone fragmentDP Lt premaxilla and maxilla fragmentDQ–DR Muzzle fragmentsDS–DT Facial bone fragmentsDU Lt mastoid area fragmentDV–DW Petrous temporal bone fragmentsDX Bag of cranial bone fragmentsDY Rt pubis and partial acetabulumDZ Iliac crest fragmentEA Pelvic fragments in bagEB Axis odontoid processEC Third and fourth cervical vertebrae fused

Table 1

894 . . ET AL.

value were recovered as the result of carefulsurface collecting, sieving and identification.The most significant of these are listedbelow.

measurement, the size of the canine tip, thegreat length of the lower P3 honing facet,and the size of the lower canines (preservedin KNM-WT 39368 CX) suggest that theupper canine was very large.

Cranium. The only portions of the craniumpreserved are a partial glenoid fossa of theleft temporal bone (KNM-WT 39368 DL),the supraciliary margin and partial orbitalroof of the left frontal bone (KNM-WT39368 DM, Figure 2) and a fragment of the

Continued


ED Fifth and six cervical vertebrae lt zygapophyses fusedEE First thoracic vertebraEF Thoracic vertebral bodyEG–EI Thoracic vertebra fragments?EJ Lumbar vertebrae, two fusedEK Lumbar vertebra bodyEL Lower thoracic vertebral bodyEM–EO Lumbar vertebral body fragmentsEP–ER Lumbar neural spinesES Lumbar vertebrae fragmentsET–EU Lower lumbar vertebrae transverse processesEV First caudal vertebraEW–EX Proximal caudal vertebraeEY Vertebrae fragmentsEZ Rt tibiaFA Rt fibulaFB Femoral head part?FC Rt clavicleFD Lt clavicle sternal headFE Lt humeral head crushedFF Lt humerus lateral epicondyleFG Lt radial headFH Humeral bone fragmentsFI Rt glenoid fossa of scapulaFJ Rt acromion processFK Long bone fragmentsFL Bag of bone fragmentsFM Two rt sesamoid bonesFN Three sesamoid bonesFO Lt digit I proximal phalanxFP Lt lunateFQ Lt scaphoidFR–FT Lt carpalsFU Rt digit I proximal phalanxFV Rt carpalFW Rt digit V middle phalanxFX Lt digit I distal phalanx

Table 1

Maxillary dentition. The only piece of themaxillary dentition preserved is the tip of theleft upper canine (KNM-WT 39368 CY).The anterior aspect of the tooth shows astrong developmental groove. The posterior(honing) aspect is very sharp. Although thebase of the tooth was not preserved for

895

Figure 2. Supraciliary margin and partial orbital roof of the left frontal bone (KNM-WT 39368 DM);(a) superior view; (b) frontal view. Approximately life size.

Figure 3. Maxilla fragment (the left margin of the narial aperture) of KNM-WT 39368. Approximately life size.

muzzle preserving the left side of the narialaperture (KNM-WT 39368 DP, Figure 3).The partial glenoid fossa of the left temporal

bone preserves the anterolateral surface onlyand adjacent surface of the temporal fossa.The orientation of the anterior margin of the

896 . . ET AL.

Figure 4. (a) and (b).

glenoid fossa is transverse. A lateral tuberclefor attachment of the temporomandibularjoint capsule is marked. The left frontalbone fragment was recognized by GeorgeChaplin from a collection of sieving frag-ments. It represents a relatively thin orbitalroof with a gracile supraciliary margin. Ashallow supraciliary notch is preserved at themedial edge of the specimen. The superiorsurface of the bone is perforated by minute

foramina, which transmitted blood vessels toand from hair follicles. The lateral aspect ofthe specimen preserves the anterior-mostlimb of the temporal line, indicating that theindividual had an anteriorly thickened M.temporalis muscle. The muzzle fragment pre-serves a narrow strip of premaxilla mediallyand a larger area of maxilla laterally. Themaxilla bears numerous small foramina andstriae and appears to have been highly

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Figure 4. (c)

Figure 4. Mandible (KNM-WT 39368 CX). (a) right lateral view; (b) left lateral view; (c) occlusal view.Approximately one-half life size.

Mandible and mandibular dentitionThe mandible (KNM-WT 39368 CX, Fig-ure 4) contains a complete lower dentitionexcept for the right lower lateral incisor thatapparently was lost ante mortem. The diag-nosis of sex and age for KNM-WT 39368was based on the mandibular dentition,which is clearly that of an old adult male.The mandibular corpora have been com-pressed together during fossilization, bring-ing the tooth rows in close proximity andgiving the bone an unnaturally narrowappearance. Matrix has not been removedfrom between the mandibular corpora, pre-cluding measurement of the thickness of themandibular symphysis. All the teeth are

heavily worn, especially those on the leftside. The dimensions of all the measurableteeth are given in Table 2. The canines areheavily worn, but were long and slenderduring life. The protolophid of the right M1

is worn below the cervix and the hypolophidonly slightly less so. The left M1 appears tohave worn through at the waist and brokeninto mesial and distal halves, resulting in theexposure of the pulp chamber of the hypo-lophid. Because of the advanced state ofdental wear, very little of the cusp mor-phology of the molars can be described,except for the M3s. On these teeth, the maincusps are large, broad-based, relativelysteep-sided and the hypoconulids are large.The mandibular corpora themselves aredeep and display deep, anteriorly situatedfossae of the mandibular corpora. The fos-sae are ovate and are deepest at the level ofthe P4 and M1. On the right, the fossa of themandibular corpus has been crushed and isabsent posteriorly to the level of the retro-molar fossa. The inferior borders of themandibular corpora are thickened into

vascularized. The small size of the fragmentprecludes reconstruction of the narial aper-ture or the contour of the muzzle. Anotherprobable muzzle fragment, KNM-WT39368 DS, shows extensive evidence of sur-face vascularization, but, unfortunately, can-not be placed in its correct anatomicalposition and is not illustrated here.

898 . . ET AL.

anterior ridges that are particularly promi-nent at the level of the P4. These substantialridges contribute to the depth of the man-dibular corpus fossae. Single, large, ellipticalmental foramina perforate the anterior sur-face of the ridges under the lower P3–P4

junction. Proceeding rostrally, the ridges ofthe inferior corpus thin under the lower P3

Dimensions of the mandible and mandibular teeth of KNM-WT 39368 CX

Measurement mm

Lower first incisor buccolingual width 6·3Lower second incisor mesiodistal length 3·5Lower second incisor buccolingual width 6·1Lower canine mesiodistal length 16·1Lower canine buccolingual width 15·8Lower third premolar mesiodistal length 25·7Lower third premolar buccolingual width 7·6Lower fourth premolar mesiodistal length 8·9Lower fourth premolar buccolingual width 7·1Lower first molar mesiodistal length 12·7*Lower first molar buccolingual width (mesial lophid) 8·8*Lower first molar buccolingual width (distal lophid) 9·7*Lower first molar buccolingual width (waist) 8·4*Lower second molar mesiodistal length 14·7Lower second molar buccolingual width (mesial lophid) 12·6Lower second molar buccolingual width (distal lophid) 13·5Lower second molar buccolingual width (waist) 10·9Lower third molar mesiodistal length 18·6Lower third molar buccolingual width (mesial lophid) 13·1Lower third molar buccolingual width (distal lophid) 11·8Lower third molar buccolingual width (waist) 10·8Lower canine to lower third molar length 89·4Lower third premolar to lower third molar length 76·8Lower fourth premolar to lower third molar length 55·5Lower first molar to lower third molar length 46·3Lower tooth row length (to alveolare) 93·6Lower tooth row length (to bite point) 92·3Lower bicanine breadth 29·5*Lower incisive alveoli width 16·6*Mandibular condyle length (anteroposterior dimension) 14·6Mandibular condyle width (mediolateral dimension) 24·9Mandibular condyle to the mesial edge of lower first molar 119·4Mandibular condyle to the distal edge of lower third molar 77·3Moment arm of the masseter 93·9Moment arm of the temporalis 29·4Mandibular length (to alveolare) 169·8Mandibular length (to bite point) 166·4Mandibular corpus height (between lower first and second molars) 49·1Mandibular symphysis length 64·3

*Indicates an estimated value.All dental measurements represent maximum dimensions; definitions of mandibular

measurements follow Jablonski (1993a).

