Randall L. Susman Recently identified postcranial remains of Department of Anatomical Paranthropus and Early Homo from Sciences, School of Medicine, Swartkrans Cave, South Africa University at Stony Brook, Stony Brook, New York, 11794-8081, U.S.A. E-mail: Fifteen newly recognized hominid postcranials from Swartkrans are [email protected] described here and compared with a sample of previously described early hominids, African apes and modern humans. Ten of the new specimens are from Member 1. Two are from Member 2 and three Darryl de Ruiter are from Member 3. Nine of the fossils are referred to Paranthropus, Palaeoanthropology Unit for three to Homo, and three specimens cannot be assigned at present. Research and Exploration, The collection of hominid postcranials from Members 1–3 at Bernard Price Institute for Swartkrans now numbers more than 70 specimens. With the Palaeontology, University of description of two new, small femoral heads, SKW 19 and SK 3121, the Witwatersrand, there are now four proximal femora from Swartkrans. When SK 82 Private Bag 3, Wits 2050, and SK 97 are compared with SKW 19 and SK 3121, the two sets Johannesburg, South Africa. offer important insights into body size and sexual dimorphism in E-mail: Paranthropus robustus. [email protected] A new distal femur, SK 1896 and other bones attributed to Homo cf. erectus, indicate that male Homo were larger than Paranthropus at Swartkrans. C. K. Brain Transvaal Museum, 2001 Academic Press P.O. Box 413, Pretoria, 0001, South Africa. E-mail: [email protected]
Received 19 April 2001 Revision received 2 July 2001 and accepted 3 July 2001
Keywords: Early Homo, hominids, Paranthropus Journal of Human Evolution (2001) 41, 607–629 robustus, postcranial fossils, doi:10.1006/jhev.2001.0510 Swartkrans Cave. Available online at http://www.idealibrary.com on
Introduction robustus and Homo cf. erectus indicated by postcranial and craniodental fossils at Fifteen newly recognized hominid speci- Swartkrans number from 90 (de Ruiter, mens identified in the faunal collection from 2000) to 132 individuals (Brain, 1993). Swartkrans offer new insights into the mor- The new postcranial remains presented phology and natural history of Paranthropus here all sample previously known parts of and early Homo. The postcranial fossils the skeleton of both hominid species at described below were initially recovered Swartkrans. The postcranial collection from between 1949 and 1986 during various Members 1–3 now totals a number of bones phases of excavation at the site. Together sufficient to assess body size and sexual with the fossils described earlier (Table 1), dimorphism in Paranthropus and Homo. The the Swartkrans hominid assemblage now new additions to the hominid collection consists of 71 postcranial elements as well as bolster our confidence in the assignment of 341 craniodental remains. The estimated postcranial remains to either of the two minimum of individuals of Paranthropus species in Members 1–3. Finally, new fossils
0047–2484/01/120607+23$35.00/0 2001 Academic Press 608 . . ET AL.
Table 1 Swartkrans Hominid Postcranial Fossils, Members 1–3
Specimen Description Provenience* Reference Taxon
SK 1896 Distal femur HR Present H SK 2598 Distal humerus HR Present P SK 3121 Femoral head & neck M2 Present P SK 25600 Distal humerus LB Present P SK 24601 Proximal radius LB Present P SKW 19 Femoral head HR Present P SKX 2045 Proximal radius M2 Present H SKX 10641 Proximal phalanx (h) LB Present P? SKX 10924 Distal humerus M3 Present H SKX 15468 Proximal phalanx (h) LB Present P SKX 16699 Proximal phalanx (f) ?LB Present — SKX 19495 Distal humerus M3 Present P SKX 38529 Metatarsal III LB Present — SKX 38653 Middle phalanx (h) M3 Present P SKX 42695 Talus LB Present — SK 82 Proximal femur HR Napier, 1964 P SK 97 Proximal femur HR Napier, 1964 P SK 18b Proximal radius M2 Broom & Robinson, 1949 H SK 3981a Thoracic vertebra HR Robinson, 1970 P SK 3981b Thoracic vertebra HR Robinson, 1970 P SK 50 Os coxae HR Broom & Robinson, 1950; P Robinson, 1970 SK 853 Lumbar vertebra HR Broom & Robinson, 1949; H Napier, 1959 SK 854 Cervical vertebra C2 HR Broom & Robinson, 1949; P Napier, 1959 SK 860 Distal humerus (baboon?) HR — SK 84 Metacarpal I HR Napier, 1959; Rightmire, 1972 P SK 85 Metacarpal IV HR Napier, 1959 H SK 14147 (SKW27) Metacarpal V HR Day & Scheuer, 1973 — SK 3155b Os coxae HR Brain et al., 1974 H SKX 5016 Distal phalanx—pollex LB Susman, 1989 P SKX 5017 Metatarsal I LB Brain, 1984; Susman & Brain, 1988 P SKX 5018 Proximal phalanx (h, IV?) LB Susman, 1989 P SKX 5019 Middle phalanx (h) LB Susman, 1989 P SKX 5020 Metacarpal I LB Susman, 1989 P SKX 5021 Middle phalanx (h) LB Susman, 1989 P SKX 5022 Middle phalanx (h) LB Susman, 1989 P SKX 3602 Distal radius LB Susman, 1989 P SKX 8761 Proximal ulna LB Susman, 1989 P SKX 8963 Distal phalanx (h) LB Susman, 1989 P SKX 9449 Middle phalanx (h) LB Susman, 1989 P SKX 13476 Middle phalanx LB Susman, 1989 P SKX 34805 Distal humerus LB Susman, 1989 H SKX 3774 Distal humerus LB Susman, 1989 P SKW 4776 Cervical vertebra C3 (C4) LB Susman, 1993 P SKX 45690 Proximal phalanx (hallux) LB Susman, 1989 P SKX 247 Metatarsal II M2 Susman, 1989 — SKX 344 Middle phalanx (f) M2 Susman, 1989 — SKX 1084 Patella M2 Susman, 1989 P SKX 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 609
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 H SKX 27431 Proximal phalanx (h, III) M3 Susman, 1989 H? SKX 27504 Distal phalanx (h) M3 Susman, 1989 — SKX 31117 Medial cuneiform M3 Susman, 1989 P SKX 33355 Middle phalanx (h) M3 Susman, 1989 — SKX 33380 Metatarsal V M3 Susman, 1989 P SKX 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. enhance the view of the positional behavior, Measurements were made on skeletal and prehensile capabilities of Paranthropus materials with dial, vernier and coordinates and early Homo at Swartkrans from 1·8to calipers and recorded to the nearest 1·0 m.y.a. 0·10 mm. Measurements of the fossils were made on the actual specimens. Photographs, a–p and m–l radiographs, and CT scans of Materials and methods selected fossils were made at University The ‘‘new’’ fossils were compared with Hospital and Medical Center at Stony previously described postcranial elements Brook (UHMC). CT scans were made with from Swartkrans (Table 1), with australo- a G.E./CTi Helical CT scanner in the pithecines and early Homo from sites else- Department of Radiology, UHMC. where in South and East Africa, and with a sample of extant ape and modern human Postcranial remains from counterparts. The comparative sample Members 1–3 consists of skeletal materials of bonobos, chimpanzees, and gorillas from the Femoral head (SK 3121) Tervuren Museum, the American Museum SK 3121 is a well-preserved right femoral of Natural History, the Cleveland Museum head retaining the proximal 23 mm of the of Natural History, the United States neck (Figure 1). The specimen was National Museum, the Dart Collection at mechanically prepared out of an ex situ the University of the Witwatersrand, and the Member 2 breccia block and had originally Department of Anatomical Sciences at been cataloged as a carnivore femur. The Stony Brook. In addition to the above, the head measures 28·8 mm in diameter (supero- human sample consists of a size range that inferiorly). From what remains of the neck it includes data on Euroamericans and Akka was apparently long given the small size of pygmies supplied by Bill Jungers. Data taken the head. The femoral neck is compressed from the literature are so acknowledged in anteroposteriorly (Table 2). It is difficult Tables 1–3. to assess the relative position of the fovea 610 . . ET AL.
