Pattern in craniofacial biology: Evidence from the old world monkeys (Cercopithecidae)

An understanding of craniofacial growth, both evolutionarily and clinically, requires an investigation of pattern -geometric relations that remain relatively constant among growing structures or components of the skull. Several craniofacial biologists have suggested that specific morphological relations remain invariant during growth and in interspecific comparisons of adults of varying size. We tested the hypothesized invariance of a series of craniofacial angles in a sample of adult Old World monkeys. Fifteen angles were determined from lateral cranial radiographs. Criteria for examining angular invariance included tests for significant correlations and regression slopes with palatal length (overall skull size), tests for significant mean differences (ANOVAs) in angular values between the two subfamilies of Cercopithecidae - Cercopithecinae and Colobinae - and the computation and ranking of standard deviations (SDs) and coefficients of variation (CVs). Results indicate that most of the cranial angles purported to be invariant do not in fact meet the criteria for acceptance. One of the few cranial angles that evinces a somewhat constant value is that between the posterior maxillary plane and the neutral axis of the orbits, providing very limited support for Enlow's (1982) claim that this region represents a fundamental anatomical interface (at least within Old World monkeys). Our analysis suggests that while there may be several relatively invariant structural relations within the skull, most of those previously discussed as representing evidence of pattern in primate-wide or mammal-wide comparisons are incorrect.