Associated cranial and forelimb remains attributed to Australopithecus afarensis from Hadar, Ethiopia

Journal of Human Evolution 48 (2005) 593e642 Associated cranial and forelimb remains attributed to Australopithecus afarensis from Hadar, Ethiopia M.S.M. Drapeau a, *, C.V. Ward b, W.H. Kimbel c, D.C. Johanson c, Y. Rak d a De partement d anthropologie, Universite de Montre al, C.P. 6128, succursale Centre-ville, Montre al QC H3C 3J7, Canada b Department of Anthropology, Department of Pathology and Anatomical Sciences, Swallow Hall, University of Missouri, Columbia, MO 65211, USA c Institute of Human Origins and Arizona State University, P.O. Box 874101, Tempe, AZ 85287, USA d Department of Anatomy, Sackler Faculty of Medicine, Tel Aviv University/Ramat-Aviv, Tel Aviv, 69978, Israel Received 13 October 2003; accepted 18 February 2005 Abstract A partial skeleton from Hadar, Ethiopia (A.L. 438-1) attributed to Australopithecus afarensis is comprised of part of the mandible, a frontal bone fragment, a complete left ulna, two second metacarpals, one third metacarpal, plus parts of the clavicle, humerus, radius, and right ulna. It is one of only a few early hominin specimens to preserve both cranial and postcranial elements. It also includes the first complete ulna from a large A. afarensis individual, and the first associated metacarpal and forelimb remains. This specimen, dated to approximately 3 Ma, is among the geologically youngest A. afarensis fossils and is also one of the largest individuals known. Its ulnar to mandibular proportions are similar to those of the geologically older and much smaller A.L. 288-1, suggesting that body size increased without disproportional enlargement of the mandible. Overall, however, analysis of this large specimen and of the diminutive A.L. 288-1 demonstrates that the functional morphology of the A. afarensis upper limb was similar at all body sizes; there is no evidence to support the hypothesis that more than one hominin species is present at Hadar. Morphologically, all apparent apomorphic traits of the elbow, forearm, wrist, and hand of A.L. 438-1 are shared uniquely with humans. Compared to humans, A.L. 438-1 does have a more curved ulna, although A.L. 288-1 does not, and it appears to have had slightly less well-developed manipulatory capabilities of its hands, although still more derived than in apes. We conclude that selection for effective arboreality in the upper limb of Australopithecus afarensis was weaker than in nonhominins, and that manipulative ability was of greater selective advantage than in extant great apes. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Australopithecus afarensis; forelimb; ulna; metacarpals; proportions * Corresponding author. E-mail addresses: m.drapeau@umontreal.ca (M.S.M. Drapeau), wardcv@missouri.edu (C.V. Ward), wkimbel.iho@asu.edu (W.H. Kimbel), johanson.iho@asu.edu (D.C. Johanson), yoelrak@post.tau.ac.il (Y. Rak). 0047-2484/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jhevol.2005.02.005

594 M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 Introduction In 1994, Kimbel and colleagues announced the discovery of a partial skeleton from Hadar, Ethiopia, attributed to Australopithecus afarensis (Fig. 1). This specimen, A.L. 438-1, is from the Kada Hadar member, situated stratigraphically approximately 12 m below the BKT-2 tuff, which has been dated to 2.94 Ma (Kimbel et al., 1994; Semaw et al., 1997). It is thus approximately 3 million years old, equivalent in age to the adult male skull A.L. 444-2 (Kimbel et al., 2004). The A.L. 438-1 specimen preserves part of the mandible, a few small maxillary bits, a frontal bone fragment, a complete left ulna, two second metacarpals, one third metacarpal, plus a part of the clavicle, humerus, radius and right ulna. The A.L. 438-1 specimen is significant for a number of reasons. It preserves both craniofacial and postcranial remains, affording an opportunity not only to compare intraskeletal cranial and postcranial proportions in A. afarensis, but also to compare two individuals attributed to that species, the other being the partial skeleton A.L. 288-1 ( Lucy ). While A.L. 288-1 is one of the smallest known A. afarensis individuals, A.L. 438-1 is among the largest. This size discrepancy between A.L. 438-1 and A.L. 288-1 provides an opportunity to test the hypothesis that two morphologically distinct species, one large and one small, are present at Hadar (e.g. Johanson and Taieb, 1976; Olson, 1981, 1985; Ferguson, 1983, 1984; Senut and Tardieu, 1985; Zihlman, 1985; Schmid, 1989; Senut, 1996, 1999) by comparing the proportions and morphology of these specimens. The A.L. 438-1 skeleton also provides the first complete ulna of A. afarensis, and the first from a large individual. The only other nearly complete ulna belongs to A.L. 288-1 (Johanson et al., 1982). Because the A.L. 438-1 ulna is comparable in size to at least two others tentatively attributable to Australopithecus sensu lato, L40-19 from the Omo valley (Howell and Wood, 1974; Howell and Coppens, 1976; McHenry et al., 1976; Howell et al., 1987) and OH 36 from Olduvai Gorge (Leakey, 1978; Day, 1986; Tobias, 1991) (Fig. 2), A.L. 438-1 permits the comparison of A. afarensis to these specimens with a minimum of confounding allometric effects. The A.L. 438-1 specimen is also the first A. afarensis fossil to include metacarpals associated with forearm elements. Because forelimb proportions are relevant to discussions of forelimb function and behavioral adaptations, this discovery stands to add important information to our understanding of forelimb functional anatomy in early hominins. In this paper, we provide detailed anatomical descriptions of the postcranial elements of this new specimen. The frontal fragment and mandible are described in detail elsewhere (Kimbel et al., 2004; C. Robinson et al., in prep.). We also put these postcranial bones into evolutionary context by comparing their morphology and proportions to those of extant and fossil hominoids. Recovery and context All of the elements attributed to A.L. 438-1 are very well preserved, bear mostly sharp, fresh breaks, and are buff colored with occasional gray to yellow mottling. The first fragments recovered (by D. Johanson on 24 February 1992) were surface finds of the left proximal ulna, two left metacarpals, a segment of humeral shaft, and the frontal fragment. During a sweep of the surface, additional hominin fragments were recovered: several ulnar and radial shaft portions, including a left ulnar fragment that articulates with the initially discovered piece to make up the proximal half of the bone; a right metacarpal; a small maxillary fragment with tooth roots; and a partial right mandible with M 2 and M 3 crowns. Subsequent excavation of 52 m 2 to a depth of up to 53 cm resulted in the recovery of several in situ fragments that can be associated with the surface finds. Two distal fragments and associated shaft splinters of the left ulna were excavated in situ a few meters from the surface finds. The fragments were discovered within 3 cm of each other, and articulate to produce the distal shaft and epiphysis of the left ulna. These pieces together articulate along a planar, lightly weathered shaft break with the two proximal portions recovered on the surface to produce the virtually complete left ulna.

M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 595 A complete listing of anatomical elements comprising the A.L. 438-1 skeleton is found in Table 1. Also from A.L. 438 are a right lower third premolar (A.L. 438-2), a fragment of left lower third premolar (A.L. 438-3), and a proximal hand phalanx (A.L. 438-4). None of these can be associated definitively with A.L. 438-1. The two premolars are from a dentally smaller and younger (judging from occlusal wear) individual than A.L. 438-1, which has large, heavily worn postcanine teeth. The hand phalanx features a more weathered surface and was found several meters from the main concentration of hominin remains at the locality; for these reasons, and given the demonstrated presence of at least two dental individuals at the locality, we withhold attribution of this postcranial specimen to the A.L. 438-1 individual. The hominin-bearing outcrop is part of a massive pebble conglomerate, indurated by CaCO 3 cement, which locally forms a resistant ledge cut by a small seasonal stream channel. Regionally, this unit is a 5 m thick sequence of laminated, crossbedded sands alternating with small pebble conglomerate lenses. The hominin specimens found on the surface at A.L. 438 eroded from a 2e5 cm thick pebbly sand with calcite concretions, which is part of the same fluvial cycle in the upper Kada Hadar Member that yielded the A.L. 444-2 skull, two mandibles from A.L. 437, a partial occipital bone from A.L. 439, and a mandible fragment with isolated lower tooth crowns from A.L. 440, all situated stratigraphically 10e12 m below BKT-2 (Kimbel et al., 2004). Taxonomic identification of A.L. 438-1 There is no consensus on the utility of postcranial remains for the taxonomic identification of early hominins. Fortunately, the A.L. 438-1 postcranial fossils are associated with taxonomically Fig. 1. Postcranial remains of A.L 438-1; (a) lateral fragment of left clavicle, A.L. 438-1v; (b) diaphyseal fragment of right humerus, A.L. 438-1c; (c) diaphyseal fragment of right ulna, A.L. 438-1m; (d) left ulna, A.L. 438-1a; (e) diaphyseal fragment of left radius, A.L. 438-1l&p; (f) right second metacarpal, A.L. 438-1f; (g) left third metacarpal, A.L. 438-1d; and (h) left second metacarpal, A.L. 438-1e.

596 M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 Fig. 2. Human, chimpanzee, and fossil early hominin ulnae; OH 36 and L40-19 are reversed for comparison. Drawing by Susan Alta Martin. valuable parts of the cranium and mandible, and thus we are confident that inferences we draw herein concerning the comparative and functional anatomy of the A.L. 438-1 forelimb and hand apply to A. afarensis. The A.L. 438-1 craniodental remains A.L. 438-1b: frontal fragment (Fig. 3) The adult frontal bone has been relatively under-sampled in the A. afarensis hypodigm (Kimbel et al., 1984); though with the recent recovery of two fairly complete skulls from the Hadar Formation (A.L. 444-2 in 1992 and A.L. 822-1 in 2000), the species distinctive adult frontal anatomy is now much better known (Kimbel et al., 1994, 2004). This newly acquired knowledge permits the taxonomic identification of even relatively small fragments of the frontal bone. Thus, the A.L. 438-1b fragment, which includes the medial portions of the supraorbital elements and anteromedial squama, preserves several features that combine to diagnose A. afarensis frontal bones (see the full description and comparison in

M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 597 Table 1 Elements comprising the A.L. 438-1 partial skeleton A.L. 438-1 a-v a: left ulna b: frontal bone fragment c: diaphyseal fragment of right proximal humerus d: left metacarpal III e: left metacarpal II f: right metacarpal II g: right mandibular corpus with partial ramus h: right lower M1 fragment i: right lower M3 fragment j: right lower I1 fragment k: right upper P4 fragment l & p: proximal left radius fragment m: diaphyseal fragment of right ulna n: humeral diaphyseal cortex fragment o: humeral diaphyseal cortex fragment q: upper molar root fragment r: distal humerus fragment, NON-HOMINID? s: maxillary fragment with partial zygomatic root t: tooth fragment, NON-HOMINID u: left lower molar root v: clavicle fragment Kimbel et al., 2004). These include 1) low, sagittally and transversely flat squama without supratoral sulcus or frontal trigone; 2) thick squamal cross-section (10.8 mm just to the right of midline, avoiding the influence of the frontal crest, approximately 35 mm posterior to the coronal plane of the superior orbital margins); 3) anterior supraorbital margins oriented ca. 90 to the sagittal plane, with more lateral portions deviating anteriorly (observed in superior view); 4) supraorbital elements vertically thicker laterally than medially (observed in anterior view). A.L. 438-1g: partial mandible (Fig. 4) The mandible associated with the A.L. 438-1 upper limb material preserves the symphyseal region, right corpus, and partial right ramus. It is one of the largest in the now extensive A. afarensis mandible series from Hadar (n Z 23 measurable specimens; Kimbel et al., 2004; Robinson et al., in preparation). Despite its large dimensions Fig. 3. Frontal fragment (A.L. 438-1b).

