Randall L. SusmanDepartment of AnatomicalSciences, School of Medicine,University at Stony Brook,Stony Brook, New York,11794-8081, U.S.A. E-mail:rsusman@uhmc.sunysb.edu

Darryl de RuiterPalaeoanthropology Unit forResearch and Exploration,Bernard Price Institute forPalaeontology, University ofthe Witwatersrand,Private Bag 3, Wits 2050,Johannesburg, South Africa.E-mail:deruitd@science.pg.wits.ac.za

C. K. BrainTransvaal Museum,P.O. Box 413, Pretoria,0001, South Africa. E-mail:brainnew@iafrica.com

Received 19 April 2001Revision received2 July 2001 andaccepted 3 July 2001

Keywords: Early Homo,hominids, Paranthropusrobustus, postcranial fossils,Swartkrans Cave.

Recently identified postcranial remains ofParanthropus and Early Homo fromSwartkrans Cave, South Africa

Fifteen newly recognized hominid postcranials from Swartkrans aredescribed here and compared with a sample of previously describedearly hominids, African apes and modern humans. Ten of the newspecimens are from Member 1. Two are from Member 2 and threeare from Member 3. Nine of the fossils are referred to Paranthropus,three to Homo, and three specimens cannot be assigned at present.

The collection of hominid postcranials from Members 1–3 atSwartkrans now numbers more than 70 specimens. With thedescription of two new, small femoral heads, SKW 19 and SK 3121,there are now four proximal femora from Swartkrans. When SK 82and SK 97 are compared with SKW 19 and SK 3121, the two setsoffer important insights into body size and sexual dimorphism inParanthropus robustus.

A new distal femur, SK 1896 and other bones attributed to Homocf. erectus, indicate that male Homo were larger than Paranthropus atSwartkrans.

� 2001 Academic Press

Journal of Human Evolution (2001) 41, 607–629doi:10.1006/jhev.2001.0510Available online at http://www.idealibrary.com on

Introduction

Fifteen newly recognized hominid speci-mens identified in the faunal collection fromSwartkrans offer new insights into the mor-phology and natural history of Paranthropusand early Homo. The postcranial fossilsdescribed below were initially recoveredbetween 1949 and 1986 during variousphases of excavation at the site. Togetherwith the fossils described earlier (Table 1),the Swartkrans hominid assemblage nowconsists of 71 postcranial elements as well as341 craniodental remains. The estimatedminimum of individuals of Paranthropus

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robustus and Homo cf. erectus indicated bypostcranial and craniodental fossils atSwartkrans number from 90 (de Ruiter,2000) to 132 individuals (Brain, 1993).

The new postcranial remains presentedhere all sample previously known parts ofthe skeleton of both hominid species atSwartkrans. The postcranial collection fromMembers 1–3 now totals a number of bonessufficient to assess body size and sexualdimorphism in Paranthropus and Homo. Thenew additions to the hominid collectionbolster our confidence in the assignment ofpostcranial remains to either of the twospecies in Members 1–3. Finally, new fossils

� 2001 Academic Press

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Table 1 Swartkrans Hominid Postcranial Fossils, Members 1–3

Specimen Description Provenience* Reference Taxon

SK 1896 Distal femur HR Present HSK 2598 Distal humerus HR Present PSK 3121 Femoral head & neck M2 Present PSK 25600 Distal humerus LB Present PSK 24601 Proximal radius LB Present PSKW 19 Femoral head HR Present PSKX 2045 Proximal radius M2 Present HSKX 10641 Proximal phalanx (h) LB Present P?SKX 10924 Distal humerus M3 Present HSKX 15468 Proximal phalanx (h) LB Present PSKX 16699 Proximal phalanx (f) ?LB Present —SKX 19495 Distal humerus M3 Present PSKX 38529 Metatarsal III LB Present —SKX 38653 Middle phalanx (h) M3 Present PSKX 42695 Talus LB Present —SK 82 Proximal femur HR Napier, 1964 PSK 97 Proximal femur HR Napier, 1964 PSK 18b Proximal radius M2 Broom & Robinson, 1949 HSK 3981a Thoracic vertebra HR Robinson, 1970 PSK 3981b Thoracic vertebra HR Robinson, 1970 PSK 50 Os coxae HR Broom & Robinson, 1950;

Robinson, 1970P

SK 853 Lumbar vertebra HR Broom & Robinson, 1949;Napier, 1959

H

SK 854 Cervical vertebra C2 HR Broom & Robinson, 1949;Napier, 1959

P

SK 860 Distal humerus (baboon?) HR —SK 84 Metacarpal I HR Napier, 1959; Rightmire, 1972 PSK 85 Metacarpal IV HR Napier, 1959 HSK 14147 (SKW27) Metacarpal V HR Day & Scheuer, 1973 —SK 3155b Os coxae HR Brain et al., 1974 HSKX 5016 Distal phalanx—pollex LB Susman, 1989 PSKX 5017 Metatarsal I LB Brain, 1984; Susman & Brain, 1988 PSKX 5018 Proximal phalanx (h, IV?) LB Susman, 1989 PSKX 5019 Middle phalanx (h) LB Susman, 1989 PSKX 5020 Metacarpal I LB Susman, 1989 PSKX 5021 Middle phalanx (h) LB Susman, 1989 PSKX 5022 Middle phalanx (h) LB Susman, 1989 PSKX 3602 Distal radius LB Susman, 1989 PSKX 8761 Proximal ulna LB Susman, 1989 PSKX 8963 Distal phalanx (h) LB Susman, 1989 PSKX 9449 Middle phalanx (h) LB Susman, 1989 PSKX 13476 Middle phalanx LB Susman, 1989 PSKX 34805 Distal humerus LB Susman, 1989 HSKX 3774 Distal humerus LB Susman, 1989 PSKW 4776 Cervical vertebra C3 (C4) LB Susman, 1993 PSKX 45690 Proximal phalanx (hallux) LB Susman, 1989 PSKX 247 Metatarsal II M2 Susman, 1989 —SKX 344 Middle phalanx (f) M2 Susman, 1989 —SKX 1084 Patella M2 Susman, 1989 PSKX 1261 Middle phalanx (f) M2 Susman, 1989 —SKX 3062 Middle phalanx (h) M2 Susman, 1989 —SKX 3342 Cervical vertebra M2 Susman, 1989 H?SKX 3498 Triquetral M2 Susman, 1989 —SKX 3699 Proximal radius M2 Susman, 1989 P

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enhance the view of the positional behavior,and prehensile capabilities of Paranthropusand early Homo at Swartkrans from 1·8 to1·0 m.y.a.

Materials and methods

The ‘‘new’’ fossils were compared withpreviously described postcranial elementsfrom Swartkrans (Table 1), with australo-pithecines and early Homo from sites else-where in South and East Africa, and with asample of extant ape and modern humancounterparts. The comparative sampleconsists of skeletal materials of bonobos,chimpanzees, and gorillas from theTervuren Museum, the American Museumof Natural History, the Cleveland Museumof Natural History, the United StatesNational Museum, the Dart Collection atthe University of the Witwatersrand, and theDepartment of Anatomical Sciences atStony Brook. In addition to the above, thehuman sample consists of a size range thatincludes data on Euroamericans and Akkapygmies supplied by Bill Jungers. Data takenfrom the literature are so acknowledged inTables 1–3.

Measurements were made on skeletalmaterials with dial, vernier and coordinatescalipers and recorded to the nearest0·10 mm. Measurements of the fossils weremade on the actual specimens. Photographs,a–p and m–l radiographs, and CT scans ofselected fossils were made at UniversityHospital and Medical Center at StonyBrook (UHMC). CT scans were made witha G.E./CTi Helical CT scanner in theDepartment of Radiology, UHMC.

