Neurocranium versus Face: A Morphometric Approach with Classical Anthropometric Variables for Characterizing Patterns of Cranial Integration in Extant Hominoids and Extinct Hominins

Abstract

The relative importance of the two main cranial complexes, the neurocranium and the splanchnocranium, has been examined in the five species of extant hominoids and in a huge sample of extinct hominins using six standard craniometric variables that measure the length, width and height of each cranial module. Factor analysis and two-block partial least squares were used for establishing the major patterns of developmental and evolutionary integration between both cranial modules. The results obtained show that all extant hominoids (including the anatomically modern humans) share a conserved pattern of developmental integration, a result that agrees with previous studies. The pattern of evolutionary integration between both cranial modules in australopiths runs in parallel to developmental integration. In contrast, the pattern of evolutionary and developmental integration of the species of the genus Homo is the opposite, which is probably the consequence of distinctive selective regimes for both hominin groups.

Fig 1. Craneometric variables used in this study.

For abbreviations, see text. BBH was estimated following [58]; in those specimens with sagittal crest, bregma was placed in the plane surrounding the cranial vault surface. BPL was estimated in the toothless specimens placing prosthion on the middle line of the skull, at the most inferior point of the maxillo-alveolar process. Note that the three variables of each cranial module are linearly independent, as each of them cannot be obtained as a linear combination of the other two.

Fig 2. Bivariate plot of the scores of the specimens analyzed on the first two factors.

Dotted lines enclose the 95% confidence ellipses for the living species.

Fig 3. Plots of the specimens’ scores for Factors I and II on their geometric means.

A) Bivariate plot of FI scores on the difference between the log₁₀-transformed geometric mean for the three neurocranial variables and the corresponding value for the three facial variables. B) Bivariate plot of FII scores on the log₁₀-transformed geometric mean for the six analyzed variables.

Fig 4. Bivariate plot of the scores for the specimens analyzed on the first two factors.

Dotted lines enclose the 95% confidence ellipses for the four hominoid groups considered in this study.

Fig 5. Two-block partial least squares plots.

Non-pooled (A) and pooled within species (B) 2B-PLS plots of the face vs. the neurocranium for the living species, respectively. Non-pooled (C) and pooled within-species (D) 2B-PLS plots of the face vs. the neurocranium for size-scaled living species, respectively. The correlations between the scores on each block are significant for all species (p < 0.05).

Fig 6. Non-pooled within-species 2B-PLS plots of the face vs. neurocranium for size-scaled adults of the living and extinct hominoid species.

The correlations between the scores on each block are significant for all species/groups (p < 0.05).

Fig 7. Non-pooled within-species 2B-PLS plots of the face vs. neurocranium for scaled specimens.

The correlations between the scores on each block are significant for all species/groups with the exclusion of microcephalic crania (p < 0.05).

Fig 8. Bivariate plots of the scores for different taxonomic sets on the first two factors.

A) Bivariate plots of FII on FI scores. Ellipses enclose the 95% confidence regions. The ellipse for australopiths was plotted excluding WT-170000; a: convex hull for habilines; a*: Dmanisi paleodeme; b: convex hull for erectines; c: convex hull for H. heidelgergensis; d: convex hull for H. neanderthalensis. B) Intraspecific allometries resulting from sexual dimorphism. C) Interspecific allometries that run in parallel to ontogenetic scaling. Note that LB1 relates with the habilines through ontogenetic scaling. D) Interspecific allometry opposed to ontogenetic scaling.