Foramen Magnum Position Variation in Pan troglodytes, Plio-Pleistocene Hominids, and Recent Homo sapiens: Implications for Recognizing the Earliest Hominids James C.M. Ahern* Department of Anthropology, University of Wyoming, Laramie, Wyoming 82071 KEY WORDS hominid origins; cranial base; Sahelanthropus; Ardipithecus; craniometrics ABSTRACT The anteroposterior position of the fora- men magnum distinguishes living Homo sapiens from apes, and has been used as evidence for the hominid status of numerable fossils in the history of human paleontology. Dur- ing the past decade, foramen magnum position has been cited as evidence of the hominid status of Ardipithecus and Sahelanthropus. Specifically, the basion of Ardpithecus is reported to be inline with the bicarotid chord, while the basion of Sahelanthropus is reported to both touch the bipo- rion chord and intersect the bicarotid chord. In order to assess the effectiveness of anteroposterior foramen magnum position in distinguishing hominids from nonhominid apes, this study examined whether or not the positions of biporion and bicarotid relative to basion sufficiently distinguished Pan troglodytes from recent Homo sapiens and Plio-Pleisto- cene hominids. The distances from basion to the biporion chord (BSBIP) and from basion to the bicarotid chord (BSBIC) were measured on samples of chimpanzee (n = 69) and recent human (n = 42) crania and a sample of Plio- Pleistocene hominid fossils (n = 8). The data were used to test the hypothesis that BSBIP and BSBIC measurements do not sufficiently distinguish P. troglodytes from hominids. While basion to biporion (BSBIP) does not effectively distin- guish P. troglodytes from Plio-Pleistocene hominids and hu- mans when used univariately, basion to bicarotid (BSBIC), when used univariately or bivariately with BSBIP, can be used to test whether or not an unknown specimen is a hominid. These results are used to evaluate the hominid status of Ardipithecus and Sahelanthropus. Am J Phys An- thropol 127:267–276, 2005. © 2005 Wiley-Liss, Inc. The position of the foramen magnum clearly dif- fers between extant humans and apes (Dean and Wood 1981, 1982; Aiello and Dean, 1990; Luboga and Wood, 1990; Strait et al., 1997; Schaefer, 1999). Humans exhibit anterior foramina magna, while apes exhibit more posterior ones. The gross differ- ence in the position of the foramen magnum likely reflects head position and locomotion posture (Dart, 1925; Broom, 1938; Le Gros Clark, 1954), although the exact nature of this relationship has not been established (Ashton and Zuckerman, 1956; Moore et al., 1973; Masters et al., 1991). The centrally located foramen magnum of humans positions the head so that it is perpendicular with the upright, bipedal body, while the posteriorly located foramen magnum of apes positions the head so that it is inline with the quadrupedal body. Rather than examining the gross difference in foramen magnum position between ex- tant humans and apes, this paper is concerned with how useful foramen magnum position is in distin- guishing early bipedal hominids from nonhominid apes. This is an especially important issue regarding discoveries of the earliest purported hominids. Dart (1925) cited an inferred anterior foramen magnum position as evidence of the hominid status for the earliest known hominid at the time, the Taung fossil. As the hominid fossil record has been pushed back to the Mio-Pliocene, foramen magnum position has continued to be used to support the hominid status of some of the earliest purported hominids, such as Sahelanthropus (Brunet et al., 2002) and Ardipithecus (White et al., 1994). According to Bru- net et al. (2002, p. 149), the Sahelanthropus fossil TM 266-01-060-1 exhibits a biporion line that touches basion and a basion that “is intersected by the bicarotid chord; the basion is posterior in large apes and anterior in some of the later hominids.” Although Brunet et al. (2002) merely described the anatomy and did not explicitly use basion’s relative position as an argument for the hominid status of Sahelanthropus, the position of the foramen magnum has figured in the debate surrounding Sa- helanthropus (e.g., Brunet, 2002; Wolpoff et al., 2002). Between 1.2–2.5 million years younger than Grant sponsor: University of Wyoming Office of Research. *Correspondence to: James C.M. Ahern, Department of Anthropol- ogy, Dept. 3431, 1000 E. University Ave., University of Wyoming, Laramie, WY 82071-3431. E-mail: jahern@uwyo.edu Received 21 June 2003; accepted 9 April 2004. DOI 10.1002/ajpa.20082 Published online 19 November 2004 in Wiley InterScience (www. interscience.wiley.com). AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 127:267–276 (2005) © 2005 WILEY-LISS, INC.