598 M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 morphological pattern. The external symphyseal profile, although relatively upright, is rounded and bulbous, with a full basal segment. The lateral corpus surface bears mild hollowing confined to the superior part of the corpus, as in other large Hadar mandibles (such as A.L. 444-2). The root of the ramus arises high on the corpus and runs anteriorly to the coronal level of mesial M 1, a morphology that is at the anterior end of the range of variation for Hadar mandibles, but which can be matched in the relatively diminutive A.L. 288-1i. Dental arcade shape is reconstructed as a blunt U-shape, with weak posterior divergence of the straight postcanine tooth rows. Anatomical description of the A.L. 438-1 postcranial fossils Each fossil is described first without specific comparative references to provide a basic inventory of the preservation and morphology (Table 2), and then is placed in comparative context. A.L. 438-1a: left ulna (Fig. 5) Fig. 4. Mandibular fragment (A.L. 438-1g). (perpendicular corpus height: 41.3 mm; minimum corpus breadth: 24.7 mm, both measured at the M 1 level), its cross-sectional shape (as gauged by the corpus breadth/height index, 59.8%) is within the range of variation for other Hadar A. afarensis mandibles, and in fact lies less than 1 standard deviation from the Hadar sample mean (57.4 G 5.8%). The A.L. 438-1g mandible displays a number of features that fit the A. afarensis Preservation This bone, recovered in four major fragments (see above), is nearly complete. It is missing through breakage only the anterior tip of the olecranon beak and superior trochlear surface, where cancellous bone is exposed. The dimensions of the break measure 15.2 mm mediolaterally by 9.1 mm proximodistally. The inferomedial corner of the trochlear surface is eroded, with part of the articular facet displaced inferiorly and posteriorly. The eroded edge runs 10.4 mm along the margin of the trochlear notch for a maximum of 4.1 mm proximodistally. The inferior margin of the radial facet is missing over an area 10.5 mm anteroposteriorly by about 2 mm proximodistally. The distal articular surface is missing a small area of surface bone about 3.5 mm in diameter. The diaphysis is broken in four pieces that join together without distortion. The first break, measured on the medial side, is located 80 mm from the proximal end, with the lateral side broken 5.5 mm more proximally than the medial side. The slightly

M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 599 Table 2 Measurements of postcranial elements of A.L. 438-1 Clavicle Measurement, mm Anteroposterior diameter at conoid 16.3 Superoinferior diameter at conoid 12.9 Humerus Maximum diameter of surgical neck 28.8 Minimum diameter of surgical neck 24.9 Ulna Maximum length 278 Physiological length 252 Max. proximodistal height of olecranon process from proximal end of trochlear notch (7.7) 1 Mediolateral width of trochlear notch, including radial notch 30.2 Height of trochlear notch from tip of coronoid and anconeal processes (23.2) Anteroposterior diameter of radial notch 16.4 Proximodistal diameter of radial notch 10.2 Anteroposterior diameter of coronoid process 31.8 Anteroposterior diameter of anconeal process (26.2) Mediolateral diameter of midshaft 14.8 Anteroposterior diameter of midshaft 17.6 Length of styloid distal to the ulnar head 3.7 Mediolateral diameter of head 17.5 Anteroposterior diameter of head 8.9 Radius Maximum transverse diameter of radial tuberosity 15.5 Maximum proximodistal diameter of radial tuberosity 25.3 Maximum mediolateral diameter of shaft below tuberosity 15.9 Minimum anteroposterior diameter of shaft below tuberosity 13.7 Metacarpal 3 Maximum length 64.8 Dorsopalmar width of head 13.9 Maximum mediolateral width of head 12.9 Dorsopalmar diameter of midshaft 9.0 Mediolateral diameter of midshaft 8.8 Dorsopalmar width of base 15.7 Mediolateral width of base 13.3 Left Right Metacarpal 2 Maximum length 66.5 66.3 Dorsopalmar width of head 13.6 13.9 Maximum mediolateral width of head 12.6 12.6 Dorsopalmar diameter of midshaft 8.3 8.5 Mediolateral diameter of midshaft 8.9 8.5 Dorsopalmar width of base 14.7 14.5 Mediolateral width of base 13.0 12.7 1 () indicates estimated measurement. weathered second break is located approximately 159 mm from the proximal end. It follows a quasitransverse plane, although the anterior surface is broken slightly more distally than the posterior surface. The third break lies about 240 mm from the proximal end. The anterior surface of this break is 10 mm further proximal than the posterior surface. Laterally, this fracture continues proximally

600 M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 Fig. 5. Left ulna (A.L. 438-1a). as a small longitudinal crack along the shaft that causes no apparent distortion. On the medial side, the anterior and posterior breaks are connected by a diagonal break. An area of surface bone about 2 mm wide is weathered away from an area adjacent to the longitudinal crack between the interosseous crest and the posterior border of the shaft. Two major pieces of cortical surface bone are missing along the most proximal break. One, on the anterolateral surface, measures 12.2 mm mediolaterally

M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 601 by 6.7 mm proximodistally and 1.7 mm deep, and the other, on the medial surface, measures 13.9 mm mediolaterally by 9 mm proximodistally and 3.1 mm deep. At the most distal break, there is a 2 by 2 mm hole anteriorly. About 10 mm proximal to the distal break, there is a 13 by 9 mm flake of surface bone missing along the anterior side. In all other places, surface bone is in excellent condition. Vascular foramina and muscle attachment sites are very well preserved. Morphology The ulna is 278 mm long from olecranon process to styloid process. The olecranon process projects proximally past the trochlear articular surface by 7.7 mm at its peak, which is located proximal to the medial margin of the trochlear surface. The posterior margin of the triceps brachii insertion site is lipped proximally. There is a prominent ridge for the posterior portion of the triceps insertion. There is no obvious subtendinous bursal area on the olecranon. The capsular ligament attachment cannot be observed proximally because this area is broken. A large foramen on the proximal surface of the olecranon measures 4.6 by 3.4 mm wide, and is 3.8 mm deep. The cavity is connected with a smaller foramen just lateral and slightly posterior to it that measures 2.5 by 1.5 mm. The trochlear notch faces anteriorly and its proximodistal length can be estimated at 23.2 mm. Proximally, the trochlear surface measures 25.7 mm mediolaterally, and distally, maximum mediolateral breadth including the radial notch is estimated at 30.2 mm. There is no extension of subchondral surface laterally to articulate with the lateral epicondyle of the humerus within the olecranon fossa. The trochlear keel is mild, with medial and lateral portions set at a 133 angle to one another distally and 131 proximally. Distally, the trochlear notch articular surface extends from the summit of the keel 14.9 mm medially and 11.7 mm laterally. Proximally, the surface extends 14.7 mm medial to the keel and 13.5 mm lateral to it. Each quadrant of the joint surface is mildly concave in all dimensions, with a faint lip around the external margins. The proximal and distal portions of the joint surface are joined in the middle by an isthmus of subchondral bone only 4 mm wide, where a very small area of bone is missing. This isthmus is flanked on both sides by an area of nonarticular bone that is raised above the articular surface and covered with small vascular foramina. The radial notch measures 16.4 mm anteroposteriorly by 10.2 mm superoinferiorly, and meets the trochlear surface along a 13.3 mm long junction extending to within 5.3 mm of the tip of the coronoid process. The notch is set at a 145 angle to the lateral portion of the distal trochlear surface in the coronal plane, and at a 30 angle to the anteroposterior axis of the joint. The notch is teardrop shaped in outline, tapering anteriorly. It is concave anteroposteriorly, and very mildly so proximodistally. Extending distally from the posterior margin of the radial notch is a strong supinator crest that runs distally about 30.5 mm before it fades into the rounded lateral margin of the bone. The crest is roughened throughout its length with irregular ridges along its posterior margin. About 10 mm distal to this crest, an irregular roughened arc of bone extends distally about 14.5 mm and curves anteriorly about 8 mm, possibly delineating either the end of the supinator attachment area or the proximal end of the interosseous membrane. Starting at a point just distal to the supinator attachment site, a blunt but distinct interosseous crest extends to the level of the pronator quadratus attachment site. Along its posterior margin, the surface bone is roughened. Distinct attachment sites for abductor pollicis longus, extensor pollicis longus and extensor indicis muscles cannot be discerned. Posteriorly, the ulna measures 18.7 mm wide mediolaterally at the middle of the trochlear notch level, and 27.6 mm wide proximally. There is a salient, roughened ridge for the posterior oblique bands of the ulnar head of flexor carpi ulnaris and the ulnar collateral ligament, producing a mildly concave surface below it, extending 13 mm distal to the olecranon. The surface of the triangular area delineated by this ridge, by another prominent ridge along the posterior border of the anconeus insertion, and by the tip of the olecranon is covered by small foramina. The confluence of these ridges is at the level of the radial notch, and the combined ridge extends distally to blend with

602 M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 the posterior border of the shaft. Distal to this point, the posterior border is a rounded ridge, which becomes roughened for a distance of about 75 mm, beginning about 82 mm from the distal margin of the radial notch to about 77 mm from the tip of the styloid process. In posterior view, the whole posterior diaphyseal border forms a mildly sinusoidal curve, laterally convex for approximately its first 45 mm, medially convex for most of its length, and then faintly laterally convex again for its distal 66 mm. Distal to the supinator crest, the lateral aspect of the shaft is nearly flat anteroposteriorly, becoming more convex distally so that the cross section immediately proximal to the head is almost circular. The medial aspect of the diaphysis is smooth and rounded. There is a roughened tubercle about 6 mm in diameter for attachment of the anterior and oblique bands of the ulnar collateral ligament adjacent to the most medially projecting part of the preserved trochlear surface. Anterior to it, the surface of the coronoid process is coarse where the flexor digitorum superficialis muscle attached. Medial to this area, the surface of the coronoid process is riddled with small nutrient foramina. The superoanterior margin of the flexor digitorum profundus attachment area forms a faint ridge proximally, adjacent laterally to, but slightly more extensive than, the brachialis insertion area. The brachialis insertion forms a deep pit measuring 21 mm proximodistally by 6 mm mediolaterally. Its posteromedial border is a sharp crest, which may also represent the margin of the pronator teres attachment site. The floor of the pit is roughened and marked by transverse ridges, and its lateral border is only mildly roughened along the anterior border of the shaft. Just distal to the brachialis insertion pit, the anterior border of the shaft becomes sharper adjacent to a piece of missing cortex and flattens rapidly distal to the most proximal break in the bone. A nutrient foramen is visible just below the break between the two proximal-most fragments, 77 mm from the proximal end of the bone, 48 mm from the tip of the keel on the inferior portion of the trochlear surface. In medial and lateral views, the shaft is almost uniformly concave anteriorly. In anterior view, the proximal three-quarters of the shaft is barely convex medially, becoming barely convex laterally in its distal quarter. Due to the rugose pronator quadratus muscle attachment, the distal lateral margin appears more convex than the medial margin is concave. The pronator quadratus crest is a low, roughened arc extending about 38 mm proximodistally from the roughened area of the styloid process where the medial collateral ligaments of the wrist attach. The distal end of the ulna measures a maximum of 20.1 mm anteroposteriorly across the head and styloid. The ulnar head measures 17.5 mm mediolaterally and 8.9 mm anteroposteriorly. There is a slight ridge of bone along its outer margin anteriorly, but this is indistinct medially and laterally. Vascular foramina are visible in the depressions between the styloid process and head. The styloid process is straight, extending 6.5 mm from the depth of triangular articular disk attachment and only 3.4 mm distal to the distalmost point on the head. The roughened attachment area for the ulnar collateral ligament of the wrist extends proximally 21 mm from the tip of the styloid process. The posterior margin of the styloid process is almost sharp along the edge of the groove for the tendon of extensor carpi ulnaris. The groove arcs anteriorly at the tip of the styloid process where it becomes visible in medial view. A.L. 438-1c: right proximal humeral shaft fragment (Fig. 6) Preservation This specimen is preserved from the surgical neck distally for a maximum of 76.9 mm. It is preserved cranially from the proximal-most point of the pectoralis major insertion site on the lateral side, to where the bone begins to flare toward the head on the medial side. Proximally, cancellous bone is visible along crystalline matrix lines, but some compact bone is visible medially. The distal break is obliquely angled at 10 to a transverse plane and only compact bone is visible. A good cross-section can be obtained distally. Numerous small longitudinal and transverse cracks traverse the specimen, but none of them causes distortion.