Table 2

and develop a rugose inferior surface thatleads to the rugosities of the mental emi-nence. The mental symphysis is very long,with its inferior shelf extending posteriorlyto the level of the lower P4. The rugositiesthat cover its rostral surface form a narrowinverted ‘‘V’’ shape as one views thesymphysis from the front. The alveoli of

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mandibular teeth, as well as the remnants ofthe teeth themselves, describe the reversedCurve of Spee that is seen in most speci-mens of Theropithecus throughout time (Eck& Jablonski, 1987; Jablonski, 1993a). Themandibular ramus is inclined at an angle ofapproximately 120� relative to the base ofthe mandibular corpus. The mandibularcondyle is robust and deep in an anteropos-terior direction. The articular surfaceextends from its superior aspect along theposterior surface of the condyle for approxi-mately 14 mm, describing an articular con-tour that is common in cercopithecoids thatdemonstrate open-mouthed gape displays(Jablonski, 1993a). The coronoid process israised only slightly above the level of themandibular condyle.

Axial skeletonA large number of ribs are preserved,although the poor condition of most of themprecludes their identification by number.Three sternal body segments, KNM-WT39368 BC, BY and BZ, are also pre-served. The skeleton also comprises severalvertebrae, but the identification of someelements has been hampered by poor pres-ervation. Two complete cervical vertebraeare preserved together in a block of matrix(KNM-WT 39368 EC). One completesuperior thoracic vertebra, T1 or T2, isalso preserved (KNM-WT 39368 EE).The lumbar vertebrae are in poor conditionand cannot be clearly identified. Mostremarkable is the preservation of a completeseries of 19 tail vertebrae, from the mostproximal (KNM-WT 39368 BX) to themost distal (KNM-WT 39368 CO). Thetail vertebrae are characteristic of those ofmodern long-tailed cercopithecids andshow no evidence of extreme muscularity.The lengths of the caudal vertebrae aregiven in Table 3. The thoracic vertebraand ribs of KNM-WT 39368 are shown inFigure 5.

Forelimb skeleton

Clavicle. The left clavicle is preserved(KNM-WT 39368 FC). Its natural curva-ture has been exaggerated greatly duringfossilization, precluding determination ofthe element’s length or natural degrees oftorsion or curvature. The sternal end of theclavicle is large and has a quadrangularoutline. The medial portion of the shaftis oval in cross-section. The shaft thinsand broadens laterally into a craniocaudallyflattened acromial end.

Table 3 Lengths of the caudal vertebrae ofKNM-WT 39368

Specimen number(suffix to KNM-WT39368)

Caudalvertebranumber Lengths (mm)

BX 1 and 2(fused)

76·0 (combinedlength)

CA 3 51·0CB 4 55·0BW 5 53·0BV 6 51·0CC 7 47·5CD 8 45·5CE 9 41·5CF 10 37·5CG 11 32·5CH 12 28·5CI 13 25·0CJ 14 21·5CK 15 16·2CL 16 13·8CM 17 10·9CN 18 8·9CO 19 7·0

The vertebrae are listed in sequence from mostproximal (KNM-WT 39368 BX) to most distal(KNM-WT 39368 CO).

Scapula. A complete left scapula is preserved(KNM-WT 39368 BB, Figure 6), as well asthe glenoid fossa (KNM-WT 39368 FE,Figure 6) and acromion (KNM-WT 39368FT) from the right. The scapula is sub-isoceles in shape (sensu Roberts, 1974). Thevertebral (medial) border is thin, relativelylong and slightly convex, supporting a large

900 . . ET AL.

Figure 5. The thoracic vertebra (T1 or T2) and ribs of KNM-WT 39368. Clots of matrix can be seenadhering to the surfaces of several of the bones. Approximately one-fourth life size.

area for the attachment of M. serratusanterior. The lateral (inferior or axillary)border is a complex region rather than asimple edge. The medial aspect of the lateralborder and the inferomedial angle of thescapula’s dorsal surface is occupied by themoderately sized attachment area of M. teresmajor. The central and lateral portions of thelateral border are dominated by a sharp crestthat extends medially from the infraglenoidtubercle and extends roughly two-thirdsalong the length of the lateral border. Thiscrest contributes to the formation of abroad axillary sulcus whose ventral marginconsists of a thickened keel or scapular pillar(Olivier & Depreux, 1954) that separates thelateral border from the costal surface. Thesurface of this sulcus is interpreted as pro-viding attachment for an accessory slip ofthe M. subscapularis (Olivier & Depreux,1954; Birchette, 1982). The superior borderof the scapula is evenly thin and rises fromthe base of the coracoid to its most superiorpoint, at about the midpoint of the margin.From this point medialward the superiormargin runs transversely to intersect thevertebral border. The costal aspect of thescapula describes an undulating, concave

surface that accommodated the attachmentof a large M. subscapularis. The supra-spinous fossa is shaped roughly like a longrectangle that has had its superior and lateralcorner obliquely trimmed. The floor of thesupraspinous fossa is on the same plane asthat of the infraspinous fossa. The infra-spinous fossa is roughly triangular in outlineand constitutes more than two-thirds of thebone’s dorsal surface area. The scapularspine is very deep for most of its length ofthe element, and has a smooth, narrow edge.Its base is oriented almost perpendicular tothe blade. Unfortunately, the spine wascrushed superiorly during or after fossiliz-ation, rendering difficult an accurate appre-ciation of its depth and contour. Theacromion is short, stout and roughly quad-rangular, being slightly flattened cranio-caudally. Laterally, the lateral borderrecurves sharply toward the superior marginof the glenoid fossa. The coracoid processrises from a wide base, which is the continu-ation of the scapula’s superior border. Theprocess itself is oblong and lies at an angle ofabout 70� to its base, with its major axisdirected ventrolaterally. The glenoid fossa(cavity) is symmetrically piriform, with a

901

Figure 6. The left scapula, KNM-WT 39368 BB; (a) dorsal view. Approximately one-half life size; (b)view of the glenoid fossa of the right scapula, KNM-WT 39368 FE, approximately life size.

well-delimited periphery. The superior mar-gin of the fossa protrudes slightly past thedeepest point of the fossa’s concavity, creat-ing a slight cranial lip. The orientation of thefossa is lateral when the vertebral border isoriented along the craniocaudal axis. Thegreatest functional thickening of the scapula

is along its inferior border, especially in thearea interpreted as being the attachmentarea for the accessory slip of M. subscapu-laris. The dimensions of the scapula, aswell as those of most of the other preservedfore- and hindlimb elements of KNM-WT39368, are given in Table 4.

902 . . ET AL.

Figure 7. The right humerus, KNM-WT 39368 BH; (a) posterior view; (b) anterior view. Note thepathological enlargement of the lateral lip of the trochlea. Slightly less than one-half life size.

