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Primate Research Institute, Kyoto University
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Harrison / Oral

Terry Harrison, Department of Anthropology, New York University, New York, NY 10003

     Proconsul, from the early and middle Miocene of East Africa, is widely regarded as the most primitive representative of the Hominoidea. However, a review of the historical development of this viewpoint shows that the proposed relationship of Proconsul to extant hominoids has long been an unquestioned assumption without adequate evidence. There appear to be relatively few characters that can be advanced in support of such a phylogenetic link, and in many cases these features are of limited or uncertain significance for determining relationships. Critical scrutiny of some of the key morphological features and complexes traditionally viewed as Proconsul+hominoid synapomorphies (i.e., relative premolar cusp height, brain size, morphology of the distal humerus, and absence of a tail) serves to illustrate the nature and complexity of this problem. For example, based on new estimates of cranial capacity in KNM-RU 7290, relative brain size in Proconsul heseloni can be shown to be close to the mean value for extant anthropoids. Also, given the range of diversity seen in modern anthropoid clades it would appear that the degree of encephalization is of limited utility as a character in phylogenetic analysis, and can be shown to be much more intimately correlated with behavioral or ecological attributes, such as diet. Similarly, vertebrae associated with the partial skeletons of Proconsul heseloni from the Kaswanga Primate Site on Rusinga Island can be interpreted as caudal vertebrae that demonstrate that Proconsul may have had a relatively long tail.
     On the other hand, there are a number of derived cranial and postcranial characters shared by all extant catarrhines that are absent in Proconsul. An important set of synapomorphies linking extant catarrhines to the exclusion of Proconsul relate to the possession of a complex of anatomical specialization associated behaviorally with a greater emphasis on the adoption of more orthograde postures and with sitting or sleeping upright on relatively small diameter perches. These include relatively large lumbar vertebrae and the development of ischial callosities. The primitive retention of small lumbar vertebrae and the absence of ischial callosities in Proconsul demonstrate that it lacked a major defining characteristic of modern catarrhines. In addition, extant catarrhines share a number of important derived features of the cranial base and facial skeleton, whereas Proconsul retains a more primitive platyrrhine-like morphology in these respects. One can conclude from these new observations that Proconsul is not a primitive hominoid, as has generally been inferred, but is rather a stem catarrhine that diverged prior to the last common ancestor of all extant members of the Catarrhini. It is important to note, however, that removal of Proconsul from consideration as a stem hominoid does not dramatically alter the configuration or timing of the major cladogenetic events in catarrhine and hominoid evolution. Morotopithecus, an East African early Miocene contemporary of Proconsul, appears to be a demonstrable stem hominoid that establishes that this clade originated prior to ~21 Ma.