published from the Usno and Shungura Formations. Here, we provide morphological and
functional arguments in favor of a taxonomic allocation of *n* = 10 specimens to the species *P. mutiwa*. Humeral, ulnar and femoral specimens with morphological resemblance to the *P. mutiwa* partial skeleton KNM-WT 16827 were found in members C, E and G of the Shungura Formation and in the White Sands locality of the Usno Formation. KNM-WT 16827 provided critical information on the substrate preferences of *P. mutiwa* and the previous functional analysis of Anderson (2021) hypothesized that it was mainly terrestrial based on, among the anatomical characters also preserved in the Omo specimens, a robust deltoid tuberosity, a retroflexed humeral epicondyle, a deep ulnar sigmoid notch, a prominent femoral greater trochanter and an asymmetrical astragalar trochlea. Our functional interpretation of *P. mutiwa* is more balanced and demonstrates that its elbow was likely adapted to tree climbing and overall, that its anatomy was consistent with mixed locomotor substrate preferences.

The most distinct morphological aspects of the *Paracolobus* cf. *mutiwa* from the Omo are their enlarged *m. brachioradialis* enthesis (and hence enlarged lateral supracondylar crest) and their deep supratrochlear fossae, features unknown in this state of development in other colobines apart for the partial skeleton of *P. mutiwa* KNM-WT 16827. A broad and proximally developed lateral supracondylar crest was interpreted in relation to manual foraging and climbing in the papionines *Mandrillus* and *Cercocebus* (Fleagle & McGraw 2002) and climbing in fossil and extant anthropoids (Fleagle & Simons 1982, Senut *et al.* 2001, Koukoubis *et al.* 1995) and carnivores (Gardin *et al.* 2021). Given the primary role of the *m. brachioradialis* as an elbow flexor (Boland and Spigelman 2008), we interpret here the developed lateral supracondylar crest of the Omo specimens and KNM-WT 16827 as evidence for climbing behaviors instead of terrestrial quadrupedalism, as proposed by Anderson (2021). Similarly, a deep sigmoid notch would have stabilized the elbow during slow and cautious climbing (Nakatsukasa *et al.* 1996; Drapeau 2008; Alba *et al.* 2012). The morphology of the supratrochlear fossae and medial epicondyle also supports our functional hypothesis regarding climbing abilities. The large size of *P. mutiwa* implies osteological and behavioral adaptations to dwell in trees. Specifically, we hypothesize that *P. mutiwa* presents osteological adaptations related to enhanced abilities to maneuver and climb on vertical arboreal supports with a flexed elbow.

While minor morphological variation is observed in *P.* cf. *mutiwa* specimens from the Omo,
substantial differences in size and mass were noticed based on geometric mean comparisons
and body mass inferences. These observations demonstrate the presence of a high degree
of sexual dimorphism and thus identification of presumably large male individuals (i.e., OMO
176-10006) and smaller female individuals (i.e., OMO 70-10042 and L 293-10004) according
to the lower and upper range of size variation of our sample. Apart from KNM-WT 16827, no canines of *P. mutiwa* are preserved (i.e., only the lower portion of the upper canine crown is preserved in KNM-ER 3843). Thus, on the sole basis of canine dimensions, we cannot confidently assess the level of sexual dimorphism in *P. mutiwa*. According to our postcranial specimens, the degree of sexual dimorphism in *P. mutiwa* would be as high as that of *Nasalis larvatus*. Sexual dimorphism in frequencies of substrate use and locomotor behaviors is known for extant cercopithecid (e.g., described for *Cercocebus agilis* in Shah 2003, for *Rhinopithecus bieti* in Isler & Grüter 2006, and for *Rhinopithecus strykeri* in Yang et al. 2021). Our functional results have significant implications on this aspect as the specimen L 293-10004, which is assumed to be from a female *P. cf. mutiwa* individual, presents a distinct morphology from the male *P. cf. mutiwa* individuals. Indeed, its overall gracility, enhanced mobility of its proximal radioulnar joint, and lack of marked stabilization of the humeroulnar joint reflect a greater utilization of arboreal substrates relative to putative male ulnar specimens. Subsequently, this suggests sexual dimorphism in locomotor substrate use in conjunction with body mass differences in the fossil species *P. mutiwa*.

