Effects of fronto-occipital artificial cranial vault modification on the cranial base and face.

Artificial reshaping of the cranial vault has been practiced by many human groups and provides a natural experiment in which the relationships of neurocranial, cranial base, and facial growth can be investigated. We test the hypothesis that fronto-occipital artificial reshaping of the neurocranial vault results in specific changes in the cranial base and face. Fronto-occipital reshaping results from the application of pads or a cradle board which constrains cranial vault growth, limiting growth between the frontal and occipital and allowing compensatory growth of the parietals in a mediolateral direction. Two skeletal series including both normal and artificially modified crania are analyzed, a prehistoric Peruvian Ancon sample (47 normal, 64 modified crania) and a Songish Indian sample from British Columbia (6 normal, 4 modified). Three-dimensional coordinates of 53 landmarks were measured with a diagraph and used to form 9 finite elements as a prelude to finite element scaling analysis. Finite element scaling was used to compare average normal and modified crania and the results were evaluated for statistical significance using a bootstrap test. Fronto-occipitally reshaped Ancon crania are significantly different from normal in the vault, cranial base, and face. The vault is compressed along an anterior-superior to posterior-inferior axis and expanded along a mediolateral axis in modified individuals. The cranial base is wider and shallower in the modified crania and the face is foreshortened and wider with the anterior orbital rim moving inferior and posterior towards the cranial base. The Songish crania display a different modification of the vault and face, indicating that important differences may exist in the morphological effects of fronto-occipital reshaping from one group to another.     

Craniofacial suture stenosis: morphologic effects

Craniofacial anomalies, such as Apert's and Crouzon's syndromes, are presumed to be related to premature growth arrest of cranial base growth sites. However, premature growth arrest at cranial vault sutures in animals appears to play a causative role in the development of cranial deformities characteristic of single-suture, or simple, craniosynostosis in humans. To study the possible causative role of cranial vault and other (interface) suture stenoses on the development of craniofacial deformity, a vault suture and an interface suture between the cranial vault and facial skeleton were simultaneously immobilized. Thirty-one New Zealand White rabbits at 9 days of age underwent implantation of dental amalgam growth markers adjacent to cranial vault and facial sutures. In the experimental group (n = 15), methylcyanoacrylate adhesive was applied over the coronal (vault) and frontonasal (interface suture between vault and facial skeleton) sutures to immobilize them. The remaining 16 animals served as sham-treated controls. All animals underwent serial radiographic cephalometry to document growth effects in the cranial vault, cranial base, and facial skeleton. Application of adhesive resulted in statistically significant (p less than 0.05) reduction in growth at the coronal and frontonasal sutures. This was accompanied by an overall significant reduction in neurocranial vault length during the first 30 days of development.(ABSTRACT TRUNCATED AT 250 WORDS)     

Asymmetric vault modification in Hopi crania

Cradleboarding was practiced by numerous prehistoric and historic populations, including the Hopi. In this group, one result of cra-dleboarding was bilateral or asymmetric flattening of the posterior occipital. We test whether cradleboarding had significant effects on the morphology of the cranial vault, cranial base, and face. Additionally, we examine associations between direction of flattening and asymmetric craniofacial growth. A skeletal sample of Hopi from the Old Walpi site includes both nonmodified (N = 43) and modified individuals (N = 39). Three-dimensional coordinates of 53 landmarks were obtained using a diagraph. Thirty-six landmarks were used to define nine finite elements in the cranial vault, cranial base, and face. Finite element scaling was used to compare average nonmodified individuals, with averages of bilaterally, right, and left modified individuals. The significance of variation among “treatment” groups was evaluated using a bootstrap test. Pearson product-moment correlations test the association of asymmetry with direction of modification. Hopi cradleboarding has a significant effect on growth of the cranial vault, but does not affect morphology of the cranial base or face. Bilateral flattening of the cranial vault leads to decreased length and increased width of the cranial vault. Flattening of the right or left cranial vault results in ipsilaterally decreased length and width coupled with a corresponding increased length and width on the contralateral side of the cranial vault. There is a significant correlation of size asymmetry with direction of modification in the cranial vault, but not with size or shape change in the cranial base or face. © 1995 Wiley-Liss, Inc.     

