Letter to the editor: Response to Holloway and Broadfield's critique of our reconstruction of the Taung virtual endocast

Without providing any support for their claim, Holloway and Broadfield (H&B) assert that ‘‘the Falk and Clarke (2007) reconstruction of Taung miscalculates the midsagittal plane, resulting in a significant reduction in cranial capacity that may call into question the taxonomic significance of the fossil’’ (Holloway and Broadfield, 2011:322). We agree that the 22 cm difference in the capacities of our respective reconstructions of the Taung endocast may be due largely to differences in our reconstructed midsagittal planes; however, we have good reason to reject the assertion that our plane is miscalculated. The Taung fossil (Australopithecus africanus) consists of a natural endocast that reproduces external morphology from most of the right and part of the left hemispheres of the brain, a separate portion of the fossilized face that articulates with the endocast, and a mandible that occludes with the maxillary dentition in the face. The right temporal and both frontal poles are separated from the endocast and embedded in the back of the facial fragment. In 2007, we published a description of a new virtual endocast of Taung in AJPA and provided a cranial capacity estimate of 382 cm, which is 22 cm smaller than an estimate of 404 cm published by Holloway in 1970. Our reconstruction incorporated, for the first time, both frontal lobes, which were extracted manually from the back of the facial fragment, imaged, and attached electronically to the posterior portion of the endocast, which had been reconstructed by mirror imaging the right hemisphere of the separate natural endocast. We, thus, did not mirror image Taung’s frontal polar region as H&B claim, and the slight asymmetry (a left frontal petalia) in that part of our reconstruction (Falk and Clarke, 2007: Fig. 2) is not surprising because asymmetries in the frontal polar region occur frequently in apes and more often in humans. More to the point, the slight left frontal petalia reproduced by our reconstruction of Taung’s endocast has recently been confirmed independently from a 3D-CT reconstruction of Taung’s frontal lobes (Fig. 1). Taung’s natural endocast has a small gap at the midline of the right cerebellar region, which (since it was missing) could not be mirror imaged. A gap was therefore left in this part of the mirrored natural endocast, which was subsequently filled manually (modeled) with Plasticine by DF. This was done by using a mirrored bony fragment of the occipital condyle that was attached to the natural endocast and a trace of an enlarged right marginal sinus as guidelines (see Falk and Clarke, 2007 for details). Because enlarged occipital-marginal (O/M) sinuses of australopithecines are more often present unilaterally than bilaterally, and on the right rather than the left side, DF decided not to mirror the traces of Taung’s enlarged right O/M sinus. For our article, we therefore did not calculate a midsagittal plane around which to mirror image either this hand-reconstructed region or the frontal polar region, and thus the asymmetries in these parts of our reconstruction are not attributable to errors in mirror imaging. Nevertheless, Holloway and Broadfield (H&B) suggest that we 1) mirror imaged Taung’s frontal lobes and 2) did so incorrectly because we failed to accurately define the midsagittal plane around which the mirror imaging was supposedly performed: ‘‘A virtual reconstruction of Taung must assume perfect symmetry, a feature called into question here in Taung’s most recent reconstruction by Falk and Clarke (2007)’’ (H&B, 2011:319). . ..‘‘The authors rely heavily on mirror imaging to produce the final endocast, but the reconstruction displays a visible lack of symmetry between right and left sides’’ (H&B, 2011:319). . ..‘‘A careful examination of Falk and Clarke’s (2007) Figure 2 (see Fig. 5) indicates that a lack of symmetry exists between left and right cerebral hemispheres. . .. issues of symmetry include. . .a left prefrontal that is not an exact duplicate of the actual right side’’ (H&B, 2011:320-322). . ..‘‘The Falk and Clarke (2007) reconstruction of Taung miscalculates the midsagittal plane. . . Again, it is mandatory that the missing left half be exactly the same as the present right half ’’ (H&B, 2011:322). Although we did not compute a midsagittal plane for Taung’s frontal polar region, we did compute one for the separate natural endocast so that we could mirror image its nearly complete right hemisphere. As H&B note, the reconstruction of the midsagittal plane is critical for obtaining an accurate cranial capacity. This is true because the position of the midsagittal plane of an object limits its volume on one side of that plane and therefore constrains the mirrored (doubled) volume. Our procedures for mirror imaging the right hemisphere of Taung’s natural endocast were described in detail, and were performed with advanced computer technology and software (Falk and Clarke, 2007). After an automated digitization process that captured 3D data from all surfaces of the Taung natural endocast, we sequentially digitized all of the points on the obvious midline (Fig. 1) that courses along the endocast’s dorsal surface (including the sagittal suture, SS) and continues ventrally midway between the orbits and medial to the frag-