Postcranial remains fromMembers 1–3

Table 1 Continued

Specimen Description Provenience* Reference Taxon

SKW 2954 Metacarpal IV M2 Susman, 1989 —SKW 3646 Metacarpal III M2 Susman, 1989 —SKX 19576 Proximal phalanx (h) M3 Susman, 1989 —SKX 22511 Proximal phalanx (h) M3 Susman, 1989 —SKX 22741 Proximal phalanx (h) M3 Susman, 1989 HSKX 27431 Proximal phalanx (h, III) M3 Susman, 1989 H?SKX 27504 Distal phalanx (h) M3 Susman, 1989 —SKX 31117 Medial cuneiform M3 Susman, 1989 PSKX 33355 Middle phalanx (h) M3 Susman, 1989 —SKX 33380 Metatarsal V M3 Susman, 1989 PSKX 35439 Middle phalanx (h) M3 Susman, 1989 —SKX 35822 Proximal phalanx (h) M3 Susman, 1989 —SKX 36712 Middle phalanx (h) M3 Susman, 1989 —

*HR=Member 1 Hanging Remnant.LB=Member 1 Lower Bank.M2=Member 2.M3=Member 3.

Femoral head (SK 3121)SK 3121 is a well-preserved right femoralhead retaining the proximal 23 mm of theneck (Figure 1). The specimen wasmechanically prepared out of an ex situMember 2 breccia block and had originallybeen cataloged as a carnivore femur. Thehead measures 28·8 mm in diameter (supero-inferiorly). From what remains of the neck itwas apparently long given the small size ofthe head. The femoral neck is compressedanteroposteriorly (Table 2). It is difficultto assess the relative position of the fovea

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capitis due to the incompleteness of the neckand lack of a shaft.

Comparative anatomy. Like SK 82 andSK 97, SK 3121 possesses a long, com-pressed femoral neck, typical of australo-pithecines. Table 2 displays the neck shapeindex of the four Swartkrans hominids, thelarge and small femora of A. afarensis, thosefrom Koobi Fora, modern humans andchimpanzees. Results illustrate the com-pressed shape of the femoral neck in theaustralopithecines.

SK 3121 is a small femoral head approxi-mately the size of A.L. 288-1ap. The twoheads measure 28·8 mm and 28·6 mm(Johanson et al., 1982), respectively. SK3121 is smaller than the two previouslyreported Swartkrans femora, SK 82 and SK97 (Napier, 1964; Robinson, 1972; Ruff

et al., 1999). It is likely that SK 82 (with asuperoinferior head diameter of 34·4 mm)and SK 97 (head diameter 37·1 mm)

represent male Paranthropus. Based on itssmall size and flat neck we assign SK 3121to Paranthropus. Its small size suggestsfurther that the SK 3121 femoral head isprobably that of a female.

Figure 1. Femoral heads SKW 19 (left) and SK 3121 (right). (a) posterior view of head and neck;(b) inferomedial view of neck cross-section.

Femoral head (SKW 19)SKW 19 is an intact right femoral head,extracted from the characteristically pinkcolored breccia of the Hanging Remnant ofMember 1. It was mechanically preparedand what is preserved of the fossil is in goodcondition. The fossil retains 8–10 mm of theproximal femoral neck. A CT scan of theneck recalls the morphology seen in SK3121 (above) and suggests that SKW 19 atone time possed an anteroposteriorly com-pressed femoral neck [Figure 1(b)]. Themost notable feature of SKW 19 is its smallhead that measures 30·7 mm in diameter.The fovea capitis is large with a prominentrim.

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Comparative anatomy. With only a smallpart of the femoral neck, assessment of thetaxonomic status of SKW 19 is somewhatproblematic. However, the a–p compressionof what little remains of the femoral neckjust proximal to the head is suggestive ofP. robustus, rather than Homo (Figure 1).

The size of SKW 19 (head diameter of30·7 mm) is close to that of SK 3121(28·8 mm). These two ‘‘new’’ femoralheads together with SK 82 and 97 suggestbimodality in femoral heads’ sizes atSwartkrans. The small size of SKW 19 andSK 3121 suggest that these two representParanthropus females, while their largercounterparts, SK 82 and SK 97 (Napier,1964; Robinson, 1972), represent males ofthis species. This contrast in female andmale femoral head diameter would be

Femoral neck shape index (a–p/si�100)

Specimen(s) Index S.D. Taxon

SK 3121 72 — Paranthropus robustus femaleSK 82 70 — Paranthropus robustus maleSK 97 75 — Paranthropus robustus maleA.L. 288-lap 70* — Australopithecus afarensis femaleA.L. 333-3 63* — Australopithecus afarensis maleKNM-ER 738 73† — ParanthropusKNM-ER 815 64‡ — ParanthropusKNM-ER 1503 62† — ParanthropusKNM-ER 1481 74† — Homo

Sub-Saharan African§Male 78 8·7Female 81 —

American black§Male (45) 79 6·4Female (8) 80 4·6

Zulu§Male (25) 83 5·7Female (25) 86 7·1

Khoisan§Male (28) 86 5·9Female (19) 87 6·1

Pan troglodytes (3) 78 —

*From Johanson et al. (1982).†Measurements taken by Stern (personal communication).‡From Walker (1973).§The human sample is from Grine et al. (1995).

Table 2

reflected in body size dimorphism compar-able to that suggested for Paranthropus atSwartkrans and at Koobi Fora (McHenry,1991, 1992a). Based on femoral head sizeshere, and using data from McHenry (1991)and Grine et al. (1995), the body massestimates for male and female Paranthro-pus at Swartkrans are 40 kg and 31 kg,respectively.

SKW 19 and SK 3121 are similar in sizeto SK 3155b, the small partial os coxae fromthe Hanging Remnant of Member 1,described by Brain et al. (1974). Brain et al.(1974) referred the SK 3155b pelvis to thegenus Homo. McHenry (1975) likenedSK 3155b to SK 50. The implications forbody size estimates in Paranthropus andHomo females at Swartkrans are discussedbelow.

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Figure 2. Distal femur SK 1896. (a) anterior, (b) medial, (c) lateral views.

Distal femur (SK 1896)SK 1896 is the distal end of a right femur,recovered from the Hanging Remnant ofMember 1 using acetic acid preparation.The postdepositional distortion of the fossilmade identification difficult; it was initiallythought to be a large felid. The entire distalend is represented although the specimen isdeformed by post mortem crushing, crack-ing and shearing (Figure 2). A minoramount of surface bone has been lost in thevicinity of the posterior limb of the lateralcondyle. The specimen is cracked in anumber of places but it retains the truemorphology of the patellar groove, andmuch of the medial and the lateral condyles.Most affected by post mortem crushingis the cross-sectional geometry of whatremains of the shaft.

The crushing and deformation of the shaftis most notable on the anteromedial surfaceof the bone, medial to the medial patellarlip. The medial condyle displays two trans-verse breaks, one anterior, and the otherposterior. There is a crack running supero-inferiorly on the uppermost extent ofthe medial condylar articular surface.The menisco–tibial surfaces are partially

obscured by cracks running through them.The profile of the medial condyle has a‘‘rounded’’ appearance with a reducedradius of curvature compared to that of theflatter lateral condyle.

The lateral condyle has two major postmortem breaks, one running mediolaterallyacross the anteroinferior surface, the otheron the posterior surface. The effect of thesetwo artefacts is an anteromedial displace-ment of the inferolateral articular surfacealong a shear plane running back to front.The lateral condyle is missing theposterolateral-most extent of the articularsurface. However, enough remains of thecondyle to observe that it is relatively flat,with an increased radius of curvature.

