The occipital bun, a distinctive convexity of the occipital squama, is often considered to be a uniquely derived Neandertal trait. Some scholars, however, consider the occipital morphology found in some early modern and extant human crania (often described as “hemi-buns”) to be homologous with Neandertal occipital buns. A number of hypotheses have been proposed to explain occipital bun/hemi-bun development, including neck muscle function, head carriage, brain growth timing, and cranial base cartilage growth timing, as well as braincase and facial integration. The feature, however, has never before been metrically quantified in a large subadult sample or studied in a well-documented growth series. The primary goal of this dissertation, therefore, was to assess hemi-bun growth and development in a combined comparative sample of extant humans amassed from the following growth series: the University of Toronto Burlington Growth Study, the Iowa Facial Growth Study, the Oregon Growth Study, the University of Oklahoma Denver Growth Study, the Wright State University Fels Longitudinal Study, and the Michigan Growth Study.
Cephalograms from these studies facilitated the collection of longitudinal cranial growth and development data. In total, measurements were collected from 468 cephalograms representing 16 males and 10 females. Measured subjects represented the ends of the range of variation in adult midsagittal occipital bone shape, including subjects with defined hemi-buns, as well as subjects lacking all evidence of hemi-bun morphology. Frontal and lateral cephalograms were measured for each subject at 9 age points, spanning from 3.0 to 20.4 years of age. A total of 16 landmarks and 153 sliding semi-landmarks were digitized at each age point. Geometric morphometric analyses, including relative warps analysis and two-block partial least squares analysis, were conducted to assess patterns of cranial covariation and sexual dimorphism in occipital bone growth and possible attendant variation in occipital bun development or absence.
In both bunned and non-bunned subjects, midsagittal occipital shape was found to be established very early in ontogeny, and then to remain largely unchanged between 3 years of age and adulthood. This result contradicts previous developmental hypotheses, which posit that occipital bunning results from a pattern of late posteriorly-directed brain growth. No evidence of sexual dimorphism in hemi-bun shape was found to exist in this extant human sample; however, defined hemi-buns were found to covary significantly with an elongated and low midsagittal neurocranial vault in both sexes. Other aspects of cranial morphology, including cranial and basicranial breadth, midcoronal vault shape, and basicranial angle, did not covary significantly with occipital bun morphology at any of the sampled age points.
These results reveal that occipital bunning, at least in this sample, is not a discrete trait, but instead develops along a continuum in association with a distinct pattern of neurocranial elongation. Previous studies have suggested that Neandertal occipital buns are similarly associated with elongated cranial vaults. While more work must be done to quantify occipital bun morphology in fossil subadults, this study finds no evidence to disprove the developmental homology of the feature in modern humans and Neandertals, and therefore further undermines the idea that occipital bunning is a unique Neandertal trait.