## Rhinocolobus turkanaensis

Hypotheses regarding the locomotor repertoire and locomotor substrate use of *R. turkanaensis* were primarily based on the partial male skeleton KNM-ER 1542 (Jablonski & Leakey, 2008b), but isolated remains from Koobi Fora (Jablonski & Leakey 2008b), Hadar (Ciochon 1986, Frost & Delson 2002) and Laetoli (Harrison 2011, but see Frost et al. 2022) are also part of the hypodigm of *R. turkanaensis*. Although the taxonomic allocations of ulnar and humeral specimens of *Paracolobus cf. mutiwa* have been confidently demonstrated, the case is noticeably different for *R. turkanaensis* since the specimens discussed here (with the exclusion of the cf. *Rhinocolobus* sp. from Laetoli) span a time interval of ca. 600,000 years for the Omo specimens and 1.84 million years for asserted and presumed *Rhinocolobus* comparative specimens.

Regarding forearm bones, the morphotype from members C and E that we likened to *Rhinocolobus* suggests increased forearm extension and rotational capabilities compared to *P. mutiwa*. Our knowledge of the postcranial anatomy of *Rhinocolobus* comes primarily from male specimens (Jablonski & Leakey 2008b). Our data support the hypothesis of sexual dimorphism to explain the differences between *Rhinocolobus* specimens from the Omo and Koobi Fora, as their size difference does not exceed that of *Nasalis larvatus*.

The *R. cf. turkanaensis* femora identified from lower Member G exhibits a morphology reminiscent to that of an isolated femur from Koobi Fora putatively identified as *Rhinocolobus*
sp. (KNM-ER 551). A nearly complete radius (OMO 2-10029) from unit G-29 has anatomical characteristics in line with a frequent use of arboreal substrates and enhanced forearm mobility. Its radial head anatomy is a perfect fit with its congruent portion on the distal part of a sub-complete humerus from G-28 (F 500-1). The nearly complete humerus F 500-1 fits the morphology and size of the comparative sample of *Rhinocolobus* from Koobi Fora, particularly to KNM-ER 45611.

As for the functional anatomy and paleoecology of *Rhinocolobus*, our analysis corroborates previous studies (Table 1) and supports: 1) increased arm extension capabilities and mobility of the humeroradial and glenohumeral joints, 2) preferential use of arboreal substrates, and 3) significant hip mobility.

## Early Colobus *from* Member L

Our knowledge of the appearance of the genus *Colobus* is based on fossils spanning from the lower Pliocene of Kanam East (if we assume the stratigraphic context is correct) to the Middle Pleistocene of the Asbole deposits (Harrison 1996, Frost & Alemseged 2007). Prominently, specimens from the Koobi Fora Formation (Okote Member) and Asbole Formation, are at least 380,000 years older and up to 450,000 years younger, respectively, than the fossil colobines from Member L. Fossils from the Okote Member represent *Co. freedmani*, a species distinct from any modern species, whereas the taxonomic status of the Asbole sample is not precisely asserted (*Piliocolobus* or *Colobus* but excluding *Co. guereza*). In any case, no definitive assignment of cranial, dental nor postcranial fossils to *Co. guereza* has been established with certainty in the Pliocene and early Pleistocene of Africa. The molecular dating of the separation of *Co. guereza* from its sister taxa *Co. polykomos* and *Co. vellerosus* to ca. 1.60 ± 0.40 Ma (Ting, 2008) overlaps the time interval corresponding to Member L deposition (1.38 Ma - 1.05 Ma).

Numerous craniodental specimens similar in morphology and size to *Colobus* have been identified in the members K and L of the Shungura Formation (Leakey 1987). The postcranial specimens of early *Colobus* described here from Member L present unambiguous morphological adaptations for arboreal locomotor substrate preferences. Nevertheless, minor morphological differences have been demonstrated in Shungura specimens, mostly related to higher abilities for leaping compared to *Co. freedmani*. A humerus from Konso (KGA 4-418), previously identified as an indeterminate Cercopithecidae by Frost (2014), is also hypothesized here to be a colobine similar in morphology to extant and fossil *Colobus*. Moreover, KGA 4-418 bears a close resemblance with the *Colobus* collection from Shungura. A precise taxonomic allocation is impossible based on the postcranial specimens recovered
so far from Shungura, and we recognize the limitation of our comparative dataset in identifying
significant postcranial differences between *Colobus* and *Piliocolobus*. Indeed, our
*Piliocolobus* sample is dominated by female individuals and by populations predominantly
coming from central Africa. Future studies focusing on the morphological distinction between
*Colobus* and *Piliocolobus* could potentially clarify the taxonomic status of Shungura
specimens.

