The most complete and best-preserved cranium of a Paleogene anthropoid ever found, that of a small female of the early Oligocene ( approximately 29-30 Ma) stem catarrhine species Aegyptopithecus zeuxis, was recovered from the Jebel Qatrani Formation (Fayum Depression, Egypt) in 2004. The specimen is that of a subadult and, in craniodental dimensions, is the smallest Aegyptopithecus individual known. High-resolution computed tomographic (microCT) scanning of the specimen's well preserved cranial vault confirms that Aegyptopithecus had relatively unexpanded frontal lobes and a brain-to-body mass ratio lower than those of living anthropoids. MicroCT scans of a male cranium recovered in 1966 [Egyptian Geological Museum, Cairo (CGM) 40237] reveal that previous estimates of its endocranial volume were too large. Thus, some amount of encephalization evolved independently in platyrrhine and catarrhine anthropoids, and the relative brain size of the last common ancestor of crown Anthropoidea was probably strepsirrhine-like or smaller. A. zeuxis shows extreme sexual dimorphism in craniodental morphology (apparently to a degree otherwise seen only in some highly dimorphic Miocene catarrhines), and the crania of female Aegyptopithecus lack a number of morphological features seen in larger males that have been accorded phylogenetic significance in catarrhine systematics (e.g., a well developed rostrum, elongate sagittal crest, and frontal trigon). Although a unique pattern of craniofacial sexual dimorphism may have characterized advanced stem and basal crown catarrhines, expression of various allegedly "discrete" craniofacial features may have been intraspecifically variable in early catarrhine species due to high levels of dimorphism and so should be treated with caution in phylogenetic analyses.