In addition to the disparity in the shape ofthe medial and lateral condyles, SK 1896has a prominent anteriorly projecting lateralpatellar lip. Despite the a–p deformation ofthe shaft, it retains a valgus orientation whenthe fragment is placed on a flat surface. Thecracks running through the medial andlateral condyles obfuscate precise measure-ment of the femoral angle but it is safe toinfer a value within the human range of6–14� (Tardieu, 1999).

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Manual proximal phalanx IV? (SKX 15468)SKX 15468 is a short, stout, hominidproximal phalanx of either finger IV or II(Figure 3). It was excavated from the LowerBank of Member 1 (E4N5, SW, 740–750 cm), but was originally not recognizedas hominid. It was derived from the lightlycalcified sediment that characterizes most ofthe Lower Bank, requiring no preparation.The subtlety of the asymmetry of the basaltubercles and the palmar surface of the bodysuggest that it is from the second or fourthdigit. The fossil is essentially complete andundistorted. The major post mortemartefacts include: (1) a small crack runningproximodistally just inside the flexor sheathimpression, (2) a more sizeable sinusoidalcrack on the distal half of the posterior

The epicondyles both display post mor-tem damage. The medial epicondyle hasbeen cracked and depressed slightly but theadductor tubercle is still discernible. Thelateral epicondyle is cracked through andappears to have been reattached to thelateral side of the condyle. The area ofbone between the lateral epicondyle andthe posterolateral-most extent of thecondylar articular surface is missing andthe cancellous network is exposed.

The anterior surface for the bursal portionof the synovial cavity is depressed above thesuperomedial margin of the trochlea. Thefovea on the posterior surface just superiorto, and between, the condyles is essentiallyintact, displaying only a minor amount ofdistortion resulting from the a–p crushingforce that appears to have come from thefront.

Comparative anatomy. The condylar pro-files are hominid-like and differ from thoseof nonhuman primates. The lateral condylehas a ‘‘flattened’’ appearance while themedial condyle is more sharply curved. Nodoubt the discrepant radii of curvatureobserved in the condyles of SK 1896resulted in a hominid-like screwing home ofthe joint upon full extension of the knee.Viewed distally, end on, the condyles appearto be elongated anteroposteriorly, like thoseof later hominids, and unlike those of greatapes (Tardieu, 1981, 1999).

The new femur displays features ascribedto Homo in the prominent lateral patellar lip.The lateral lip resembles that of humans andKNM-ER 1472 and 1481 more than TM1513, Sts 34, or other australopithecines. Ifthe valgus orientation of the femoral shaft isa reasonable approximation of its expressionduring life, then the femoral angle may alsobe closer to that of Homo than that ofParanthropus and Australopithecus.

SK 1896 represents a large femur asjudged by the broad patellar groove andlarge condylar articular surfaces. The newfemur is larger than both TM 1513 and Sts

34. Mediolaterally, from peak to peak, thepatellar groove of SK 1896 measures35.9 mm. Those of TM 1513 and Sts 34measure 24·0 mm and 26·8 mm, respect-ively. Accurate measurement of the bi-epicondylar diameter of SK 1896 is notpossible, nor is a mediolateral breadth of thelateral articular surface. It is, however, poss-ible to obtain a maximum m–l diameter ofthe medial condyle of 23·6 mm. This com-pares with 22·3 mm for TM 1513 and22·1 mm for Sts 34. The mediolateralbreadth of 23·6 mm yields a body massestimate of 57 kg (based on a least squaresregression with a standard error of the esti-mate of 7·36 kg) for SK 1896 based on themediolateral breadth of the medial femoralcondyle in sample 18 Euro-Americans fromthe Cleveland Museum of Natural History.This estimate is larger than any other wehave obtained for Swartkrans hominids (seealso McHenry, 1991, 1992a). It thusappears that based on morphology and com-parisons with the distal femur of modernhumans and other early hominids (includingthose of australopithecines and early Homo)that SK 1896 probably represents a verylarge, male Homo in Member 1.

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Figure 3. Manual proximal phalangeal curvature of Swartkrans hominids SKX 15468, SKX 5018, SKX22741 and extant hominoids. A larger included angle indicates a greater curvature. The comparativesample includes 146 human phalanges, and those of 88 gorillas, 63 chimpanzees and 38 bonobos. Verticallines are the group means; long bars are the 95% confidence limits of the populations.

Table 3 Manual proximal phalanx, SKX 15468(measurements in mm; included angle in degrees)

Length* 34·9M-L midshaft diameter 9·3A-P midshaft diameter* 6·3M-L base diameter 12·0M-L trochlear diameter 8·7Max. m–l diameter 9·3Epitrochlear diameter 9·3Interarticular length 32·9Coordinate height* 5·5Included angle 30·7

*Measurements used to calculate included angle (seeSusman et al., 1984).

surface, and (3) a patch of erosion of thecortex on the dorsal surface of the body andon the dorsal rim of the proximal articularsurface. The post mortem damage doesnot affect significantly the measurementsin Table 3.

The phalanx has a maximum length of34·9 mm, shorter than its counterpart, andoutside of the ranges, in bonobos (malesand females, mean=50 mm, n=14),chimpanzees (males and females, mean=56 mm, n=33), and humans (males andfemales of the Cleveland Museum ofNatural History collection, mean=43 mm,n=40). Its mediolateral diameter at mid-shaft is 9·3 mm, similar to that of humans,bonobos, and small chimpanzees. It has welldeveloped basal tubercles, prominent but

depressed flexor sheath margins and ananteflexed, well-differentiated trochlea witha prominent central sulcus.

Comparative anatomy. SKX 15468 repre-sents a small hand. Based on maximumlength the phalanx is most similar to that ofgibbons among extant apes and humans; itis considerably shorter than its counterpart(either II or IV) in bonobos, chimpanzees,or humans. Overall, SKX 15468 bears astriking resemblance to another manualproximal phalanx from the Lower Bank ofMember 1 at Swartkrans, SKX 5018(Susman, 1988 et seq.). SKX 15468 andSKX 5018 are roughly similar in size(maximum lengths of 34·9 and 33·4 mm,respectively). Both fossils display a similarstaining and patina, and they share similareffects of surface weathering in the form ofsurface cracks and cortical exfoliation. BothSwartkrans fossils are closer to each other inoverall morphological pattern than either isto Stw 28, a relatively intact proximalphalanx from Sterkfontein Member 4.

The included angle, a measure of curva-ture (Susman et al., 1984; Stern et al., 1995)related to arboreality and terrestriality, com-pares favorably in SKX 15468, SKX 5018and SKX 22741 from Swartkrans (Fig-ure 3). These fossils fall within the 95%confidence limits of the mean for thehumans sampled (Susman et al., 1984) and

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at the low end of the gorilla range. The cur-vatures of the fossils lie outside the ranges ofbonobos and chimpanzees. Despite theirhuman-like curvatures, the Swartkransphalanges display a more ‘‘primitive,’’ ape-like shape. Save for their curvatures, they donot resemble closely the manual proximalphalanges of modern humans.

Figure 4. Anterior view of left distal humeri, (a) SKX 10924, (b) SKX 19495, (c) SK 24600, and(d) SK 2598. Note the carnivore tooth mark (arrow) on the anterior left aspect of SKX 19495.

Distal humerus (SKX 10924)SKX 10924 is an essentially intact distal endof a hominid humerus [Figure 4(a)]. It wasrecovered from the decalcified Member 3deposit (W6S4, 0–100 cm), and bears thedark manganese staining that typifies thismember. There is a longitudinal crack onthe posterior surface of the shaft that ends ina ‘‘Y’’ with one limb heading to the medialpillar and the other limb toward the lateralpillar. The specimen has suffered postmortem damage to the posterior surface ofthe medial epicondyle and to the lateral edgeof the trochlea on its posterior surface.