M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 603 compacta missing along the lateral margin of the proximal end. It is very lightly weathered with few hairline cracks through the surface bone. Fig. 6. Diaphyseal fragment of proximal right humerus (A.L. 438-1c). Morphology This bone fragment measures 28.8 mm anteroposteriorly and 24.9 mm mediolaterally at the surgical neck. The pectoralis major attachment forms a sharp crest along the lateral border of the intertubercular groove, starting about 10 mm from the proximal break. The latissimus dorsi attachment area is visible on the floor of the groove, separated from that of the pectoralis by a smooth sulcus. The pectoralis major crest grades into the proximal end of the deltoid tuberosity, as a thick raised ridge visible in cross-section at the distal break. The teres major insertion forms a pit-like depression along the anteromedial border of a ridge extending distally from the lesser tuberosity. Posteriorly, the attachment of the lateral head of the triceps brachii forms a low but sharply demarcated crest descending the length of the fragment towards the lateral side of the bone. At its inferior-most preserved point, this triceps crest is still about 24 mm from the deltoid tuberosity. The distal break is roughly triangular in cross-section. A.L. 438-1d: left metacarpal 3 (Fig. 7) Preservation This specimen is complete and perfectly preserved except for some small bits of surface Morphology This metacarpal (MC) measures 64.8 mm in proximodistal length. The proximal end of the bone has no styloid process. The base measures 15.7 mm dorsopalmarly, and dorsally it measures 13.3 mm mediolaterally. The medial and lateral sides of the base are concave and converge palmarly, so that the base is only 6.6 mm wide palmarly. Dorsopalmarly, its capitate surface area is mostly convex, with slight concavoconvexities. The MC2 facet is bilobate but continuous and concave dorsopalmarly, the two lobes forming a 140 angle with each other. The MC4 facet is restricted to the dorsal portion of the bone. The prominent extensor carpi radialis brevis attachment forms a low, roughened tubercle dorsally at the proximolateral corner. Attachment for the flexor carpi radialis and/or the oblique head of the adductor pollicis occupies the whole palmar aspect of the proximal end, and is particularly robust. The shaft is short, roughly circular in crosssection, and only mildly curved in medial or lateral view. The attachment for the transverse head of adductor pollicis makes a low, roughened ridge along the entire length of the palmar surface of the shaft. The faint dorsal interosseous muscle attachment ridges are contiguous, but remain separate throughout their length. In medial or lateral view, the distal articular surface is almost uniformly convex. There are barely perceptible parasagittal grooves. From the distal-most point on the bone, the dorsal side of the articular facet extends proximally only 6.0 mm while the palmar side extends 12.1 mm laterally and 12.7 mm medially. Palmarly, between the medial and lateral projections of this articular surface, the bone is concave and covered with vascular foramina. The articular surface is asymmetric dorsopalmarly; palmarly, it is 12.9 mm wide mediolaterally but only 9.0 mm wide dorsally. In distal view, the medial margin runs in a sagittal plane but the lateral side arcs medially towards its dorsal edge. There is no ridge along the dorsal

604 M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 Fig. 7. Metacarpals of A.L. 438-1; from left to right for all views, right second metacarpal (A.L. 438-1f), left second metacarpal (A.L. 438-1e) and left third metacarpal (A.L. 438-1d). margin of the head. The collateral ligament pits are large, measuring about 8.5 mm proximodistally on both sides. The collateral ligament attachments have well-developed tubercles, and the bone measures 13.7 mm mediolaterally across them. A.L. 438-1e: left metacarpal 2 (Fig. 7) Preservation This left metacarpal is perfectly preserved, with only few superficial hairline cracks.

M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 605 Morphology This metacarpal measures 66.5 mm in proximodistal length. The proximal end of the bone is roughly triangular, but with convex medial and dorsal sides. The base is 13.0 mm mediolaterally (measured dorsally) and 14.7 mm dorsopalmarly. In proximal view, the trapezoid facet occupies most of the proximal surface, although a continuous surface for the capitate, 3 mm wide, is visible because it is set at an angle of 30 to the long axis of the shaft. The trapezium facet is also visible, as it is set at 12 to the long axis of the shaft and 35 to its dorsopalmar axis, and measures 4.4 mm in its proximodistal and dorsopalmar diameters. The two halves of the trapezoid facet are set at 100 to one another. The larger medial half measures 14.1 mm dorsopalmarly by 7.0 mm mediolaterally, and the smaller lateral half is only 7.7 mm dorsopalmarly by 6.4 mm mediolaterally. The nonarticular surface dorsal and palmar to the junction between the two portions of the trapezoid facet has small pits. The trapezium facet is visible in lateral view. A prominent dorsal metacarpal ligament attachment area forms a rugose tubercle distal to the trapezium facet at the dorsopalmar center of the bone. There is a distinct tubercle for attachment of extensor carpi radialis longus along the dorsomedial aspect of the proximal shaft, demarcated from the ligament attachment by a shallow furrow. Medially, the capitate facet has a convex distal contour that is continuous across the bone. The MC3 facet is bilobate and only barely continuous dorsopalmarly. Both portions measure about 3.2 mm proximodistally and are set at 140 to one another. Palmarly, there is a distinct tubercle for flexor carpi radialis and another one more medially and barely more proximally for the oblique head of adductor pollicis, with a small vascular foramen interposed between them. The shaft is straight in palmar view and mildly curved in medial and lateral views. Its crosssection is almost circular, with only faint dorsal interosseous ridges that are contiguous but do not meet. The second palmar interosseous muscle attachment forms a thin ridge running the length of the palmar diaphysis. The head is asymmetrical in palmar view. Its medial side straight and in a sagittal plane and the lateral side is broader proximally, swelling slightly before it arcs medially towards its distalmost extent. Palmarly, the articular facet is mildly concave with two shallow grooves medially and laterally. In distal view, the dorsopalmar midline of the head s articular surface is almost parallel to the dorsopalmar long axis of the base, so that there is only slight medial torsion of the head. Head torsion appears exaggerated because the palmar portion of the head is broader than the dorsal portion, measuring 12.6 and 7.7 mm respectively, and because the medial side is close to the sagittal plane while the lateral side tapers medially toward its dorsal margin. This appearance is accentuated by the convex MC3 facet. Palmarly, the head s articular surface extends proximally about 13.2 mm medially and 11.2 mm laterally, and in between there is a depressed area with numerous vascular foramina. The articular surface is more extensive palmarly than dorsally, where it extends only 6.5 mm proximally from the distal-most point on the bone. The contour of the head is almost evenly rounded in medial or lateral view. There is no ridge along the dorsal articular margin. The collateral ligament pits are large, measuring about 6.5 mm in diameter, and are flanked by pronounced tubercles; the bone measures 12.7 mm mediolaterally across them. A.L. 438-1f: right metacarpal 2 (Fig. 7) Preservation This bone is perfectly preserved, except for a missing flake of surface bone on the palmar side of the lateral portion of the head, and a smaller one on the palmolateral margin of the base, just palmar to the trapezium facet. Morphology This bone is nearly a perfect mirror image of A.L. 438-1e, although on the right bone there is a small amount of anterior lipping along the palmar edge of the head, especially on the medial side.

606 M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 A.L. 438-1 l & p: left radius shaft fragment (Fig. 8) Preservation This fragment is broken from the cranial end of the tuberosity, and is 95.9 mm long. It was preserved in six pieces that fit together. The main proximal portion is about 56 mm long and includes the full shaft circumference. Proximally, it is broken through the neck and through a point 31 mm distal to the radial tuberosity. The lower section is a 40 mm long strip that preserves the interosseous crest. Small longitudinal cracks are apparent, but cause no distortion. The bone is very mildly weathered and its preserved parts are in very good condition. A reasonable cross-section is obtainable at the distal end of the proximal fragment. Morphology The radial tuberosity is about 15.5 mm in maximum width and 23.5 mm proximodistally as preserved. The tuberosity is raised above the shaft surface anteriorly, but its posterior edge is rounded and blends with the shaft; it is demarcated by discoloration and somewhat coarser texture than the shaft. No oblique line is visible. The interosseous border is rounded proximally but becomes more sharply crested distally with a roughened surface. The crest is flanked by grooves on either side that are deepest anteriorly. The rest of the bone is smooth and featureless. A.L. 438-1m: right ulnar shaft fragment (Fig. 1) Preservation This fragment is 71.3 mm long extending cranially from about the level of the middle break of the left ulna. It shows numerous tiny longitudinal cracks and is slightly weathered. The distal end is broken transversely, but the proximal break is oblique, oriented about 12 off of the longitudinal axis. At the proximal extremity, the medullary cavity is filled with dark matrix, while at the distal end the cavity is ringed by crystalline matrix. Morphology This bone is morphologically very similar to the more complete left ulna (A.L. 438-1a). A.L. 438-1v: left clavicle fragment (Fig. 9) Fig. 8. Diaphyseal fragment of left radius (A.L. 438-1l&p), anterior view. Preservation This fragment is 61.7 mm long and consists of part of the lateral third of the clavicle, fractured obliquely at both ends and missing the most of the acromial end. The entire circumference of the bone is preserved throughout the length of the fragment. Its surface is generally smooth with only a few hairline cracks and very slightly abraded cortex at the medial break on the anteroinferior margin. There is no evidence of distortion.

M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 607 Fig. 9. Lateral fragment of left clavicle (A.L. 438-1v). Morphology At its lateral margin, at the level of the conoid tubercle, the cross section of the fragment measures 12.9 mm superoinferiorly and about 16.3 mm anteroposteriorly (Table 2). The origin of the deltoid muscle is marked as a rough and wide, raised ridge on the anterosuperior surface, measuring 22.6 mm wide and 6.1 mm anteroposteriorly. The superior surface medial to the deltoid muscle origin is flat and even becomes concave toward the medial break. At that level, the posterior margin of the bone is mildly rugose at the trapezius muscle attachment site. Inferiorly, the conoid tubercle is not well developed. Instead, the inferior and posterior surfaces of the shaft are separated by a sharp crest that extends medially from the break for 25.8 mm where it merges with the posteroinferior margin of the bone. A nutrient foramen is present immediately anterior to that crest, about 23.7 mm from the lateral break. Materials and methods We compared the anatomical elements of A.L. 438-1 to those of extant African apes and humans to place its morphology in comparative context. Here we present a comparative skeletal metric analysis of the most complete elements, the ulna and metacarpals, a discriminant function analysis of proximal ulnar form, and an analysis of proportions among forearm segment lengths. We include descriptive comparisons to all relevant and available hominin fossils of Australopithecus afarensis. We also compare the ulnae to two other specimens, OH 36 and L40-19. The hominin ulna OH 36 is of uncertain taxonomic affiliation, though it is often considered to belong to Australopithecus boisei (Walker and Leakey, 1993; Aiello et al., 1999), or even to Homo erectus (Leakey, 1978; Day, 1986; Tobias, 1991). The L40-19 ulna is an isolated specimen from Member E of the Shungura Formation, Omo River basin, Ethiopia, and has been tentatively referred to a robust australopith taxon (Howell and Wood, 1974; Howell and Coppens, 1976; McHenry et al., 1976; Howell et al., 1987). Comparative morphology Most standard measurements are from Martin and Saller (1957) or Knussmann (1967). Some unique ulnar measures were taken to reflect functionally relevant skeletal form (Fig. 10) following Drapeau (2001, 2004). Because the olecranon is the site of insertion of the triceps brachii muscle, olecranon form determines the lever arm of this muscle. To understand the functional significance of olecranon length, it is necessary to consider olecranon morphology in terms of triceps brachii muscle leverage, using both triceps lever arm length and angle relative to the rest of the bone (Drapeau, 2001, 2004). To quantify triceps leverage, we consider the triceps bony lever length (olecranon length), load arm length (physiological ulna length), and olecranon orientation (Fig. 10, Table 3). This enables us to compare triceps leverage of a to that of humans, great apes, and other fossil hominins. To reflect its anatomical orientation relative to the forearm, we quantified trochlear notch orientation as the angle between two lines; one drawn between the two anterior-most points of the olecranon beak and the coronoid process, and the other between the deepest point on the trochlear

608 M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 UTH UDT URP UCP OO UL UAS OL Fig. 10. Measurements used in the olecranon analysis and to compute the size surrogate (see Tables 3 and 4), modified from Drapeau (2004). articular surface and the center of the ulnar head. This method has the advantage of being independent of diaphyseal curvature. Keeling of the trochlear notch is measured as the angle between the medial and lateral articular surfaces of the notch. Some specimens have protuberant, non-articular bone extending laterally and medially from the central portion of the keel. As a result, the tip of the keel is not articular and the profile of the joint at that level is irregular. For this reason, keeling was measured in the proximal third of the notch. Ulnar diaphyseal curvature as measured by Knussmann (1967) is influenced by diaphyseal robusticity, i.e., the thicker the diaphysis is anteroposteriorly, greater the curvature will be. For that reason, we measured ulnar curvature as the maximum height of the arc formed by the posterior diaphyseal profile (Martin and Saller, 1957; Aiello et al., 1999; see Table 3). Both ulnae of A.L. 288-1 are missing some part of the diaphysis, though the left ulna is missing only a short section (about 3 cm, as inferred from the length estimate of Kimbel et al., 1994). Both proximal and distal fragments are very straight, and they were oriented relative to each other in a range of likely anatomical positions. Attempts to exaggerate the curvature or to reduce it were not satisfactory past a certain point and resulted in non-natural looking reconstructions. Measures of minimum and maximum curvature were calculated from the two reconstructions at the limit of the estimates of likely morphology. These values are only estimates of curvature for A.L. 288-1 and should therefore be interpreted with due caution. The geometric mean of linear measurements is used as a size surrogate (SS) in this study (Jolicoeur, 1963; Mosimann, 1970; Mosimann and James, 1979; Darroch and Mosimann, 1985; Jungers et al., 1995; Richmond et al., 1998). All measurements were taken on the ulna and were selected specifically so that isolated fossil ulnae could be included in the analysis. Five measurements (Fig. 10 and Tables 3, 4) were selected because they scale isometrically with body mass, and in a natural log space are highly correlated with published body masses among primates as a whole and large apes in particular (Drapeau, 2004). For each of these measurements, least square regression of mean sex- and species-specific SS values of our sample against sex- and speciesspecific body weights published in the literature (Smith and Jungers, 1997) yields a Pearson s R of 0.99 and a slope of 0.95 G 0.05. There is no