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. capitis due to the incompleteness of the neck represent male Paranthropus. Based on its and lack of a shaft. small size and flat neck we assign SK 3121 Comparative anatomy. Like SK 82 and to Paranthropus. Its small size suggests SK 97, SK 3121 possesses a long, com- further that the SK 3121 femoral head is pressed femoral neck, typical of australo- probably that of a female. pithecines. Table 2 displays the neck shape index of the four Swartkrans hominids, the Femoral head (SKW 19) large and small femora of A. afarensis, those SKW 19 is an intact right femoral head, from Koobi Fora, modern humans and extracted from the characteristically pink chimpanzees. Results illustrate the com- colored breccia of the Hanging Remnant of pressed shape of the femoral neck in the Member 1. It was mechanically prepared australopithecines. and what is preserved of the fossil is in good SK 3121 is a small femoral head approxi- condition. The fossil retains 8–10 mm of the mately the size of A.L. 288-1ap. The two proximal femoral neck. A CT scan of the heads measure 28·8 mm and 28·6mm neck recalls the morphology seen in SK (Johanson et al., 1982), respectively. SK 3121 (above) and suggests that SKW 19 at 3121 is smaller than the two previously one time possed an anteroposteriorly com- reported Swartkrans femora, SK 82 and SK pressed femoral neck [Figure 1(b)]. The 97 (Napier, 1964; Robinson, 1972; Ruff most notable feature of SKW 19 is its small et al., 1999). It is likely that SK 82 (with a head that measures 30·7 mm in diameter. superoinferior head diameter of 34·4 mm) The fovea capitis is large with a prominent and SK 97 (head diameter 37·1 mm) rim. 611
Table 2 Femoral neck shape index (a–p/si100)
Specimen(s) Index S.D. Taxon
SK 3121 72 — Paranthropus robustus female SK 82 70 — Paranthropus robustus male SK 97 75 — Paranthropus robustus male A.L. 288-lap 70* — Australopithecus afarensis female A.L. 333-3 63* — Australopithecus afarensis male KNM-ER 738 73†—Paranthropus KNM-ER 815 64‡—Paranthropus KNM-ER 1503 62†—Paranthropus KNM-ER 1481 74†—Homo Sub-Saharan African§ Male 78 8·7 Female 81 — American black§ Male (45) 79 6·4 Female (8) 80 4·6 Zulu§ Male (25) 83 5·7 Female (25) 86 7·1 Khoisan§ Male (28) 86 5·9 Female (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).
Comparative anatomy. With only a small reflected in body size dimorphism compar- part of the femoral neck, assessment of the able to that suggested for Paranthropus at taxonomic status of SKW 19 is somewhat Swartkrans and at Koobi Fora (McHenry, problematic. However, the a–p compression 1991, 1992a). Based on femoral head sizes of what little remains of the femoral neck here, and using data from McHenry (1991) just proximal to the head is suggestive of and Grine et al. (1995), the body mass P. robustus, rather than Homo (Figure 1). estimates for male and female Paranthro- The size of SKW 19 (head diameter of pus at Swartkrans are 40 kg and 31 kg, 30·7 mm) is close to that of SK 3121 respectively. (28·8 mm). These two ‘‘new’’ femoral SKW 19 and SK 3121 are similar in size heads together with SK 82 and 97 suggest to SK 3155b, the small partial os coxae from bimodality in femoral heads’ sizes at the Hanging Remnant of Member 1, Swartkrans. The small size of SKW 19 and described by Brain et al. (1974). Brain et al. SK 3121 suggest that these two represent (1974) referred the SK 3155b pelvis to the Paranthropus females, while their larger genus Homo. McHenry (1975) likened counterparts, SK 82 and SK 97 (Napier, SK 3155b to SK 50. The implications for 1964; Robinson, 1972), represent males of body size estimates in Paranthropus and this species. This contrast in female and Homo females at Swartkrans are discussed male femoral head diameter would be below. 612 . . ET AL.
Figure 2. Distal femur SK 1896. (a) anterior, (b) medial, (c) lateral views.