The articular features are not sharplydefined [Figure 4(a)]. The capitulum ispositioned on the anterior surface. The fossilhas an indistinct zona conoidea between thetrochlea and the capitulum. The centralsulcus of the trochlea is not deeply grooved.The supracondylar ridge is evident but notpronounced. The olecranon fossa is large,ellipsoidal and lacks a sharp crest, or edge,on its lateral wall.

The shaft is broken and retains only37 mm of bone proximal to the biepicon-dylar axis. The biepicondylar diameter ofthe specimen measures 43·8 mm across.The trochlea measures 15·0 mm on itsanterior surface and the capitulum measures13·8 mm. The lateral epicondyle does notrise prominently above, or proximal to, thesuperomedial edge of the capitulum.

Comparative anatomy. SKX 10924appears to have a suite of features thatrecalls that of Homo. This includes the rela-tively indistinct zona conoidea, the distalpositioning of the lateral epicondyle relativeto the capitulum, and the greater relativeanteroposterior diameter of the distalhumeral shaft (Figure 5). The capitulum ispositioned anteriorly as in humans, ratherthan distal and posterior as in chimpanzees.The shape of the olecranon fossa is alsomore reminiscent of humans than apes.Notwithstanding the above, it is importantto note that all of these features vary withinextant taxa.

Both McHenry (1973, 1975) and Senut(1981) have commented on the difference inshape of the olecranon fossa in apes andhumans. In both shape of the fossa and inthe expression of its lateral border, SKX10924 resembles humans rather thanapes. Also more human- than ape-like is theunremarkable supracondylar ridge and theincreased relative anteroposterior diameter

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of the humeral shaft inparting the appear-ance of a more ‘‘rounded’’ cross-section[Figure 5(a)]. Senut (1978a:7) commentedon the ‘‘conspicuous triangular shape’’ ofthe shaft of the distal humerus in humans.By contrast, Senut (1978b:7) describedthe diaphysis of the distal humerus inchimpanzees as ‘‘flat, anteroposteriorly.’’The unremarkable zona conoidea is anadditional human-like trait in SKX 10924.More primitive and ape-like is the fact thatin SKX 10924 the lateral epicondyle risesabove the upper margin of the capitulum.

SKX 10924 is considerably smaller thanTM 1517, a P. robustus humerus fromKromdraai. It is also considerably smallerthan SKW 3774, a Paranthropus distalhumerus from Swartkrans Member 1, and itis much smaller than KNM-ER 739, a pur-ported P. boisei from Koobi Fora. It isslightly smaller than SKX 34805, a distalhumerus from Swartkrans Member 1(Susman, 1989 et seq.). SKX 34805 wasinitially referred to the genus Paranthropusby Susman (1989). We now believe, basedon the cross-sectional shape of the shaft and

comparisons with the expanded sample ofdistal humeri at Swartkrans, that it shouldbe re-assigned to Homo. It appears likely,based on its morphological pattern, thatSKX 10924 represents Homo. On the basisof its small size in comparison to SKX34805 (a presumptive male Homo) SKX10924 is likely to be a female.

Figure 5. CT scans of distal humeri. Scans are taken proximal to the biepicondylar line at approximatelythe same point on each specimen (indicated below). Note the ‘‘rounded’’ shape of SKX 10924 [(a) left]compared with the a–p flattened cross-sections of the other three distal humeri, (b) SKX 19495, (c) SK24600, and (d) SK 2598 left to right. While slight differences in the points at which the individualhumerus fragments were scanned accounts for some of the variation observed, the shape of SKX 10924differs from that of its counterparts.

Distal humerus (SK 24600)SK 24600 is an intact left distal humerusfrom the Lower Bank of Member 1 [Figure4(c)]. It most probably comes from the sameindividual as SK 24601, a proximal radius(see below). SK 24600 and SK 24601 wereboth recovered from a box labeled ‘‘SKDump,’’ making their provenience initiallyuncertain since they were not derived fromthe in situ excavations. However, from thesame box were recovered a deciduous molar(Rdm2) which had an accompanying prov-enience label indicating the Lower Bank(E3N4, 450–500 cm), as well as a premolar(LP3) that was refit via interproximal con-tact to a maxilla previously recorded fromthe Lower Bank (de Ruiter & Brain, n.d.).

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This maxilla was also derived from the sameexcavation unit as the deciduous molar(E3N4, 450–500 cm). The matrix adheringto the other fossils in the box (including SK24600 and SK 24601) was consistent withthe Lower Bank, and as such, SK 24600 andSK 24601 have both been attributed to theLower Bank of Member 1.

The specimen preserves the same portionof the distal humerus and is only slightlylarger than SKX 10924 [Figure 4(a)]. Thebiepicondylar diameter of the fossil is44·7 mm. SK 24600 has a mottled appear-ance. The medial epicondyle is intact butthere appears to be some erosion of thelateral epicondyle. There is some minorerosion of the anterolateral edge of thecapitulum and the anteromedial edge of thetrochlea. On the posterior surface there isdamage to the medial and lateral edges ofthe trochlea and to the posterior surface ofthe lateral epicondyle. On the posterior sur-face of the shaft, just above the olecranonfossa, there are some longitudinal surfacecracks. The olecranon fossa is damaged,resulting in what is probably a post mortemperforation. There is matrix adhering to theedges of the fenestra.

The trochlea has a prominent medial edgeand a well-defined sulcus. The capitulumsits mainly on the anterior surface. Lateral tothe trochlea is a prominent zona conoidea.The capitulum is well defined and what isleft of the lateral epicondyle appears to riseabove the upper edge of the capitulum. Theolecranon fossa itself is ellipsoidal.

Comparative anatomy. SK 24600 isslightly larger than SKX 10924 as deter-mined by the biepicondylar diameters ofeach specimen (44·7 mm and 43·8 mm,respectively). Both specimens are small,only slightly larger in biepicondylar diameterthan that of A.L. 288-1 (biepicondylardiameter of 41·1 mm). They are smallin comparison to other distal humerifrom Swartkrans (SKW 3774) and fromKromdraai (TM 1517, 54·0 mm). The

capitulum in SK 24600 has a human-like,anterior orientation, similar to that of SKX10924.

While the two specimens are similar inoverall size and shape, there are some subtledifferences suggesting that they may repre-sent the two hominid species at Swartkrans.SK 24600 has a shaft that in cross-sectionappears broader mediolaterally than that ofSKX 10924 (Figure 5). The zona conoideaand the trochlea are more prominent in SK24600 than in SKX 10924. These differ-ences, and comparisons with TM 1517,KNM-ER 271 as well as with modernhumans, chimpanzees and bonobos, suggestthat SK 24600 samples Paranthropus. Basedon its small size in relation to SKX 3774, apresumptive male Paranthropus, we believeSK 24600 to be that of a female.

Distal humerus (SKX 19495)SKX 19495 is a left distal humerusfragment, lacking articular surfaces and thelateral epicondyle [Figure 4(b)]. The fossil isfrom decalcified Member 3 (W2S5, NE,160–170 cm) and is uniformly dark-stainedby manganese. A distinct carnivore toothscore is visible on the anterior surface[arrow, Figure 4(b)], while several probablepunctate depressions are visible on themedial pillar of the posterior aspect.

Only the superior-most portions of themedial epicondyle and the uppermost halfto two-thirds of the olecranon fossa arepresent. Otherwise, the entire distal entity,including most of the medial epicondyle, thelateral epicondyle, the trochlea, and thecapitulum are missing.

The two most notable features of SKX19495 are its small size, similar to the othernew distal humeri (Figure 4), and thepresence of tooth marks left by predators orscavengers at, or around, the time of death.

Comparative anatomy. Although thefossil has a superficial likeness to SKX10924, SKX 19495 is larger with a differentshaft cross-sectional shape [Figure 5(b)].