M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 609 Table 3 Measurements used in comparative metric analyses Ulna UL Physiological length from proximodistal center of the trochlear notch to the distalmost point on the head (Knussmann, 1967; see Fig. 10) OL Olecranon length from the axis of rotation of the ulna to the most distal point of the triceps insertion site (see Fig. 10) OO Olecranon orientation measured as the angle between the axis of olecranon length and long axis of the bone (see Fig. 10) OH Proximodistal olecranon height measured from the anterior tip of the anconeal process (see Fig. 11) UKN Keeling of trochlear notch measured as the angle between the articular surfaces of the trochlear notch on each side of the keel (see Fig. 15) on the proximal part of the trochlear notch UNO Trochlear notch orientation measured as the angle between two lines; one drawn between the two anterior-most points of the anconeal and the coronoid processes, and the other between the deepest point on the trochlear articular surface and the center of the ulnar head (Knussmann, 1967; see Fig. 14) UC Ulnar diaphyseal curvature measured as the maximum distance from the posterior-most margin of the bone and a line drawn between two inflexion points on the posterior margin of the diaphysis, one at the level of the radial notch and the other at the level of minimal distal circumference (Martin and Saller, 1957; Aiello et al., 1999; see Fig. 16) UAS 1 Maximum anteroposterior diameter at midshaft (see Fig. 10) UMA Maximum mediolateral diameter of at midshaft UAH Anteroposterior diameter of head UMH Maximum mediolateral diameter of head UTH 1,2 Trochlear notch height measured as the minimum distance between the anterior-most projection of the anconeal and coronoid processes (Martin and Saller, 1957; Knussmann, 1967; McHenry et al., 1976; Senut, 1981; Richmond et al., 1998; Aiello et al., 1999; see Fig. 10) UDT 1,2 Trochlear notch depth measured as the maximum distance between the cord measure in UTH and the deepest point on the keel of the trochlear notch (Knussmann, 1967; Aiello et al., 1999; see Fig. 10) URP 1 Maximum proximodistal height of radial notch (Knussmann, 1967; Feldesman, 1979; Richmond et al., 1998, see Fig. 10) URA 2 Anteroposterior length of radial notch (Begun, 1992; Richmond et al., 1998) UCP 1 Maximum anteroposterior distance from the tip of the coronoid to the posterior margin of the ulna (McHenry et al., 1976; Senut, 1981; Begun, 1992; see Fig. 10) UNS 2 Mediolateral breadth of proximal portion of the articular surface of the trochlear notch (Feldesman, 1979; Ruff, 2002) UTW 2 Mediolateral breadth of middle portion of the articular surface of the trochlear notch (McHenry et al., 1976; Senut, 1981; Richmond et al., 1998; Aiello et al., 1999) UNR 2 Mediolateral breadth of trochlear notch measured immediately posterior to radial notch ULM 2 Proximodistal length of articular trochlear notch measured medially (Ruff, 2002) ULL 2 Proximodistal length of articular trochlear notch measured laterally (Ruff, 2002) UTR 2 Mediolateral breadth of articular trochlear and radial notches (Begun, 1992; Richmond et al., 1998) UTB 2 Mediolateral breadth of articular trochlear notch anterior to radial notch (Begun, 1992; Richmond et al., 1998) UPC 2 Distance from posterior margin of ulna to the maximum depth of the keel of the trochlear notch (McHenry et al., 1976; Feldesman, 1979; Senut, 1981; Begun, 1992; Richmond et al., 1998; Aiello et al., 1999) UAP 2 Anteroposterior length of anconeal process (McHenry et al., 1976; Senut, 1981; Richmond et al., 1998) UOM 2 Maximum mediolateral breadth of olecranon process (Richmond et al., 1998; Aiello et al., 1999) USM 2 Mediolateral breadth of diaphysis immediately distal to radial notch (Richmond et al., 1998) UNL 2 Minimum distance between the posterior margin of the ulna and the articular surface of the trochlear notch measured laterally (Richmond et al., 1998) Metacarpals ML Maximum length measured as the maximum distance from head to the proximal articular facet MBD Maximum dorsopalmar diameter of proximal end MBM Maximum mediolateral diameter of proximal end MMD Maximum dorsopalmar diameter at midshaft MMM Maximum mediolateral diameter at midshaft MHA Maximum dorsopalmar diameter of distal articulation MHP Palmar mediolateral diameter of distal articulation MHD Dorsal mediolateral diameter of distal articulation MTA Trapezium facet angle relative to sagittal plane, measured as the angle the facet makes to long axis of diaphyses in dorsal view (see Fig. 23b) (continued on next page)

610 M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 Table 3 (continued) PTA Trapezium facet angle relative to sagittal plane measured as the angle the facet makes to transverse axis of the base in proximal view (see Fig. 23a) Mandible CH Corpus height measured as the maximum superoinferior height at M1 CB Corpus breadth measured as the maximum mediolateral breadth at M1 1 Measurement used to calculate the size surrogate. 2 Measurement used in the discriminant function analysis. apparent bias in the estimation of the body size for all species included in the analysis, and we think, therefore, that the SS provides a reliable estimate of body mass for the comparative sample. Data in bivariate plots are logged to control for potentially nonlinear, allometric, or heteroscedastic data. The fossils are compared to samples of other large hominoid species (Tables 4, 5, 6). All apes were wild shot and all specimens were adult and free from obvious pathologies. Discriminant function analysis We also present a discriminant function analysis, which was designed to identify overall morphological patterns that might be useful in distinguishing extant and fossil hominoid proximal ulnae. On a trait-by-trait basis, only subtle differences exist among hominoids, despite wide variation in locomotor and manipulative behaviors involving the upper limb. Discriminant functions were calculated using 18 different measurements of Table 4 Means and standard deviations for measurements 1 of the ulna used in comparative metric analyses Extant species/ Specimens UL OL OO OH UKN UNO UC UAS UMS UAH UMH UTH UDT URP UCP SS 2 Homo sapiens 238 30.3 110.0 4.2 123 21.4 2.9 15.1 13.9 10.7 17.3 23.2 12.1 11.1 34.8 17.1 n Z 30 3 19 2.7 7.3 1.5 8 5.5 1.9 2.1 2.0 1.3 2.4 2.4 1.6 2.1 3.0 1.9 Pan paniscus 254 28.3 105.4 2.0 109 19.9 11.4 13.5 12.6 7.0 16.7 20.2 10.6 10.0 32.6 15.6 n Z 18 4 14 2.0 5.7 2.5 10 3.8 1.8 1.4 1.3 1.1 2.8 1.7 0.9 1.6 2.1 1.1 P. troglodytes 262 32.6 102.0 3.5 105 20.5 12.4 15.6 15.1 8.1 18.6 22.1 11.5 11.5 36.5 17.4 n Z 41 5 17 3.4 7.2 2.3 8 7.7 2.8 1.9 2.1 0.7 1.5 3.5 1.8 1.6 3.4 1.8 G. gorilla 309 39.2 97.4 1.9 116 27.3 14.8 20.0 19.9 11.7 25.0 28.8 14.0 18.3 45.9 23.3 n Z 50 6 29 6.6 7.0 2.3 7 5.2 4.0 3.2 3.4 1.9 3.2 4.7 2.1 2.3 6.6 3.0 Pongo pygmaeus 343 28.0 96.6 2.1 110 25.7 12.6 17.3 17.6 9.9 21.7 20.4 11.6 14.5 34.0 18.2 n Z 25 7 25 4.0 8.9 1.2 7 6.2 3.4 3.4 2.7 1.7 3.3 4.0 2.5 1.7 5.6 2.8 A.L. 438-1 252 31.9 125 7.7 131 10.5/13.3 8 7.5 17.6 14.8 8.9 17.5 23.2 11.0 10.2 31.8 17.1 A.L. 288-1 181/206 8 24.4 118 5.1 130/132 9 9 2.5/4.1 8 12.3 7.7 5.9 11.9 15.7 7.9 6.8 22.5 11.8 L40-19 295 34.4 111 5.1 121 2/9.5 12.5 14.8 11.7 11.2 21.0 21.2 13.2 12.9 35.8 18.0 OH 36 247/277 8 33.1 122/125 8 12.0 102 7/10 8 13.9/15.7 8 17.3 12.7 17.3 10.0 9.3 36.1 15.9 A.L. 333-12 7.7 18.3 1 Measurements described in Table 3. 2 SS: size surrogate calculated as the geometric mean of measurements UAS, UTH, UDT, URP, and UCP. 3 For the variable UKN, n Z 28. 4 For the variables OL, OO, OH, and UNO, n Z 20; for UKN, n Z 17. 5 For the variables OL, OO, UKN, and UNO, n Z 28. 6 For the variables OL and OO, n Z 31; for UKN, n Z 34; for UNO, n Z 22. 7 For the variable UKN, n Z 24. 8 Minimum and maximum estimates. 9 Values for A.L. 288-1t and A.L. 288-1n.

Table 5 Means and standard deviations for the measurements 1 of metacarpals used in comparative metric analyses Extant species/specimens 2ML 2MBD 2MBM 2MMD 2MMM 2MHA 2MHP 2MHD MTA PTA 3ML 3MBD 3MBM 3MMD 3MMM 3MHA 3MHP 3MHD Homo sapiens 69.1 15.7 15.1 9.3 8.3 14.2 14.2 9.7 37 38 68.9 13.8 14.1 9.6 8.5 14.4 14.0 10.5 n Z 30 5.0 1.5 1.6 1.0 0.9 1.2 1.3 0.9 5.9 5.6 5.1 3.0 1.1 1.0 0.6 0.9 1.3 0.8 Pan paniscus 88.4 15.0 12.1 7.4 7.4 12.9 10.9 9.7 87.5 13.7 13.0 8.2 7.7 14.3 11.8 12.0 n Z 18 3.5 1.0 1.2 0.6 0.6 1.0 1.1 1.0 3.1 3.5 1.1 0.6 0.6 0.9 1.4 1.2 Pan troglodytes 89.3 15.1 12.3 8.3 7.9 15.2 12.2 10.5 10 19 87.9 15.5 13.1 9.0 8.5 16.0 13.2 13.1 n Z 28 6.4 1.2 0.9 0.6 0.7 1.5 0.9 1.0 6.5 7.9 5.6 1.6 1.4 1.0 0.5 1.4 0.8 1.2 Gorilla gorilla 89.1 19.1 16.9 13.3 11.6 18.2 15.7 14.4 13 38 87.2 18.3 15.6 12.4 11.0 18.9 16.4 17.1 n Z 34 2 9.8 2.2 3.0 1.3 1.9 2.0 2.3 2.1 6.5 7.1 9.8 2.2 1.9 1.4 2.2 2.1 2.4 2.3 Pongo pygmaeus 102.1 14.8 12.3 8.1 7.3 14.6 12.9 11.0 99.8 15.4 12.7 8.5 7.4 15.3 13.4 11.9 n Z 24 10.8 2.1 1.8 1.4 1.1 2.3 1.5 1.4 10.4 1.9 1.9 1.4 1.0 2.3 1.8 1.6 A.L. 438-1 left 66.5 14.7 13.0 8.3 8.9 13.6 12.6 7.7 19 40 64.8 15.7 13.3 9.0 8.8 13.9 12.9 9.0 A.L. 438-1 right 66.3 14.5 12.7 8.5 8.5 13.9 12.6 8.1 19 42 A.L. 333-48 61.5 13.0 12.0 7.3 7.8 11.9 11.3 7.2 20 44 A.L. 333-15 3.1 11.4 18 38 A.L. 333-153 11.5 11.7 A.L. 333w-6 13.3 12.9 A.L. 333-65 13.9 11.5 A.L. 333-16 59.6 12.3 11.0 7.5 7.2 11.5 11.8 7.7 1 Measurements described in Table 3. 2 For variable MTA, n Z 28. M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 611