Distal femur (SK 1896) obscured by cracks running through them. SK 1896 is the distal end of a right femur, The profile of the medial condyle has a recovered from the Hanging Remnant of ‘‘rounded’’ appearance with a reduced Member 1 using acetic acid preparation. radius of curvature compared to that of the The postdepositional distortion of the fossil flatter lateral condyle. made identification difficult; it was initially The lateral condyle has two major post thought to be a large felid. The entire distal mortem breaks, one running mediolaterally end is represented although the specimen is across the anteroinferior surface, the other deformed by post mortem crushing, crack- on the posterior surface. The effect of these ing and shearing (Figure 2). A minor two artefacts is an anteromedial displace- amount of surface bone has been lost in the ment of the inferolateral articular surface vicinity of the posterior limb of the lateral along a shear plane running back to front. condyle. The specimen is cracked in a The lateral condyle is missing the number of places but it retains the true posterolateral-most extent of the articular morphology of the patellar groove, and surface. However, enough remains of the much of the medial and the lateral condyles. condyle to observe that it is relatively flat, Most affected by post mortem crushing with an increased radius of curvature. is the cross-sectional geometry of what In addition to the disparity in the shape of remains of the shaft. the medial and lateral condyles, SK 1896 The crushing and deformation of the shaft has a prominent anteriorly projecting lateral is most notable on the anteromedial surface patellar lip. Despite the a–p deformation of of the bone, medial to the medial patellar the shaft, it retains a valgus orientation when lip. The medial condyle displays two trans- the fragment is placed on a flat surface. The verse breaks, one anterior, and the other cracks running through the medial and posterior. There is a crack running supero- lateral condyles obfuscate precise measure- inferiorly on the uppermost extent of ment of the femoral angle but it is safe to the medial condylar articular surface. infer a value within the human range of The menisco–tibial surfaces are partially 6–14 (Tardieu, 1999). 613
The epicondyles both display post mor- 34. Mediolaterally, from peak to peak, the tem damage. The medial epicondyle has patellar groove of SK 1896 measures been cracked and depressed slightly but the 35.9 mm. Those of TM 1513 and Sts 34 adductor tubercle is still discernible. The measure 24·0 mm and 26·8 mm, respect- lateral epicondyle is cracked through and ively. Accurate measurement of the bi- appears to have been reattached to the epicondylar diameter of SK 1896 is not lateral side of the condyle. The area of possible, nor is a mediolateral breadth of the bone between the lateral epicondyle and lateral articular surface. It is, however, poss- the posterolateral-most extent of the ible to obtain a maximum m–l diameter of condylar articular surface is missing and the medial condyle of 23·6 mm. This com- the cancellous network is exposed. pares with 22·3 mm for TM 1513 and The anterior surface for the bursal portion 22·1 mm for Sts 34. The mediolateral of the synovial cavity is depressed above the breadth of 23·6 mm yields a body mass superomedial margin of the trochlea. The estimate of 57 kg (based on a least squares fovea on the posterior surface just superior regression with a standard error of the esti- to, and between, the condyles is essentially mate of 7·36 kg) for SK 1896 based on the intact, displaying only a minor amount of mediolateral breadth of the medial femoral distortion resulting from the a–p crushing condyle in sample 18 Euro-Americans from force that appears to have come from the the Cleveland Museum of Natural History. front. This estimate is larger than any other we Comparative anatomy. The condylar pro- have obtained for Swartkrans hominids (see files are hominid-like and differ from those also McHenry, 1991, 1992a). It thus of nonhuman primates. The lateral condyle appears that based on morphology and com- has a ‘‘flattened’’ appearance while the parisons with the distal femur of modern medial condyle is more sharply curved. No humans and other early hominids (including doubt the discrepant radii of curvature those of australopithecines and early Homo) observed in the condyles of SK 1896 that SK 1896 probably represents a very resulted in a hominid-like screwing home of large, male Homo in Member 1. the joint upon full extension of the knee. Viewed distally, end on, the condyles appear Manual proximal phalanx IV? (SKX 15468) to be elongated anteroposteriorly, like those SKX 15468 is a short, stout, hominid of later hominids, and unlike those of great proximal phalanx of either finger IV or II apes (Tardieu, 1981, 1999). (Figure 3). It was excavated from the Lower The new femur displays features ascribed Bank of Member 1 (E4N5, SW, 740– to Homo in the prominent lateral patellar lip. 750 cm), but was originally not recognized The lateral lip resembles that of humans and as hominid. It was derived from the lightly KNM-ER 1472 and 1481 more than TM calcified sediment that characterizes most of 1513, Sts 34, or other australopithecines. If the Lower Bank, requiring no preparation. the valgus orientation of the femoral shaft is The subtlety of the asymmetry of the basal a reasonable approximation of its expression tubercles and the palmar surface of the body during life, then the femoral angle may also suggest that it is from the second or fourth be closer to that of Homo than that of digit. The fossil is essentially complete and Paranthropus and Australopithecus. undistorted. The major post mortem SK 1896 represents a large femur as artefacts include: (1) a small crack running judged by the broad patellar groove and proximodistally just inside the flexor sheath large condylar articular surfaces. The new impression, (2) a more sizeable sinusoidal femur is larger than both TM 1513 and Sts crack on the distal half of the posterior 614 . . ET AL.
Figure 3. Manual proximal phalangeal curvature of Swartkrans hominids SKX 15468, SKX 5018, SKX 22741 and extant hominoids. A larger included angle indicates a greater curvature. The comparative sample includes 146 human phalanges, and those of 88 gorillas, 63 chimpanzees and 38 bonobos. Vertical lines are the group means; long bars are the 95% confidence limits of the populations.
Table 3 Manual proximal phalanx, SKX 15468 depressed flexor sheath margins and an (measurements in mm; included angle in degrees) anteflexed, well-differentiated trochlea with a prominent central sulcus. Length* 34·9 M-L midshaft diameter 9·3 Comparative anatomy. SKX 15468 repre- A-P midshaft diameter* 6·3 sents a small hand. Based on maximum M-L base diameter 12·0 length the phalanx is most similar to that of M-L trochlear diameter 8·7 Max. m–l diameter 9·3 gibbons among extant apes and humans; it Epitrochlear diameter 9·3 is considerably shorter than its counterpart Interarticular length 32·9 (either II or IV) in bonobos, chimpanzees, Coordinate height* 5·5 Included angle 30·7 or humans. Overall, SKX 15468 bears a striking resemblance to another manual *Measurements used to calculate included angle (see proximal phalanx from the Lower Bank of Susman et al., 1984). Member 1 at Swartkrans, SKX 5018 (Susman, 1988 et seq.). SKX 15468 and surface, and (3) a patch of erosion of the SKX 5018 are roughly similar in size cortex on the dorsal surface of the body and (maximum lengths of 34·9 and 33·4 mm, on the dorsal rim of the proximal articular respectively). Both fossils display a similar surface. The post mortem damage does staining and patina, and they share similar not affect significantly the measurements effects of surface weathering in the form of in Table 3. surface cracks and cortical exfoliation. Both The phalanx has a maximum length of Swartkrans fossils are closer to each other in 34·9 mm, shorter than its counterpart, and overall morphological pattern than either is outside of the ranges, in bonobos (males to Stw 28, a relatively intact proximal and females, mean=50 mm, n=14), phalanx from Sterkfontein Member 4. chimpanzees (males and females, mean= The included angle, a measure of curva- 56 mm, n=33), and humans (males and ture (Susman et al., 1984; Stern et al., 1995) females of the Cleveland Museum of related to arboreality and terrestriality, com- Natural History collection, mean=43 mm, pares favorably in SKX 15468, SKX 5018 n=40). Its mediolateral diameter at mid- and SKX 22741 from Swartkrans (Fig- shaft is 9·3 mm, similar to that of humans, ure 3). These fossils fall within the 95% bonobos, and small chimpanzees. It has well confidence limits of the mean for the developed basal tubercles, prominent but humans sampled (Susman et al., 1984) and 615
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. at the low end of the gorilla range. The cur- The shaft is broken and retains only vatures of the fossils lie outside the ranges of 37 mm of bone proximal to the biepicon- bonobos and chimpanzees. Despite their dylar axis. The biepicondylar diameter of human-like curvatures, the Swartkrans the specimen measures 43·8 mm across. phalanges display a more ‘‘primitive,’’ ape- The trochlea measures 15·0mmonits like shape. Save for their curvatures, they do anterior surface and the capitulum measures not resemble closely the manual proximal 13·8 mm. The lateral epicondyle does not phalanges of modern humans. rise prominently above, or proximal to, the superomedial edge of the capitulum. Distal humerus (SKX 10924) Comparative anatomy. SKX 10924 SKX 10924 is an essentially intact distal end appears to have a suite of features that of a hominid humerus [Figure 4(a)]. It was recalls that of Homo. This includes the rela- recovered from the decalcified Member 3 tively indistinct zona conoidea, the distal deposit (W6S4, 0–100 cm), and bears the positioning of the lateral epicondyle relative dark manganese staining that typifies this to the capitulum, and the greater relative member. There is a longitudinal crack on anteroposterior diameter of the distal the posterior surface of the shaft that ends in humeral shaft (Figure 5). The capitulum is a ‘‘Y’’ with one limb heading to the medial positioned anteriorly as in humans, rather pillar and the other limb toward the lateral than distal and posterior as in chimpanzees. pillar. The specimen has suffered post The shape of the olecranon fossa is also mortem damage to the posterior surface of more reminiscent of humans than apes. the medial epicondyle and to the lateral edge Notwithstanding the above, it is important of the trochlea on its posterior surface. to note that all of these features vary within The articular features are not sharply extant taxa. defined [Figure 4(a)]. The capitulum is Both McHenry (1973, 1975) and Senut positioned on the anterior surface. The fossil (1981) have commented on the difference in has an indistinct zona conoidea between the shape of the olecranon fossa in apes and trochlea and the capitulum. The central humans. In both shape of the fossa and in sulcus of the trochlea is not deeply grooved. the expression of its lateral border, SKX The supracondylar ridge is evident but not 10924 resembles humans rather than pronounced. The olecranon fossa is large, apes. Also more human- than ape-like is the ellipsoidal and lacks a sharp crest, or edge, unremarkable supracondylar ridge and the on its lateral wall. increased relative anteroposterior diameter 616 . . ET AL.