Effects of annular cranial vault modification on the cranial base and face

Artificial modification of the cranial vault was practiced by a number of prehistoric and protohistoric populations, frequently during an infant's first year of life. We test the hypothesis that, in addition to its direct effects on the cranial vault, annular cranial vault modification has a significant indirect effect on cranial base and facial morphology. Two skeletal series from the Pacific Northwest Coast, which include both nonmodified and modified crania, were used: the Kwakiutl (62 nonmodified, 45 modified) and Nootka (28 nonmodified, 20 modified). Three-dimensional coordinates of 53 landmarks were obtained using a diagraph, and 36 landmarks were used to define nine finite elements in the cranial vault, cranial base, and face. Finite element scaling was used to compare average nonmodified and average modified crania, and the significance of the results were evaluated using a bootstrap test. Annular modification of the cranial vault produces significant effects on the morphology of the cranial base and face. Annular modification in the Kwakiutl resulted in restrictions of the cranial vault in the medial-lateral and superior-inferior dimensions and an increase in anterior-posterior growth. Similar dimensional changes are observed in the cranial base. The Kwakiutl face is increased anterior-posteriorly and reduced anterior-laterally to posterior-medially. Similar effects of modification are observed in the Nootka cranial vault and cranial base, though not in the face. These results demonstrate the developmental interdependence of the cranial vault, cranial base, and face. © 1993 Wiley-Liss, Inc.     

The influence of artificial cranial vault deformation on the expression of cranial nonmetric traits: its importance in the study of evolutionary relationships

Nonmetric cranial traits have been commonly used in evolutionary relationship studies. They develop during the growth and development of an individual, and for this reason its expression presents different sources of genetic and nongenetic variation. However, the use of these features in evolutionary relationship studies carries the implicit assumption that much of the nonmetric trait variation is essentially genetic. Among the nonheritable factors, cranial vault deformation has been the most studied in human populations. Because of the widespread distribution and elevated rate of artificial cranial vault deformation found in America, and the importance of nonmetric traits in evolutionary relationship studies in this area, the objectives of this paper are as follows: (a) to study the influence of artificial cranial vault deformation on the presence of nonmetric traits within samples of human craniofacial remains; and (b) to establish artificial cranial vault deformation influence on evolutionary relationships between local populations on a regional scale. Our results indicate that artificial cranial vault deformations alter the variation and covariation of metric and nonmetric traits in some samples. Wormian bones, placed in cranial vault sutures, are the most influenced by this factor. However, our results suggest that when all nonmetric traits were used the artificial cranial vault deformation did not influence the basic pattern of variation among samples. The exclusion or inclusion of wormians bones in evolutionary relationships analysis did not modify the results, but using only wormians bones lead to inconsistent results indicating that these traits have little value on these kind of studies.     

Blocking endogenous FGF-2 activity prevents cranial osteogenesis

Normal growth and morphogenesis of the cranial vault reflect a balance between cell proliferation in the sutures and osteogenesis at the margins of the cranial bones. In the clinical condition craniosynostosis, the sutures fuse prematurely as a result of precocious osteogenic differentiation and craniofacial malformation results. Mutations in several fibroblast growth factor receptor (FGFR) genes have now been identified as being responsible for the major craniosynostotic syndromes. We have used a grafting technique to manipulate the levels of endogenous FGF-2 ligand in embryonic chick cranial vaults and thereby perturb morphogenesis. Implantation of beads loaded with FGF-2 did not affect normal cranial development at physiological concentrations, although they elicited a morphogenetic response in the limb. Implantation of beads loaded with a neutralising antibody to FGF-2 generated a concentration-dependent response. When a single bead was implanted, the grafts grew to a massive size as a result of increased cell division in the tissue. With greater inactivation of FGF-2 protein (two to three beads implanted), all further bone differentiation and cell proliferation was blocked. These data further support the emerging idea that the intensity of FGF-mediated signalling determines the developmental fate of the skeletogenic cells in the cranial vault. High and low levels correlate with differentiation and proliferation, respectively. A balance between the two ensures normal cranial vault morphogenesis. This is consistent with the observation that several FGFR mutations causing craniosynostosis result in constitutive activation of the receptor.     