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SKX 19495 is broad (m–l) and flat (a–p)similar in shape to SK 24600, and unlikeSKX 10924. In this feature SKX 19495 andSK 24600 resemble TM 1517 and are morelike bonobos and chimpanzees than theyare like modern humans. Thus, theyare assigned to Paranthropus rather thanHomo. The small size is suggestive ofthe fact that this fossil may represent afemale.

Distal humerus (SK 2598)SK 2598 is left humerus fragment fromthe Hanging Remnant of Member 1 [Figure4(d)]. In addition to being the smallest ofthe six distal humeri from SwartkransMembers 1–3, SK 2598 is also the leastcomplete (Figure 4). As a result of itsincompleteness, SK 2598 was not initiallyrecognized as hominid, and was placed in an‘‘Indeterminate Fossil’’ box from the Hang-ing Remnant. It has grayish mottling, differ-ent from the other humeri. Only a portion ofthe medial epicondyle is present and it hassuffered a spawling-off of the posterior sur-face. There are numerous small weatheringcracks also on the posterior surface. Roughlyhalf of the olecranon fossa is preserved.What remains of the shaft has a flattenedappearance.

Comparative anatomy. The shaft of SK2598 is similar in cross-section to those ofSK 24600 and SKX 19495 suggestingParanthropus affinity [Figure 5(d)]. The fos-sil is small, but close in size to SK 24600 andto SKX 10924. It is interesting to note thatall four of the distal humeri reported hererepresent left elbows. Two previouslydescribed right distal humerus fragments(SKX 34805 and SKW 3774, Susman,1989 et seq.) represent larger individuals.

Pedal proximal phalanx (SKX 16699)SKX 16699 is a pedal proximal phalanx,likely to be that of the fifth toe (Figure 6).The bone is of questionable provenience,but thought to have come from the Lower

Bank of Member 1. It was derived from atrench dug by miners searching for lime-stone deposits in the area of the LowerBank, and its preservation is consistent withbeing derived from the lightly calcifiedLower Bank. The proximal phalanx is intactand undistorted save for half a dozen or sosmall spots of surface damage on the baseand on the distal articular surface, probablyincurred following recovery of the speci-mens. None of these artefacts affect signifi-cantly the morphology or measurement ofthe specimen.

The bone is small, measuring 20·3 mm inmaximum length. It has an ‘‘hour-glass’’shape with a well-developed base, a narrowbody and an expanded distal end. There is awell-developed basal collar for the attach-ment of the metatarsophalangeal (MTP)joint capsule. The basal articular surface isround in proximal view with a dorsal con-cavity implying an enhanced range ofdorsiflexion at the MTP joint. The plantarsurface of the base and proximal body is welldeveloped. The fossil has only a very modestcurvature as indicated by an included angleof 22�.

Comparative anatomy. SKX 16699 hasthe ‘‘hour-glass’’ shape typical of humanpedal proximal phalanges. The includedangle is also contained within the 95%fiducial limits of the human mean, and liesoutside those of great apes (Susman et al.,1984). This relatively straight phalanx is likethat of humans and unlike apes and otherearly hominids (e.g., Stw-355 which has anincluded angle of 36�, and A.L. 288-1y, withan included angle of 36�). Other human-likefeatures of SKX 16699 include a concave,‘‘excavated’’ dorsal edge of the proximalarticular surface, and a base with anenhanced development on its plantarsurface.

The phalanx is small compared tomodern humans. If SKX 16699 indeed rep-resents a fifth toe, then the individual isapproximately the same size as A.L. 288.

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SKX 16699 measures 20·3 mm in lengthcompared with a pedal proximal phalanxlength in A.L. 288-1y of 22·6 mm. If A.L.288-1y is from a second or fourth toe andSKXX 16699 is that of a fifth toe, then thetwo individuals may have been approxi-mately equal in body size. It is not poss-ible to assign this bone to Paranthropus orHomo.

Figure 6. Pedal proximal phalanx SKX 16699 (right), manual middle phalanx SKX 38653 (center), andmanual proximal phalanx SKX 10641 (left). (a) Mediolateral view, (b) dorsal view. SKX 10641 is that ofa subadult and is lacking its epiphysis.

Manual proximal phalanx (SKX 10641)SKX 10641 is a subadult manual proximalphalanx from the Lower Bank of Member 1(W1S4, SW+SE, 50–100 cm). It lacks theepiphysis (Figure 6). The specimen is darklystained by manganese. The weakly devel-oped secondary bony features such as theflexor sheath margins, basal tubercles, cur-vature, and distal trochlea are the result of

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the young age of this individual and itsrelatively poor state of preservation. Thereare surface cracks and some spots wherecortical bone has spawled off. On the palmarsurface of the trochlea a proximal corner hasbeen lost.

Comparative anatomy. The young onto-genetic age (including the lack of a basalepiphysis) of SKX 10641 precludes an accu-rate assessment of length and curvature.These factors, in turn, hinder taxonomicassignment. Nonetheless, SKX 10641 bearsa closer morphological resemblance to SKX5018 and SKX 15468 than it does to SKX22741 and SKX 27431 (Susman, 1988et seq.) and this may argue for its inclusionin Paranthropus.

Manual middle phalanx (SKX 38653)SKX 38653 is a manual middle phalanxfrom Member 3 (N wall, 700 cm). It has amottled gray appearance and is in poorcondition due to both pre- and post mortemalterations (Figure 6). Its poor conditionmade the fossil difficult to identify initially,and it was originally thought to be a baboonfossil. The bone measures 21·3 mm inlength. This measurement is probably closeto the actual length, being altered onlyslighly by loss of bone on the proximal-most point on the dorsal articular rim. Themaximum breath (m–l) of 8·9 mm is, like-wise, close to, but not precisely, the truevalue due to slight post mortem bone losscoupled with the presence of a small osteo-phyte on the left side (in palmar view).The trochlea has been altered by the depo-sition of bone on the palmar surface. Anadditional set of changes is observed in thepost mortem loss of bone surrounding boththe proximal and distal articular surfaces.Adding to the poor state of preservation is aseries of weathering cracks and spots oferosion. The bone has very protuberantflexor sheath margins, the apices of which liecloser to the proximal end than to the distalend.

Comparative anatomy. SKX 38653 bearsa close resemblance to SKX 5021 and othermanual middle phalanges from Member 1(Susman, 1988 et seq.). Among bonobos,chimpanzees, and humans, SKX 38653 ismost similar to its human counterparts.Nonetheless, the fossil differs from humansin the increased dorsopalmar ‘‘thickness’’ ofthe distal portion of the body and the pro-tuberant flexor sheath margins. Whilehumans most often have a distal concavityon the palmar surface of the body just proxi-mal to the trochlea, the fossil displays amediolateral convexity on the distal half ofthe palmar surface. As is the case with themiddle phalanges in humans, SKX 38653 isessentially straight, lacking in longitudinalcurvature. We hesitate to assign thisMember 3 fossil to either Homo or Paran-thropus on morphological grounds. Others(see Grine, 1988; Brain, 1993) have attrib-uted the entire hominid craniodental samplefrom Member 3 at Swartkrans to Paranthro-pus. In this case, it is most likely that SKX38653 represents P. robustus.

Third metatarsal (SKX 38529)SKX 38529 is a proximal fragment of a leftthird metatarsal (Figure 7). The fossil wasrecovered from the Lower Bank of Member1 (W5S4, 610–660 cm). This specimenwas initially labeled as being a potentialhominid, but this identification was not con-firmed until the present study. The speci-men consists of the proximal 26·9 mm of thebone. What exists of the bone is in relativelygood shape as far as surface morphologygoes. The distal extent displays a clean,sharp-edged break that exposes a narrowmedullary cavity. There is erosion on theplantar aspect of the base and on its medialsurface. On the medial side of the base botharticular facets are eroded. On the lateralside of the base a single articular facet forthe fourth metatarsal is present but it isdamaged by erosion of its periphery. Thelarge tubercle on the lateral side for the

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intermetatarsal ligament is intact. Much ofthe rim of the lateral cuneiform (proximal)articular surface has been lost. SKX 38529has an actual mediolateral basal breadth of12·3 mm. The true measure was slightlygreater by 0·5 mm or so, as judged by a lossof bone on the medial side of the base.

