Distinctiveness of <i>Ugandapithecus</i> from <i>Proconsul</i>

The decision to create the genus Ugandapithecus by Senut et al., 2000 has been criticised, either directly and in detail by MacLatchy & Rossie (2005b) who argued that it is a junior synonym of Proconsul, or indirectly without providing reasons, firstly by Harrison (2001) who wrote that he did not retain it as a genus distinct from Proconsul, and then by Suwa et al. (2007) who employed the name “Ugandapithecus” with inverted commas, implying some degree of doubt about its validity as a genus, but without providing details. More recently Harrison & Andrews (2009) have recognised the Meswa sample as a separate species but they argue that it should be maintained within Proconsul, despite the morphological differences that it has from other species of the genus. We here re-examine the question by comparing, on the one hand, the holotype maxilla of Proconsul africanus, the type species of the genus, with the upper dentition of Ugandapithecus major, and, on the other hand, the holotype mandible of Ugandapithecus major with the lower dentition and mandibles previously attributed to Proconsul africanus. We conclude that the differences between the known upper and lower dentitions of P. africanus and U. major are of such a degree that the two taxa warrant generic separation, and that the differences are not related to sexual dimorphism. Where Proconsul africanus differs from Ugandapithe- cus major, it approaches Proconsul nyanzae and Proconsul heseloni from Rusinga.

Furthermore, the range of morphometric variation within the fossil samples previously attributed to Ugandapithecus major is so great that it far surpasses variation in any other hominoid, fossil or extant. Previously this great amount of variation was interpreted to mean that U. major was extremely dimor- phic, with huge males and small females, but if this is true, then U. major would be unique among homi- noids in having females in which the cheek teeth fall completely outside the range of variation of male cheek teeth. All other known male and female hominoid species possess cheek teeth whose ranges of variation overlap strongly. Bivariate plots of the teeth attributed to Ugandapithecus reveal three consis- tent non-overlapping clusters of points among the Early Miocene specimens, which we interpret to represent three distinct species. A fourth species of the genus, U. gitongai, was previously defined at the Middle Miocene site of Kipsaraman, Kenya. We conclude that Ugandapithecus was a lineage of great ape distinct from Proconsul, and its main evolutionary trend was an increase in dimensions from basal Early Miocene Ugandapithecus meswae (21.5-19 Ma) to late Early Miocene species Ugandapithecus leg- etetensis nov. sp. (20-19 Ma) and Ugandapithecus major (19-18 Ma), and culminating in late Middle Miocene Ugandapithecus gitongai (ca. 14.5 Ma).