Metopic suture of Taung ( Australopithecus africanus ) and its implications for hominin brain evolution

Abstract

The type specimen for Australopithecus africanus (Taung) includes a natural endocast that reproduces most of the external morphology of the right cerebral hemisphere and a fragment of fossilized face that articulates with the endocast. Despite the fact that Taung died between 3 and 4 y of age, the endocast reproduces a small triangular-shaped remnant of the anterior fontanelle, from which a clear metopic suture (MS) courses rostrally along the midline [Hrdlicka A (1925) Am J Phys Anthropol 8:379-392]. Here we describe and interpret this feature of Taung in light of comparative fossil and actualistic data on the timing of MS closure. In great apes, the MS normally fuses shortly after birth, such that unfused MS similar to Taung's are rare. In humans, however, MS fuses well after birth, and partially or unfused MS are frequent. In gracile fossil adult hominins that lived between ∼3.0 and 1.5 million y ago, MS are also relatively frequent, indicating that the modern human-like pattern of late MS fusion may have become adaptive during early hominin evolution. Selective pressures favoring delayed fusion might have resulted from three aspects of perinatal ontogeny: (i) the difficulty of giving birth to large-headed neonates through birth canals that were reconfigured for bipedalism (the "obstetric dilemma"), (ii) high early postnatal brain growth rates, and (iii) reorganization and expansion of the frontal neocortex. Overall, our data indicate that hominin brain evolution occurred within a complex network of fetopelvic constraints, which required modification of frontal neurocranial ossification patterns.brain size evolution | virtual endocast | fossil hominins | frontal cortex | obstetrics

New high-resolution computed tomography data of the Taung partial cranium and endocast and their bearing on metopism and hominin brain evolution

“…The form of metopic suture indicated in the high-resolution image data set of the Taung partial cranium is inconsistent with the form of metopic suture and open anterior fontanelle described by Falk and colleagues (2). A persistent metopic suture in the region between nasion and glabella would contradict an anterior to posterior zipper model of closure suggested by Falk and colleagues.…”

“…Moreover, using the high-resolution image data set of the Taung natural endocast, we find no evidence of a distinct superiorly protruding ridge on the Taung natural endocast indicating a possible open (or partially fused) metopic suture rostral to bregma. Thus, neither of the predicted features necessary to support the form of metopic suture in the Taung Child suggested by Falk and colleagues (2) are observed in the present study. Standard medical CT images do not provide spatial resolution equivalent to that of the high-resolution image data set generated in the present study (Fig.…”

“…endocasts from the collection of R. (2), one would have to conclude that the bonobo prefrontal was reorganized and more advanced than the prefrontal of several australopiths. Moreover, when augmenting the bonobo sample with an additional 35 chimpanzee (Pan troglodytes) endocasts from the collection of R.L.H., the combined mean EV becomes 364 mL (SD, 43 mL), with a combined mean maximum prefrontal breadth between Broca's caps (third inferior convolutions) of 68.5 mm (SD, 4.3 mm).…”

“…Here we offer direct evidence for verifying the purported metopic suture and open anterior fontanelle (2, 7), evidence that is less limited than the image data available to Falk and colleagues (2) and less affected by the aforementioned well-known preservation issues. Specifically, we use these data to assess whether the sharp-edged remnant of cortical bone covering much of the left half of the frontal region of the natural endocast reflects an open metopic suture, as has been suggested (2, 7).…”

“…Because the Taung Child is the only australopith put forth in the adaptationist argument for metopism in hominins (2), critical evaluation of this specimen is necessary to verify the accuracy and evolutionary significance of these claims. Here we offer direct evidence for verifying the purported metopic suture and open anterior fontanelle (2, 7), evidence that is less limited than the image data available to Falk and colleagues (2) and less affected by the aforementioned well-known preservation issues.…”