Figure 5. CT scans of distal humeri. Scans are taken proximal to the biepicondylar line at approximately the 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) SK 24600, and (d) SK 2598 left to right. While slight differences in the points at which the individual humerus fragments were scanned accounts for some of the variation observed, the shape of SKX 10924 differs from that of its counterparts. of the humeral shaft inparting the appear- comparisons with the expanded sample of ance of a more ‘‘rounded’’ cross-section distal humeri at Swartkrans, that it should [Figure 5(a)]. Senut (1978a:7) commented be re-assigned to Homo. It appears likely, on the ‘‘conspicuous triangular shape’’ of based on its morphological pattern, that the shaft of the distal humerus in humans. SKX 10924 represents Homo. On the basis By contrast, Senut (1978b:7) described of its small size in comparison to SKX the diaphysis of the distal humerus in 34805 (a presumptive male Homo) SKX chimpanzees as ‘‘flat, anteroposteriorly.’’ 10924 is likely to be a female. The unremarkable zona conoidea is an additional human-like trait in SKX 10924. Distal humerus (SK 24600) More primitive and ape-like is the fact that SK 24600 is an intact left distal humerus in SKX 10924 the lateral epicondyle rises from the Lower Bank of Member 1 [Figure above the upper margin of the capitulum. 4(c)]. It most probably comes from the same SKX 10924 is considerably smaller than individual as SK 24601, a proximal radius TM 1517, a P. robustus humerus from (see below). SK 24600 and SK 24601 were Kromdraai. It is also considerably smaller both recovered from a box labeled ‘‘SK than SKW 3774, a Paranthropus distal Dump,’’ making their provenience initially humerus from Swartkrans Member 1, and it uncertain since they were not derived from is much smaller than KNM-ER 739, a pur- the in situ excavations. However, from the ported P. boisei from Koobi Fora. It is same box were recovered a deciduous molar slightly smaller than SKX 34805, a distal (Rdm2) which had an accompanying prov- humerus from Swartkrans Member 1 enience label indicating the Lower Bank (Susman, 1989 et seq.). SKX 34805 was (E3N4, 450–500 cm), as well as a premolar initially referred to the genus Paranthropus (LP3) that was refit via interproximal con- by Susman (1989). We now believe, based tact to a maxilla previously recorded from on the cross-sectional shape of the shaft and the Lower Bank (de Ruiter & Brain, n.d.). 617
This maxilla was also derived from the same capitulum in SK 24600 has a human-like, excavation unit as the deciduous molar anterior orientation, similar to that of SKX (E3N4, 450–500 cm). The matrix adhering 10924. to the other fossils in the box (including SK While the two specimens are similar in 24600 and SK 24601) was consistent with overall size and shape, there are some subtle the Lower Bank, and as such, SK 24600 and differences suggesting that they may repre- SK 24601 have both been attributed to the sent the two hominid species at Swartkrans. Lower Bank of Member 1. SK 24600 has a shaft that in cross-section The specimen preserves the same portion appears broader mediolaterally than that of of the distal humerus and is only slightly SKX 10924 (Figure 5). The zona conoidea larger than SKX 10924 [Figure 4(a)]. The and the trochlea are more prominent in SK biepicondylar diameter of the fossil is 24600 than in SKX 10924. These differ- 44·7 mm. SK 24600 has a mottled appear- ences, and comparisons with TM 1517, ance. The medial epicondyle is intact but KNM-ER 271 as well as with modern there appears to be some erosion of the humans, chimpanzees and bonobos, suggest lateral epicondyle. There is some minor that SK 24600 samples Paranthropus. Based erosion of the anterolateral edge of the on its small size in relation to SKX 3774, a capitulum and the anteromedial edge of the presumptive male Paranthropus, we believe trochlea. On the posterior surface there is SK 24600 to be that of a female. damage to the medial and lateral edges of the trochlea and to the posterior surface of Distal humerus (SKX 19495) the lateral epicondyle. On the posterior sur- SKX 19495 is a left distal humerus face of the shaft, just above the olecranon fragment, lacking articular surfaces and the fossa, there are some longitudinal surface lateral epicondyle [Figure 4(b)]. The fossil is cracks. The olecranon fossa is damaged, from decalcified Member 3 (W2S5, NE, resulting in what is probably a post mortem 160–170 cm) and is uniformly dark-stained perforation. There is matrix adhering to the by manganese. A distinct carnivore tooth edges of the fenestra. score is visible on the anterior surface The trochlea has a prominent medial edge [arrow, Figure 4(b)], while several probable and a well-defined sulcus. The capitulum punctate depressions are visible on the sits mainly on the anterior surface. Lateral to medial pillar of the posterior aspect. the trochlea is a prominent zona conoidea. Only the superior-most portions of the The capitulum is well defined and what is medial epicondyle and the uppermost half left of the lateral epicondyle appears to rise to two-thirds of the olecranon fossa are above the upper edge of the capitulum. The present. Otherwise, the entire distal entity, olecranon fossa itself is ellipsoidal. including most of the medial epicondyle, the Comparative anatomy. SK 24600 is lateral epicondyle, the trochlea, and the slightly larger than SKX 10924 as deter- capitulum are missing. mined by the biepicondylar diameters of The two most notable features of SKX each specimen (44·7 mm and 43·8 mm, 19495 are its small size, similar to the other respectively). Both specimens are small, new distal humeri (Figure 4), and the only slightly larger in biepicondylar diameter presence of tooth marks left by predators or than that of A.L. 288-1 (biepicondylar scavengers at, or around, the time of death. diameter of 41·1 mm). They are small Comparative anatomy. Although the in comparison to other distal humeri fossil has a superficial likeness to SKX from Swartkrans (SKW 3774) and from 10924, SKX 19495 is larger with a different Kromdraai (TM 1517, 54·0 mm). The shaft cross-sectional shape [Figure 5(b)]. 618 . . ET AL.