Humerus. A complete right humerus(KNM-WT 39368 BH, Figure 7), a lefthumeral head (KNM-WT 39368, FE) andleft lateral epicondyle (KNM-WT 39368FF) are preserved. The right humerus wascrushed extensively, especially in midshaft,but has been reconstructed to restore thebone’s natural contours. The nearly hemi-spherical head of the humerus is directeddorsally and slightly craniad. It is slightlywider in its mediolateral dimension than it is

long. The greater tuberosity is broad, butdoes not extend superiorly past the level ofthe humeral head. A large, pitted facet forthe insertion of M. infraspinatus is welldemarcated on the lateral surface of thetuberosity, but a well-defined facet for theinsertion of M. supraspinatus on its superioraspect is lacking. Ventrally, this tuberosityhas a sharp, elevated edge that forms theproximal end of the lateral lip of the inter-tubercular groove. The lesser tuberosity lies

903

Figure 8. The left ulna, KNM-WT 39368 AP; (a) medial view; (b) lateral view. Note the pathologicalexaggeration of the margins of the sigmoid notch, probably due to osteoarthritis. Slightly less than one-halflife size.

below the level of the humeral head. Acurved, smooth area on its medial surfacemarks the attachment area of M. subscapu-laris. The ventral surface of the tuberosityforms a rounded edge to the proximal end ofthe medial lip of the intertubercular groove.The groove itself is deep and narrow, and

remains well defined below the surgicalneck. The deltoid crest, which is a continu-ation of the lateral lip of the intertuberculargroove, is very strongly developed, andextends to midshaft. A large delto–triceps–brachialis crest runs distally on the ventralsurface from the level of the surgical

904 . . ET AL.

neck and then winds around the lateralaspect of the shaft until it intersects first theintradeltoid crest and then the deltoid crestat midshaft. The insertion for M. deltoideuson the humeral shaft is large and rugose atits margins. The shaft of the humerus is verystraight; unfortunately, damage to the boneat midshaft precludes assessment of theshape of the midshaft cross-section. Thelateral supracondylar ridge (supinator crest)is raised and develops into slight flange thatcurves posteriorly from the lateral epi-condyle to the lateral aspect of the distalshaft. The medial epicondyle is large andstrongly dorsiflexed, exhibiting almost notrue medial projection. An elliptical facet onits distomedial surface accommodated theattachments of strong tendons of the wristand digital flexors. The lateral epicondylepresents a large muscular attachment areaand a prominent tubercle for the attach-ments of Mm. extensor carpi radialis brevisand extensor digitorum communis. The capitu-lum exhibits a characteristically semi-ovoidarticular surface that extends considerablyventrally of the concave surface of the troch-lea. The details of the true trochlea aredifficult to discern because of damage, but

on the ventral surface of the distal humerusit is narrower than the capitulum. Viewedposteriorly, the trochlea is a deep and rela-tively narrow gutter, the medial margin ofwhich develops a large trochlear flange. Thedistal extremity of this feature has, unfortu-nately, been damaged and its full extentcannot be assessed. Despite this damage, thetrochlear flange still extends distally beyondthe lowest point on the capitulum andappears to have undergone pathologicalenlargement during life, possibly as theresult of osteoarthritis. The olecranon fossais damaged, but appears to have been deepand relatively narrow.

Ulna. Both ulnae are preserved, but sufferedfrom crushing. Fortunately, neither bonewas badly deformed and both have beennearly completely reconstructed. On theright (KNM-WT 39368 W, Figure 8), theproximal ulna is a separate element whilethe distal ulna has been preserved in itsin situ position along with the right distalradius and right carpus (Figure 9). The leftulna (KNM-WT 39368 AP) is preserved asa separate element. The proximal end ofthe ulna is distinguished by a retroflexed

Figure 9. Drawing of the articulated forelimb skeletons of KNM-WT 39368 as they were found in situ atLomekwi site LO 5. Drawing by Peter Gaede. Approximately one-fourth life size.

905

Table 4 Dimensions of the preserved elements of the appendicular skeleton of the KNM-WT 39368

Measurement Value (mm)

Scapular length (transverse dimension) 135·1Scapular width (vertebral border length) 106·6Glenoid fossa length 31·0Glenoid fossa width 21·9Angle of the scapular spine to the glenoid fossa 88�Angle of the lateral (inferior) border to the glenoid fossa 127�Angle of the lateral (inferior) border to the scapular spine 37�Humeral head diameter (superior-inferior dimension) 27·3Humeral head diameter (medial-lateral dimension) 31·7Humeral length (including trochlear flange) 230·9Humeral length (to trochlea) 223·8Distance from the superior margin of the humeral head to the distal-most extent of thedeltoid tuberosity

112·3

Biepicondylar breadth 47·5Distal humeral articular surface width 36·3Medial epicondylar width (measured on the ventral surface) 5·8Medial epicondylar width (measured on the dorsal surface) 15·4Trochlear breadth (ventral) 14·5*Trochlear breadth (dorsal) 15·3*Capitulum breadth 22·8Lateral epicondylar breadth (dorsal) 12·8Capitulum anteroposterior diameter 12·1Humeral midshaft diameter (dorsoventral) 17·8*Humeral midshaft diameter (mediolateral) 24·2Ulnar length (including styloid process) 260*Olecranon length 20·4Sigmoid (trochlear) notch length 18·1Sigmoid (trochlear) notch breadth (at its midpoint) 21·8Coronoid process protrusion from the base of the sigmoid notch 18·4Proximal ulnar breadth at the radial notch 28·1Ulnar midshaft diameter (dorsoventral) 13·9Ulnar midshaft diameter (mediolateral) 11*Angle of inclination of the sigmoid notch relative to the long axis of the ulna 17�Angle of inclination of the olecranon relative to the long axis of the ulna 48�Radial length 233*Radial midshaft diameter (dorsoventral) 11·1*Radial midshaft diameter (mediolateral) 17·5Radial neck length (to the inferior margin of the bicipital tuberosity) 46·9Radial neck diameter (dorsoventral) 15·6Radial head diameter (mediolateral) 19·5Radial head diameter (mediolateral) 22·3Tibial length 219·4Tibial midshaft diameter (dorsoventral) 26·5*Tibial midshaft diameter (mediolateral) 13·6Fibular length 203

*Indicates an estimated value.Measurements of the proximal ulna follow Conroy (1974) and Jolly (1972).

olecranon of moderate length that developsa rounded, superomedially directed flange.This flange develops into a rounded crestthat extends distally down the posterioraspect of the shaft for over one-third of

its length. The proximal end of the ole-cranon, including its extensive flange andcrest, are rugose and clearly accommodatedthe insertion of a large M. triceps brachiitendon. The medial aspect of the olecranon

906 . . ET AL.

Radius. The complete right radius (KNM-WT 39368 X) is preserved, but its distal endremains in an in situ block with the rightdistal ulna and right carpus (Figure 9). Onthe left, only the radial head (KNM-WT39368 FG) and distal epiphysis (KNM-WT39368 AP) are preserved. As with the otherforelimb long bones, the radius has beencrushed, but well reconstructed. The shaftappears to have been straight. The head isnearly circular in outline, showing only aslightly greater mediolateral than dorso-ventral dimension. The shallow, centrallylocated depression on its superior surfaceaccommodated the humeral capitulum. Themedial edge of the head is slightly higherthan is the lateral, resulting in a slightmediolateral obliquity to its orientation. Theneck of the radius, although somewhat dam-aged, is robust, and appears exceptionallythick at the proximal end of the bicipital(radial) tuberosity. The bicipital (radial)tuberosity is large in area, but the details ofits surface cannot be discerned because ofdamage. The size of the tuberosity suggestsa large insertion for M. biceps brachii. Theinterosseous crest is prominent and isaligned approximately with the medialborder of the radial tuberosity. The antero-medial surface of the radius is bounded bythe anterior border and the interosseouscrest. In ventral view, the concavity of thissurface has been exaggerated by post mor-tem crushing. The radial styloid process issharply pointed, but is not strongly distallyprojecting.

