Summary

1. In this paper, the phylogenetic relationships of Theropithecus to other cercopithecid genera are considered and the phyletic relationships within the genus are examined.

2. Theropithecus is distinguished from other Papionini by a large number of derived gross anatomical features associated mostly with its adaptations to a highly terrestrial habitus and ‘manual grazing’.

3. Available neontological and palaeontological evidence suggests that, among the Papionini, Theropithecus shared a common ancestor most recently with Papio, between 7 and 3.5 Ma ago.

4. Theropithecus comprises three distinct species groups: one including the modern gelada, one connecting T. darti and T. oswaldi, and one containing T. brumpti and its predecessors. It is thought that these lineages arose within a relatively short space of time, between 4.0 and 3.5 Ma. Delson (chapter 5) has suggested that the anatomical distinctions of T. brumpti and its putative forebears are sufficient to warrant their placement in the subgenus Omopithecus, leaving the subgenus Theropithecus to accommodate T. gelada, T. darti, and T. oswaldi. This scheme is not universally accepted, however, and two alternative views are presented.

5. Theropithecus gelada exhibits a mixture of plesiomorphic and autapomorphic features, some of the latter possibly related to its adaptation to the cold, dry environment of the Ethiopian highlands. This combination of features is not particularly useful in determining the origin of its lineage relative to others.

6. Theropithecus darti and T. oswaldi comprise a single continuous lineage of animals adapted to a highly terrestrial, grass-eating existence in the lowland grasslands of Africa. The lineage is characterized by several trends, including marked increase in body size and marked diminution of incisor and canine size. The absence of morphological discontinuity within the lineage suggests that the differences between T. darti and T. oswaldi may best be expressed as subspecific designations within T. oswaldi.

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