SKX 19495 is broad (m–l) and flat (a–p) Bank of Member 1. It was derived from a similar in shape to SK 24600, and unlike trench dug by miners searching for lime- SKX 10924. In this feature SKX 19495 and stone deposits in the area of the Lower SK 24600 resemble TM 1517 and are more Bank, and its preservation is consistent with like bonobos and chimpanzees than they being derived from the lightly calcified are like modern humans. Thus, they Lower Bank. The proximal phalanx is intact are assigned to Paranthropus rather than and undistorted save for half a dozen or so Homo. The small size is suggestive of small spots of surface damage on the base the fact that this fossil may represent a and on the distal articular surface, probably female. incurred following recovery of the speci- mens. None of these artefacts affect signifi- Distal humerus (SK 2598) cantly the morphology or measurement of SK 2598 is left humerus fragment from the specimen. the Hanging Remnant of Member 1 [Figure The bone is small, measuring 20·3mmin 4(d)]. In addition to being the smallest of maximum length. It has an ‘‘hour-glass’’ the six distal humeri from Swartkrans shape with a well-developed base, a narrow Members 1–3, SK 2598 is also the least body and an expanded distal end. There is a complete (Figure 4). As a result of its well-developed basal collar for the attach- incompleteness, SK 2598 was not initially ment of the metatarsophalangeal (MTP) recognized as hominid, and was placed in an joint capsule. The basal articular surface is ‘‘Indeterminate Fossil’’ box from the Hang- round in proximal view with a dorsal con- ing Remnant. It has grayish mottling, differ- cavity implying an enhanced range of ent from the other humeri. Only a portion of dorsiflexion at the MTP joint. The plantar the medial epicondyle is present and it has surface of the base and proximal body is well suffered a spawling-off of the posterior sur- developed. The fossil has only a very modest face. There are numerous small weathering curvature as indicated by an included angle cracks also on the posterior surface. Roughly of 22. half of the olecranon fossa is preserved. Comparative anatomy. SKX 16699 has What remains of the shaft has a flattened the ‘‘hour-glass’’ shape typical of human appearance. pedal proximal phalanges. The included Comparative anatomy. The shaft of SK angle is also contained within the 95% 2598 is similar in cross-section to those of fiducial limits of the human mean, and lies SK 24600 and SKX 19495 suggesting outside those of great apes (Susman et al., Paranthropus affinity [Figure 5(d)]. The fos- 1984). This relatively straight phalanx is like sil is small, but close in size to SK 24600 and that of humans and unlike apes and other to SKX 10924. It is interesting to note that early hominids (e.g., Stw-355 which has an all four of the distal humeri reported here included angle of 36, and A.L. 288-1y, with represent left elbows. Two previously an included angle of 36). Other human-like described right distal humerus fragments features of SKX 16699 include a concave, (SKX 34805 and SKW 3774, Susman, ‘‘excavated’’ dorsal edge of the proximal 1989 et seq.) represent larger individuals. articular surface, and a base with an enhanced development on its plantar Pedal proximal phalanx (SKX 16699) surface. SKX 16699 is a pedal proximal phalanx, The phalanx is small compared to likely to be that of the fifth toe (Figure 6). modern humans. If SKX 16699 indeed rep- The bone is of questionable provenience, resents a fifth toe, then the individual is but thought to have come from the Lower approximately the same size as A.L. 288. 619
Figure 6. Pedal proximal phalanx SKX 16699 (right), manual middle phalanx SKX 38653 (center), and manual proximal phalanx SKX 10641 (left). (a) Mediolateral view, (b) dorsal view. SKX 10641 is that of a subadult and is lacking its epiphysis.
SKX 16699 measures 20·3 mm in length Manual proximal phalanx (SKX 10641) compared with a pedal proximal phalanx SKX 10641 is a subadult manual proximal length in A.L. 288-1y of 22·6 mm. If A.L. phalanx from the Lower Bank of Member 1 288-1y is from a second or fourth toe and (W1S4, SW+SE, 50–100 cm). It lacks the SKXX 16699 is that of a fifth toe, then the epiphysis (Figure 6). The specimen is darkly two individuals may have been approxi- stained by manganese. The weakly devel- mately equal in body size. It is not poss- oped secondary bony features such as the ible to assign this bone to Paranthropus or flexor sheath margins, basal tubercles, cur- Homo. vature, and distal trochlea are the result of 620 . . ET AL. the young age of this individual and its Comparative anatomy. SKX 38653 bears relatively poor state of preservation. There a close resemblance to SKX 5021 and other are surface cracks and some spots where manual middle phalanges from Member 1 cortical bone has spawled off. On the palmar (Susman, 1988 et seq.). Among bonobos, surface of the trochlea a proximal corner has chimpanzees, and humans, SKX 38653 is been lost. most similar to its human counterparts. Comparative anatomy. The young onto- Nonetheless, the fossil differs from humans genetic age (including the lack of a basal in the increased dorsopalmar ‘‘thickness’’ of epiphysis) of SKX 10641 precludes an accu- the distal portion of the body and the pro- rate assessment of length and curvature. tuberant flexor sheath margins. While These factors, in turn, hinder taxonomic humans most often have a distal concavity assignment. Nonetheless, SKX 10641 bears on the palmar surface of the body just proxi- a closer morphological resemblance to SKX mal to the trochlea, the fossil displays a 5018 and SKX 15468 than it does to SKX mediolateral convexity on the distal half of 22741 and SKX 27431 (Susman, 1988 the palmar surface. As is the case with the et seq.) and this may argue for its inclusion middle phalanges in humans, SKX 38653 is in Paranthropus. essentially straight, lacking in longitudinal curvature. We hesitate to assign this Manual middle phalanx (SKX 38653) Member 3 fossil to either Homo or Paran- SKX 38653 is a manual middle phalanx thropus on morphological grounds. Others from Member 3 (N wall, 700 cm). It has a (see Grine, 1988; Brain, 1993) have attrib- mottled gray appearance and is in poor uted the entire hominid craniodental sample condition due to both pre- and post mortem from Member 3 at Swartkrans to Paranthro- alterations (Figure 6). Its poor condition pus. In this case, it is most likely that SKX made the fossil difficult to identify initially, 38653 represents P. robustus. and it was originally thought to be a baboon fossil. The bone measures 21·3mm in Third metatarsal (SKX 38529) length. This measurement is probably close SKX 38529 is a proximal fragment of a left to the actual length, being altered only third metatarsal (Figure 7). The fossil was slighly by loss of bone on the proximal- recovered from the Lower Bank of Member most point on the dorsal articular rim. The 1 (W5S4, 610–660 cm). This specimen maximum breath (m–l) of 8·9 mm is, like- was initially labeled as being a potential wise, close to, but not precisely, the true hominid, but this identification was not con- value due to slight post mortem bone loss firmed until the present study. The speci- coupled with the presence of a small osteo- men consists of the proximal 26·9mmofthe phyte on the left side (in palmar view). bone. What exists of the bone is in relatively The trochlea has been altered by the depo- good shape as far as surface morphology sition of bone on the palmar surface. An goes. The distal extent displays a clean, additional set of changes is observed in the sharp-edged break that exposes a narrow post mortem loss of bone surrounding both medullary cavity. There is erosion on the the proximal and distal articular surfaces. plantar aspect of the base and on its medial Adding to the poor state of preservation is a surface. On the medial side of the base both series of weathering cracks and spots of articular facets are eroded. On the lateral erosion. The bone has very protuberant side of the base a single articular facet for flexor sheath margins, the apices of which lie the fourth metatarsal is present but it is closer to the proximal end than to the distal damaged by erosion of its periphery. The end. large tubercle on the lateral side for the 621