Comparative anatomy. The SKX 38529fragment has a decidedly human appear-ance. It differs from the third metatarsals ofbonobos and chimpanzees. In size, SKX38529 is roughly equivalent to that of smallhuman females in our sample of Euro- andAfrican-Americans. SKX 38529 is similarin both size and morphology to SKX 247, aleft second metatarsal from SwartkransMember 2 (Susman, 1989 et seq.).1 Bothfossils preserve approximately the same partof the metatarsal, viz., the initial 25–35% ofthe bone. The mediolateral basal breadth ofSKX 38529 measures 12·3 mm comparedto SKX 247 with a mediolateral basalbreadth of 11·7 mm. At a point 25 mm fromthe proximal end, the mediolateral breadthof the shaft of SKX 38529 measures 7·1 mmwhile that of SKX 247 measures 7·0 mm.The dorsopalmar diameters of SKX 38529and SKX 247 measure 9·6 mm and 9·3 mm,respectively. SKX 38529 is also compat-ible in size with SKX 33380, a left fifth

metatarsal from Member 3 (Susman, 1988et seq.). This bone cannot be assigned toHomo or Paranthropus based on mor-phological criteria, but the overwhelmingproportion of Paranthropus craniodentalremains in the Member 1 sample makesit likely that SKX 38529 samplesParanthropus.

1In the original, and subsequent, descriptions of SKX247 it was referred to as metatarsal III. SKX 247 is theproximal end of a second metatarsal.

Figure 7. Third metatarsal SKX 38529 (a–d, left). (a) Medial, (b) lateral, (c) plantar, and (d) dorsal views.

Talus (SKX 42695)SKX 42695 consists of a right posteriortalus fragment, probably from the LowerBank of Member 1 (Figure 8). The speci-men was initially catalogued as coming fromMember 5. In fact, the fossil was recoveredduring the removal of disturbed sedimentnear the interface between the Lower Bankof Member 1 and Member 5, just to the eastof the Member 5 Channel. The talus doesnot evince the geochemical characteristicsthat typify Member 5 fossils. Rather, itmatches the preservation and texture ofLower Bank fossils, and as such has beenattributed to the Lower Bank of Member 1.

The fossil retains the posterior two-thirdsof the trochlea including the intact fibulararticular surface, a small posterior remnantof the tibial malleolar surface, the sulcusfor the tendon of flexor hallucis longus(FHL), and a large, intact posterior subtalarfacet. The head and neck are missing. Thetrochlear surface measures approximately25 mm across (m–l). Despite the fact that

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the medial and lateral talar tubercles aredamaged slightly, they do not appear to havebeen prominent.

Comparative anatomy. The size of SKX42695 is within the ranges of both humansand chimpanzees. The trochlea widens an-teriorly, is relatively broad, and has relativelyequal medial and lateral sides. These arefeatures in which the fossil resembleshumans more than chimpanzees or bono-bos. The lateral edge of the trochlea is onlyslightly more elevated than the medial edge,a feature seen more often in humans than inapes. The vertical orientation of the sulcusfor the FHL is more like that of humansthan it is like chimpanzees or bonobos.

SKX 42695 has a substantial mediolateralbreadth of 25 mm as noted above. As such itis larger than TM 1517, Stw-88, Stw-102,Stw-347, OH 8-A, KNM-ER 813A,KNM-ER 1464, and KNM-ER 1476A. It issmaller than KNM-ER 5428. Based on theequation presented in McHenry (1992a) foran all-human least squares regression oftalar breadth and body mass, SKX 42695yields an estimate of 53 kg. Because of thestrong morphological similarities of hominidtali and the incompleteness of SKX 42695,it is not possible to assign this talus toParanthropus or Homo on morphologicalgrounds.

Figure 8. Talus SKX 42695. (a) Posterior, (b) superior view.

Proximal radius (SK 24601)SK 24601 [Figure 9(c)] is a small, leftproximal radius from the Lower Bank ofMember 1 (E3N4, 450–500 cm). SK 24601is a likely mate for the left distal humerusfragment, SK 24600. Both elements comefrom the Lower Bank of Member 1, are thesame size and articulate well, have the samepatina, and both display a continuous chan-nel of damage across their posterolateralsurfaces at the point at which the radial headapposes the capitulum of the humerus. Thisfossil has been attributed to the Lower Bankfor the same reasons outlined above for SK24600.

With a head diameter of 17·3 mm,SK 24601 is smaller than SKX 2045 (seebelow) and the previously described radialhead, SKX 3699 (Susman, 1989 et seq.).The overall length of what remains of SK24601 is 47·3 mm. The small remainingportion the specimen precludes assessmentof the interosseous border and determi-nation of the relative position of the radialtuberosity.

Comparative anatomy. SK 24601 hassuffered damage to its posterior surfacebut what remains indicates the samedonut-shaped (vs. cylinder-shaped) patternas that observed in SKX 3699, viz. a tri-partite articular surface with contact for a

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well-developed zona conoidea (Senut, 1986;Rose, 1988). In anterior view, the rim taperslaterally yielding a ‘‘cap-like’’ appearance.The neck appears thin and more constrictedthan that of humans.

Based on a least squares regression ofradial head diameter (17·3 mm) on mass, ina sample of 35 modern humans (including18 Euro-Americans and 17 African pyg-mies) SK 24601 yields a body mass estimatefor the fossil of 36 kg. This comports wellwith an estimate of 37 kg when the leastsquares regression equation for humans ofMcHenry (1992a) is applied. These bodymass estimates are equivocal as to the malevs. female status of SK 24601, but the suiteof morphological features (above), suggeststhat SK 24601 (+SK 24600) is most likelyto represent Paranthropus.

Figure 9. Proximal radii. (a) SKX 3699, (b) SKX 2045, (c) SK24601, (d) Stw-431-a, and (e) SK 18b.

Proximal radius (SKX 2045)SKX 2045 is the proximal 40% of a rightradius from Member 2 [Figure 9(b)]. Thefossil measures 123·7 mm in length and is ingood condition. There are numerous surfacecracks the most prominent of which arefound on the medial surface of the shaft andthe interosseous border or ‘‘crest.’’ Thesurface is mottled by manganese stain.Although undoubtedly derived from in situMember 2, the exact provenience of thisfossil within the Member is uncertain. It wasoriginally thought to be a baboon radius.

The head is relatively large, measuring21·6 mm in diameter. There is a small spotof erosion in the center of the fovea andthree small spots of exposed cancellousbone on the periphery of the head. The headhas only a faint hint of a ‘‘donut’’ shape

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(Figure 9), and only a slight bevel for articu-lation with the zona conoidea of thehumerus. Thus, while the fovea for the headof the capitulum and the ‘‘rim’’ for articu-lation with the ulnar notch and annularligament are in evidence, the ‘‘third’’ articu-lar surface for the zona conoidea is poorlyrepresented in SKX 2045.

The bicipital tuberosity is unremarkable.The interosseous crest, although affected bypost mortem cracking, is prominent, as isthe oblique line. The anterior surface for theorigin of flexor pollicis longus is flat and‘‘blade-like.’’ The pronator teres insertionside on the lateral aspect of the shaft isevident, but it has been partly obscured bysurface weathering and truncated by thebreak in the bone prior to midshaft. Theshaft has a thick cortex at the point at whichit is exposed.

