Cercopithecids became diverse in the African Plio–Pleistocene (Jablonski, 2002; Jablonski and Frost, 2010). They also comprise a major proportion of the Pliocene eastern African fauna as documented from localities such as Middle Awash (Frost, 2001a, 2001b; Hlusko, 2006), Hadar (Eck, 1993; Frost, 2001a; Frost and Delson, 2002), Woranso-Mille (Frost et al., 2014), Gona (Frost et al., 2020b), and Omo (Eck, 1987) in Ethiopia; Koobi Fora (Jablonski et al., 2008), Kanapoi (Frost et al., 2020a) in Kenya and Laetoli in Tanzania (Leakey and Delson, 1987; Harrison, 2011). Although the fossils cover a range of time from ∼4.4 to 3.0 Ma, there are gaps in the record. Fossil remains that fill these gaps improve our understanding of cercopithecid evolution. For example, Theropithecus oswaldi is known from deposits as old as 3.7 Ma at Woranso-Mille (Frost et al., 2014), but specimens older than 3.7 Ma have been judged to be unassignable to any particular Theropithecus species below the genus level (Frost et al., 2014, 2020a). Even those assigned to particular species are the subject of a number of taxonomic assignments (Freedman, 1957; Delson, 1993; Jablonski, 2002; Getahun et al., 2023). Moreover, phylogenetic relationships between cercopithecid taxa are also not fully resolved (Gilbert, 2013; Pugh and Gilbert, 2018). Therefore, the discovery of additional materials from sites like Galili is crucial to address cercopithecid taxonomic and phylogenetic questions.

The fossil-bearing deposits at Galili are the southern-most Early Pliocene-aged paleontological site located in the Afar region of Ethiopia (Fig. 1). Fieldwork at Galili has been conducted since 1999, with teams led by Yohannes Haile-Selassie (1999), Horst Seidler (2000–2010) and most recently (ongoing since 2016) by Scott Simpson (Haile-Selassie and Asfaw, 2000; Kullmer et al., 2008, 2022; Simpson et al., 2017). The geology of the Mt. Galili Formation has been well studied (Kullmer et al., 2008, 2022; Hujer et al., 2015; Quade, 2018). The Formation includes different fossiliferous members from older to younger: Lasdanan (5.37 ± 0.04 to >4.43 Ma), Dhidinley (<4.43 to 3.94 Ma) and Shabeley Laag that ranges from 3.94 ± 0.03 to <3.87 ± 0.02 (Hujer et al., 2015; Quade, 2018). Both Hujer et al. (2015) and Kullmer et al. (2022) have recognized Godiray as a valid member located between the Dhidinley and the Shabeley Laag members. However, it was dropped in the current stratigraphic and dating work in progress because it is too thin to be of use as a meaningful member (Fig. 2; Quade, 2018). The fossil-rich members (n > 4000 Number of Identified Specimens) have yielded primarily terrestrial taxa including cercopithecid and hominin fossils. The hominin fossils have been assigned to Australopithecus cf. anamensis (Haile-Selassie and Asfaw, 2000; Simpson et al., 2017), cf. Australopithecus afarensis, Australopithecus sp. (Kullmer et al., 2008; Simpson et al., 2017), Au. cf. afarensis (Macchiarelli et al., 2004) and possibly Ardipithecus (Simpson et al., 2017).

Cercopithecids are the third most common faunal groups at Galili, after bovids and suids (Kullmer et al., 2008, 2022; Simpson et al., 2017). The cercopithecid fossils include 655 identifiable specimens representing multiple taxa from both cercopithecid subfamilies, with some of the earliest known fossils of T. oswaldi (Kullmer et al., 2008, 2022; Stadlmayr, 2009; Simpson et al., 2017; Reda et al., 2022).

Galili is one of the few African fossil sites that sample the critical time between 4.4 and 3.8 Ma (Fig. 2), overlapping the last appearance of Ardipithecus and first appearance of Australopithecus (White et al., 2009; Simpson et al., 2017). Both Ardipithecus ramidus and Au. anamensis have been recognized from several sites in Ethiopia and Kenya, including Galili. Materials of Ar. ramidus were published from the 4.4 Ma Aramis sediments (White et al., 1994, 2009) and from the 4.8–4.3 Ma As Duma member of the Sagantole Formation at Gona (Semaw et al., 2005; Simpson et al., 2019). Australopithecus anamensis fossils have been recovered from the 4.2–4.1 Ma Asa Issie in the Middle Awash (White et al., 2006) and the 3.8 Ma Woranso-Mille horizons (Haile-Selassie et al., 2019) in Ethiopia and from Kanapoi and Allia Bay in Kenya (Leakey et al., 1995). However, the evolutionary relationship between Ardipithecus and Australopithecus remains unclear despite their geographic proximity and geological age successions. Therefore, given its geological contemporaneity and proximity to sites that include Ar. ramidus and Au. Anamensis fossils, the analysis of fauna recovered from Galili can provide an important paleoecological context in the 4.4–3.8 Ma time range.

Cercopithecid-bearing fossil sites that partially overlap 4.4–3.8 Ma include Laetoli, Tanzania (Leakey and Delson, 1987; Harrison, 2011), Koobi Fora (Jablonski et al., 2008) and Kanapoi (Harris et al., 2003; Frost et al., 2020a) in Kenya, and Gona (Frost et al., 2020b), the Middle Awash (Frost, 2001a, 2001b; Hlusko, 2006) and Woranso-Mille (Frost et al., 2014) from Ethiopia. To date, only three mandibles and 50 isolated cercopithecid teeth from one locality at Galili have been studied (Stadlmayr, 2009). In the current study we conducted a comprehensive craniodental description and taxonomic identification of 362 Galili fossil cercopithecids recovered as of 2019. Because cercopithecids have been used successfully as paleoecological indicators (WoldeGabriel et al., 1994; Elton, 2006; White et al., 2006, 2009; Frost, 2007), the study of Galili cercopithecids will contribute to our understanding of the paleoecology of Galili and allow comparisons with other penecontemporaneous sites.