Carpus. The identification of all the indi-vidual carpal bones from the left side hasproved highly problematic, except for thelarge scaphoid (KNM-WT 39368 FQ) andlunate (KNM-WT 39368 FP). These bones

overshadowed by the flange is a deep fossathat accommodated the origins of Mm.flexor carpi ulnaris and flexor digitorum profun-dus, responsible for radial deviation of thewrist and flexion of the fingers, respectively.The anconeal and coronoid processes arerobust and project strongly ventrally, con-tributing to the creation of a constrictedsigmoid (trochlear) notch. In ventral view,the sigmoid notch is narrow and resemblesa twisted parallelogram, with the sides andthe articular surfaces of the anconeal andcoronoid processes directed craniolaterally.The ventral surface of the sigmoid notchfolds around the medial and lateral surfacesof the anconeal process and the medialsurface of the coronoid process. Theenlargement of the coronoid process and theextensive folding over of the articular surfaceof the sigmoid notch onto the medial surfaceof the process is asymmetrically developed,being considerably larger on the left sidethan on the right. This is interpreted here asbeing another indication of bony pathologydue to osteoarthritis. The radial notchis shallow, slightly concave centrally, andprojects slightly laterally. The surface area ofthe notch cannot be assessed on the rightdue to damage and has been expandedby the pathological enlargement of thecoronoid process on the left . The shaft ofthe ulna is relatively straight, showing only aslight medialward bowing at midshaft. Theinterosseous crest is visible as a blunt ridgebeginning immediately distal to the radialnotch. Its distal extent cannot be assessedbecause of extensive damage to the bone atmidshaft. The supinator fossa, which isinterposed between the interosseous crestand the anterior border of the ulna, isshallow. On the lateral surface of the ulnabetween the interosseous crest and theposterior border is a broad fossa that accom-modated the attachments of the long pollicalflexor and extensor and that of M. extensordigiti secundi et tertii proprius. The styloidprocess is stout, rounded and only slightly

projecting. (Measurements of the angularinclination of the olecranon process incli-nation and sigmoid notch are included inTable 4.)

907

can also be seen interposed between thedistal radius and the metacarpals in theblock of articulated elements from the rightside (see Figure 10).

Figure 10. Posterior (a) and anterior (b) views of the right wrist area. Aproximately one-half life size.

Metacarpals. The metacarpals and phalangesof both hands are complete and well pre-served (Figure 11). Having been foundarticulated in situ (and mostly preserved inthat condition on the right side), there hasbeen no question as to the proper identifi-cation of digital number. The specimen

numbers and dimensions of these elementsare given in Table 5. The description here isbased on the anatomy of the elements of theleft hand, which were carefully disarticu-lated and labeled during preparation. Intheir morphology and proportions, themetacarpals and phalanges of KNM-WT39368 closely resemble those of T. gelada.Metacarpal I is a long, slender bone. Itsexpanded proximal articular surface is ovate,concave and slightly expanded ventrallyto fit the articular process of the trapezoid.

908 . . ET AL.

Figure 11. Posterior (a) and anterior (b) views of the left manus of KNM-WT 39368. Bony elementsshown approximately one-half life size, with spaces between the bones exaggerated in order to clearlydisplay the anatomy.

The head of the first metacarpal issmall, rounded and slightly mediolaterallycompressed. Metacarpal II is a robust bonewith a stout base, which is dominated by thearticular facet for the trapezium. Like meta-carpal I, its head is small, rounded and

mediolaterally compressed. Metacarpal IIIis virtually identical in length to metacarpalII, but exhibits a significantly more robustshaft. Its base bears a broad, flat facet forarticulation with the capitate, and its head isbroad and rounded. Metacarpal IV, only

909

slightly shorter than metacarpal III, alsoexhibits a robust shaft. Its base is narrowand dominated by the dorsoventrally ori-ented facet for articulation with the hamate.Its head is somewhat narrower than that ofmetacarpal III. Metacarpal V, only slightlyshorter than IV, is, again, a robust bone.The mediolaterally beveled base bears apoorly defined facet for articulation withthe pisiform. The head is as describedfor metacarpal IV. The shafts of all of themetacarpals are round in cross-section andare straight.

Phalanges. The proximal phalanx of digit Ihas been slightly crushed, but the details ofits anatomy have not been obscured. It bearsa broad base and a slender shaft that iselliptical in cross-section. Viewed from theside, a slight dorsoventral curve of the shaftcan be discerned. The proximal phalanges ofdigits II–V exhibit broad bases and flat-

Hindlimb skeleton

Os coxa. The largest piece of the pelvispreserved is from the left side and whichcomprises most of the left pubis (KNM-WT39368 DY). The specimen has been badlycrushed and distorted. The obturator fora-men is preserved, but its natural outlinecannot be accurately reconstructed. Only asmall portion of the anterior aspect of theacetabulum is preserved and the diameterof the fossa cannot be estimated. A largecollection of pelvic fragments were recov-ered at LO 5, but could not be accuratelyre-assembled.

Table 5 Lengths of the metacarpals andphalanges of the left manus of KNM-WT 39368

Specimennumber(suffix toKNM-WT39368) Element

Measurement(mm)

BS Ray I metacarpal 40·1FO Ray I proximal phalanx 17·7FX Ray I distal phalanx 9·0BU Ray II metacarpal 50·5AH Ray II proximal phalanx 23·7AA Ray II middle phalanx 13·0Z Ray II distal phalanx 9·4BJ Ray III metacarpal 48·9AD Ray III proximal phalanx 27·8AC Ray III middle phalanx 16·5*Y Ray III distal phalanx 11·3AI Ray IV metacarpal 47·1AE Ray IV proximal phalanx 27·2AF Ray IV middle phalanx 17·8AG Ray IV distal phalanx 11·7AJ Ray V metacarpal 47·1AK Ray V proximal phalanx 23·5AL Ray V middle phalanx 14·1AM Ray V distal phalanx 9·4

*Estimated due to damage.

Tibia. Only the right tibia (KNM-WT39368 EZ, Figure 12) is preserved. Theshaft of the bone has been crushed andreconstructed. The general anatomy of thetibia is conservative among cercopithecids,and the present specimen conforms to thispattern. The superior articular surface of thetibia is oriented nearly perpendicular to thelong axis of the shaft. The lateral aspect ofthe lateral tibial condyle has been partiallyeroded and displaced slightly superiorly dur-ing or after fossilization. Despite this, it isstill clear that the lateral aspect of the tibialplateau was smaller in area and oriented ona more superior plane than the medial.The tubercle for attachment of the cruciateligaments is strongly raised. A posteriorenlargement of the lateral condyle may bedue to the fusion of the sesamoid bone ofthe lateral head of M. gastrocnemius to thatprocess. The tibial shaft is robust dorso

tened, slightly dorsoventrally curved shafts.Of the middle and distal phalanges, only thedistal phalanx of digit I warrants comment.This small bone has a broad, flattenedbase that tapers distally to a dorsoventrallycompressed and mediolaterally roundedterminus; in appearance the bone closelyresembles that of T. gelada.

