In their analysis, Pallas and colleagues identify 32 postcranial elements from the Plio-Pleistocene collections of the Lower Omo Valley, Ethiopia as colobine (Pallas et al., 2024). This is a valuable contribution towards understanding colobine evolution, Plio-Pleistocene environments of the Turkana Basin, Kenya and Ethiopia, and how the many large-bodied catarrhines, including at least three hominins, four colobines, and three papionins, all with body masses over 30 Kg shared this ecosystem.

Today, colobine monkeys have greater diversity in Asia than in Africa, where they are represented by three small to medium-sized forms: olive, red, and black and white colobus (Grubb et al., 2003; Roos and Zinner, 2022). In the Pliocene and Pleistocene, however, they were significantly more diverse, with at least four additional large-bodied genera that varied considerably in body size, and as evidenced by multiple proxies, their preferred habitats, diets, and locomotor behaviors (Frost et al., 2022 and references therein). The highly fossiliferous sediments of the Shungura and Usno Formations in the Lower Omo Valley span the period from 3.75 to 1.0 Ma (Heinzelin, 1983; McDougall et al., 2012; Kidane et al., 2014) and have contributed greatly to understanding human and mammalian evolution during the African Plio-Pleistocene (Howell and Coppens, 1974; Boisserie et al., 2008), including the enigmatic large-bodied colobines (Leakey, 1982; 1987). Despite large samples of postcranial material from the Lower Omo Valley (Eck, 1977), most of our knowledge of fossil colobine postcrania is based on a relatively few associated skeletons from other eastern African sites (Birchette, 1982; Frost and Delson, 2002; Jablonski et al., 2008; Anderson, 2021). This is because the vast majority of postcrania from the Lower Omo Valley are not directly associated with taxonomically diagnostic elements.

Based on qualitative and quantitative comparison with an extensive database of extant cercopithecoid postcrania, Pallas et al. (2024) identify 32 long bones of the fore- and hindlimbs as colobine. These range in age from approximately 3.3 to 1.1 Ma. They made their identifications using a combination of body mass estimation and comparison with associated skeletons of Plio-Pleistocene and extant taxa. In this way, they tentatively allocate some of the larger material dated to 3.3. to 2.0 Ma to taxa previously recognized from craniodental remains, especially Rhinocolobus cf. turkanaensis and Paracolobus cf. mutiwa; and the smaller ca. 1.1 Ma to cf. Colobus. Interestingly, they also identify several specimens, especially from Members B and C, that are unlikely to represent taxa previously described for the Lower Omo Valley and make a possible link to Cercopithecoides meaveae, otherwise only known from the Afar Region, Ethiopia (Frost and Delson, 2002).

Based on these identifications, Pallas et al. (2024) hypothesize that Rhinocolobus may have been adapted to more suspensory postures compared to Cercopithecoides and Paracolobus which are estimated to have been more terrestrial. Additionally, they suggest that the possibly semi-terrestrial Paracolobus mutiwa may show adaptations for vertical climbing. These are novel observations, and if they are correct give further clues as to how these primates seemingly managed to co-exist in the same area for nearly a million years (Leakey, 1982; 1987; Jablonski et al., 2008). Better understanding the locomotor and positional behaviors of these taxa will also make them more useful in reconstructions of the paleoenvironments represented by the Shungura and Usno Formations.

References

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