Cranial capacity/cranial base relationships and prediction of vault form: A canonical correlation analysis

Canonical correlation analysis was used to test an hypothesized morphological relationship between vault form and cranial capacity relative to length of the chondrocranium. Ninety-five adult male Czech skulls were measured for vault form expressed as length, width and height of the brain case; the chondrocranium was represented by nasion-basion and basion-opisthion lengths. In terms of explained variation, the first and most important dimension of covariation between vault and chondrocranial variables was size. The second most significant dimension of covariation expressed the hypothesized shape relationships—i.e., overall size being equal, the shorter the chondrocranial base relative to cranial capacity, the shorter and wider the vault. Furthermore, the competing hypothesis that vault form is determined by facial length proved untenable since facial length was predictive of vault shape only when measured as prosthion-basion, a measure that incorporates basal length. When corrected for basal length, facial length is unrelated to vault form. The results are consistent with the assumption that phylogenetic and microevolutionary trends toward brachycephaly in man stem from changes in the relationship between two components of skull growth, the chondrocranial base and the brain.     

Nasal capsular cartilage is required for rat transpalatal suture morphogenesis

In the cranial vault, suture morphogenesis occurs when the growing cranial bones approximate and overlap or abut one another. Patency of developing sutures is regulated by the underlying dura mater. Once cranial sutures form, bone growth proceeds from the sutures in response to growth signals from the rapidly expanding neurocranium. Facial sutures do not develop in contact with the dura mater. It was therefore hypothesized that facial suture morphogenesis and bone growth from facial sutures are regulated by tissues with an equivalent role to the dura mater. The present study was designed to test this hypothesis by characterizing the morphology and growth factor expression in developing transpalatal (TP) sutures and their surrounding tissues, and then assessing the role of the overlying nasal capsular (NC) cartilages in maintaining suture patency. TP sutures develop as overlapping sutures, similar to cranial coronal sutures, and expression of Tgf-betas in TP sutures was similar to their distribution in cranial coronal sutures. To establish whether NC cartilages play a role in regulating TP suture morphogenesis, fetal rat TP sutures were cultured with associated attached NC cartilages or with NC cartilages removed. Sutures cultured for upward of 5 days with intact NC cartilages remained patent and maintained their cellular and fibrous components. However, in the absence of NC cartilages, the cellular nature of the sutures was not maintained and they became progressively acellular, with bony bridging across the suture. This finding is similar to that for cranial vault sutures cultured in the absence of dura mater, indicating that NC cartilages play an equivalent role to dura mater in maintaining the patency of developing sutures. These studies indicate that tissue interactions likely regulate morphogenesis of all cranial and facial sutures.     

Cranial skeletogenesis and osteology of the redeye tetra Moenkhausia sanctaefilomenae

The skeletogenesis and osteology of the syncranium of the redeye tetra Moenkhausia sanctaefilomenae is described. Skeletal development is rapid, with many elements of the chondrocranium and splanchnocranium well formed prior to the onset of ossification. The chondrocranium develops from an initial set of cartilaginous precursors, and continued elaboration proceeds from a series of processes which expand and converge to form the floor of the cranial vault, the otic capsule, the supraorbital bridge and the ethmoid region. Prodigious growth is observed for a number of splanchnocranial elements, including the Meckel's cartilage and the ceratohyal cartilage. Ossification occurs in overlapping phases with initial ossification of the jaws and neurocranial floor followed by the splanchnocranium, the supraorbital bridges and the ethmoid and cranial vault. Teeth are observed primarily on the premaxilla and dentary, while a single tooth is present on the maxilla. Particular cartilages, which had originally formed in the early larva, appear to degenerate and have no ossified representative in the adult syncranium. The cranial development for M. sanctaefilomenae is compared to those of other characiforms.     

