Medeiros et al. (1997) also proposed that A. chamek represents the most basal form of the genus, which they argued originated in the southwest- ern Amazon basin and extended eastwards and northwards, giving rise to A. marginatus in the central Amazon and to A. belzebuth in the northwest. The authors also concluded that Ateles fusciceps (including A. rufiventris) may be reproductively isolated from A. geoffroyi and A. hybridus and therefore considered a different spe- cies (Fig. 1) (Medeiros et al., 1997).