612 M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 Table 6 Means and standard deviations for measurements 1 of the mandible used in comparative metric analyses Extant species/specimens CB CH Pan troglodytes 13.9 30.4 n Z 21 1.1 2.8 Gorilla gorilla 19.1 37.1 n Z 12 1.7 3.7 A.L. 438-1 23.5 37.1 A.L. 288-1 17.1 29.9 1 Measurements described in Table 3. Analysis of proportions among segments Length measurements are described in Table 3. Estimated ulnar length for A.L. 288-1 is taken from Kimbel et al. (1994). Only results from the analysis of the second metacarpals are presented because of the high correlation between second and third metacarpals, with no significant interspecific variation (Drapeau, 2001). Comparative morphology the proximal ulna (Tables 3 and 7). Two analyses were performed, one on raw measurements and one on size-controlled measurements, the latter of which were obtained by dividing the raw data by the geometric mean of the 18 measurements (Aiello et al., 1999). The comparative sample was divided into four groups: Homo sapiens (n Z 30), Pan (P. paniscus and P. troglodytes are pooled as in Aiello et al., 1999) (nz 62), Gorilla gorilla (n Z 50), and Pongo pygmaeus (n Z 25). Fossils formed a fifth group (n Z 5; A.L. 288-1t, A.L. 438-1a, L40-19, OH 36, StW 113). The proximal ulna StW 113 is from Sterkfontein, South Africa, but as it is formally undescribed at present we do not discuss its morphology elsewhere in the paper. Clavicle The clavicle of A.L. 438-1 is larger and more robust than other A. afarensis specimens preserving the same region. The anteroposterior and superoinferior diameters of A.L. 438-1v at the conoid tubercle are 16.3 and 12.9 mm, compared with 12.4 and 9.3 mm in A.L. 333x-6/9 (Lovejoy et al., 1982). Although more robust, A.L. 438-1v is very similar in morphology to the other A. afarensis clavicles. Lateral curvature is comparable in A.L. 438-1, A.L. 333x-6/9, chimpanzees and humans, and appears more curved than typical for gorillas (Schultz, 1930) in parts preserved. The deltoid muscle insertion site is particularly well Table 7 Means and standard deviations of all measurements used in discriminant function analyses Extant species/ UTH UDT UNS UTW UNR ULM ULL UTR UTB URA URP UCP UPC UAP UOM USM UNM UNL Specimens Homo sapiens 23.2 12.1 25.1 15.3 26.9 29.1 26.9 31.1 20.3 17.4 11.1 34.8 17.9 25.1 21.5 22.1 14.1 13.3 n Z 30 2.4 1.6 2.9 2.4 3.0 2.7 2.9 4.1 2.4 2.7 2.1 3.0 1.5 2.5 2.5 3.2 1.5 1.8 Pan paniscus 20.2 10.7 20.3 15.1 21.6 25.6 24.9 23.0 19.8 14.3 10.0 32.7 18.5 26.7 17.4 15.5 14.2 12.8 n Z 20 1.6 0.7 1.7 2.6 2.1 2.4 2.1 3.0 2.2 1.4 1.5 2.1 1.7 2.8 1.4 1.3 1.5 2.4 Pan troglodytes 22.0 11.5 22.0 15.6 24.7 29.0 27.7 30.0 21.0 15.8 11.5 36.5 21.8 30.3 22.3 18.2 16.9 14.8 n Z 42 3.5 1.8 2.0 2.9 2.6 3.1 2.5 3.5 2.4 2.3 1.5 3.4 2.3 2.8 2.8 2.0 2.2 2.7 Gorilla gorilla 28.8 14.0 31.5 31.9 38.8 35.6 34.0 47.7 30.9 24.8 18.3 45.9 26.8 35.1 28.9 28.3 20.2 17.3 n Z 50 4.7 2.1 5.1 5.7 5.8 4.9 4.7 7.5 4.7 3.5 2.3 6.6 4.3 5.9 5.4 4.2 3.8 4.4 Pongo pygmaeus 20.4 11.6 22.6 17.4 31.7 28.7 25.8 36.4 23.6 19.8 14.5 34.0 18.3 24.0 22.5 19.6 13.5 11.4 n Z 25 4.0 2.5 3.4 4.3 3.9 3.7 3.4 5.2 2.9 2.8 1.7 5.6 2.7 3.7 3.6 3.0 2.4 3.0 A.L. 438-1 23.2 11.0 25.7 11.4 23.0 26.3 24.7 30.2 18.5 16.4 10.2 31.8 16.2 26.2 27.8 19.6 12.6 10.2 A.L. 288-1 15.7 7.9 16.1 12.3 16.0 21.5 21.5 19.0 16.0 9.8 6.8 22.5 12.4 18.4 13.4 15.2 10.0 9.3 L40-19 23.1 11.9 26.2 17.3 24.7 35.2 31.9 26.5 20.3 13.8 12.9 33.4 20.8 30.1 28.2 23.9 14.8 13.5 OH 36 17.3 10.0 23.8 15.8 24.7 30.5 28.5 28.2 21.2 15.8 9.3 36.1 23.6 30.1 24.0 18.1 17.1 10.7 StW 113 20.3 11.3 21.7 13.5 22.2 29.5 25.2 22.8 18.4 10.0 8.0 31.0 17.4 26.4 21.6 17.8 13.4 11.8

M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 613 pronounced, as is typical for humans (Olivier, 1951a, b, c, 1952, 1954a, b, c, 1955), matching the similar robustness of its humeral insertion (see below). The conoid tubercle, however, is weak compared with that of most humans (Olivier, 1951a, b, c, 1952, 1954a, b, c, 1955), as it is in A.L. 333x-6/9. Despite this, the tubercle morphology is fundamentally human-like, placed on the inferior aspect of the bone. Chimpanzees tend to have a tubercle that is situated in the plane of the acromial end, giving the cross section of the bone a flattened oval shape (Partridge et al., 2003). In A.L. 438-1v, the cross-sectional shape of the shaft at the conoid is triangular, reflecting the inferior placement of the conoid relative to the acromial end of the bone, with distinct posterior and superior surfaces as is typical of humans and australopithecines (Lovejoy et al., 1982; Partridge et al., 2003). This specimen is insufficiently preserved to assess acromial end length distal to the conoid tubercle, or torsion of the bone (Oxnard, 1984). If clavicular morphology reflects variation in shoulder orientation and function (Aiello and Dean, 1990), similarities between this fragment and clavicles of other hominins suggest a broad similarity in upper torso organization. Apparent differences in conoid ligament insertion site morphology may suggest differences from chimpanzees, but such a conclusion must remain tentative given the fragmentary nature of this specimen. Humerus Based on the preserved proximal segment, the humerus is very large, and appears to have belonged to one of the largest known Australopithecus afarensis individuals. There are two other A. afarensis humeri that preserve the surgical neck in good enough condition to allow accurate measurements, A.L. 333-109 and A.L. 288-1 (the cortex of A.L. 333-107 is eroded). Specimen A.L. 333-109 measures 25.5 mm anteroposteriorly by 22.3 mm mediolaterally at that location (compared to 28.8 and 24.9 mm for A.L. 438-1). The diminutive A.L. 288-1, which can be measured only mediolaterally, is 17.8 mm wide. The distal portion of the A.L. 438-1c fragment appears to correspond to the proximal portion of A.L. 137-50 (Kimbel et al., 1994). In comparable areas, these specimens appear similar in size and robusticity. The maximum cross-sectional diameter across the attachment for the lateral head of triceps brachii is about 29 mm for both specimens. They also appear to have similar cortical thicknesses, although the obliquely broken end of A.L. 137-50 makes this difficult to assess with certainty or to quantify accurately. These fossils are also similar in the degree to which the narrow ridge of the latissimus dorsi muscle insertion angles medioinferiorly. Neither appears to differ in length over this short distance, although we recognize this is only a crude estimate of similarity. Still, their apparent similarity in size makes it reasonable to combine these two bones and compute a humeral length estimate. When this is done, the length estimate for A.L. 137-50 is close to 300 mm, only slightly greater than the 295 mm estimate reported by Kimbel et al. (1994). Comparing this with the A.L. 438-1 ulna measurement yields an ulna/ humerus length ratio of 89e90%, similar to the previous estimate based on these fossils (w91%; Kimbel et al., 1994). The preserved muscle markings on the A.L. 438-1c humeral fragment are quite dramatic, as are those on A.L. 137-50, A.L. 333-107, A.L. 333-109, and reported for MAK VP-1/3 (White et al., 1993). The A.L. 438-1 specimen confirms what appears to be typical of large A. afarensis humeri: in the rugosity of their muscle markings, they are most comparable to the humeri of robust humans and to some chimpanzees among extant hominoids. In contrast to A.L. 333-109, the bicipital groove of A.L. 438-1 is shallow, and human-like in conformation: the lateral border of the groove is low, but sharply crested along the pectoralis major insertion site, becoming rounded and swollen distally. Chimpanzees, on the other hand, tend to have a higher, rounded swelling forming the posterior border throughout its length. The degree to which these differences might reflect differences in humeral head torsion is unclear, but since bicipital groove form does not appear to be closely associated with torsion within species (Napier and Davis, 1959; Fleagle and Simons, 1982; Rose, 1989;

614 M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 Begun, 1994; Larson, 1996), it is unlikely to be closely related to variation in torsion among the species either. Torsion cannot be predicted accurately from this small fragment of humerus. Ulna The A.L. 438-1 ulna is large and robust. In olecranon height, A.L. 438-1, like A.L. 288-1 and L40-19, have relatively tall olecranons. They overlap the human, chimpanzee and gorilla distributions (Fig. 11), and A.L. 288-1n overlaps even the orangutan range. The olecranon of OH 36 is higher than that of all other specimens in our comparative sample. Olecranon height varies little across taxa, but this does not necessarily signify similarity in function. The olecranon process is the insertion site of the triceps brachii muscle, but olecranon process orientation also contributes to length of the lever arm of the triceps brachii muscle, confounding the use of olecranon height to infer triceps lever arms. Humans appear to have slightly longer olecranon processes than do apes, except Pan troglodytes (Fig. 11), but do not necessarily have a greater triceps lever arm length because they differ in olecranon process orientation (Fig. 12). Humans have a more proximally oriented triceps brachii lever arm than do great apes, whose bony levers are inclined more posteriorly. A proximally oriented olecranon appears to be the hominin pattern, as both A. afarensis fossils exceed the sample ranges for all the apes, as does OH 36. These fossil hominins resemble only humans near the highest end of the sample range, while L40-19 approaches the mean for humans, but is also in the upper end of the ranges of all great apes in the sample. A more retroflexed olecranon process enhances triceps leverage when the arm is at or near full extension (Jolly, 1967). Because orangutans resemble African apes in this feature, a more posteriorly oriented triceps lever arm cannot be linked exclusively to knuckle walking, but instead 12 OH 36 Olecranon height (mm) 8 4 0 L40-19 AL 288-1 -4 H. sapiens P. paniscus P. troglodytes G. gorilla P. pygmaeus Fossils Fig. 11. Boxplot of olecranon proximodistal height (OH). The dot represents the median, the box represents the interquartile range (25e75%), the whiskers represent the range excluding the outliers, the empty circles represent the outliers (more than 1.5 times the interquartile range from the median), and the asterisks represent the extreme outliers (more than 3 times the interquartile range). Although there is some overlap among species, humans have proximodistally longer olecranon processes (T-test, p! 0.05) than all other large hominoids except P. troglodytes. The A.L. 438-1 point falls only within outliers for humans and chimpanzees, and outside of the sample ranges for all other taxa. The OH 36 ulna has a particularly long olecranon process, but A.L. 288-1 and L40-19 fall within both ape and human ranges.

