William J. Sanders Comparative morphometric study of the Museum of Paleontology, australopithecine vertebral series The University of Michigan, Stw-H8/H41 1109 Geddes Avenue, Ann Arbor, MI 48109. Email: [email protected] Lower spinal structure correlates well with positional behavior among mammals. Nonetheless, the functional morphology of the axial post- Received 20 April 1997 crania of australopithecines has received less attention than their Revision received appendicular skeletons. This paper presents a detailed description 15 September 1997 and and comparative morphometric analysis of the australopithecine accepted 22 September thoracolumbar vertebral series Stw-H8/H41, and examines spinal 1997 mechanics in early hominids. Stw-H8/H41 is an important specimen, as the australopithecine vertebral sample is small, and vertebral series Keywords: australopithecine, are more useful than isolated elements for the interpretation of Stw-H8/H41, vertebrae, spinal function. Results of the study support the interpretation that spinal function, positional australopithecine species are highly sexually dimorphic. The study behavior. also reveals a considerable amount of morphometric variation other than size among australopithecine vertebrae, though the sample is too small and incomplete to ascertain whether this indicates signifi- cant interspecific differences in spinal function. Most importantly, structural and metric observations confirm that the morphology of the lower spine in australopithecines has no modern analogue in its entirety. Aspects of zygapophyseal structure, numerical composition of the lumbar region, and centrum wedging suggest that the australopithecine vertebral column was adapted to human-like intrinsic lumbar lordosis and stable balance of the trunk over the pelvis in sustained bipedal locomotion. However, relative centrum size in australopithecines indicates that either they had a different mechanism for channeling vertical forces through the vertebral column than humans, or differed behaviorally from humans in ways that produced smaller increments of compression across their centra. These findings have important implications for hypotheses of australopithecine positional behavior, and demonstrate that larger samples and more complete vertebral series are needed to improve our understanding of australopithecine spinal function. ? 1998 Academic Press Limited Journal of Human Evolution (1998) 34, 249–302 Introduction trial bipeds mechanically and kinematically similar to modern humans (Lovejoy et al., Bipedal adaptations in the postcranial 1973; Lovejoy, 1974, 1988; Tague & skeleton of australopithecines date to at least Lovejoy, 1986; Latimer et al., 1987; Latimer the early Pliocene (Leakey et al., 1995). & Lovejoy, 1989, 1990a,b; Latimer, 1991). However, there is no consensus about the Proponents of this view explain variations in effectiveness or relative importance of postcranial anatomy between australo- bipedal stance and progression in their pithecines and humans as a consequence positional repertoires (see McHenry, 1982; of differences in obstetric mechanisms, or Stern & Susman, 1983; Susman et al., as (in australopithecines) functionally- 1985; Berge & Kazmierczak, 1986; Berge, unimportant retentions of ancestral traits 1994). In one view, it is hypothesized (Tague & Lovejoy, 1986; Lovejoy, 1988; that australopithecines were obligate terres- Latimer & Lovejoy, 1989; Tague, 1991). 0047–2484/98/030249+54 $25.00/0/hu970193 ? 1998 Academic Press Limited 250 . Alternatively, it is suggested that australo- biomechanical aspects of the australo- pithecines were either more quadrupedal pithecine vertebral column, complementing and arboreal or employed a different, less prior investigations by Robinson (1972), proficient form of bipedalism than humans, Rose (1975), and Velte (1984, 1987). These or both (Oxnard, 1975; Zihlman, 1978; studies demonstrate the significance of Jungers, 1982; Stern & Susman, 1983; vertebrae for appraising bipedal abilities in Susman et al., 1984; Susman et al., 1985; australopithecines. McHenry, 1986, 1991a; Tompkins, 1986; This paper presents a comparative Rak, 1991; Schmid, 1991; Heinrich et al., morphological and metrical analysis of the 1993; Berge, 1994; Duncan et al., 1994; thoracolumbar vertebral series Stw-H8/H41 Hunt, 1994; Spoor et al., 1994; Abitbol, (Figure 1), assigned to Australopithecus 1995; Clarke & Tobias, 1995; Berger & africanus (Tobias, 1973, 1978, 1980; Tobias, 1996; MacLatchy, 1996). Howell, 1978). Stw-H8 is comprised of four Undoubtedly, the unique morphology of articulated lumbar vertebrae from the their postcrania, which exhibits mosaics of hominid-bearing locality of Sterkfontein, primitive features common to apes, derived South Africa (Tobias, 1973). Oakley et al. traits shared with humans, and autapo- (1977, p. 131) noted that while these morphies (McHenry, 1991a, 1994; Susman vertebrae were ‘‘removed [by Tobias in & Stern, 1991; McHenry & Berger, 1996), 1969] from R. M. Cooper’s display has complicated attempts to reconstruct cabinet,’’ they had been ‘‘excavated australopithecine positional behavior. It is many years earlier.’’ Although their exact evident that more comprehensive surveys provenience is uncertain, adherent matrix of information about individual skeletal shows that the Stw-H8 vertebrae derive elements and joint complexes are required from Member 4 (Oakley et al., 1977), which to better clarify the locomotor and postural is dated by biochronological correlation and abilities of australopithecines (see Stern & geomorphological and geophysical methods Susman, 1991). to ca. 2·6–2·8 Myr (Delson, 1984, 1988; It is well established that among modern Vrba, 1985; Berger & Tobias, 1996). The mammals, structural variations in the spine Stw-H41 vertebrae (two articulated lower correspond closely with differences in thoracics) were found by Tobias in 1975 in columnar function and positional behavior Dump 18 at Sterkfontein (Oakley et al., (Hatt, 1932; Slijper, 1946; Hildebrand, 1977), and are thought to form a continuous 1959; Howell, 1965; Gambaryan, 1974; series (T11-L4) with Stw-H8, based on Curtis, 1995; Shapiro, 1993a, 1995). None- ‘‘concordance of size, shape, state of theless, disproportionately less attention has preservation, colouration and matching been given to the functional morphology areas of damage’’ (Tobias, 1978, p. 385). It of the axial postcranial skeleton of australo- is likely that Stw-H8/H41 was placed in pithecines than to their appendicular Australopithecus africanus (Tobias, 1973, elements, despite the evident utility of 1978, 1980; Howell, 1978) because it is the vertebral anatomy for evaluating posture only hominid species usually recognized in and locomotion in fossil taxa (e.g., Zhou Sterkfontein Mbr. 4 (Partridge, 1982; Day, et al., 1992; Sanders & Bodenbender, 1994; 1986; Berger & Tobias, 1996; but see Giffin, 1995). A number of recent studies Clarke, 1994). (Sanders, 1990, 1994, 1995, 1996; Schmid, Besides Stw-H8/H41, the only other 1991; Shapiro, 1991, 1993b; Ward & australopithecine vertebral series are from Latimer, 1991) have sought to address this A. africanus partial skeletons Sts-14 and disparity by focusing on architectural and Stw-431, A. afarensis individual A.L. 288-1 -8/41 251 Figure 1. Thoracolumbar vertebrae of Australopithecus africanus, specimens Stw-H41 (T11–T12) and Stw-H8 (L1–L4). Abbreviations: cvf=costovertebral facet; o=osteophyte. (a) Right lateral view. Dorsal is to the top of the page and cranial is to the right. Arrows point to costovertebral facets in T11–T12, and an osteophyte on the ventral face of L2. (b) Ventral view. Cranial is to the right. (c) Dorsal view. Cranial is to the right. (d) Outlines of cranial centrum surfaces. Reconstructed portions are indicated by dotted lines. (‘‘Lucy’’), and Paranthropus robustus indi- thoracics and two lumbars, along with a vidual SKW 14002. Sts-14 preserves sacrum (Johanson et al., 1982). A small thoracics T4–T12, lumbars L1–L6, and number of isolated vertebrae are also known parts of sacrals S1–S2 (Robinson, 1972). for A. afarensis (Lovejoy et al., 1982) and A. Vertebrae T9–T12, L1–L6, and a sacrum africanus (Robinson, 1972). The vertebral are present in the Stw-431 skeleton, which is series of SKW 14002 is also undescribed, undescribed (McHenry, 1994; McHenry and represents either L1–L4 or L2–L5 from & Berger, 1996). Vertebral fragments from an immature individual (personal communi- A.L. 288-1 represent portions of seven cation, H. McHenry; Susman & Stern, 252 . 1991). These vertebrae supplement an Paranthropus robustus specimens SK 3981a otherwise meager vertebral sample for (T12), SK 3981b (L6), and SK 853 Paranthropus robustus (Brain, 1970; (middle? lumbar); and Homo erectus ver- Robinson, 1970, 1972; Rose, 1975) and tebrae KNM-WT 15000Y, AR+BA, P. boisei (Grausz et al., 1988). AA+AV, Z+BW, AB, BM, and AC (T11, There are no detailed published accounts L1–L6). Observations made on original of Stw-H8/H41, and a previous study of specimens (Sts-14; Stw H8/H41) and from these vertebrae by Ankel & Tobias (Oakley photographs and good quality casts (Sts 14; et al., 1977) was apparently abandoned. It Stw-H8/H41; A.L. 288-1; A.L. 333-73; is an important specimen because few KNM-WT 15000) were supplemented by, australopithecine vertebrae have been col- and checked for accuracy against, notes and lected, and serial patterns in associated measurements taken by other researchers on vertebrae are more informative than isolated original specimens of all individuals in