Comparative Morphology of Mammals Exhibiting Facial Reduction

Abstract Body : Facial reduction, or the marked reduction of the viscerocranium resulting in a brachycephalic morphotype, is a phenomenon described extensively in the evolutionary, clinical, and veterinary literature. Despite the fact that this phenomenon applies to a variety of species and can even be found in the clinical medical and veterinary literature, it has not been analyzed comprehensively across major clades. For example, in primate taxa, there are a variety of cranial morphologies spanning from the long-snouted Lemur catta to the short-snouted Presbytis femoralis. However, not only primates exhibit facial reduction. Bats and dogs also exhibit a distinctive reduction of the viscerocranium. This study focuses on the examination of specific patterns of localized shape change in regions of the cranium that are deemed important due to their modular nature and ontogenetic significance. Specifically, we examined facial reduction as a morphological phenomenon in six specific cranial modules. We recorded three-dimensional coordinate data for 55 cranial landmarks that allowed us to analyze differences in shape across the entire cranium of three representative taxa (primates n = 88, bats n = 64, dogs n = 81).

Through geometric morphometric analyses, we determined that in both dogs and bats, the variance on PC1 correspond with cephalic index (CI), indicating that the majority of variance is associated with craniofacial proportions. In primates, however, this was not the case. When broken out by module, several interesting patterns emerged. In module 1 (face), PC1 appeared to correlate with CI in both bats and dogs, while this was not the case in primates. In all three taxa, there is a lateral and posterior displacement of the landmarks of the nasal opening associated with facial reduction. Bats and dogs have a nasal aperture that is wider in the medial-lateral direction than it is long in the superior-inferior direction. The inverse is true in primates. In module 2 (orbit), CI is only correlated with PC1 in dogs, but not in bats and primates. The mean orbit shape is anteriorly projected in the primates, while in the bats and dogs it is positioned more on the side of the head. Thus, in dogs, facial reduction is associated with anteriorly displaced orbits. In module 3 (zygomatic), a similar pattern emerges where only dogs have a CI that is correlated with PC1. In all three taxa, as PC1 score increases, the zygomatic arch is compressed anterior-posteriorly. In module 4 (cranial vault), CI does not appear to correlate with PC1 in bats but does correlate with PC1 in primates. In dogs, PC2 appears to correlate with CI. The shape change seen in bats and dogs is similar (although not associated with CI), but in primates, there is an extreme posterior projection of inion. In module 5 (cranial base), only dogs have CI values that correlate with PC1 score. The shape appears to be conserved in all three taxa, with very little observed displacement of the cranial base landmarks. Finally, in module 6 (oral), bats and dogs have CI values that correlate directly with PC1 score. Both primates and bats have a parabolic hard palate shape as their mean shape, while the mean hard palate shape is more spade-shaped in the dogs. In conclusion, the shape changes associated with CI and facial reduction differed across bats, primates, and dogs in almost all modules. These findings suggest that brachycephaly is achieved differently in all three taxa.