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.