As forest-dependent cercopithecids, contraction and expansion of forest cover as well as
changes in the hydrographic system may have significantly influenced the taxonomy and
distribution of early *Colobus* representatives. If we consider Koobi Fora, Asbole and Omo
specimens to have different taxonomic status, then this taxonomic diversity may reflect
distinct forest refugia, similar to the pattern inferred from molecular data in Cercopithecini
(Tosi 2008). Future studies of craniodental fossils of the small colobines from Member L may
resolve the issue of the taxonomy of the earliest Shungura *Colobus* representatives.

*Taxonomically indeterminate specimens from Usno and the Member B*

Previously identified as a colobine (Ciochon 1993), the humerus OMO 3/O-1968-1410 has a
puzzling mosaic of characters and its taxonomic assignment is treated with caution here. If
we accept the colobine status of this specimen, then it provides evidence of a partly terrestrial
colobine in the time frame covered by unit B-12 (ca. 2.92 Ma). The recognition of a new
partially terrestrial colobine will further add to the ecomorphological diversity documented
hitherto among this subfamilly (Jablonski & Leakey 2008a&b, Pallas et al. 2019). Until now,
evidence for the ulnar anatomy of early medium-sized colobines from the late Pliocene in
eastern Africa has been meager (Frost & Delson 2002, Hlusko 2006, 2007).

Ulnar specimens of a medium-sized colobine from the lower part of the Member B (P 732-1) and Usno (B-818A) demonstrate the presence of arboreal colobines during this period in the northern part of the Turkana Depression.

*Taxonomically indeterminate specimens from the Member C*

Intriguing postcranial specimens that differ in size and shape from *Rhinocolobus* and *Paracolobus* were identified in Member C. The morphological distinctiveness of OMO 165-1973-608, OMO 18-1967-135 and OMO 18-1971-702 confirms taxonomic diversity among the colobine paleocommunity of Shungura Member C, a period that also includes *Rhinocolobus* and *Paracolobus*. These specimens may represent the same taxon as Colobinae gen. indet. sp. indet. known from isolated dental specimens in Member C (n = 13 specimens spanning units C-4 to C-8, and from locality OMO 18 according to Leakey 1987).
This last point is strengthened by the congruence of body masses inferred from postcranial
and dental data.

In functional terms, the morphology of the humerus of the indeterminate colobine from the Member C reflects a combination of mobility and stabilization of the humeroradial and humeroulnar joint. The closest morphological similarity is with *Ce. meaveae* and with specimens from Lemudong'o assigned to *P. enkorikae* (Hlusko 2007), although the Omo specimens are larger than the latter. Considering the meager postcranial data we have for this Colobinae gen. indet. sp. indet., it seems difficult to assess taxonomic hypotheses with confidence but further comparisons with *Ce. meaveae* are needed.

IMPLICATIONS FOR THE PALEOENVIRONMENTS, PALEOECOLOGY, AND EVOLUTION OF THE PLIO-
PLEISTOCENE FOSSIL FAUNA

The presence of taxonomically distinct, mainly arboreal, large colobines in members C, E,
Lower G, and Upper G and the existence of small colobines in Member L have implications
for our interpretations of contemporaneous paleoecosystems. Although cercopithecids with
terrestrial postcranial traits (e.g., *Allenopithecus*) can be found in closed environments
(Maisels et al. 2006), the presence of large arboreal colobines adds a further line of evidence
for paleoenvironmental hypotheses and allows discussion of niche partitioning among the
catarrhine fauna, and early hominins in particular. As with fossil colobines, great taxonomic
diversity is documented for early hominins during the Plio-Pleistocene (Wood and Boyle,
2016), leaving open the question of niche partitioning between arboreal colobines and partly
arboreal hominins.

The depositional contexts of Member C correspond to a fluvial environment (i.e., the
meandering paleo-Omo river) and floodplains along with paleosols indicating a drier climate
compared to pre-2.9 Ma (Haesaerts et al. 1983). This drying trend is confirmed by the C4
enriched diets of bovids, suids and hippopotamids (Bibi et al. 2012; Negash et al. 2020), but
the abundance of the bovid Tragelaphini and its mixed feeding habits rather indicate the
presence of closed and mesic habitats (Reed 1997; Bobe and Eck 2001; Alemseged 2003;
Negash et al. 2015; Blondel et al. 2018). The presence of fossilized wood and a palynological
spectrum dominated by grasses, from C-9 to C-7, further confirms the presence of a mosaic
of habitats in Member C (Bonnefille and Deschamps 1983). Fossil specimens attributed to
*Australopithecus* sp. indet. and *Paranthropus aethiopicus* have been documented in Member
C (Suwa et al. 1996; Wood and Leakey 2011).
The depositional contexts of Member E are similar to those of Member C, although the environment is drier in E-1, as shown by pedogenic data (Haeserts et al. 1983). Compared with Member D, *Theropithecus* show a shift in diet towards one enriched in C₄ plants, similar to tragelaphins (Blondel et al. 2018) and in line with the presence of bovid ankle remains adapted to open environments (Barr 2015). Fossilized woods are documented in conjunction with a high percentage of grass pollen (Bonnefille and Deschamps 1983). The presence of *Homo*-like dental remains in E-2 and E-5 also testifies to change in the hominin fossil record (Suwa et al. 1996; Wood and Leakey 2001).