910 . . ET AL.

ventrally, and only slightly bowed in thatdirection. The tibial tuberosity is wellmarked, and the anterior aspect of the shaftdistal to the tuberosity is rugose to midshaft.The medial malleolus is robust, but themorphology of the distal tibia is otherwisedifficult to reconstruct due to extensivecrushing. The most interesting part of the

tibia in KNM-WT 39368 is its distalextremity, which exhibits an angulatedmedial malleolus and a prominent notch forthe passage of the tendon of M. tibialisposterior.

Figure 12. (a) and (b).

Fibula. The right fibula (KNM-WT 39368FA) is preserved. The shaft of the bone

911

is straight, and the lateral malleolus isbroad.

Figure 12. (c).

Figure 12. The right tibia, KNM-WT 39368 EZ; (a) posterior view; (b) anterior view; (c) lateral view.Approximately one-half life size.

Comparative and functionalanatomical results and discussion

Prior to the discovery and study ofKNM-WT 39368, the postcranium of T.brumpti was known mostly from composite‘‘individuals’’ representing animals quitewidely separated in time and space. With therecognition of KNM-WT 39368, the oppor-tunity exists to compare the anatomy of anacknowledged individual skeleton with thatof composite individuals of its own speciesand with skeletons of other extinct andextant cercopithecid taxa.

The preserved portions of the skull ofKNM-WT 39368 are, in general, very simi-lar to those described for other specimens ofT. brumpti. If there is one regret about thespecimen, however, it is that the craniumitself or portions of it are not better pre-served. The fragment of left frontal bone is

the most interesting element of the craniumbecause its supraciliary ridge is significantlymore gracile and less superiorly projectingthan the prominent ‘‘spectacle rim’’ mor-phology seen in most specimens of T.brumpti. In this feature, KNM-WT 39368most closely resembles the type specimenof T. baringensis, KNM-BC 2, fromChemeron, which Eck & Jablonski (1984)have suggested is representative of the basalancestor of the T. brumpti lineage. The rela-tively gracile construction and nonprojectingmorphology of the supraciliary ridge inKNM-WR 39368 also recalls that seen inmost specimens of P. hamadryas subspecies.The supraciliary ridge also bears numerousminute foramina that were probably associ-ated with the blood vessels supplying thebases of hair follicles. This morphology ischaracteristic of animals who developsupraorbital hypertrichy. The distal muzzlefragment (representing a portion of theborder of the narial aperture) is small, buttantalizing in its morphology, especially, in

912 . . ET AL.

Figure 13. Reconstruction of the skull and of a male T. brumpti, in two views, based on a composite ofcranial and mandibular specimens from the Omo and West Turkana. The nostrils are shown in an anteriorposition, as in modern Papionina with long muzzles and comparable nasal morphology. The skull inthree-quarters view (top left) shows the anterolateral expansion of the zygomatics typical of the species,which in turn implies the presence of large M. masseterici, the bulk of which would be visible under the skinand fur of the living animal (bottom left). The morphology of the craniomandibular articulation, as wellas other craniodental features, reflects an adaptation to wide gape displays (top right). In life, thesedisplays would likely be emphasized by showing the light-colored eyelids, as in modern papionins (bottomright). A dark muzzle and facial mask would provide for a dramatic contrast against the light-colored teethand gums.

its preservation of a rugose surface indicativeof a highly vascularized muzzle integument.This is also seen in another probable muzzlefragment (KNM-WT 39368 DS), the ana-tomical placement and orientation of whichcannot be determined. The anatomical evi-dence provided by the surfaces of the supra-ciliary ridge and muzzle fragments suggestthat KNM-WT 39368 sported shiny, color-ful skin on the surface of its muzzle and

areas of hypertrichy on its face. Thisinformation was then combined with paleo-environmental evidence of the species’preference for forest habits, and the knowl-edge, derived from other specimens, of T.brumpti’s uniquely enlarged zygomaticarches and associated chewing muscles (Eck& Jablonski, 1987; Jablonski, 1993a). Thismade possible the reconstruction of thehead of KNM-WT 39368 as relatively

913

dramatically colored and patterned (Figure13). This configuration would encouragevisibility by conspecifics under the reduced-light conditions of a forest or forest-marginhabitat, as in modern guenons and man-drills.

The mandible and mandibular dentitionof KNM-WT 39368 are very similar tothose of other large males of T. brumpti,described and compared in detail by Eck &Jablonski (1987). Its only significant depar-ture from this condition is in the mandibulardentition, which although heavily worn, dif-fers slightly from that seen in later repre-sentatives of the species from West Turkanaor the Omo. KNM-WT 39368 presents thesmall incisors characteristic of T. brumpti.The molars of KNM-WT 39368 and otherearly representatives of T. brumpti from theTulu Bor Member of the Koobi ForaFormation and the Lower LomekwiMember of the Nachukui Formation arecharacterized by lower, more broadly basedcusps with fewer enamel infoldings. Simplermolar morphology has been observed byLeakey (1993) to be characteristic of allearly specimens of T. brumpti. This mor-phology, which resembles that of T. barin-gensis, appears to precede the more ‘‘typical’’appearance of the species, as representedby specimens from Members C-G of theShungura Formation of the Omo or theUpper Lomekwi Member of the NachukuiFormation. This finding parallels theintriguing description of the supraorbitalmargin of KNM-WT 39368, which appearsto represent a more primitive condition thanthat of later T. brumpti. Both morphologiesstrengthen the evolutionary connectionbetween T. baringensis and T. brumpti, andsupport the suggestion that the former isancestral to the latter (Eck & Jablonski,1984).

Quadrupedal primates exhibit variationsin scapular morphology associated withpreferences for an arboreal or a terrestrialhabitus (Ashton & Oxnard, 1964; Roberts,

1974; Kimes et al., 1981). Changes inscapular blade morphology are also linkedwith body size differences such that largerprimates of closely related groups may haverelatively wider scapulae (Roberts, 1974)without necessarily exhibiting differences inlocomotor behavior. In KNM-WT 39368,the scapula is narrow, having a ratio ofscapular length to width of 126·7, wellwithin the range of terrestrial cerco-pithecines and terrestrially adapted colo-bines (Ashton & Oxnard, 1964; Birchette,1982). Although the absolute sizes of thesupraspinous and infraspinous fossae werenot measured on KNM-WT 39368, theconsiderably greater area of the latter allies itto those cercocopithecoids, such as Papioand Erythorcebus, exhibiting large percent-ages of terrestrial behaviors (Roberts, 1974).Of great interest on the scapula ofKNM-WT 39368 is the wide axillary sulcusthat marks the lateral two-thirds of thescapula’s lateral border. The interpretationof this feature is difficult. If it accom-modated a large accessory slip of M. sub-scapularis, the muscle may have played asignificant role in humeral abduction androtation. The last major features of impor-tance on the scapula are the shape andorientation of the glenoid fossa. The sym-metrical piriform shape of the fossa inKNM-WT 39368 that lacks collateralindentations denotes a large area of contactbetween the fossa and the head of thehumerus. This indicates that the fossa waslargely responsible for transmission of bodyweight and muscular compressive forces,as it is in other terrestrial cercopithecoids(Birchette, 1982). The angle of the glenoidfossa to the scapular spine has been shownby Jolly (1965), Ashton & Oxnard (1964)and others to distinguish the scapulae ofterrestrial monkeys, especially baboons,from more arboreal species. In KNM-WT39368, the glenoid fossa is oriented laterallyand the aforementioned angle is 88�, withinthe range of modern terrestrial baboons

914 . . ET AL.

