A Systematic Revision of the Late Miocene Cercopithecid Monkeys From the Lemudong'o Formation, Kenya

The late Miocene Lemudong'o Formation in southwestern Kenya (6.2-6.0 Ma) samples an important period in Africa when early representatives of modern catarrhine clades (Hominidae and Cercopithecidae) appear. Fossils collected from this formation before 2007 include three species of colobine monkey. However, several new fossils have been recovered, including a partial cranium, calling for a reassessment of Lemudong'o Formation cercopithecid taxonomy.

This study describes 79 new cercopithecid specimens from collection years 2007-2016 and 2019-2024. These specimens, together with previously published material from the Lemudong'o Formation, are compared to fossil cercopithecids from other contemporaneous sites in Africa as well as a sample of extant cercopithecids.

New material supports the presence of the three previously identified colobine taxa and identifies a novel fourth colobine species. The novel colobine species is described as Olotimcolobus narokensis gen. et sp. nov., and the previously recognized small indeterminate colobine is here named Lemucolobus orkimusei gen. et sp. nov. Existing hypodigms are refined in light of this new taxonomy. A papionin is also identified from this formation for the first time, adding to the scant late Miocene cercopithecine record.

The Lemudong'o Formation is remarkable for its high cercopithecid diversity in the Late Miocene. The predominance of colobines here and at other contemporaneous sites (e.g., Lukeino, Lothagam, Nakali, Toros Menalla) suggests that colobines were more diverse than cercopithecines through the early part of their radiation. Possible ecological factors are discussed in the context of early cercopithecid diversity and evolution.