Since the mid-1990s, various types of genetic rather than morphological data have been used to revisit the evolutionaryrelationships among the spider monkeys. For example, Medeiros et al. (1997) used karyotype data to divide the spider monkeys into four karyomorphs ([1] A. geoffroyi + A. hybridus; [2] A. fusciceps + A.rufiventris; [3] A. belzebuth + A. chamek + A. marginatus; and [4] A.paniscus), a schema similar to that of Froehlich et al. (1991), apart from the positions of A. hybridus and A. paniscus. Medeiros et al.(1997) also proposed that A. chamek represents the most basal form of the genus, which they argued originated in the southwestern
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 species(Fig. 1) (Medeiros et al., 1997).