华北人欧洲人和澳洲土著人的头骨厚度

The thickness of the cranial vault at the midline on the mid-frontal squama, pre-bregmatic einence, frontal at bregma, parietal at vertex, occipital at lambda and the external occipita1 protuberance was recorded in 40 male and 7 female Northern Chinese crania, 47 male and 52 female Australian Aboriginal crania and 13 male European crania using specially nodified vernier calipers. Comparison of vault thickness data obtained through direct measurement with those obtained fron lateral radiographs indicated that direct measurenent provided consistently more accurate results.
Male and fermale samples were processed separately so that the extent of sexbased variation could be examined.Student's t test was used to compare the sample means and the percentage of sexual dimorphism for each dimension was calculated according to Garn et al, (1964).The possibility of an allometric association between the thickness of the bones within the cranial vault, size of the cranial vault and stature was examined using Spearman's rank correlation coefficient and the Australian Aboriginal sample.
All but one of the mean thickness dimensions in the Australian Aboriginal male sample is significantly greater than the Northern Chinese and European means. The female results support those obtained with the males.In both males and females thickness at the external occipital protuberance, in all of the populations examined,did not correlate highly with that obtained from other parts of the cranial vault.This reflects the high degree of morphological variation in the position of the internal occipital protuberance and its influence on cranial vault thickness dimensions recorded at the external occipital protuberance.The European and Northern Chinese samples have similar cranial vault thickness dimensions. The Spearman's rank correlation coefficient matrix scores provide sone support for a biological association between vault thickness and overall cranial size. However, there appears to be little support for an association between stature and cranial vault thickness. The difference between the male and female mean vault thickness dimensions were significant at bregma, vertex and the external occipital protuberance in Australian Aboriginals and lambda and the external occipital protuberance in Northern Chinese. Some caution is needed in the interpretation of the Northern Chinese female data as the sample is extremely small.
Evidence of trauma, supressed fractures, is extremely common on the vaults of Australian Aboriginal crania from southern and central Australia. Traditionally Australian Aboriginals, males and females, involved in agressive dispute will use a substantial wooden implement and strike to the head of thir opponent(Meggitt 1962).The injuries that result from this are more common in females than in male. This form of social interaction must have rigorously selected against those individuals with thinner bones in their cranial vaults. To a large degree this may explain the greatly thickened vaults in Australian Aboriginals relative to Europeans and Northern Chines.This may also provide a clue to the factors resulting in the development of marked cranial vault thickness in Homo erectus.
     

Cranial vault modification as a cultural artifact: a comparison of the Eurasian steppes and the Andes.

This paper details the practice of intentional cranial vault modification in the Eurasian steppes as well as in the pre-Columbian Andes focusing on the similarities and differences in how the practice was used to respond to changes in society. The appearance of vault modification in the steppes and the forms seen in the cemeteries of the Syr Darya and Amu Darya River deltas are discussed. Temporal changes in the pattern of modification are also investigated, especially the dramatic homogenization of the custom resulting from the conquests of the Huns. This is contrasted with incidences of cranial modification in the south-central Andes, including the appearance of deliberate head shaping as well as shifts in the practice during the expansion of the Bolivian Altiplano state of Tiwanaku. Similarities in the use of cranial vault modification between these unrelated areas and in the alterations of the practice resulting from foreign contact are considered in light of vault modification's role as a malleable cultural artifact.     

Skull base expansion: craniofacial effects

In order to determine what effect the anterior cranial base has on the developing craniofacial skeleton, mechanical expansion of the growth of one segment of the anterior cranial base was performed. New Zealand white rabbits were used for this study. A sham-treated group (n = 16) underwent implantation of dental amalgam markers to either side of the frontonasal, coronal, and lambdoid sutures at 9 days of age to serve as markers of vault growth. The experimental group (n = 7) underwent the same marker placement at 9 days of age, but, in addition, at 30 days of age these animals underwent placement of a mechanical spring, unilaterally, at the frontosphenoid suture. A second control group (n = 8) underwent the same exposure of the frontosphenoid suture, but the spring was laid only on the surface of the bone. All animals were followed by radiographic cephalometry at 9, 30, 60, and 90 days of age. The experimental group demonstrated statistically significant expansion of the cranial base and ipsilateral coronal suture. The midface skeletal dimensions were unchanged by spring distraction of the cranial base. These findings indicate that cranial base sutural growth can be manipulated mechanically and that growth changes can be attained secondarily in the cranial vault skeleton.     

