A study done in 1987 linked Plesiadapiformes with adapids and omomyids through nine shared-derived features, six of which are cranial or dental : ( 1 ) auditory bulla inflated and formed by the petrosal bone, ( 2 ) ectotympanic expanded laterally and fused medially to the wall of the bulla, ( 3 ) promontorium centrally positioned in the bulla, and large hypotympanic sinus widely separating promontorium from the basisphenoid, ( 4 ) internal carotid entering the bulla posteriolaterally and enclosed in a bony tube, ( 5 ) nannopithex fold on the upper molars, and ( 6 ) loss of one pair of incisors.
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Its autapomorphies ( unique characteristics ) are the prossession of a sharp central ridge on the supraoccipital ( the bone above the occipital ), elongated frontal bones that are 2.86 times as long as they are wide, slender basipterygoid projections on the basisphenoid, a bone of the lower braincase, an atlas intercentrum that is horseshoe-shaped in cross-section, slender diapophyses, thin spines of the dorsal vertebrae and sacral vertebrae, the possession of a clear keel at the bottom of the sacral vertebrae, and an ilium with on the inner side a distinct ridge along the edge of the hip joint.