Comparative anatomy. The radial head ofSKX 2045 differs somewhat from the othertwo proximal radii from Swartkrans (SK24601 and SKX 3699). Although SKX2045 is similar in size to the Stw-431a,an australopithecine from Member 4 atSterkfontein (Figure 9), the two radii differin shape. Stw-431a is slightly larger with ahead diameter of 22·2 and appears to have athinner neck and a thicker surface for thezona conoidea than does SKX 2045. Theradial heads of Paranthropus, both SK 24601and SKX 3699, are like that of Stw-431aand unlike SKX 2045. This is reflectedin the proximal view of the fovea, whichreveals an exaggerated ‘‘donut’’ shape in theaustralopithecines [Figure 9(a), (c), (d)].The relatively thick radial neck ofSKX 2045 is more like that of humans thanchimpanzees and bonobos.

The bicipital tuberosity is not as welldeveloped as it appears in apes. Napier &Davis (1959: 33) observed that the radialtuberosity in chimpanzees is displaced pos-teriorly, relative to the interosseous border.We observed a similar position of the radialtuberosity in chimpanzees and bonobos. In

humans the tuberosity is essentially pos-itioned anterior to the interosseous border.SKX 2045 is human-like in this regard.Stw-431a recalls the condition in Pan. Thebicipital tuberosity is not well preserved ineither of the Paranthropus radii, SK 24601 orSKX 3699.

The anterior surface of SKX 2045, fromwhich the flexor pollicis longus (FPL) takesits origin, is also human- rather thanchimpanzee-like. This portion of the radiusis marked by a sharp interosseous bordermedially and a flat to slightly concaveanterior surface in both humans and inSKX 2045. On the contrary, this portion ofthe radius in chimpanzees and bonobos isless ‘‘blade-like’’ and often convex ratherthan flat or concave. A comparison of extanthumans and African apes suggests a differ-ence in the area on the radius for the originof the flexor pollicis longus in humans andits homologue (the radial moiety of flexordigitorum profundus) in apes. Humanshave a broader, flat to concave surface forthe origin of the FPL than do bonobos,chimpanzees and gorillas. SKX 2045 hasa surface similar to that of humans. Inthis feature the radius of Stw-431a fromMember 4 at Sterkfontein, appears toresemble chimpanzees more than humans.

The radial head diameter yields a bodyweight of 53 kg (58 kg using McHenry’sleast squares equation for humans, 1992a).The significance of this large estimate is seenin the comparison of this fossil with SK24601, from the Lower Bank of Member 1,which yields a body mass estimate of 36 kg(see above). The large size of SK 2045 andits human-like morphology when comparedto Swartkrans radii SK 24601 and SKX3699, suggest that it samples a large (male?)Homo cf. erectus in Member 2.

Discussion

Because two hominid taxa have beenknown at Swartkrans since 1949 (Broom &

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Robinson, 1949), the first question asked ofany new find is whether it belongs to Paran-thropus or to Homo cf. erectus. This issue isespecially vexing when discussion involvespostcranial remains that, until recently, havebeen few compared with the remains ofskulls and teeth. The problem is confoundedfurther by the exigencies of cave taphonomy.Since hominid fossils are often the frag-mented remains of carnivore feedingbehavior, even when postcranials are seem-ingly associated with jaws and teeth, theassociations are never certain.

In order to attempt to understand thecomparative behavioral ecology of theSwartkrans hominids, the first item of busi-ness is to assign individual elements to eitherParanthropus or Homo. In order to do so, it isnecessary to align individual elements withtheir australopithecine counterparts or withthose of Homo. This was a difficult under-taking in the early days at Swartkrans asBroom was clearly less interested in post-cranial fossils than he was in skulls andteeth. Nowhere is this more evident thandespite the fact that the SK 97 femora werediscovered in 1949, save for a brief men-tion by Broom et al. (1950), they werenot described and analyzed until Napierundertook the task in 1964.

As a result, Broom & Robinson (1949)offered a fairly narrow perspective on thecomparative morphology of Parathropus andHomo at Swartkrans. When Robinson(1972) wrote his synthesis on the locomotoranatomy of the early hominids from SouthAfrica he had but ten or so fossils to workwith. Of these, only a pair of femora, SK 82and SK 97 represented duplicates of thesame bone, permitting only a faint glimpseof the intraspecific variation and compara-tive anatomy of the two hominid species atSwartkrans. Early on, the species assign-ments were tentative for all but a few ofthe Swartkrans postcranials. In most cases,the assignment of a fossil to either Paran-thropus or to Homo was influenced by

preconceptions about the presumptive‘‘derived’’ morphology of Homo versus themore ‘‘primitive’’ morphology of Paranthro-pus (Robinson, 1972: 4).

In the period of excavation from 1979–1986 dozens of additional postcranial fossilswere recovered at Swartkrans (Brain, 1993).The fossils described here represent 15remains recovered between 1949 and 1986.The hominid postcranial collection nownumbers over seventy elements. Moreover,the expanded collection contains four proxi-mal femora (with diagnostic features of thehead and neck), six distal humeri (withfeatures that may identify the two species atSwartkrans), four proximal radii, two polli-cal metacarpals (representing two differentspecies, Susman 1988, 1991; Trinkaus &Long, 1990), and multiples of a number ofhand and foot bones. The greatly expandedcollection of fossils from Swartkransimproves our understanding of what isParanthropus and what is Homo, and itallows us a more reliable view of variationand sexual dimorphism within these taxa.

Species attribution, body size and sexualdimorphism in P. robustusBody size is one of the most importantcharacteristics of an animal’s biology andecological status. Among primates, whenstock is taken of body size and sexual dimor-phism, insight is gained into importantaspects of behavior and social organization.Within primates (e.g., families, subfamilies),larger species live on the ground; smallerones tend to be more arboreal. In old worldmonkeys and apes (save for orang-utans)sexually dimorphic species tend to be moreterrestrial than their more arboreal kin.Characteristics of sociality and social organ-ization are also reflected in degree of sexualdimorphism (Clutton-Brock & Harvey,1977).

McHenry (1992b) has pointed out themany caveats and pitfalls of estimatingbody size from isolated fossil fragments. He

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suggests that among the many approaches tothe problem of reconstructing body size inearly hominids, the best predictors of bothbody size and stature in early hominids arefeatures of the hindlimb joints measured in abroad size range of modern humans. Lessconsistent predictions result from estimatesbased on joints of the upper limb and fromequations based on measurements takenfrom extant apes, or all hominoid regres-sions (McHenry, 1992b).

Elements in the Swartkrans collectionnow include six distal humeri representingtwo large and four small hominids. Mor-phology of the humeri suggests the likeli-hood that one large specimen (SKX 34805)is attributable to Homo (contra Susman,1988) and one large distal humerus (SKX3774) is that of Paranthropus. The mor-phology of the small humeri suggests thatthree are possibly those of Paranthropus (SK2598, SK 24600, and SKX 19495).

Of the two proximal radii described herefor the first time, SK 24601 is likely, onmorphological grounds, to represent Paran-thropus. It is small in size compared with bothSKX 3699 and SKX 2045. SK 24601, to-gether with its mate (SK 24600), is likely tobe that of a female Paranthropus while thelarger, but morphologically similar, SKX3699, is likely to be that of a male. McHenry(1991, 1992a), notes that body mass esti-mates for early hominids are more reliablewhen they are based on measurements of thelower limb. Nonetheless, a general sense ofcomparative body mass in the Swartkranshominids can be glimpsed from a comparisonof SKX 2045, SKX 3699, and SK 24601.When the fossils are compared with a sampleof Euro-Americans and African pygmies,body mass estimates consistent with esti-mates from other joint surfaces are observed.For example, estimates from the diameter ofthe femoral heads suggest that male andfemale body masses of Paranthropus clusteraround 40 kg and 30 kg, respectively.Females of Homo cf. erectus appear to have

been similar to those of Paranthropus in size,viz. around 30 kg. However, males of Homo atSwartkrans appear, based on evidence of thefemur (SK 1896), radius (SKX 2045) (andpossibly the talus, SKX 42695), to have beenlarger than their Paranthropus counterparts.