Delayed cranial vault reconstruction for sagittal synostosis in older children: an algorithm for tailoring the reconstructive approach to the craniofacial deformity

An algorithm for the management of sagittal synostosis in older children who underwent delayed cranial vault reconstruction is presented. This algorithm tailors the surgical approach to the specific craniofacial deformity present in each case. The scaphocephalic deformity characteristic of sagittal synostosis varies significantly when presentation is delayed beyond the first year of life, the time during which reconstruction is usually performed. Sixteen patients with sagittal synostosis who presented after 12 months of age, and were a mean of 3.2 years of age at the time of cranial vault reconstruction, were reviewed. Four patients demonstrated preoperative symptoms and objective findings indicative of increased intracranial pressure, including frequent headaches and emesis, papilledema, or digital markings on computed tomographic scan. Each of the 16 patients underwent either (1) single-stage total vault reconstruction with or without concomitant fronto-orbital expansion; (2) two-stage total vault reconstruction with anterior two-thirds vault expansion followed by transverse occipital expansion and recession a mean of 8.7 months later; or (3) anterior two-thirds vault reconstruction with or without fronto-orbital expansion. In each case, the extent of the scaphocephalic deformity determined the procedure used. The presence of severe frontal bossing associated with transverse restriction of the orbitotemporal region was an indication for fronto-orbital expansion in addition to vault reconstruction, whereas significant occipital protrusion was an indication for transverse posterior vault expansion and recession in addition to anterior two-thirds vault reconstruction. Excellent aesthetic results were obtained in all cases regardless of the type of reconstruction performed. However, it is essential that the extent of the deformity be carefully evaluated preoperatively to permit selection of the appropriate technique for reconstruction.     

A factor analysis of cranial base and vault dimensions in children

Lengths within the cranial base and vault were measured in cephalometric radiographs of 220 boys and 177 girls ranging in age from 0 to 15 years; all these children are participants in The Fels Longitudinal Growth Study. The present study is based on mixed longitudinal data derived from 1640 radiographs for boys and 1260 radiographs for girls. Factor analysis was applied separately for boys and girls for each age group; i.e., 0–3, 4–6, 7–9, 10–12, and 13–15 years. For the 0–3 year age group, two factors were extracted in each sex, whereas four factors were extracted in the rest of the age groups. The factor structures are similar in the three older age groups of boys (7–9, 10–12, and 13–15 years). The first four factors for these groups are labelled, respectively: cranial vault size, posterior cranial base length, presphenoid length, and basisphenoid length. The order of the third and fourth factors is reversed in the 7–9 year olds. For girls, the factors extracted were also the same in both the 7–9 and 10–12 year age groups, even though the order of factors was different between age groups; i.e., anterior cranial base length, cranial vault size, basisphenoid length, and basioccipital length. Differential growth rates among cranial base dimensions probably cause changes in factor patterns. Obliteration of the spheno-occipital synchondrosis is suggested as the mechanism responsible for the change of factor pattern in the girls. Closure of this synchondrosis would have occurred too late to affect the patterns in boys.     

Maxillary changes and occlusal traits in crania with artificial fronto-occipital deformation

Artificial fronto-occipital deformation of the cranial vault was typical of pre-Columbian cultures in the central Andean coastal regions. We have studied the influence of this deformation on maxillary and mandibular morphology. Measurements were performed on 86 adult Ancon skulls with anteroposterior deformation. Undeformed skulls from the area of Makatampu (n = 52) were used as the control group. To explore the influence of the deformity on occlusion, the skulls were categorized using the Angle classification and the alignment of the interincisor midline. In the group of deformed skulls, there was an increase in lateral growth of the vault and of the base of the skull (P < 0.001), giving rise to a greater interpterygoid width of the maxilla (P < 0.001), and an increase in the transverse diameter of the palatal vault. The mandible presented an increase in the length of the rami (P < 0.001) and in the intercondylar width, with no alteration of mandibular length. The deformed skulls had normal (class I) occlusion, with no displacement of the midline. The difference in the asymmetry index between the two groups was not statistically significant. Artificial fronto-occipital deformation of the cranial vault provoked compensatory lateral expansion of the base that was correlated with the transverse development of the maxilla and mandible. Occlusion and sagittal intermaxillary position were not affected by the cranial deformity. These results provide evidence of the integration between the neurocranium and the viscerocranium in craniofacial development, and support the hypothesis of a compensatory effect of function.     

Osseous expansion of the cranial vault by craniotasis.

A study of cranial vault lengthening using a custom expandable fixation-distraction (craniotatic) appliance was performed in the young-adult rabbit model. Ten 24-week-old rabbits underwent circumferential suturectomy plus expansion (expanded group), 10 underwent circumferential suturectomy without expansion (sham control group), and 10 served as normal controls. The appliance was lengthened at a rate of 2.5 mm per week for 5 weeks. Serial lateral cephalometry, comparative dry-skull anthropometric measurements, and histologic examinations were performed. The expanded group demonstrated a significantly longer skull, cranial vault, anterior cranial base, posterior face, and orbit as compared with the control groups (p less than 0.05). Callus bone filled the distracted suturectomy and united the frontofacial complex to the posterior cranium. In conclusion, skull lengthening by distraction osteogenesis is possible in the rabbit model and offers a new technique for future investigation in the treatment of coronal synostosis.     