(Jolly, 1965). In scapular morphology, thepicture of KNM-WT 39368 is principallyone of a terrestrially adapted baboon, inwhich fore-aft movements of the forelimb inprotraction and retraction dominated. Theonly interesting exception to this is the largesize of the axillary gutter, which may indi-cate that KNM-WT 39368 could engage inrather more humeral abduction and lateralrotation than is the case with most otherbaboons. This exception is one of severalinteresting anatomical oddities that denotethat T. brumpti possesses a forelimb that ismore flexible at the shoulder joint than arethose of other theropiths or large papionins(Krentz, 1993).

The proximal humerus of KNM-WT39368 presents an interesting suite of char-acteristics that distinguish it from those ofother theropiths and Papio baboons, includ-ing the low degree of protrusion of thegreater tuberosity above the humeral headand a deep insertion for the M. infraspinatuson the lateral aspect of the greater tuberosity(Krentz, 1993). The proximal prolon-gation of the greater tuberosity has longbeen related to terrestrial quadrupedalism(Savage, 1957), and is directly related toincreasing the moment arm of the supra-spinatus about the shoulder joint. Larson &Stern (1989) have pointed out, however,that differences in the height of the greatertuberosity more accurately reflect the role ofM. supraspinatus as an elevator of the fore-limb against gravity. They interpreted the‘‘lowering of the tuberosity’’ as actually rais-ing the head upward for the purposes ofincreasing overall mobility at the gleno-humeral joint. The relatively small size andlack of superior protrusion of the greatertuberosity of KNM-WT 39368 is in contrastto that seen in the highly terrestrial extantpapionins, T. gelada and P. hamadryas sub-species, as well as in the Plio-Pleistocenetheropiths, T. darti and T. oswaldi. Theprominence of the insertion of M. infra-spinatus signifies the importance of the

muscle in shoulder stabilization as well as inlateral rotation of the humerus. This inser-tion tends to be more prominent in arborealspecies, reflecting the role of M. infra-spinatus in enhancing shoulder stability, andless prominent in terrestrial ones in whichthe muscles of the rotator cuff in generaltend to be relatively smaller (Krentz, 1993).

The condition presented by the distalhumerus contrasts with that presented bythe proximal portion of the bone. Here, thepicture is more consistently of featuresassociated with a terrestrial locomotor habi-tus. In KNM-WT 39368, the most import-ant of these features is the breadth of thedistal articular surface relative to the bi-epicondylar breadth, the retroflexion of themedial epicondyle, the narrowness of thetrochlea and the steepness of its bonymargins, and the distal protrusion of thetrochlea. Although the latter two featuresappear to have been exaggerated inKNM-WT 39368 as a result of osteo-arthritis, these conditions contradict thosedescribed for the distal humerus of T.brumpti by Krentz (1993). The index thatexpresses the breadth of the articular surfaceof the distal humerus relative to the bi-epicondylar width is 76·4 in KNM-WT39368, greater than the average value of73·4 for Papio baboons (Jolly, 1972) and forother theropiths studied by Krentz (1993),including his sample of T. brumpti (T.gelada=71·6; T. oswaldi=70·1, and T.brumpti=72·4). All of the theropiths, includ-ing KNM-WT 39368 cross Jolly’s (1972)rubicon of 70% that distinguishes terrestrialfrom arboreal cercopithecids. The large sizeand posterior projection of the medial epi-condyle bespeaks two important conditionsrelevant to locomotion and manipulation inKNM-WT 39368. Firstly, the large sizeindicates that the animal possessed largecarpal and distal flexors that would haveenhanced its abilities to grasp fine objectsduring feeding (Jolly, 1970). Secondly, themedial epicondyle protrudes more strongly

915

posteriorly than medially, in order toenhance the action of the pronators andflexors (Fleagle & Simons, 1982). Thus,KNM-ER 39368 appears to have possessedlarge carpal pronators and digital and carpalflexors, the origins of which were situated toenhance their mechanical advantage. Theimportance of the narrowness of the trochleaand the distal extension of the trochlearflange in KNM-ER 39368 is difficult toassess because of the arthritic pathology thatappears to have accentuated their expres-sion. Inspection of other specimens of thedistal humerus of T. brumpti indicates, how-ever, that the condition of KNM-ER 39368is only a slight exaggeration of that observedin other examples of the species from KoobiFora and West Turkana. The narrowness ofthe trochlea is interpreted then as an indi-cation of the degree of sharing between thehumeroulnar and humeroradial joints in therole of transmitting compressive and tensileforces across the elbow joint (Birchette,1982). In arboreal species, the surface areaof the former articulation is considerablylarger than the latter, an arrangement thatfrees the latter to maneuver more freelyin pronation and supination. The distalextension of the trochlear flange has beeninterpreted similarly by Jolly (1965), as acharacteristic of terrestrial species whichrequire greater elbow stability by counter-acting the forces that would tend either todisplace the ulna medially or the humeruslaterally. In summary, the humerus ofKNM-WT 39368 and other representativesof T. brumpti presents a suite of features,especially in the distal humerus, thatare generally characteristic of terrestrialpapionins, but which emphasize greaterflexibility at the glenohumeral joint andstrength of the digital and carpal flexors.

The ulna is a particularly informativebone in terms of providing insight into thelocomotor habits of Old World monkeys.The most important features of the ulna thatwill be considered here are the relative

length of the olecranon, the inclination ofthe olecranon process relative to the ulnarshaft (olecranon retroflexion), the configur-ation of the medial surface of the olecranonprocess, and the width and orientation of thearticular surface of the sigmoid notch. Theimportant features of coronoid process pro-jection, shape of the radial notch and ulnarshaft robustness were not possible to accu-rately assess for KNM-WT 39368 becauseof pathology and damage. The relativelength of the olecranon process is related tothe action of M. triceps brachii and is consid-ered a measure of the lever arm of thatmuscle (Gray, 1968). A short process per-mits full extension of the elbow, whichincreases the length of the forelimb andwhich, in turn, increases the length of stride.A longer olecranon process increases theleverage of M. triceps brachii and increasesthe mechanical efficiency of the musclewhen the forearm is flexed (Gray, 1968;Conroy, 1974). A long olecranon process ischaracteristic of arboreal primates, while arelatively lower process is characteristicof terrestrial primates that normally havetheir forelimbs extended during locomotion(Oxnard, 1963; Jolly, 1967; Ashton et al.,1976). The olecranon process of KNM-WT39368 is long and the index expressingits relative length is 7·8, just exceedingthe value of 7·6 for Colobus reported byBirchette (1982). Krentz (1993), utilizing adifferent index of relative olecranon processlength, also found that T. brumpti had rela-tively longer processes than Papio baboonsor other theropiths except for T. darti. Thisfinding is curious when one considers thatthe olecranon of KNM-WT 39368 andother specimens of T. brumpti is highlyretroflexed. Ulnar retroflexion has beenfound to consistently differentiate arborealfrom terrestrial cercopithecids becauseolecranon process inclination is related tothe different requirements of M. tricepsbrachii for maximum efficiency in climbingversus moving over a level surface (Jolly,

916 . . ET AL.