Figure 7. Third metatarsal SKX 38529 (a–d, left). (a) Medial, (b) lateral, (c) plantar, and (d) dorsal views. intermetatarsal ligament is intact. Much of metatarsal from Member 3 (Susman, 1988 the rim of the lateral cuneiform (proximal) et seq.). This bone cannot be assigned to articular surface has been lost. SKX 38529 Homo or Paranthropus based on mor- has an actual mediolateral basal breadth of phological criteria, but the overwhelming 12·3 mm. The true measure was slightly proportion of Paranthropus craniodental greater by 0·5 mm or so, as judged by a loss remains in the Member 1 sample makes of bone on the medial side of the base. it likely that SKX 38529 samples Comparative anatomy. The SKX 38529 Paranthropus. fragment has a decidedly human appear- ance. It differs from the third metatarsals of Talus (SKX 42695) bonobos and chimpanzees. In size, SKX 38529 is roughly equivalent to that of small SKX 42695 consists of a right posterior human females in our sample of Euro- and talus fragment, probably from the Lower African-Americans. SKX 38529 is similar Bank of Member 1 (Figure 8). The speci- in both size and morphology to SKX 247, a men was initially catalogued as coming from left second metatarsal from Swartkrans Member 5. In fact, the fossil was recovered Member 2 (Susman, 1989 et seq.).1 Both during the removal of disturbed sediment fossils preserve approximately the same part near the interface between the Lower Bank of the metatarsal, viz., the initial 25–35% of of Member 1 and Member 5, just to the east the bone. The mediolateral basal breadth of of the Member 5 Channel. The talus does SKX 38529 measures 12·3 mm compared not evince the geochemical characteristics to SKX 247 with a mediolateral basal that typify Member 5 fossils. Rather, it breadth of 11·7 mm. At a point 25 mm from matches the preservation and texture of the proximal end, the mediolateral breadth Lower Bank fossils, and as such has been of the shaft of SKX 38529 measures 7·1mm attributed to the Lower Bank of Member 1. while that of SKX 247 measures 7·0 mm. The fossil retains the posterior two-thirds The dorsopalmar diameters of SKX 38529 of the trochlea including the intact fibular and SKX 247 measure 9·6 mm and 9·3 mm, articular surface, a small posterior remnant respectively. SKX 38529 is also compat- of the tibial malleolar surface, the sulcus ible in size with SKX 33380, a left fifth for the tendon of flexor hallucis longus (FHL), and a large, intact posterior subtalar facet. The head and neck are missing. The 1In the original, and subsequent, descriptions of SKX 247 it was referred to as metatarsal III. SKX 247 is the trochlear surface measures approximately proximal end of a second metatarsal. 25 mm across (m–l). Despite the fact that 622 . . ET AL.
Figure 8. Talus SKX 42695. (a) Posterior, (b) superior view. the medial and lateral talar tubercles are Proximal radius (SK 24601) damaged slightly, they do not appear to have SK 24601 [Figure 9(c)] is a small, left been prominent. proximal radius from the Lower Bank of Comparative anatomy. The size of SKX Member 1 (E3N4, 450–500 cm). SK 24601 42695 is within the ranges of both humans is a likely mate for the left distal humerus and chimpanzees. The trochlea widens an- fragment, SK 24600. Both elements come teriorly, is relatively broad, and has relatively from the Lower Bank of Member 1, are the equal medial and lateral sides. These are same size and articulate well, have the same features in which the fossil resembles patina, and both display a continuous chan- humans more than chimpanzees or bono- nel of damage across their posterolateral bos. The lateral edge of the trochlea is only surfaces at the point at which the radial head slightly more elevated than the medial edge, apposes the capitulum of the humerus. This a feature seen more often in humans than in fossil has been attributed to the Lower Bank apes. The vertical orientation of the sulcus for the same reasons outlined above for SK for the FHL is more like that of humans 24600. than it is like chimpanzees or bonobos. With a head diameter of 17·3 mm, SKX 42695 has a substantial mediolateral SK 24601 is smaller than SKX 2045 (see breadth of 25 mm as noted above. As such it below) and the previously described radial is larger than TM 1517, Stw-88, Stw-102, head, SKX 3699 (Susman, 1989 et seq.). Stw-347, OH 8-A, KNM-ER 813A, The overall length of what remains of SK KNM-ER 1464, and KNM-ER 1476A. It is 24601 is 47·3 mm. The small remaining smaller than KNM-ER 5428. Based on the portion the specimen precludes assessment equation presented in McHenry (1992a) for of the interosseous border and determi- an all-human least squares regression of nation of the relative position of the radial talar breadth and body mass, SKX 42695 tuberosity. yields an estimate of 53 kg. Because of the Comparative anatomy. SK 24601 has strong morphological similarities of hominid suffered damage to its posterior surface tali and the incompleteness of SKX 42695, but what remains indicates the same it is not possible to assign this talus to donut-shaped (vs. cylinder-shaped) pattern Paranthropus or Homo on morphological as that observed in SKX 3699, viz. a tri- grounds. partite articular surface with contact for a 623
Figure 9. Proximal radii. (a) SKX 3699, (b) SKX 2045, (c) SK24601, (d) Stw-431-a, and (e) SK 18b. well-developed zona conoidea (Senut, 1986; Proximal radius (SKX 2045) Rose, 1988). In anterior view, the rim tapers SKX 2045 is the proximal 40% of a right laterally yielding a ‘‘cap-like’’ appearance. radius from Member 2 [Figure 9(b)]. The The neck appears thin and more constricted fossil measures 123·7 mm in length and is in than that of humans. good condition. There are numerous surface Based on a least squares regression of cracks the most prominent of which are radial head diameter (17·3 mm) on mass, in found on the medial surface of the shaft and a sample of 35 modern humans (including the interosseous border or ‘‘crest.’’ The 18 Euro-Americans and 17 African pyg- surface is mottled by manganese stain. mies) SK 24601 yields a body mass estimate Although undoubtedly derived from in situ for the fossil of 36 kg. This comports well Member 2, the exact provenience of this with an estimate of 37 kg when the least fossil within the Member is uncertain. It was squares regression equation for humans of originally thought to be a baboon radius. McHenry (1992a) is applied. These body The head is relatively large, measuring mass estimates are equivocal as to the male 21·6 mm in diameter. There is a small spot vs. female status of SK 24601, but the suite of erosion in the center of the fovea and of morphological features (above), suggests three small spots of exposed cancellous that SK 24601 (+SK 24600) is most likely bone on the periphery of the head. The head to represent Paranthropus. has only a faint hint of a ‘‘donut’’ shape 624 . . ET AL.