How and why humans grow thin skulls: Experimental evidence for systemic cortical robusticity

To what extent is cranial vault thickness (CVT) a character that is strongly linked to the genome, or to what extent does it reflect the activity of an individual prior to skeletal maturity? Experimental data from pigs and armadillos indicate that CVT increases more rapidly in exercised juveniles than in genetically similar controls, despite the low levels of strain generated by chewing or locomotion in the neurocranium. CVT increases in these individuals appear to be a consequence of systemic cortical bone growth induced by exercise. In addition, an analysis of the variability in vault thickness in the genus Homo demonstrates that, until the Holocene, there has been only a slight, general decrease in vault thickness over time with no consistent significant differences between archaic and early anatomically modern humans from the Late Pleistocene. Although there may be some genetic component to variation in CVT, exercise-related, non-genetically heritable stimuli appear to account for most of the variance between individuals. The thick cranial vaults of most hunter-gatherers and early agriculturalists suggests that they may have experienced higher levels of sustained exercise relative to body mass than the majority of recent, post-industrial humans. © 1996 Wiley-Liss, Inc.     

Developmental stress and cranial hypostosis by epigenetic trait occurrence and distribution: an exploratory study on the Italian Neandertals

The occurrence and distribution of 35 cranial epigenetic traits in the Italian Neandertals (Saccopastore 1 and 2, Guattari 1) were examined according to morpho-functional cranial regions and with respect to a distinction between hypostosis (i.e., weak osseous development, arrested morphogenesis, retention of infantile features) and hyperostosis (i.e. excess of ossification, not reaching the pathological condition). The results, expressed ashypostotic scores, showed higher levels of hypostosis in these Neandertal specimens than in recent European samples. The highest expressions of hypostotis were observed in those regions of the Neandertal cranium where disequilibrium between skeletal and cerebral growth factors was expected. These results—interpreted as expression of developmental stress—are consistent with a heterochronic interpretation of the development of the Neandertal cranium; namely, a faster ossification of the cranial vault relative to brain growth rates.     

Early Holocene human remains from the Argentinean Pampas: Additional evidence for distinctive cranial morphology of early South Americans

The cranial morphology of Early Holocene American human samples is characterized by a long and narrow cranial vault, whereas more recent samples exhibit a shorter and wider cranial vault. Two hypotheses have been proposed to account for the morphological differences between early and late‐American samples: (a) the migratory hypothesis that suggests that the morphological variation between early and late American samples was the result of a variable number of migratory waves; and (b) the local diversification hypothesis, that is, the morphological differences between early and late American samples were mainly generated by local, random (genetic drift), and nonrandom factors (selection and phenotypic plasticity). We present the first craniometric study of three early skulls from the Argentinean Pampas, dated ～8,000 cal. years BP (Arroyo Seco 2, Chocorí, and La Tigra), and one associated with mega‐faunal remains (Fontezuelas skull). In addition, we studied several Late Holocene samples. We show that the skulls from the Argentinean Pampas are morphologically similar to other Early Holocene American skulls (i.e., Lagoa Santa from Brazil, Tequendama, Checua, and Aguazuque from Colombia, Lauricocha from Peru, and early Mexicans) that exhibit long and narrow cranial vaults. These samples differ from the Late Holocene American samples that exhibit a shorter and wider cranial vault. Our results underscore the important differences in cranial morphology between early and late‐American samples. However, we emphasize the need for further studies to discuss alternative hypotheses regarding such differences. Am J Phys Anthropol 143:298–305, 2010. © 2010 Wiley‐Liss, Inc.     

Occipital bunning among later pleistocene hominids

Occipital bunning is a posterior projection of the occipital squama, which occurs in varying frequencies in samples of archaic Homo sapiens, Upper Pleistocene anatomically modern humans, and recent humans. It can be best interpreted as a product of the timing of posterior cerebral growth relative to the growth of the cranial vault bones. It is not a feature that was unique to the Neandertals.