The Skull of Australopithecus Afarensis

The Skull of Australopithecus Afarensis

The Skull of Australopithecus Afarensis

The Skull of Australopithecus Afarensis

Synopsis

The book is the most in-depth account of the fossil skull anatomy and evolutionary significance of the 3.6-3.0 million year old early human species Australopithecus afarensis. Knowledge of this species is pivotal to understanding early human evolution, because 1) the sample of fossil remains of A. afarensis is among the most extensive for any early human species, and the majority of remains are of taxonomically inormative skulls and teeth; 2) the wealth of material makesA. afarensis an indispensible point of reference for the interpretation of other fossil discoveries; 3) the species occupies a time period that is the focus of current research to determine when, where, and why the human lineage first diversified into separate contemporaneous lines of descent. Uponpublication of this book, this species will be among the most thoroughly documented extinct ancestors of humankind. The main focus of the book - its organizing principle - is the first complete skull of A. afarensis (specimen number A.L. 444-2) at the Hadar site, Ethiopia, the home of the remarkably complete 3.18 million year old skeleton known as "Lucy," found at Hadar by third author D. Johanson in 1974. Lucy and other fossils from Hadar, together with those from the site of Laetoli inTanzania, were controversially attributed to the then brand new species A. afarensis by Johanson, T. White and Y. Coppens in 1978. However, a complete skull, which would have quickly resolved much of the early debate over the species, proved elusive until second author Y. Rak's discovery of the 444 skullin 1992. The book details the comparative anatomy of the new skull (and the cast of its brain, analyzed by R. Holloway and M. Huan) , as well as of other skull and dental finds recovered during the latest, ongoing field work at Hadar, and analyzes the evolutionary significance of A. afarensis in the context of other critically important discoveries of earliest humans made in recent years. In essence, it summarizes the state of knowledge about one of the central subjects of currentpaleoanthropological investigation.

Excerpt

The 1970s collection of hominin cranial remains from Hadar is notoriously weak in its representation of the frontal bone. Besides the complete but distorted frontal of the A.L. 333-105 juvenile (Kimbel et al., 1982), only two very incomplete adult specimens provided glimpses of frontal morphology: A.L. 288-1 (Johanson et al., 1982b) and A.L. 333-125 (Asfaw, 1987). With the recovery of the almost complete frontal bone of A.L. 444-2, we are able to fill one of the last remaining gaps in our knowledge of the Hadar hominin adult skull. Another frontal specimen, A.L. 438-1b, contributes important information on the glabellar and supraglabellar regions, which are missing or poorly preserved in A.L. 444-2.

Superior View

The A.L. 444-2 frontal bone features prominent, laterally projecting supraorbital bars, strongly convergent temporal lines, and a transversely broad squama with only moderate postorbital constriction (Figures 3.31 and 5.1). the minimum distance between the temporal lines (30 mm) in the plane of the postorbital constriction is much smaller than the postorbital constriction itself (77 mm), creating on each side an extensive, almost horizontally inclined facies temporalis that, in coronal section, slopes gradually from the inferior temporal lines to the medial wall of the temporal fossa. in between the temporal lines, the supraglabellar region bears a mild hollow that grades smoothly onto the superior surface of the supraorbital bars. Neither a supratoral sulcus nor a trigonum frontale is present.

The supraorbital bars are wide anteroposteriorly, measuring 16 mm at the right lateral break, about 42 mm lateral to the midline. the preserved portions of the anterior supraorbital margins are aligned coronally, forming right angles with the midsagittal line. At the lateral break on each side, the margin actually occupies a slightly more anterior plane than the middle part of the margin, suggesting an anteriorly prominent superolateral corner of the orbit. At the medial break through the left supraorbital, about 22 mm lateral to the midline, the anterior margin begins to swing out toward glabella (this area is damaged on the right side). the extent of anterior glabellar protrusion is suggested by the preserved supraglabellar plate, whose superior surface projects in the midline about 5 mm beyond the anterior supraorbital margins. As this supraglabellar surface shows no evidence of dropping over on to the anterior face of the glabellar mass, the actual position of glabella was at least a few mm anterior to this point.

Strong temporal crests parallel the anterior supraorbital margins, demarcating and truncating the supraorbital bars posteriorly. On the right, the temporal crest stands about 1 mm above the superior surface of the bar, rather than forming a sharp-edged, posteriorly extended shelf that overhangs the anterior wall of the temporal fossa (Figure 5.1). Indeed, on the right side, the anterior wall of the fossa actually encroaches on the upper surface of the supraorbital bar just at the point where the temporal crest begins to curve on to the squama.

The temporal crests travel medially to a point 26 mm lateral to the midline (slightly medial to midorbit), then arc broadly onto the squama and steadily converge posteriorly . . .

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