Node

Following on from the primitive hominid condition is the node representing the last common ancestor with Gorilla. From the primitive hominid condition, we now see the emergence of a strong barlike torus, with a developed frontal sulcus. The palate has decreased in depth and the orbits are now circular-rhomboid in shape. Upper incisor heteromorphy has decreased and molar enamel has decreased in thickness. Molar cusps and cristae have increased in overall development and, finally, the mandibular torus is now weaker than the superior torus.

The emergence of Gorilla is defined by a number of apomorphies. The supraorbital is thick, the glabella is inflated, and the sulcus has increased in depth. There is an absolute increase in postorbital constriction. The supraglenoid gutter has increased in breadth. Anterior tympanic is now aligned with porion and the external auditory meatus has increased in size. Facial hafting is exaggerated; i.e., the cranium is set low, relative to the face. Nasal bones are now defined by a developed nasal keel. Orbital shape is now rectangular, broader than long, and the inferolateral orbital margin is rounded. Facial depth has increased (inferior orbital margin is well above superior nasal aperture margin). The zygomatic bone is convex and is now inserted more posteriorly. The diagonal malar length has increased in size, and finally the molars tend to be square.

Overall, much of the condition observed in Gorilla is simply an exaggeration of the primitive hominid condition defined above — an increase in neuro-orbital disjunction, which is associated with a likely increase in extension of the cranial base. This will impact increased development of the temporal fossa, facial hafting and development of the supraorbital region, and the facial frame in general (see Shea, 1985, D. Lieberman, 2000). While we can recognize an overall pattern, describing the reason underlining its development is a much more difficult task. It may be associated with a requirement to increase masticatory apparatus by increasing the temporalis muscle (increased postorbital constriction and temporal fossa), which contributed to the overall morphological form observed. Masticatory considerations probably also lie behind the reduction in molar enamel thickness, which will assist in defining more developed cusps and cristae, for an increased "shearing action" as well as increasing foveae depth to assist in the collection of juices from plant material (see Teaford, 2000). It remains possible that Graecoptheus belongs to the Gorilla lineage.

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