(Figure 9), and only a slight bevel for articu- humans the tuberosity is essentially pos- lation with the zona conoidea of the itioned anterior to the interosseous border. humerus. Thus, while the fovea for the head SKX 2045 is human-like in this regard. of the capitulum and the ‘‘rim’’ for articu- Stw-431a recalls the condition in Pan. The lation with the ulnar notch and annular bicipital tuberosity is not well preserved in ligament are in evidence, the ‘‘third’’ articu- either of the Paranthropus radii, SK 24601 or lar surface for the zona conoidea is poorly SKX 3699. represented in SKX 2045. The anterior surface of SKX 2045, from The bicipital tuberosity is unremarkable. which the flexor pollicis longus (FPL) takes The interosseous crest, although affected by its origin, is also human- rather than post mortem cracking, is prominent, as is chimpanzee-like. This portion of the radius the oblique line. The anterior surface for the is marked by a sharp interosseous border origin of flexor pollicis longus is flat and medially and a flat to slightly concave ‘‘blade-like.’’ The pronator teres insertion anterior surface in both humans and in side on the lateral aspect of the shaft is SKX 2045. On the contrary, this portion of evident, but it has been partly obscured by the radius in chimpanzees and bonobos is surface weathering and truncated by the less ‘‘blade-like’’ and often convex rather break in the bone prior to midshaft. The than flat or concave. A comparison of extant shaft has a thick cortex at the point at which humans and African apes suggests a differ- it is exposed. ence in the area on the radius for the origin Comparative anatomy. The radial head of of the flexor pollicis longus in humans and SKX 2045 differs somewhat from the other its homologue (the radial moiety of flexor two proximal radii from Swartkrans (SK digitorum profundus) in apes. Humans 24601 and SKX 3699). Although SKX have a broader, flat to concave surface for 2045 is similar in size to the Stw-431a, the origin of the FPL than do bonobos, an australopithecine from Member 4 at chimpanzees and gorillas. SKX 2045 has Sterkfontein (Figure 9), the two radii differ a surface similar to that of humans. In in shape. Stw-431a is slightly larger with a this feature the radius of Stw-431a from head diameter of 22·2 and appears to have a Member 4 at Sterkfontein, appears to thinner neck and a thicker surface for the resemble chimpanzees more than humans. zona conoidea than does SKX 2045. The The radial head diameter yields a body radial heads of Paranthropus, both SK 24601 weight of 53 kg (58 kg using McHenry’s and SKX 3699, are like that of Stw-431a least squares equation for humans, 1992a). and unlike SKX 2045. This is reflected The significance of this large estimate is seen in the proximal view of the fovea, which in the comparison of this fossil with SK reveals an exaggerated ‘‘donut’’ shape in the 24601, from the Lower Bank of Member 1, australopithecines [Figure 9(a), (c), (d)]. which yields a body mass estimate of 36 kg The relatively thick radial neck of (see above). The large size of SK 2045 and SKX 2045 is more like that of humans than its human-like morphology when compared chimpanzees and bonobos. to Swartkrans radii SK 24601 and SKX The bicipital tuberosity is not as well 3699, suggest that it samples a large (male?) developed as it appears in apes. Napier & Homo cf. erectus in Member 2. Davis (1959: 33) observed that the radial tuberosity in chimpanzees is displaced pos- Discussion teriorly, relative to the interosseous border. We observed a similar position of the radial Because two hominid taxa have been tuberosity in chimpanzees and bonobos. In known at Swartkrans since 1949 (Broom & 625
Robinson, 1949), the first question asked of preconceptions about the presumptive any new find is whether it belongs to Paran- ‘‘derived’’ morphology of Homo versus the thropus or to Homo cf. erectus. This issue is more ‘‘primitive’’ morphology of Paranthro- especially vexing when discussion involves pus (Robinson, 1972: 4). postcranial remains that, until recently, have In the period of excavation from 1979– been few compared with the remains of 1986 dozens of additional postcranial fossils skulls and teeth. The problem is confounded were recovered at Swartkrans (Brain, 1993). further by the exigencies of cave taphonomy. The fossils described here represent 15 Since hominid fossils are often the frag- remains recovered between 1949 and 1986. mented remains of carnivore feeding The hominid postcranial collection now behavior, even when postcranials are seem- numbers over seventy elements. Moreover, ingly associated with jaws and teeth, the the expanded collection contains four proxi- associations are never certain. mal femora (with diagnostic features of the In order to attempt to understand the head and neck), six distal humeri (with comparative behavioral ecology of the features that may identify the two species at Swartkrans hominids, the first item of busi- Swartkrans), four proximal radii, two polli- ness is to assign individual elements to either cal metacarpals (representing two different Paranthropus or Homo. In order to do so, it is species, Susman 1988, 1991; Trinkaus & necessary to align individual elements with Long, 1990), and multiples of a number of their australopithecine counterparts or with hand and foot bones. The greatly expanded those of Homo. This was a difficult under- collection of fossils from Swartkrans taking in the early days at Swartkrans as improves our understanding of what is Broom was clearly less interested in post- Paranthropus and what is Homo, and it cranial fossils than he was in skulls and allows us a more reliable view of variation teeth. Nowhere is this more evident than and sexual dimorphism within these taxa. despite the fact that the SK 97 femora were discovered in 1949, save for a brief men- Species attribution, body size and sexual tion by Broom et al. (1950), they were dimorphism in P. robustus not described and analyzed until Napier Body size is one of the most important undertook the task in 1964. characteristics of an animal’s biology and As a result, Broom & Robinson (1949) ecological status. Among primates, when offered a fairly narrow perspective on the stock is taken of body size and sexual dimor- comparative morphology of Parathropus and phism, insight is gained into important Homo at Swartkrans. When Robinson aspects of behavior and social organization. (1972) wrote his synthesis on the locomotor Within primates (e.g., families, subfamilies), anatomy of the early hominids from South larger species live on the ground; smaller Africa he had but ten or so fossils to work ones tend to be more arboreal. In old world with. Of these, only a pair of femora, SK 82 monkeys and apes (save for orang-utans) and SK 97 represented duplicates of the sexually dimorphic species tend to be more same bone, permitting only a faint glimpse terrestrial than their more arboreal kin. of the intraspecific variation and compara- Characteristics of sociality and social organ- tive anatomy of the two hominid species at ization are also reflected in degree of sexual Swartkrans. Early on, the species assign- dimorphism (Clutton-Brock & Harvey, ments were tentative for all but a few of 1977). the Swartkrans postcranials. In most cases, McHenry (1992b) has pointed out the the assignment of a fossil to either Paran- many caveats and pitfalls of estimating thropus or to Homo was influenced by body size from isolated fossil fragments. He 626 . . ET AL. suggests that among the many approaches to been similar to those of Paranthropus in size, the problem of reconstructing body size in viz. around 30 kg. However, males