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Abstract
Brain shape asymmetries or petalias consist of the extension of one cerebral hemisphere
beyond the other. A larger frontal or caudal projection is usually coupled with a
larger lateral extent of the more projecting hemisphere relative to the other. The
concurrence of these petalial components is characteristic of hominins. Studies aimed
at quantifying petalial asymmetries in human and great ape endocasts rely on the definition
of the midline of the endocranial surface. Studies of brain material show that, at
least in humans, most of the medial surface of the left occipital lobe distorts along
the midline and protrudes on to the right side, making it difficult for midline and
corresponding left and right reference point identification. In order to accurately
quantify and compare brain shape asymmetries in extant hominid species, we propose
here a new protocol based on the objective definition of cranial landmarks. We describe
and quantify for the first time in three dimensions the positions of frontal and occipital
protrusions in large samples of Pan paniscus, Pan troglodytes and Gorilla gorilla.
This study confirms the existence of frontal and occipital petalias in African apes.
Moreover, the detailed analysis of the 3D structure of these petalias reveals shared
features, as well as features that are unique to the different great ape species.