| The lack of experimental models of brain expansion together with a dearth of information regarding fetal brain development detracts from a complete understanding of cranial integration and its relevance to the ontogenetic and interspecific patterning of the craniofacial morphology. To address this shortcoming, this study uses two datasets to examine the changes in cranial form associated with an increase in encephalization.;First, mice expressing a stabilized form of β-catenin were used to isolate the effects of encephalization on the development of the basicranium and neurocranium. These transgenic mice have an increased number of neural precursors, which results in the development of an enlarged brain. Morphological differences between groups were predicted to ultimately result from variation in encephalization.;Second, morphological data for fetal modern humans with normal and pathological conditions were analyzed. Like the mouse sample, the human sample was intraspecific. However, the degree of encephalization was not the only factor separating specimens: cranial morphology was also presumably influenced by environmental and genetic factors.;The transgenic mice had larger neurocrania and an altered basicranial morphology. Although it was expected that morphological differences would be correlated with histological changes, there were few differences between wild-type and transgenic basicrania at the cellular level. An unexpected result was that increased fetal encephalization might result in a compensatory decrease in cranial ossification.;The results of the human analyses showed that, contrary to expectations, a tall and rounded neurocranium was consistently correlated with a flat basicranial angle, a long and narrow foramen magnum, and a relatively small cranial base. A more narrow and elongated neuro-cranium was correlated with the opposite traits.;The mouse and human specimens differed in their responses to changes in encephalization, but cranial morphology differed among small-brained and large-brained groups in both species. Because encephalization was the primary difference between transgenic and wild-type mice, traits seen in transgenic mice can be considered to be caused by the increased encephalization, either directly as a result of the larger brain size or indirectly as a result of correlated changes in other aspects of cranial morphology. |