1967). A retroflexed olecranon process pro-vides great mechanical advantage to M. tri-ceps brachii when the forearm is alreadyextended, as in the final part of the propul-sive phase of terrestrial locomotion. Theolecranon of KNM-WT 39368 exhibits anangle of retroflexion of 48�, higher than thatreported for other specimens of T. brumpti(39·5�) (Krentz, 1993), exceeding those forT. oswaldi (43·6�) (Krentz, 1993), andwithin the range reported by Jolly (1967) forPapio (40–60�). The olecranon process ofKNM-WT 39368 and, to lesser extents, ofother theropiths is unusual in that it com-bines the relatively great length of arborealcercopithecids with the extreme retroflexionof terrestrial ones. This unique combinationmay contribute to the tremendous alacrityshown by geladas as they ascend near-vertical cliffs in powerful bounds, from posi-tions in which their elbows are alreadyextended (personal observation by NGJbased on observation of video footage).

In T. brumpti, the combination may havebeen related to a mode of locomotion orforaging for which there is no living analog.The medial surface of the olecranon ofKNM-WT 39368, like that of other mem-bers of its species and other theropiths,accommodated a large mass of muscles

Figure 14. The lengths of manual rays I, II, IV and V expressed as percentages of the length of manual rayIII. Data for living species are from Etter (1973).

Table 6 Opposability indices for KNM-WT39368, L865-2 (T. brumpti from the Omo reportedby Jablonski, 1986), and other cercopithecines (asreported by Etter, 1973)

Specimen or species Opposability index

KNM-WT 39368 69·2L865-2 70·6*Theropithecus gelada (n=5) 69·7Papio h. hamadryas (n=12) 60·1Papio h. cynocephalus (n=8) 61·7Erythrocebus patas (n=9) 51·3Cercopithecus aethiops (n=12) 52·0Macaca mulatta (n=15) 52·9

*Indicates an estimated value. See Jablonski (1986)for details.

917

related to radial deviation of the wrist andflexion of the fingers. This pattern, which isrelated to grasping in arboreal species, is notseen in other terrestrial papionins. Its con-sistent occurrence in Theropithecus, however,has been convincingly linked by Jolly (1972)and Maier (1972) to extreme manual dex-terity and an adaptation for manual grazing.The articular surface of the sigmoid notch inKNM-WT 39368 is narrow and resembles atwisted parallelogram, with the sides and thearticular surfaces directed craniolaterally.This configuration occurs in primates thatmove with extended elbows, in which theangulation of the sigmoid notch facilitatesstraighter fore and aft movements of theforearm (Krentz, 1993). The narrownessof the sigmoid notch of KNM-WT 39368,as in the case of the humeral trochlea,indicates that the humeroulnar jointwas not responsible for the transmission ofmost of the body weight and muscularforces, as it is in arboreal species (Conroy,1974).

The morphology of the radius has beenfound to be less consistently related to loco-motion than has the morphology of otherforelimb elements. Few consistent differ-ences in radial shaft curvature, robustness,diaphyseal cross-section, or neck length existbetween species of highly divergent loco-motor modes (Olivier & Soutoul, 1960;Jolly, 1965). Birchette (1982) and Krentz(1993) were, however, able to recognizeinteresting and, possibly, functionally sig-nificant differences in radial head shape andorientation in their comparative studies.Like other cercopithecids, KNM-WT 39368exhibits an elliptical radial head, a featurethat is associated with habitually pronogradeanimals, with limited capacities for supina-tion. Terrestrial cercopithecids would beexpected to have a more elliptical head (andhigher values for its associated index) thando arboreal ones. In KNM-WT 39368, theindex of radial head shape is 125, within therange of terrestrial cercopithecines (Conroy,

1974). Krentz (1993) reported that theradial head shape in T. brumpti was highlyvariable, with some individuals exhibitingquite round heads. One of us (NGJ) has alsoobserved this and has found that rounderheads tend to be found on smaller bones,presumably from smaller individuals. Radialhead shape may, thus, be related to bodysize or to the greater arboreal proclivities ofsmaller animals. The orientation of thearticular surface of the radial head is also ofpossible functional significance. The radialhead of KNM-ER 39368 and of other speci-mens of T. brumpti is only slightly angulated,an arrangement that enhances supinationand pronation. In the case of the radius, aswith the ulna, the two features of the radiusof putative functional significance conveydifferent messages. The elliptical shape ofthe radial head is generally related to higherdegrees of terrestriality, while the lack ofangulation of the head denotes an enhancedability for pronation and supination of thewrist. This seeming contradiction is entirelyconsistent, however, with the functional pic-ture provided by the distal humerus andulna of KNM-WT 39368, which indicatethat T. brumpti had an essentially terrestri-ally adapted forelimb that was also special-ized for fine manipulation and grasping withthe fingers, activities that require adeptpronation and supination of the wrist.

The region of the postcranium of Thero-pithecus that has attracted the most interestand speculation has been the hand. This isbecause of a series of findings beginningwith Napier & Napier (1967) that indicatedthat the living gelada had a high opposabilityindex, and that this was related to thespecies’ habit of manual grazing (Maier,1972; Etter, 1973). This arrangement isproduced by the relative elongation of thethumb, especially the pollical metacarpal,and the relative abbreviation of the proximaland middle phalanges of the index finger(Jablonski, 1986). In KNM-WT 39368, asin other cercopithecines, the digital formula

918 . . ET AL.

is III>IV>II>V>1. When the length ofdigits I, II, IV and V is expressed as apercentage of digit III, the greater relativelength of digit I in KNM-WT 39368 andT. gelada is readily apparent (Figure 14).Although many indices have been devised toexpress the relative length of the compo-nents of the digits and the relationships of

digits to one another, the opposability indexof Napier & Napier (1967) remains the mosteconomical mode of conveying the relation-ship between the lengths of the pollexand the index finger and their potentialfor opposition. The opposability indicesfor KNM-WT 39368 and other cerco-pithecines, including that of the fossil hand

919

specimen of T. brumpti from the Omo(Jablonski, 1986), were computed as thetotal length of digital ray I (X 100)divided by the total length of digital ray II.The results are presented in Table 6. Theopposability indices for all the theropiths areessentially the same, ranging from 69·2 forKNM-WT 39368 to 70·6 for the Omo T.brumpti, with an intermediate value of 69·7for T. gelada. This calculation demonstratesconvincingly that the high degree of oppos-ability between the index finger and thumbis a primitive character for Theropithecus(Jablonski, 1986). When this information isadded to that on the function of the shoul-der and elbow joints in Theropithecus, asdescribed here and by Krentz (1993), it isclear that the hand proportions of the genusreflect an adaptation for strong and precisepincer-like movements of digits I and IIduring foraging. The demonstration of verysimilar opposability indices in the differ-ent species of Theropithecus does not implythat the species had similar diets, ratherthat they had physically powerful precisiongrips that permitted them to efficientlyharvest and manipulate even the smallestfoodstuffs.

The final element of the postcranium ofKNM-WT 39368 requiring discussion is thetibia. As mentioned above, the tibia of T.brumpti and other theropiths is distinguishedfrom that of other papionins by its angulatedmedial malleolus and a prominent notch forthe passage of the tendon of M. tibialisposterior. Maier (1972) and Krentz (1993)have interpreted these features as beingrelated to greater flexibility of the foot andspecifically, inversion of the foot, duringforaging and locomotion. Maier (1972)reported that T. gelada inverts its foot muchof the time while feeding, and it appears thatthis habit and its attendant morphology isprimitive for the genus.