M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 615 Olecranon orientation (degrees) 125 115 105 95 85 OH 36 AL 288-1tu L40-19 75 H. sapiens P. paniscus P. troglodytes G. gorilla P. pygmaeus Fossils Fig. 12. Boxplot of olecranon process orientation (OO), measurement illustrated in Fig. 10. A larger angle reflects a more proximallyoriented olecranon process. Humans have more proximally oriented olecranon processes than other large hominoids (T-test, p! 0.05). All fossils, except L40-19, fall at the top end of the human range, with very proximally-oriented olecranon processes. is more plausibly associated with habitually extended forelimb postures during climbing, hanging, and other arboreal activities. The proximally directed olecranon seen in hominins, including Australopithecus afarensis, provides greater triceps leverage when the elbow is flexed near 90,and would appear to maximize power in flexed forelimb postures. Assuming that no flexed-forearm quadrupedalism was practiced by Australopithecus, the proximally-oriented olecranon suggests that the triceps muscle in early hominins was optimized for use in manipulatory activities, perhaps like those observed in Homo. Since no stone tools are known to be associated with A. afarensis, this morphology cannot be conclusively associated with tool-making (but see Tuttle and Basmajian, 1974). This similarity among hominins, however, suggests a similar pattern of forelimb use in A. afarensis and humans that is not typical for extant great apes. If extant great ape morphology represents the primitive condition from which the hominin pattern evolved (Tuttle and Basmajian, 1974; Harrison 1987), then A. afarensis appears derived in the direction of humans. Until we can be more certain of the polarity of this feature, especially given the extensive homoplasy among hominoids (Lockwood and Fleagle, 1999, and references therein), any such conclusion must be considered tentative. The relative triceps leverage (lever arm compared with load arm) of A.L. 438-1a and A.L. 288-1t/u is similar to that of humans and extant great apes, differing only from that of orangutans (Fig. 13), which clearly have the shortest relative triceps lever arms. Our data suggest that all other large hominoids, including humans, are broadly similar in lever/load arm proportions (see also Knussmann, 1967). Trochlear notch orientation has been observed to differ among hominoids (Fisher, 1906; White et al., 1994; Aiello et al., 1999), and has been interpreted to be related to locomotor adaptation (Rose, 1997; Zylstra, 1999). All fossil hominins have trochlear notches that are more anteriorly oriented than those of extant hominoids (Fig. 14). When overlapping, they are at the extreme inferior range of other species. Humans, chimpanzees, and bonobos are similar to one another, with slightly proximally-facing trochlear notches (although, on average, not as much as observed in the fossils),

616 M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 Ln (olecranon length, mm) 3.8 3.4 AL 288-1 Olecranon length OH 36 L40-19 3.0 5.1 5.5 5.9 Ln (ulnar length, mm) Fig. 13. Triceps lever length (OL) to ulnar length (UL); gray diamonds, modern H. sapiens; black triangles, P. paniscus; open triangles, P. troglodytes; open squares, G. gorilla; open circles, P. pygmaeus. All extant and fossil hominoids have similar proportions except P. pygmaeus. whereas gorillas and orangutans have the most proximally facing notches (Aiello et al., 1999). These data do not support the hypothesis that trochlear notch orientation reflects terrestrial locomotion with an extended forelimb (Zylstra, 1999), but do suggest that fossil hominins are derived relative to other hominoids in notch orientation (White et al., 1994; Rose, 1997). In Australopithecus afarensis, the anteriorly oriented notch appears to be related structurally to the proximally oriented olecranon, a trait for which the fossil is at the periphery of the human distribution (Fig. 12). Neandertals, like early hominins, are also characterized by anteriorly oriented trochlear notches and proximally oriented olecranon processes relative to those of modern Homo sapiens (Churchill et al., 1996; Hambu cken, 1998), supporting the hypothesis that olecranon and trochlear notch orientations are linked. There is no reason to believe that increased loading without changes in habitual forelimb posture would result in a more anteriorly oriented trochlear notch, so the morphology seen in the fossils may reflect greater duration and/or frequency of elbow loading in flexed postures than typical of modern humans (Trinkaus and Churchill, 1988). The A.L. 438-1 ulna has a relatively broad, mildly crested trochlear notch, most similar to that of modern humans (Fig. 15). Sample ranges of all taxa overlap considerably, but humans are characterized Trochlear notch orientation (degrees) 30 20 10 0 H. sapiens P. paniscus P. troglodytes G. gorilla P. pygmaeus Fossils AL 288-1 OH 36 L40-19 Fig. 14. Boxplot of trochlear notch orientation (UNO). The fossils have more anteriorly-oriented trochlear notches than apes or humans.

M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 617 Keeling of notch (degrees) 150 140 130 120 110 100 90 AL 288-1 L40-19 OH 36 H. sapiens P. paniscus P. troglodytes G. gorilla P. pygmaeus Fossils Fig. 15. Boxplot of keeling of proximal portion of trochlear notch (UKN). Extant hominoids overlap in this feature, but humans tend to have less keeled trochlear notches than do great apes (T-test, p! 0.05). The A.L. 438-1 ulna and most other fossils fall closest to the center of the human range, although they overlap with that of at least some ape species. In contrast, OH 36 is only like apes. by a less keeled trochlear notch than any other hominoids. Similar observations were made on the humerus (Senut, 1980; Zylstra, 1999; Bacon, 2000). The morphology of A. afarensis is most similar to what is observed in humans, although it overlaps with the range of G. gorilla as well. Unlike the other hominins, OH 36 falls outside of the human range, and is similar only to apes in this respect. Preuschoft (1973) interprets the keeling of the hominoid trochlear as a means to resist lateral sliding of the ulna on the humerus during contraction of muscles crossing the elbow that have a lateral component (such as the flexor digitorum superficialis, flexor carpi radialis, flexor carpi ulnaris and pronator teres; see Rose, 1993). Functionally, therefore, a flatter keel could be interpreted to suggest the presence of smaller or less developed wrist and superficial finger flexors. Ulnar diaphyseal curvature of A.L. 438-1a is more pronounced than that of humans on average, falling at the high end of the observed human sample range and at the low end of the great ape ranges (Fig. 16). It falls between the human and great ape means. The A.L. 288-1 ulnar diaphysis appears to be straighter, like that of humans. If ulnar curvature of A.L. 438-1 and A.L. 288-1 is representative of normal variation in A. afarensis, then the middle range value for of this species suggests a less curved ulna than typical for great apes and more in line with what is seen in modern humans. The functional significance of ulnar curvature is poorly understood. Forelimb bone curvature in hominoids has been suggested to alter the lever arms of the pronator and supinator muscles (Miller, 1933; Oxnard, 1963; Ziegler, 1964; Knussmann, 1967; Swartz, 1990), or to increase the size of the interosseous membrane to which important digital flexor muscles are attached (Miller, 1933; Knussmann, 1967). It appears to be associated with below-branch arboreality and grasping strength of the hands. If so, strong ulnar diaphyseal curvature suggests powerful forearm musculature and finger flexors in A. afarensis, at least in large individuals, compared with humans, but reduced muscularity compared with apes. In contrast, OH 36 and L40-19 each have very curved diaphysis, comparable to apes only. The A.L. 438-1 ulna has a distinct, but mild, interosseous crest, also seen in A.L. 288-1t and all other known hominin ulnae. African apes have reduced crests compared with other extant hominoids (Knussmann, 1967). The genetic and epigenetic influences on development of an interosseous crest are poorly understood. Crest development does not appear to be related to overall

618 M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 Ulnar diaphyseal curvature (mm) 18 12 6 0 OH 36 L40-19 AL 288-1 H. sapiens P. paniscus P. troglodytes G. gorilla P. pygmaeus Fossils Fig. 16. Boxplot of ulnar diaphyseal curvature (UC). Humans have straighter ulnae than that of other extant hominoids (T-test, p! 0.05). All large fossils, including A.L. 438, have more curved ulnae than do humans. Only the A.L. 288-1 ulna is as straight as that of humans. Considered with A.L. 438-1, the range of A. afarensis data is most similar to that of humans, although not entirely coincident. muscularity among hominoid species, as gorillas and chimpanzees have mild interosseous crests while humans usually have a well marked one. External measurements of the ulnar midshaft that include the crest are similar among apes and humans (Fig. 17), so it may be that chimpanzees and gorillas have filled in bone anterior and posterior to the crest, strengthening the diaphysis (but see Pauwels, 1979). The distal end of the ulna does not vary extensively among hominoids. While A.L. 438-1a has a large crest for attachment of the pronator quadratus muscle, as do L40-19 and OH 36, this does not separate early hominins from apes or humans. Humans have anteroposteriorly deeper ulnar heads relative to their mediolateral diameter than do extant apes, however. The specimens A.L. 438-1a and L40-19 are intermediate in shape, and cannot be differentiated from either group, while A.L. 333-12 is most similar to ape ulnae of similar size (Fig. 18). Nonetheless, this specimen appears to have an intermediate morphology, but resembles apes more closely than humans. As a group, the Australopithecus afarensis specimens do not differ from extant apes, even though A.L. 438-1 falls within the modern human distribution. This suggests that A. afarensis had a morphology intermediate between that of humans and extant apes. Discriminant function analysis of proximal ulnar morphology Discriminant function analyses illustrate overall morphological patterns among extant and fossil hominoid proximal ulnae. Both analyses, one using raw measurements and one using sizecontrolled measurements, produced four significant discriminant functions (Table 8). Analysis of raw measurements. Specimens used in this analysis (see Materials and Methods) are classified within their respective group with 97% accuracy. Only four Pan specimens were classified with the fossil hominin sample. There were four significant functions (Table 8). The first discriminant function in the analysis of raw data, which accounts for 62% of the variance,

M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 619 Ulnar midshaft dimensions A Ln (AP depth of ulnar midshaft, mm) 3.1 2.7 OH 36 2.3 AL 288-1 2.2 2.6 3.0 Ln (ML width of ulnar midshaft, mm) B C Ln (ML width of ulnar midshaft, mm) 3.0 2.6 2.2 2.4 OH 36 AL 288-1 2.8 3.2 Ln (size surrogate, mm) Ln (AP depth of ulnar midshaft, mm) 3.1 2.7 OH 36 AL 288-1 2.3 2.4 2.8 3.2 Ln (size surrogate, mm) Fig. 17. Bivariate plots of (A) anteroposterior depth to mediolateral width of ulnar midshaft, (B) mediolateral width of ulnar midshaft to the size surrogate and (C) anteroposterior depth to the size surrogate. Legend as in Fig. 13. Midshaft dimensions are similar among extant apes, humans, and Australopithecus afarensis, although A. afarensis has a particularly anteroposteriorly deep ulnar midshaft, only like some orangutans and chimpanzees. probably reflects size (Fig. 19a, Table 9). All fossil hominins group with humans and Pan. The following metrics load heavily on this function: articular mediolateral width in the middle of the notch (UTW), height of the radial notch (URP), mediolateral width of the articular trochlear notch posterior to the radial notch (UNR), mediolateral width of the trochlear and radial articular surfaces

620 M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 Ln (AP depth of articular ulnar head, mm) 2.8 2.4 2.0 Ulnar head shape L40-19 AL 288-1 AL 333-12 Discriminant scores from function 2 5 A AL 288-1 0 Stw 113 L40-19 OH 36-5 -4 0 4 8 Discriminant scores from function 1 1.6 2.3 2.7 3.1 Ln (ML width of ulnar head, mm) Fig. 18. Bivariate plot of anteroposterior depth to mediolateral width of the ulnar head articular surface. Legend as in Fig. 13. Humans have anteroposteriorly deeper metacarpal heads than do great apes. Two Australopithecus afarensis specimens (A.L. 288-1 and A.L. 438-1) are intermediate between apes and humans, while A.L. 333-12 is more similar to apes. Overall, the range of A. afarensis values is most like that of apes, though barely overlapping with humans. Discriminant scores from function 4 4 2 0-2 B OH 36 L40-19 Stw 113 AL 288-1 combined (UTR), width of the radial notch (URA), width of the articular trochlear notch anterior to the radial notch (UTB), and mediolateral width of the shaft immediately distal to the radial notch (USM). The second function, accounting for 23% of the variance, separates African apes from H. sapiens Table 8 Results of discriminant function analysis Function Eigenvalue % of variance Canonical correlation Analysis based on raw data 1 6.74 61.6 0.93 2 2.56 23.4 0.85 3 1.46 13.4 0.77 4 0.17 1.6 0.38 Analysis based on size-controlled data 1 5.21 53.7 0.92 2 2.64 27.2 0.85 3 1.61 16.6 0.78 4 0.25 2.5 0.45-4 -2 0 2 4 6 Discriminant scores from function 3 Fig. 19. Results of discriminant function analyses of the proximal ulnar morphology/raw data. Legend as in Fig. 13 (P. paniscus and P. troglodytes are pooled). (A) Discriminant scores from function 1 and 2 and (B) from function 3 and 4. The fossils, as a group, are unique among hominoids in having mediolaterally wide olecranons (UOM). Two fossils, A.L. 438-1 and L40-19, are more similar to humans than great apes, while OH 36 and StW 113 are more similar to Pan. The diminutive A.L. 288-1 has a morphology intermediate between humans and Pan. and Pongo (Fig. 19a), which are characterized by an anteroposteriorly shorter anconeal beak (UAP), less buttressing posterior to the trochlear notch (UPC), a mediolaterally narrow middle trochlear notch (UTW), and by a smaller distance between the posterior margin of the ulna and the articular surface of the trochlear notch on the medial side (UNM). For that second function, the fossil group overlaps all taxa distributions, but

