Hierarchical nature of morphological integration and modularity in the human posterior face

Abstract

Morphological integration and modularity are important points of intersection between evolution and the development of organismal form. Identification and quantification of integration are also of increasing paleoanthropological interest. In this study, the "posterior face," i.e., the mandibular ramus and its integration with the associated midline and lateral basicranium, is analyzed in lateral radiographs of 144 adult humans from three different geographic regions. The null hypothesis of homogenously pervasive morphological integration among "posterior-face" components is tested with Procrustes geometric morphometrics, partial least squares, and singular warps analysis. The results reveal statistically significant differences in integration. Only loose integrative relationships are found between midline and lateral components of the basicranium, which may indicate the presence of at least two different basicranial modules. This modularity can be interpreted in terms of spatiotemporal dissociation in the development of those basicranial structures, and gives support to hypotheses of independent phylogenetic modifications at the lateral and midline basicranium in humans. In addition, morphological integration was statistically significantly stronger between the middle cranial fossa and the mandibular ramus than between the ramus and the midline cranial base. This finding confirms previous hypotheses of a "petroso-mandibular unit," which could be a developmental consequence of well-known phylogenetic modifications in coronal topology of the posterior face and base in hominoid evolution, related to middle cranial fossa expansion. This unit could be involved in later evolutionary tendencies in the hominid craniofacial system.

Forelimb to Hindlimb Shape Covariance in Extant Hominoids and Fossil Hominins

“…The degree of covariation is the amount of variance of the sample that is explained by the first axes of each block, whereas the correlation coefficient (r) is a measure of how well the two blocks covary on the first axis. The first axis will explain the greatest proportion of the covariance in the sample and is not constrained by being orthogonal to the previous axis (Rohlf and Corti, 2000;Bastir and Rosas, 2005). The RV coefficient is analogous to the Pearson's correlation coefficient, but for high-dimensional data sets, and it can be interpreted in the same was as any coefficient of correlation.…”

Morphological integration of mandible and cranium: Orthodontic implications

“…[11][12][13][14] Some studies found significant features of integration between the cranium and mandible or some of its elements. 15 However, the idea that the mandible is relatively independent of the cranium remains pervasive. In a study of adolescents without major malocclusion, McKane and Kean 14 found minor or no covariation among the shapes of parts of the facial skeleton.…”

Variations in skull base morphology during phylogenesis and different hypotheses

“…• Bastir and Roses 4 , based on a sample of modern men (n = 144), suggest the hypothesis of a "petromandibular" unit which would be the consequence of changes in the phylogenetic skull base configuration and in posterior face during hominization. Their study shows that in modern man, this morphological integration is organized hierarchically.…”