Although a complete roster of limbindices for KNM-WT 39368 cannot be cal-culated because of the absence of bothfemora, the brachial index can be deter-mined. This index for KNM-WT 39368(calculated as the length of the radiusdivided by the length of the humerus) is100·9, very similar to that determined byKrentz (1993) for samples of T. darti, T.oswaldi and T. gelada, but significantlyhigher than the figure calculated by him forother specimens of T. brumpti (84·1), which

Figure 15. Reconstruction of the somatic musculature (top) and external appearance of T. brumpti, basedon the skeleton of KNM-WT 39368. Information on the myology of living papionins was taken fromHartmann & Strauss (1933) and Swindler & Wood (1973), as well as from personal observations by oneof us (M.A.) of a dissected specimen of P. hamadryas hamadryas. Reconstruction of the color pattern wasbased on the reference of the extant species most closely related to T. brumpti, that is, members of theAfrican Papionina, which include the genera Theropithecus, Papio, Mandrillus, Cercocebus and Lophocebus(baboons sensu lato and mangabeys). A body mane like those of extant geladas and hamadryas baboonswas rejected as unlikely development in a riverine baboon like T. brumpti. Therefore, the species was givenbody hair of moderate length. However, a sagittal ridge of longer hair on the top of the skull and massesof long hair growing backwards and outwards from the cheeks are common among both open-country andriverine papionins, including the mangabeys, and were, thus, incorporated into the reconstruction. A darkfacial mask surrounded by light-colored fur is very widespread among papionins and other cercopithecids,and the pattern is exaggerated in such riverine species as the drill and the red-capped mangabey, both ofwhich make extensive use of gape displays to expose their impressive canine teeth. Light-colored skin inand around the eyelids is a widespread papionin feature, while a bright band along the top of the muzzleis seen in the mandrill and in a variety of cercopithecine monkeys. The described pattern is useful foremphasizing facial expression in the low-light environment of the riverine woodland. For the body color,a moderate contrast between an olive-brown-gray dorsum and a light-colored, nearly white, ventrum waschosen. This pattern is seen in many riverine papionins, in contrast to the more uniform coloration ofstrictly savannah-dwelling baboons. The remarkably long tail of T. brumpti is here given a light tip, as inseveral mangabey species which also display long tails and use them in their repertoire of body language.

920 . . ET AL.

was based on a sample of composite individ-uals. When an index is calculated based onthe length of the radius relative to the lengthof the tibia of KNM-WT 39368, a figure of106·2 is obtained, strongly suggesting thatthe forelimbs of the specimen were onlyslightly longer than the hindlimbs.

The body size of T. brumpti has beenestimated, using various methods, by Krentz(1993) and by Delson et al. (2000), resultingin estimates for male body mass rangingfrom 39·4 kg and 44·8 kg (Krentz, 1993)to 60 kg (Delson et al., 2000). Using theequations of Delson and colleagues (2000),an estimate of body mass in KNM-WT39368 was calculated based on humeruslength (for male cercopithecines), one of themost accurate predictors of body mass. Thiscalculation yielded an estimate of body massfor KNM-WT 39368 of 43·8 kg. T. brumptiis also recognized as being an extremelysexually dimorphic monkey, in its canineand P3 dimensions, as well as in body mass(Eck & Jablonski, 1987; Delson et al.,2000).

Conclusions

Careful examination and comparison of theanatomy of the partial skeleton, KNM-WT39368, of T. brumpti with that of otherextinct and extant cercopithecines indicatesthat the species is characterized by a uniquemixture of postcranial features that typifyboth highly terrestrial and arboreal species.This is not the contradiction that it may, atfirst, appear. This conclusion echoes that ofKrentz (1993), who found that all thero-piths were distinguished by a unique suite ofpostcranial characters associated with fore-limb flexibility and an adaptation for dexter-ous manipulation between the first andsecond digits. The main characteristics ofthe postcranium of KNM-WT 39368 thatally it to arboreal cercopithecines are thosethat are related to flexibility and enhancedmobility at the shoulder joint, enhanced

capabilities for radial deviation and flexionof the wrist, and well-developed graspingabilities of the fingers. These charactersare, in our opinion, imprinted on a funda-mentally terrestrially adapted locomotorskeleton, in which powerful fore-aft move-ments of the pectoral and pelvic girdlesdominated locomotion. The large bony sur-face areas of contact between the elementsof the shoulder and elbow joints rein-force this interpretation. The anatomy ofevery species is an evolutionary compromisebetween sometimes conflicting functionaldemands, and that of T. brumpti is noexception. Its skeleton reflects the effects ofnatural selection operating to enhance theefficiency of the hand in foraging whileretaining the overall mechanical efficiency ofthe limbs in terrestrial locomotion.

The diet of T. brumpti has been consid-ered by several workers, based on consider-ations of jaw and jaw adductor morphology(Jablonski, 1993a) and molar tooth anatomyand microwear (Benefit & McCrossin,1990; Teaford, 1993). Based on theirexamination of the lengths of molar shearingcrests and other aspects of molar cusp mor-phology, Benefit & McCrossin (1990) pre-dicted that T. brumpti, like other earlytheropiths, exhibited longer shearing crestsand a higher potential for frugivory than T.oswaldi. Their inspection of the molar wearstriations of T. brumpti confirmed this pre-diction by revealing an absence of deeptransverse striations that may have beencaused by the grit often consumed in a dietof grasses. They concluded that T. brumptimay have been a true papionin folivore.

Based on the anatomy described here forKNM-WT 39368, the known preference ofT. brumpti for riverine forest or forest-margin habitats throughout its evolutionarytenure, and its presumed diet, we recon-struct the species as being a generallyterrestrial—but highly dexterous—verylarge-bodied, sexually dimorphic, andpossibly folivorous papionin. The species

921

appears to have been best adapted for pro-pulsive quadrupedal locomotion over gener-ally even ground, and yet was highly adept atmanual foraging. Based on the estimates forvery large body mass, especially in males, forthe species, it is doubtful that it was highlyarboreal, although a partly arboreal habitusmay have characterized juveniles or females.

T. brumpti is known to have had adramatic facial appearance, owing to itslarge zygomatic arches and greatly enlargedand anteriorly protruding Mm. masseterici,which facilitated the animal’s gape displays(Eck & Jablonski, 1987; Jablonski, 1993a).Other important characteristics of thespecies, such as the coloration of its facialskin and hair and bodily pelage, cannot beas accurately reconstructed as those of itsmasticatory and locomotor systems, but stillcan be generally predicted on the basis of theappearance of living species today. On thebasis of such analogies, we suggest thatT. brumpti would have evolved conspicuouspatterns of facial coloration, involving itsmuzzle skin, its facial hair, or both. Thecolorful and species-specific patterns offacial and perineal skin and hair seenin the modern, forest-dwelling species ofCercopithecus are obvious referents here, asare the brightly colored, iridescent muzzleand perineum of the large, forest-dwellingpapionin, Mandrillus sphinx.

Thus, in its completeness, KNM-WT39368 has given us a clearer glimpse of oneof primate evolution’s most intriguing andundoubtedly beautiful species (Figure 15).

Acknowledgements

We thank Wambua Mangoa and othermembers of the 1998 survey and excavationteam at West Turkana for sighting andrecovering KNM-WT 39368. Peter Gaedeis thanked for drawing Figure 9, and theFellows Fund of the California Academy ofSciences is acknowledged for providingfinancial support for his illustration work.

Dr Francisco Pastor of the University ofValladolid, Spain, is thanked for permittingone of us (MA) to undertake a dissection ofa hamadryas baboon in preparation for thisreconstruction. George Chaplin is thankedfor many productive discussions, and for hisassistance in data recording and the identi-fication of fragments recovered from sieving.Eric Delson and one anonymous reviewerare thanked for their constructive com-ments on the manuscript. Terry Harrison iswarmly thanked for his constructive com-ments, attention to detail, and patiencethroughout the review process.

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