This node represents the last common ancestor to the Paranthropus clade and the more derived hominins (Australopithecus, Kenyanthropus, and Homo). Compound crests are variable in development and the petrous is more coronally orientated. This is clearly associated with increased breadth of the cranial base. The glenoid fossa has increased in depth, though the postglenoid process has decreased in size. The tympanic crest is defined with a vertical plate. Cranial capacity has increased and palate prognathism is reduced. Finally, the nasal entrance is defined by a slope (as opposed to step) and the mandible is more parabolic in shape.

From the morphological condition emerging from nodes 7 and 8, we can observe an increase in cranial capacity, while also documenting a reduction in postorbital constriction and temporal fossa size, a less inclined nuchal plane, and a realignment of the petrous. These later features may be associated with a reduction in temporalis and differential development of neck musculature. Indeed, increasing coronal orientation of the petrous bone, reduced basioccipital length, and repositioning of basion relative to the bi-tympanic have all been equated with the necessary reconfiguration of the cranial base in regards to increased basial flexure, accompanied by a reduction in cranial base length (see Weidenreich, 1943; DuBrul, 1977; Olson, 1985; Aiello & Dean, 1990). As noted by Walker et al. (1986) and Dean (1988), however, P. walkeri, while having an extended cranial base, is also characterized by coronally orientated petrous bones and an anteriorly positioned foramen magnum (see Strait et al., 1997); i.e., the exact opposite of what should be expected. In addition, Cramer (1977) argues that such a "correlation" does not always hold true for extant hominids. Finally, Dean (1988) has also suggested that the orientation of the petrous bone at least may not be correlated with increased cranial base flexion, but rather the result from prenatal flattening of the skull base as the cerebellum expands faster than the posterior part of the cranium is able to elongate.

0 0

Post a comment