Primitive Condition of the Middle and Late Miocene Hominids

The overall primitive condition of the hominids is defined as follows. The supraorbital torus is a strong barlike feature, moderately thick, with intermediate glabella swelling. The frontal bone is enclosed by temporal lines with a strong anteromedial incursion, which in males, at least, usually results in a strong sagittal crest. The parietal has no overlap at asterion, with an asterionic notch. Postorbital constriction is developed and the temporal fossa is large. The cranium has developed compound temporal crests. The supraglenoid gutter is intermediately developed, while the mastoids are not overtly inflated. The temporal squama is extensively pneumatized. The external cranial base is extended. The rear of the cranium at the nuchal plane is defined by a steep inclination. The small auditory meatus tends to be medial to porion, and the petrous bone is sagittally aligned; associated with this alignment is the intermediate breadth of the cranial base. The articular tubercle is large. The basioccipital is long. The glenoid fossa is large in size, though with reduced depth. The postglenoid process is large and unfused to the tubular tympanic, which has a small vaginal process. A eustachian process is present and well developed. The distance between the TMJ and dental complex is increased. The insertion region of the digastric muscle along the base of the cranium is broad and shallow, and the longus capitis muscle insertion is long and oval in shape. The foramen magnum is oval in shape and located posterior to the bi-tympanic line as well as being inclined posteriorly. Overall cranial capacity is small, cerebellar morphology is laterally flaring with posterior protrusion, and an occipitomarginal sinus is infrequent.

The upper face is set high, relative to the frontal, with a developed frontal sinus. Interorbital is intermediate in breadth, and the lacrimal fossa is located within the orbital region as part of the interorbital. The mid face is defined by a well developed premaxilla with a "snout-like" appearance, emphasized by its strong palate prognathism. The upper- and mid-face are intermediate in overall breadth, as well as depth (the inferior orbital margin is aligned to the superior nasal aperture margin). The orbits are oval/rhomboid in shape, and the orbital fissure configuration is round. The inferolateral orbital margin is rounded, and a maxillary trigon is absent. Nasal bones are projected and tapered with no sagittal keel. The zygo-matic/malar (viewed laterally) is near vertical in orientation, and its anterior insertion to the alveolar border is relatively high and tends to be at the M1. The maxillary sinus is large. The infraorbital foramen(-ina) is/are located within the upper 50% of malar height. Diagonal malar length is intermediate. The angle of the convex subnasal region is relatively low, though the incisor alveolar border is prognathic, well beyond the bi-canine line. In terms of length, the subnasal region is intermediate. Anterior pillars do not define the inferolateral nasal aperture borders, though a well-developed canine fossa is present. The nasal entrance is stepped, with an undeveloped incisive canal. The anterior palate is moderately deep, while in breadth the palate is intermediate. The palate is thin. The upper incisor complex is of intermediate size, with developed incisor heteromorphy. Male canines are robust and daggerlike. Upper premolar complex, relative to the molar complex, is intermediate in size. Molars are relatively small, broader than long, with intermediate enamel thickness, and weak lingual cingulum. Cusps do not crowd the occlusal surface and are inflated with limited cristae development.

The mandibular symphysis recedes and is moderately robust. Superior and inferior mandibular tori are of similar development. The corpus is moderately robust. The mandible is U-shaped. The mental foramen opening is variable, located within a shallow hollow. The mandibular extramolar sulcus is broad. The P3 does not have a metaconid, and it has a developed mesiobuccal expansion.

In summary, the primitive hominid condition is characterized by strong neuro-orbital disjunction, which is emphasized by the features associated with the development of the supraorbital, postorbital constriction, and the low cranium, relative to facial height (the brain is pushed back from the face). Partly associated with these characters is the klinorynchous condition, with anterior cranial base extension, which can explain to varying degrees the development of the prognathic "snoutlike" premaxilla and the subnasal morphology (Weidenreich, 1941, 1943, 1951; Shea, 1985, 1988, 1993; Lieberman, 2000). The face and cranial base are of intermediate breadth. The posterior position of the foramen magnum, orientation of the petrous, and the steep nuchal plane can be associated with the pattern of suspension and "knuckle walking," as opposed to bipedal locomotion. The heteromorphic status of the incisors as well as molar occlusal morphology and relatively thin molar enamel can be related to dietary requirements, though as discussed above, such "functional" features cannot be dismissed outright in terms of phylogenetic significance, given the concept of "phylogenetic niche conservatism."

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