M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 621 Table 9 Correlations of prediction variables with discriminant functions based on raw measurements 1 Variable Function 1 Function 2 Function 3 Function 4 UTW 0.65 ÿ0.46 0.26 0.09 URP 0.62 ÿ0.20 ÿ0.03 ÿ0.03 UNR 0.60 ÿ0.06 0.12 ÿ0.04 UTR 0.57 ÿ0.14 0.14 ÿ0.04 URA 0.56 ÿ0.08 0.13 ÿ0.21 UTB 0.52 ÿ0.27 0.07 0.02 USM 0.49 ÿ0.13 0.53 0.03 UNS 0.40 ÿ0.17 0.44 0.03 UCP 0.34 ÿ0.33 0.21 ÿ0.25 UOM 0.31 ÿ0.16 0.15 0.29 UPC 0.31 ÿ0.48 0.10 ÿ0.08 ULM 0.30 ÿ0.18 0.21 0.10 ULL 0.30 ÿ0.30 0.25 0.01 UTH 0.29 ÿ0.23 0.34 ÿ0.22 UNM 0.23 ÿ0.43 0.14 ÿ0.18 UDT 0.22 ÿ0.10 0.18 ÿ0.20 UAP 0.22 ÿ0.50 0.14 ÿ0.07 UNL 0.15 ÿ0.29 0.17 ÿ0.33 1 Pooled within-group correlations between each variable and each discriminant function. Bold face marks the largest absolute correlation between each variable and any discriminant function. the A. afarensis specimens fall only within the H. sapiens and Pongo distributions. The third function (accounting for 13% of the variance) discriminates H. sapiens from Pongo, which has a narrower shaft when measured immediately distal to the trochlear notch (USM), and a proximally narrower articular trochlear notch Table 10 Posterior probabilities that the fossils belong to each comparative group Fossil H. sapiens Pan Gorilla Pongo Fossils Analysis based on raw data A.L. 438-1 0.10 1 0 0 0 0.90 A.L. 288-1 0.04 0.05 1 0 0 0.91 L40-19 0 1 0 0 0 1.00 OH 36 0 0.68 0 0 0.32 1 StW 113 0 0 1 0 0 1.00 Analysis based on size-controlled data A.L. 438-1 0.03 1 0 0 0 0.97 A.L. 288-1 0.01 1 0 0 0 0.99 L40-19 0 1 0 0 0 1.00 OH 36 0 0.80 0 0 0.20 1 StW 113 0 0 1 0 0 1.00 1 Second highest posterior probability. Discriminant scores from function 2 Discriminant scores from function 4 6 4 2 0-2 -4-6 -4-2 0 2 4 6 Discriminant scores from function 1 2 0-2 -4-8 A L40-19 Stw 113 AL 288-1 OH 36 B OH 36 L40-19 Stw 113-4 0 4 Discriminant scores from function 3 AL 288-1 Fig. 20. Results of discriminant function analyses of the proximal ulnar morphology/size-controlled data. Legend as in Fig. 13 (P. paniscus and P. troglodytes are pooled). (A) Discriminant scores from function from 1 and 2 and (B) from function 3 and 4. As for the raw data analysis, the fossils are all uniquely characterized by mediolaterally wide olecranons (UOM). This analysis suggests that the fossils have anteroposteriorly long coronoid processes as well (UPC). Three fossils, A.L. 288-1, A.L. 438-1, and L40-19, are more similar to humans than great apes, while OH 36 and StW 113 are more similar to Pan. (UNS) (Fig. 19b). Australopithecus afarensis is most similar to humans and to gorillas along this axis. The fourth discriminant function separates the fossil sample from most large hominoids, but more distinctively from humans (Fig. 19b). It is the only function that clearly discriminates the fossils ulnae from those of humans, although this function accounts for less than 2% of the variance structure of the data. Correlations between measurements and the fourth function indicate that the fossil

622 M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 Table 11 Correlations of prediction variables with discriminant functions based on size-corrected measurements 1 Variable Function 1 Function 2 Function 3 Function 4 UAP ÿ0.61 ÿ0.31 ÿ0.10 ÿ0.02 UTR 0.57 ÿ0.02 ÿ0.04 ÿ0.04 UNR 0.52 0.14 ÿ0.27 ÿ0.04 UPC ÿ0.38 ÿ0.42 ÿ0.08 0.05 URA 0.37 0.09 ÿ0.15 0.26 UCP ÿ0.36 0.04 ÿ0.16 0.44 URP 0.36 ÿ0.19 ÿ0.21 ÿ0.02 UNM ÿ0.34 ÿ0.22 0 0.20 UTW 0.35 ÿ0.42 0.47 ÿ0.22 ULL ÿ0.34 0.16 ÿ0.18 ÿ0.21 UNL ÿ0.23 ÿ0.04 0.04 0.35 ULM ÿ0.23 0.29 ÿ0.28 ÿ0.33 USM 0.20 0.40 0.44 ÿ0.22 UDT ÿ0.19 0.29 ÿ0.22 0.20 UTH ÿ0.17 0.19 0.12 0.20 UTB 0.16 ÿ0.15 ÿ0.19 ÿ0.18 UOM ÿ0.03 0.12 ÿ0.13 ÿ0.59 UNS ÿ0.01 0.39 0.26 ÿ0.22 1 Pooled within-group correlations between each variable and each discriminant function. Bold face marks the largest absolute correlation between each variable and any discriminant function. group tends to be characterized by a greater distance between the posterior margin of the ulna and the articular surface of the trochlear notch on the lateral side (UNL), and by a mediolaterally wide olecranon (UOM). Posterior probabilities for the fossils show that OH 36 is more similar to Pan than to the other fossils (Table 10). For the other fossils, if the possibility of belonging to the fossil group is removed, the posterior probabilities indicate that StW 113 is clearly most similar to Pan among living hominoids, while A.L. 288-1 is slightly more similar to Pan than to Homo. On the other hand, A.L. 438-1 and L40-19 are most similar to H. sapiens. Analysis of size controlled data. This analysis also discriminates the five groups very well (Fig. 20, Table 8). Specimens used in this study are correctly classified with 97% accuracy. Three Pan ulnae were classified with the fossil sample, one Pongo specimen was classified as a gorilla, and OH 36 was classified as Pan (Table 10). The first size-controlled function, which accounts for 54% of the variance, discriminates Pan from Gorilla and Pongo (Fig. 20a). This suggests that this function reflects size due to allometric relations of certain variables (Table 11). All the fossil hominins group with humans and Pan. Anteroposterior thickness of the anconeal process (UAP) partially reflects trochlear notch orientation; a thicker process results in a more anteriorly oriented notch. Since notch orientation is more anterior in the smaller species, it is not surprising that UAP has the largest negative correlation with the first discriminant function, with Pan having an anteroposteriorly deeper anconeal process than Gorilla or Pongo. The results also suggest that this function reflects absolute size more closely than it does locomotor specialization. Gorilla and Pongo are each also characterized by a wider breadth across the trochlear notch than are the other species, measured either including the radial notch (UTR) or immediately posterior to the radial notch (UNR). The second discriminant function, accounting for 27% of the variance, discriminates H. sapiens and most fossils from all other large hominoids (Fig. 20a). Factor loading of this discriminant function reveals that H. sapiens and the fossils (except OH 36), when compared to other taxa, have less buttressing posterior to the trochlear notch (UPC), a narrower trochlear notch (measured across its middle) (UTW), a large mediolateral diaphyseal diameter immediately distal to the radial notch (USM), and a wide trochlear notch measured proximally (UNS). The third function (17% of variance) discriminates Pongo from Gorilla (Fig. 20b). Correlation factors suggest that Pongo has a narrower middle of its trochlear notch (UTW), a mediolaterally narrow shaft just distal to the radial notch (USM), and a proximodistally long notch measured on the medial side (ULM). The fourth function (2.5% of variance) discriminates the fossil sample from most other large hominoids. Correlation factors show that the fossils are characterized by mediolaterally wide olecranon processes (UOM) and short anteroposterior coronoid processes (UCP). In summary, Pan and Gorilla differ in proximal ulnar morphology, suggesting that knuckle-walking behavior does not necessarily leave a distinctive imprint on this part of the forelimb. Some of the

M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 623 observed differences between Pan and Gorilla may be allometric rather than functional, but this warrants further investigation. Posterior probabilities for the fossils (Table 10) show that A.L. 438-1a, A.L. 288-1t, L40-19, and StW 113 are more similar to each other than to any extant taxon. If that possibility is removed, however, A.L. 438-1a, A.L. 288-1t, and L40-19 are more similar to H. sapiens than to any other extant large hominoid, while StW 113 is most similar to Pan. As observed previously by Aiello et al. (1999), OH 36 is more similar to Pan than to the other fossils or to H. sapiens. Summary. Only the analysis of size-controlled data discriminates H. sapiens from all other extant hominoids with only one function. The wider shaft distal to the radial notch in H. sapiens reflects the distinctively large supinator crest. This proximal shaft robusticity is not a consequence of distal trochlear notch size, as H. sapiens has narrower articular facets than other hominoids (i.e., UTW, UTB). In humans, reduced buttressing posterior to the trochlear notch (UPC) and a mediolaterally wide proximal portion of the trochlear notch (UNS) reflect olecranon form. The insertion of the triceps brachii muscle posterior to the trochlear notch (see olecranon section above) results in a thickening posterior to the notch. On the other hand, insertion of the triceps brachii more proximally, as is the tendency in H. sapiens, instead of immediately more posterior to the notch, is more likely to result in a mediolaterally wide proximal portion of the trochlear notch. In the great apes, particularly Gorilla and Pongo, the posterior anchoring of the triceps frees the proximal trochlear notch of architectural constraints, which allows for a narrower notch proximally. Similarly, the proximal insertion of the triceps, as observed in H. sapiens, frees the area posterior to the notch, resulting in a less buttressed appearance. In both discriminant analyses, the large A.L. 438-1a and L40-19 ulnae are more similar to modern human ulnae than to those of any other extant large hominoids. The small A.L. 288-1 ulna resembles those of Pan and Homo in the analysis of raw data, but when size is controlled, it is similar only to the ulnae of H. sapiens, suggesting that its similarity to Pan in the raw data analysis may be related to the small body size of this individual. Both OH 36 and StW 113 clearly resemble Pan. The morphological similarity of OH 36 to Pan ulnae has been observed previously by Aiello et al. (1999), whose analysis included diaphyseal and distal ulnar measurements as well. These authors interpret this similarity as evidence of great upper limb muscularity in this fossil. However, A.L. 438-1 is as robust but does not resemble Pan in its proximal ulnar morphology, which indicates that there is functional and/or taxonomic significance to the chimpanzee-like anatomy of OH 36 compared to the other fossil hominins. Nonetheless, with the exception of OH 36, the fossils are morphologically homogeneous despite the fact that they span a period of nearly a million years. Radius In its large size, the A.L. 438-1l/p radius is similar to A.L. 333w-33. Maximum anteroposterior and minimum mediolateral diameters of the shaft distal to the tuberosity are 15.9 and 13.7 for A.L. 438-1l/p, and 16.4 and 13.9 for A.L. 333w-33 (compared with 11.0 and 10.4 for A.L. 288-1p). Although morphologically similar to A.L. 333w- 33, A.L. 438-1l/p differs slightly in having a more well-defined tuberosity with a medial portion that projects further than does the lateral portion. Because bony relief of muscle insertions increase with chronological age in humans (Robb, 1998; Wilczak, 1998; Weiss, 2003), the difference between A.L. 438-1 and A.L. 333w-33 may be attributable to the fully adult status of the first and the subadult status of the latter. In A.L. 438-1l/p, as in other radii attributed to A. afarensis (Johanson et al., 1982; Lovejoy et al., 1982), A. anamensis (Ward et al., 2001), and Neandertals (Trinkaus and Churchill, 1988), the tuberosity is more medially directed than in modern humans. Trinkaus and Churchill (1988) propose that Neandertals emphasized the maintenance of strength over a wide range of pronation/ supination positions, and possibly habitual loading with the elbow in a more flexed position. It is