of Homo at early hominids, the best predictors of both Swartkrans appear, based on evidence of the body size and stature in early hominids are femur (SK 1896), radius (SKX 2045) (and features of the hindlimb joints measured in a possibly the talus, SKX 42695), to have been broad size range of modern humans. Less larger than their Paranthropus counterparts. consistent predictions result from estimates The evidence of the proximal femur at based on joints of the upper limb and from Swartkrans reinforces the picture of sexual equations based on measurements taken dimorphism in Paranthropus. The australo- from extant apes, or all hominoid regres- pithecine femur is described as having a suite sions (McHenry, 1992b). of features that includes a relatively small Elements in the Swartkrans collection head and long neck, low neck-shaft angle, now include six distal humeri representing anteroposteriorly compressed neck, a greater two large and four small hominids. Mor- trochanter that does not flare out from the phology of the humeri suggests the likeli- shaft, a high femoral condylar angle, and a hood that one large specimen (SKX 34805) relatively short overall length (Walker, 1973). is attributable to Homo (contra Susman, Amongst the earlier described Swartkrans 1988) and one large distal humerus (SKX fossils (Napier, 1964; Robinson, 1972) are 3774) is that of Paranthropus. The mor- two large Paranthropus femora, SK 82 and SK phology of the small humeri suggests that 97 (McHenry, 1991). They have australo- three are possibly those of Paranthropus (SK pithecine grade characters including small 2598, SK 24600, and SKX 19495). heads with long, anterposteriorly compressed Of the two proximal radii described here femoral necks. Two new small femoral heads, for the first time, SK 24601 is likely, on SK 3121 and SKW 19, represent small, pre- morphological grounds, to represent Paran- sumably female, individuals. Altogether, the thropus. It is small in size compared with both four femora indicate the presence of two large SKX 3699 and SKX 2045. SK 24601, to- males and two small females of P. robustus. gether with its mate (SK 24600), is likely to The two size groupings suggest males with be that of a female Paranthropus while the body masses around 40+kg (SK 82 and SK larger, but morphologically similar, SKX 97) and females with body masses at 30+kg 3699, is likely to be that of a male. McHenry (SK 3121 and SKW 19). This degree of (1991, 1992a), notes that body mass esti- sexual dimorphism is greater than that ob- mates for early hominids are more reliable served in modern humans, chimpanzees, or when they are based on measurements of the bonobos, but less than that observed in lower limb. Nonetheless, a general sense of orang-utans and gorillas. This finding com- comparative body mass in the Swartkrans ports with that of McHenry (1991, 1992a). hominids can be glimpsed from a comparison The new and old postcranials together sup- of SKX 2045, SKX 3699, and SK 24601. port McHenry’s proposition that P. robustus When the fossils are compared with a sample was considerably lighter in body mass than of Euro-Americans and African pygmies, the 90 kg suggested by Robinson (1972) and body mass estimates consistent with esti- earlier workers. mates from other joint surfaces are observed. For example, estimates from the diameter of Body size and sexual dimorphism in Homo the femoral heads suggest that male and cf. erectus female body masses of Paranthropus cluster The small, human-like distal humerus, SKX around 40 kg and 30 kg, respectively. 10924, represents one small individual of Females of Homo cf. erectus appear to have early Homo. The morphology of the shaft 627 and the weak zona conoidea suggest Homo in the ‘‘large’’ category. At the very least, affinities. The small size, roughly equal SKX 5016 and SKX 5020 belong in the to SK 2598, SK 24600 and SKX 19495 same size category. SKX 5018, a well- indicates that it likely represents a female. preserved manual proximal phalanx, is likely A larger, presumably male-sized cluster of to be that of a small individual. In overall fossils is represented by the Homo-like proxi- length it is comparable in size to modern mal radius, SKX 2045 [Figure 9(b)] and the gibbons. It is shorter than its counter- Homo-like distal femur, SK 1896 (Figure 2). parts in bonobos, chimpanzees and Euro- The large body mass estimate obtained from Americans. SK 85 and SKX 247 also appear a regression based on radial head diameter to be those of the smaller of the Swartkrans suggests that male Homo at Swartkrans hominids. SKX 31117, a partial medial might have been larger in mass than its cuneiform, appears to be that of a large Paranthropus counterpart represented by individual. The patellar fragment, SKX SKX 3699. The large distal femur, SK 1084 (Susman, 1988 et seq.), placed in the 1896, a presumptive Homo based on the large group by McHenry, is a poor match for derived patellar trochlea and human-like the trochlea of the large femur, SK 1896. condylar morphology, also suggests a body We would place the patella in the range of size for male Homo greater than that of the smaller Swartkrans hominids. Paranthropus. When the above adjustments are made, McHenry’s ‘‘medium’’ category shrinks and Previous estimates of body size what remains of the medium-sized hominids McHenry (1991) characterized Swartkrans are only metapodials and phalanges. Of hominids as either ‘‘small,’’ ‘‘medium’’ or course the relative size of these and their ‘‘large’’ in body size. Fossils that were simi- placement in small, medium and large lar in size to counterparts in a 28 kg female groups depends to some extent on which Akka pygmy defined the ‘‘small’’ group. The digit they represent. Thus, it seems to us ‘‘medium’’ group was likened to a 45 kg that the medium-sized fossils clustering Bantu female and the ‘‘large’’ group was around 45 kg become fewer in number. defined by its similarity to the skeleton of a While we view the constituents of 54 kg female from the Schultz collection. McHenry’s three groups somewhat differ- McHenry placed the largest number of fos- ently, the Swartkrans fossils may, in fact, fall sils in the medium size category. Our view of into three size groupings. McHenry’s size groupings results in the shift Based on both the old and the new fossils of a number of specimens from one group- it appears that both species of Swartkrans ing to another. The more significant of these hominids exhibited sexual dimorphism. include SKX 5016, SKX 8761, SK 85, SKX Females of both Paranthropus and Homo 247, and SKX 31117. averaged around 30 kg in body mass. As for SKX 5016 is an intact pollical distal pha- males, however, the postcranial evidence lanx. McHenry (1991) places this phalanx suggests that male Homo were larger than in the medium category while he places a their Paranthropus counterparts. The esti- pollical metacarpal, SKX 5020, in the large mate of 57 kg for SK 1896 is consistent with category. Susman (1988) has shown, based the estimate of 53 kg obtained from the large on metrical comparison of SKX 5016 with talus, SK 42695. The size of SK 1896 both SKX 5020 and the smaller SK 85, that indicates that Homo cf. erectus males at SKX 5016 is a ‘‘good fit’’ with SKX 5020, Swartkrans were larger than their Paranthro- the large pollical metacarpal. We would pus counterparts. The latter weighed, on place SKX 5016, together with SKX 5020, average, around 42 kg. We can envision 628 . . ET AL. three size groups at Swartkrans encompass- Schottler of the Department of Radiology of ingadifferent group of fossils than the three the UHMC provided technical support. 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