The evidence of the proximal femur atSwartkrans reinforces the picture of sexualdimorphism in Paranthropus. The australo-pithecine femur is described as having a suiteof features that includes a relatively smallhead and long neck, low neck-shaft angle,anteroposteriorly compressed neck, a greatertrochanter that does not flare out from theshaft, a high femoral condylar angle, and arelatively short overall length (Walker, 1973).Amongst the earlier described Swartkransfossils (Napier, 1964; Robinson, 1972) aretwo large Paranthropus femora, SK 82 and SK97 (McHenry, 1991). They have australo-pithecine grade characters including smallheads with long, anterposteriorly compressedfemoral necks. Two new small femoral heads,SK 3121 and SKW 19, represent small, pre-sumably female, individuals. Altogether, thefour femora indicate the presence of two largemales and two small females of P. robustus.The two size groupings suggest males withbody masses around 40+kg (SK 82 and SK97) and females with body masses at 30+kg(SK 3121 and SKW 19). This degree ofsexual dimorphism is greater than that ob-served in modern humans, chimpanzees, orbonobos, but less than that observed inorang-utans and gorillas. This finding com-ports with that of McHenry (1991, 1992a).The new and old postcranials together sup-port McHenry’s proposition that P. robustuswas considerably lighter in body mass thanthe 90 kg suggested by Robinson (1972) andearlier workers.

Body size and sexual dimorphism in Homocf. erectusThe small, human-like distal humerus, SKX10924, represents one small individual ofearly Homo. The morphology of the shaft

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and the weak zona conoidea suggest Homoaffinities. The small size, roughly equalto SK 2598, SK 24600 and SKX 19495indicates that it likely represents a female.

A larger, presumably male-sized cluster offossils is represented by the Homo-like proxi-mal radius, SKX 2045 [Figure 9(b)] and theHomo-like distal femur, SK 1896 (Figure 2).The large body mass estimate obtained froma regression based on radial head diametersuggests that male Homo at Swartkransmight have been larger in mass than itsParanthropus counterpart represented bySKX 3699. The large distal femur, SK1896, a presumptive Homo based on thederived patellar trochlea and human-likecondylar morphology, also suggests a bodysize for male Homo greater than that ofParanthropus.

Previous estimates of body sizeMcHenry (1991) characterized Swartkranshominids as either ‘‘small,’’ ‘‘medium’’ or‘‘large’’ in body size. Fossils that were simi-lar in size to counterparts in a 28 kg femaleAkka pygmy defined the ‘‘small’’ group. The‘‘medium’’ group was likened to a 45 kgBantu female and the ‘‘large’’ group wasdefined by its similarity to the skeleton of a54 kg female from the Schultz collection.McHenry placed the largest number of fos-sils in the medium size category. Our view ofMcHenry’s size groupings results in the shiftof a number of specimens from one group-ing to another. The more significant of theseinclude SKX 5016, SKX 8761, SK 85, SKX247, and SKX 31117.

SKX 5016 is an intact pollical distal pha-lanx. McHenry (1991) places this phalanxin the medium category while he places apollical metacarpal, SKX 5020, in the largecategory. Susman (1988) has shown, basedon metrical comparison of SKX 5016 withboth SKX 5020 and the smaller SK 85, thatSKX 5016 is a ‘‘good fit’’ with SKX 5020,the large pollical metacarpal. We wouldplace SKX 5016, together with SKX 5020,

in the ‘‘large’’ category. At the very least,SKX 5016 and SKX 5020 belong in thesame size category. SKX 5018, a well-preserved manual proximal phalanx, is likelyto be that of a small individual. In overalllength it is comparable in size to moderngibbons. It is shorter than its counter-parts in bonobos, chimpanzees and Euro-Americans. SK 85 and SKX 247 also appearto be those of the smaller of the Swartkranshominids. SKX 31117, a partial medialcuneiform, appears to be that of a largeindividual. The patellar fragment, SKX1084 (Susman, 1988 et seq.), placed in thelarge group by McHenry, is a poor match forthe trochlea of the large femur, SK 1896.We would place the patella in the range ofthe smaller Swartkrans hominids.

When the above adjustments are made,McHenry’s ‘‘medium’’ category shrinks andwhat remains of the medium-sized hominidsare only metapodials and phalanges. Ofcourse the relative size of these and theirplacement in small, medium and largegroups depends to some extent on whichdigit they represent. Thus, it seems to usthat the medium-sized fossils clusteringaround 45 kg become fewer in number.While we view the constituents ofMcHenry’s three groups somewhat differ-ently, the Swartkrans fossils may, in fact, fallinto three size groupings.

Based on both the old and the new fossilsit appears that both species of Swartkranshominids exhibited sexual dimorphism.Females of both Paranthropus and Homoaveraged around 30 kg in body mass. As formales, however, the postcranial evidencesuggests that male Homo were larger thantheir Paranthropus counterparts. The esti-mate of 57 kg for SK 1896 is consistent withthe estimate of 53 kg obtained from the largetalus, SK 42695. The size of SK 1896indicates that Homo cf. erectus males atSwartkrans were larger than their Paranthro-pus counterparts. The latter weighed, onaverage, around 42 kg. We can envision

628 . . ET AL.

three size groups at Swartkrans encompass-ing a different group of fossils than the threegroups proposed by McHenry (1991).Females of both hominid species weighed inat around 30 kg. Male Paranthropus aver-aged around 42 kg, while male Homo werearound 55 kg.

Taphonomy of the Swartkrans hominidsThe greatest number of hominid fossils inMembers 1–3 at Swartkrans sample P.robustus. Within Paranthropus the number ofsmaller (female) individuals exceeds thenumber of larger individuals (presumptivemales). If hominids were finding their wayinto the paleoanthropological record atSwartkrans and elsewhere by virtue of pre-dation by leopards (Brain, 1970) or othercarnivores (Marean, 1989), then one mightexpect that the smaller females, rather thanthe larger males, would be easier prey. Inturn, one might expect further that bothmale and female P. robustus might have beenmore vulnerable to predation than theirHomo counterparts, especially if Paranthro-pus males were smaller than those of Homo.

Acknowledgements

We would like to thank Francis Thackerayand Heidi Fourie of the Northern FlagshipInstitution (formerly Transvaal Museum,Pretoria) for facilitating study of thesefossils. We also thank Lee Berger for his helpin the identification of fossils as well ascomments on an earlier draft of this paper.Partial funding was provided by the Palaeo-anthropology Unit for Research and Explor-ation, University of the Witwatersrand, andby the Palaeo-Anthropology Scientific Trust(P.A.S.T.). We also thank The NationalHeritage Council of South Africa for per-mission to study the Swartkrans Fossils. DrKathleen Finzel, Department of Radiology,University Hospital at Stony Brook MedicalCenter (UHMC) provided for X-ray andCT images of the fossils. Lucy Diaz and Bill

Schottler of the Department of Radiology ofthe UHMC provided technical support. Wethank Bill Jungers for comparative data onhumans (Euro-Americans and Africanpygmies). We thank Bob Randall of theAmerican Museum of Natural History foraccess to the primate and human skeletalcollections. Bill Jungers read and offeredhelpful comments on the manuscript asdid two anonymous reviewers. Ms LucilleBetti-Nash executed the illustrations andassembled the figures.

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