624 M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 also likely that a medially oriented tuberosity is simply related to overall muscularity of the upper limb, as the radial tuberosity is also medially oriented in great apes. Metacarpals The A.L. 438-1 metacarpals are similar to others attributed to A. afarensis (MC2: A.L. 333-15, A.L. 333-48, and A.L. 333w-23; MC3: A.L. 333-16, A.L. 333-65, A.L. 333-144 and A.L. 333w- 6; see Bush et al., 1982). The A.L. 438-1 specimens are among the largest A. afarensis metacarpals known, measuring 66.4 and 66.5 in maximum proximodistal length for the right and left MC2s, respectively, and 64.9 for the MC3. In comparison, A.L. 333-48 (MC2) measures 61.5 and A.L. 333-16 (MC3) measured 59.6 in length. The A.L. 438-1 metacarpals have correspondingly rugose muscle attachment markings. For example, the tubercles for the flexor carpi radialis longus and for the oblique head of adductor pollicis on the MC2 are more clearly demarcated from each other than in any other Hadar MC2. Compared to great ape metacarpals, the A.L. 438-1 specimens are short and nearly straight (Fig. 7), as in humans. They resemble human and gorilla metacarpals in their short length, with shafts that are thicker for their length than those of chimpanzees, bonobos, or orangutans (Fig. 21a, b), which results from an expanded dorsopalmar diameter (Fig. 21c, d). We were unable to quantify more precise measures of diaphyseal cross-sectional geometry for A.L. 438-1 because these specimens are unbroken, and radiographic study of these bones has not yet been undertaken. Measuring actual cross-sectional bending and torsional rigidities on other A. afarensis metacarpals that had transversely broken during fossilization, Coffing (1998) found that A. afarensis metacarpals had roughly 23% stronger diaphyses than did modern human homologues. Our diaphyseal breadth data and calculated section areas do not appear to support this conclusion, but instead place A. afarensis well within the human distribution, which overlaps that of gorillas for both metacarpals (Fig. 21). This apparent discrepancy could be due to the fact that we were unable to include measures of internal morphology, which may vary significantly among extant and fossil taxa. Even given Coffing s results, because other pre-modern hominins tend to have higher skeletal robusticity than do modern humans in both upper and lower limbs (e.g., Musgrave, 1971, 1973; Stoner and Trinkaus, 1981; Trinkaus, 1983, 1989, 1997; Ruff et al., 1993; Churchill, 1997, 1998; Niewoehner et al., 1997), the morphology of A. afarensis seems to fit the general pre-modern tendency toward hominin postcranial robusticity. Certainly, the robust muscle attachment morphology of A. afarensis, particularly in the larger individuals, supports this hypothesis. The detailed diaphyseal morphology of the A.L. 438-1 metacarpals is similar to that of humans, with evidence of muscle attachment on the dorsal and palmar aspect of the shaft. The Hadar metacarpals have crests for the dorsal interosseous muscle that are closely approximated, but remain separate throughout their lengths. In African apes, particularly chimpanzees, these ridges almost always meet for some portion of their length distally to form distinct midline crests along the shaft, something seen occasionally in muscularly robust humans. In contrast to other Hadar metacarpals, the A.L. 438-1 MC3 diaphysis shows a palmar ridge for attachment of the transverse head of adductor pollicis (Fig. 7), as seen in humans but not in apes, suggesting greater development of the thumb musculature in that specimen compared to apes. The palmar aspect of the shaft immediately proximal to the articular surface of the head is hollowed out in the Hadar specimens (Fig. 7), as in apes, rather than being flat as in modern humans. The functional significance of this morphology is unclear. The MC2 base in A.L. 438-1 is narrower than is typical for humans and gorillas, but is somewhat wider than typical for the other apes, falling in the area of overlap between humans and chimpanzees (Fig. 22a). This variation reflects, at least in part, a laterally expanded carpometacarpal joint in humans, which in turn may be related to development of the first ray (Alba et al., 2003). The fact that A. afarensis is not fully human-like in this regard is likely related to an intermediate development of grip ability and hand function (Marzke, 1997). Metacarpal 3 shows much more overlap

M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 625 5.0 A Metacarpal diaphyseal dimensions B Ln (MC2 midshaft area, mm 2 ) 4.0 AL 333-48 Ln (MC3 midshaft area, mm 2 ) 5.2 4.2 3.0 4.0 4.4 4.8 Ln (maximum length of MC2, mm) AL 333-16 3.2 4.0 4.4 4.8 Ln (maximum length of MC3, mm) C 2.8 D Ln (DP depth of MC2 midshaft, mm) 2.5 2.1 AL 333-48 1.7 4.0 4.4 4.8 Ln (maximum length of MC2, mm) Ln (DP depth of MC3 midshaft, mm) 2.3 AL 333-16 1.8 4.0 4.4 4.8 Ln (maximum length of MC3, mm) Fig. 21. Bivariate plots of calculated mid-diaphyseal cross-sectional area (p/4 ) anteroposterior diameter ) mediolateral diameter) to maximum bone length for (A) MC3 and (B) MC2, and of dorsopalmar depth to length for (C) MC3 and (D) MC2. Legend as in Fig. 13. Australopithecus afarensis is similar to humans. among taxa in base dimensions (Fig. 22b), but, as a group, A. afarensis is more similar to apes. In its metacarpal base morphology, A.L. 438-1 is a primarily, but not exclusively, human-like. The relative size of the trapezium facet on MC2 does not differ among African hominoids, but variation in the width of the MC2 base is due to the different orientation of this facet among African

626 M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 Metacarpal base shape A 3.0 B Ln (ML width of MC2 base, mm) 3.0 2.6 AL 333-48 Ln (ML width of MC3 base, mm) 2.6 AL 333-153 AL 333w-6 AL 333-65 2.2 AL 333-15 2.3 2.7 3.1 Ln (DP depth of MC2 base, mm) AL 333-16 2.2 2.2 2.6 3.0 Ln (DP depth of MC3 base, mm) Fig. 22. Bivariate plots of mediolateral width to dorsopalmar depth of the base of (A) MC2 and (B) MC3. Legend as in Fig. 13. In general, Australopithecus afarensis has metacarpal bases that are wider than those of great apes, but narrower than those of humans. hominoids. In all taxa, the trapezium facet on MC2 faces proximolaterally and slightly palmarly, but in gorillas and humans, the trapezium facet faces about 40 palmarly, while in chimpanzees it faces almost directly laterally (Fig. 23a). In this respect, chimpanzees are autapomorphic in relation to gorillas, humans, and A. afarensis. Other variation is related to the morphology of the trapezoid facets and the palmar aspect of the trapezoid, which is narrower and more wedgeshaped (in proximal or distal view) in chimpanzees and bonobos as compared to other large hominoid taxa (Fig. 24). The palmarly broader trapezoid of gorillas compared to chimpanzees has been related by Sarmiento (1994) to the greater amount of weight borne on digit 2 by gorillas during knucklewalking (Tuttle, 1967, 1969; Inouye, 1992, 1994). Humans are palmarly broadest of all, which is related to the greater development of the thumb and supporting carpus, and particularly to a more palmarly oriented first carpometacarpal (CMC) joint (Fig. 24). The broader trapezoid of humans is correlated with a palmar re-orientation of the trapezium facet. Gorillas also have a more palmarlyoriented first CMC joint than do chimpanzees (Sarmiento, 1994), although not as much as in humans, implying that other factors, in particular trapezium morphology, affect first CMC joint orientation. In A. afarensis the orientation of the trapezium facet resembles that of humans and gorillas. Given the observed correlation between trapezium shape and first CMC joint in living hominoids, the morphology of A. afarensis suggests that it had a more palmarly-oriented first CMC joint than chimpanzees. Furthermore, since australopiths were not knuckle-walkers, this morphology can be related to development of the first ray and thumb rather than to weight bearing during locomotion. An alternative explanation would be that it is a plesiomorphic character retained from a knuckle-walking heritage (Richmond and Strait, 2000; Richmond et al., 2001), in which case chimpanzees would then be autapomorphic. The trapezium facet also varies in the degree to which it angles proximally. This facet faces more laterally than proximally in both chimpanzees and gorillas (Fig. 23b), whereas in humans it faces much more proximally (Marzke, 1983). This proximal orientation contributes to variation in

M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 627 Palmar orientation (degrees) 45 30 15 0 A H. sapiens P. troglodytes G. gorilla Fossils Trapezium facet orientation B AL 333-48 AL 333-15 Proximal orientation (degrees) 40 20 0 H. sapiens P. troglodytes G. gorilla AL 333-48 AL 333-15 Fossils Fig. 23. Boxplots of orientation of the trapezium facet relative to a sagittal plane in (A) proximal view (PTA), and (B) dorsal or palmar view (MTA). (A) Chimpanzees differ from humans and gorillas (T-test, p! 0.05) in having a more laterally-facing trapezium facet in proximal view, and A. afarensis resembles humans and gorillas with a more palmarly-facing facet. (B) Humans have trapezium facets that are oriented more proximally than those of chimpanzees or gorillas (T-test, p! 0.05). Trapezium facet orientation of A.L. 438-1 is intermediate between humans and African apes, but overlaps only with apes. the width of the metacarpal base among extant taxa (Fig. 22a), but in A. afarensis there is a unique combination of morphologies. Although the fossils fall in the lower end of the human distribution and upper end of the ape distribution for width of the MC2 base, their trapezium facets face almost entirely laterally, only about 19 proximally, and thus are similar to gorillas in this respect. The degree of palmar and proximal orientation of the trapezium facet reflects variation in thumb development. The intermediate morphology of the fossils supports previous interpretations of an intermediate thumb size in A. afarensis (Bush, 1980; Marzke, 1983; Coffing, 1998; but see Alba et al., 2003). Morphology of the proximal MC2 also suggests a thumb that in size and morphology matches neither that of humans nor that of African apes. The morphology of the contact with the capitate is also unique in the MC2s of Australopithecus afarensis and A. africanus. As noted previously, humans can rotate the MC2 up to 20 (Lewis, 1973, 1977; Dubosset, 1981; Marzke, 1983, 1997; Marzke and Shackley, 1986), enabling precision grips and pad-to-pad and three-jaw-chuck grips (Long et al., 1970; Marzke, 1983, 1997). This is due in part to the continuous MC2-capitate facet with no interposed carpometacarpal ligament, and to the oblique orientation of this facet. In humans, the facet is convex dorsopalmarly and slightly convex mediolaterally. Great apes, in contrast, have a flat, straight MC2-capitate joint oriented in a sagittal plane, and a substantial carpometacarpal ligament that clearly divides the MC2-MC3 joint into an anterior and a posterior portion. This ligament restricts motion between the MC2 and the capitate. Most Australopithecus capitates (A.L. 333-50, A.L. 288-1, KNM-WT 22944, and TM 1526) are intermediate in orientation between the African ape and human conditions, are concave in distal and medial contours, and lack pits for the carpometacarpal ligament (Broom and Schepers, 1946; Le Gros Clark, 1947; Lewis, 1973; Marzke, 1983, 1997; McHenry, 1983; Leakey and Ward, 1997; Leakey et al., 1998; Ward et al., 1999, 2001). The only exception to this morphology among the known fossil hominins is KNM-KP 31724, a capitate attributed to A. anamensis (Leakey and Ward, 1997; Leakey et al., 1998; Ward et al., 1999, 2001), which is ape-like, with a laterally facing MC2 facet and evidence of the carpometacarpal ligament. The A. afarensis MC2 specimens (Bush et al., 1982),

628 M.S.M. Drapeau et al. / Journal of Human Evolution 48 (2005) 593e642 Fig. 24. Distal carpal rows in distal view (palmar is down and medial is right), after Sarmiento (1994). Arrows reflect trapezium facet orientation. The palmar aspect of the trapezoid is broadest in humans and in gorillas, and narrow in chimpanzees. This appears to be related to the greater development of the thumb and associated carpus and more coronally oriented first carpometacarpal joint in humans, and to weight-bearing through digit 2 during knuckle-walking in gorillas (see also Fig. 23a). including those of A.L. 438-1, lack evidence of a carpometacarpal ligament because of the dorsopalmar continuity of the articular facet with the capitate. They evince a proximal rotation of the articular facet for the capitate reflecting a capitate morphology intermediate between that of humans and extant apes. This suggests a more human-like capability for MC2 rotation in most Australopithecus hands (but see Ricklan, 1988), facilitating more diverse grips than available to apes. The morphology of the MC3 base is distinctive in A.L. 438-1. As in all known Australopithecus afarensis fossil MC3s, A.L. 438-1 lacks a styloid process. A styloid process is found in most, but not all, humans, and is generally lacking in African apes (Marzke and Shackley, 1986; Marzke and Marzke, 1987; Ricklan, 1988; Ward et al., 1999). The functional significance of the styloid process is unclear, although it has been linked by Marzke and collaborators (Marzke, 1983; Marzke and Shackley, 1986; Marzke and Marzke, 1987; see also Ricklan, 1987) to stabilizing the joint against mechanical loads generated during power grips when making and using tools. If this is true, its absence in Australopithecus may reflect the lack of later hominin-like tool use or tool making. Most A. afarensis MC3 specimens (A.L. 333w-6, A.L. 333-144, A.L. 333-153, A.L. 438-1d) have a continuous dorsopalmar articular facet with MC2, indicating the absence of a carpometacarpal ligament in those specimens. This is consistent with the morphology observed on the MC2 and supports the hypothesis that A. afarensis had some MC2 pronation capabilities (Marzke, 1983, 1997). The A.L. 438-1 capitate facet on the MC3 is human-like. African apes have a broad, concavoconvex proximal articular surface for the capitate and corresponding morphology of the distal capitate (Tuttle, 1969; McHenry, 1983). This provides increased joint surface area and would limit eccentric motion during knuckle-walking. Orangutans and humans lack this morphology. The capitate facets of some early hominin specimens, such as A.L. 438-1, A.L. 333-144, and A.L 333-153, are smoothly convex dorsopalmarly (Ward et al., 1999), while others, such as A.L. 333-16, A.L. 333-65, and A.L. 333w-6, have a more irregular surface. The shape of the capitate facet in A.L. 438-1 and A.L. 333w-6 is narrower palmarly than typical for humans and great apes, while in the other Hadar fossils, it is broader than in humans but narrower than in apes. The A.L. 438-1 metacarpal heads have large, prominent collateral ligament pits and adjacent tubercles, and the articular facets have two mild parasagittal grooves (Fig. 7), morphology found only in humans among extant hominoids (Lewis, 1977, 1989). Relative to their dorsopalmar depth,