Summary

1. The Turkana Basin provides a unique and relatively continuous record of part of the evolution of two extinct lineages of Theropithecus. Representatives of the T. brumpti lineage are found in deposits ranging in age from approximately 3.5 to 2.0 Ma. T. oswaldi replaces T. brumpti after 2 Ma as the common cercopithecoid in the later part of the Koobi Fora Formation. The two species are found in contemporary deposits only in the Shungura Formation between Units E-3 and G-12 (approximately 2.4 to 2.0 Ma).

2. Compared with later material, the early T. brumpti specimens are smaller and have less complex molar teeth, they lack a reversed curve of Spee and expanded flaring zygomatic arches, and they have less developed maxillary and mandibular corpus fossae, mental ridges and mental rugosity. All these features are more progressively developed in the later specimens from the lower Omo Valley. The T. brumpti lineage includes specimens previously referred to T. baringensis and T. quadratirostris. Two subspecies of T. brumpti are now recognized: T. b. baringensis for plesiomorphic material from earlier horizons and T. b. brumpti for apomorphic material from later horizons. Specimens of intermediate evolutionary status are referred to T. brumpti subspecies indeterminate.

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