The depositional contexts of the Lower G Member are distinct from those of the members C and E, with deltaic conditions in G-11 and G-13 and lacustrine conditions in G-12. Paleobotanical data indicate the presence of a marshy gallery forest bordered by savannah (Bonnefille and Deschamps 1983) and the bovid Reduncini shows grazing habits (Blondel et al. 2022). Lower G witnesses the appearance of *Paranthropus boisei* in Shungura (Suwa et al. 1996; Wood and Leakey, 2011). The Upper G Member is characterized by a major shift in depositional context, with lacustrine conditions up to G-27 but deltaic conditions from G-28 onwards (Haesaerts et al. 1983). A return to fluviatile conditions is seen in the lower part of the Member L, with loessic sediments demonstrating dry conditions. Ostracods in the upper part of Member L, indicative of lacustrine conditions, contrast with those of the lower part of Member L. Bovids shows marked grazing habits (Negash et al. 2020) throughout the Member L sequence, and palynological spectra are dominated by grass pollens.

The description of climbing and arboreal quadrupedal traits on humeral and ulnar specimens attributed to *P*. cf. mutiwa and *R*. cf. *turkanaensis* from members C, E, Lower G and Upper G agree with the presence of at least a moderate forest cover during these time intervals. This result mitigates the idea of dry environments in Member E, specifically in E-1, E-3 and E-4 where we documented humeral and ulnar remains of *Paracolobus* and *Rhinocolobus*. Similarly, the presence of a small colobine attributed to *Colobus* in Member L support the presence of forested environments, in agreement with lacustrine conditions, but mitigating the biogeochemical results obtained from ungulate teeth. Based on dental and postcranial data, *Paracolobus* and *Rhinocolobus* are documented in presumed sympatry from Member C to Upper G, with the addition of a third, smaller, sympatric colobine in Member C. The presence in Member C of at least three colobine taxa with an estimated body mass superior to 10 kg, and reaching over 30 kg in some cases, implies the presence of a highly productive ecosystem to sustain such a diversity of colobines, not to mention the diversity of cercopithecines and hominins. The results of this study also illustrate that Shungura has no modern analog in African ecosystems. The presence of climbing-related traits in *Paracolobus*
and elbow extension in *Rhinocolobus*, particularly visible on the ulna, demonstrate subtle
differences in locomotor behaviors but the lack of data on other articular complexes precludes
any definitive conclusion. Nevertheless, our results provide new insights into the niche
partitioning of colobines and early hominins. *Paranthropus* and early *Homo*, which are
documented in presumed sympatry with *Paracolobus* and *Rhinocolobus*, have upper limb
features compatible with climbing, presumably for sheltering or nesting in trees (Ruff 2009;
Green et al. 2018; Lague et al. 2019; Richmond et al 2020; Alemseged 2023). The arboreal
adaptations of *Paracolobus* and *Rhinocolobus* demonstrated here imply that competition for
access to forest cover between catarrhines should not be neglected and may have
engendered selective pressures for a greater reliance on ground-based activities in early
hominins, especially in periods of climatic fluctuations when forest cover availability
diminished.

CONCLUSION

In the present study, we described fore- and hindlimb fossils with close morphological
affinities to associated postcranial specimens of *Rhinocolobus turkanaensis* and *Paracolobus*
*mutiwa*, adding to the knowledge of the functional anatomy and paleoecology of these large
extinct colobines. A diversity of size and morphologies is highlighted in our *Rhinocolobus* cf.
*turkanaensis* sample while our description of isolated specimens presumably assigned to *P.*
*mutiwa* provides valuable information on the functional aspect of the postcranial anatomy of
this species, particularly in regard to its climbing abilities. Among others, we described a
partial elbow of a possible *Paracolobus mutiwa* individual (L 236-1a&b), sub-complete humeri
of specimens possibly belonging to *Paracolobus mutiwa* and *Rhinocolobus turkanaensis*, the
second most complete radius of a large colobine and a sub-complete tibia of a colobine
similar in size to *Colobus*. Evidence for enhanced foream extension capabilities were
highlighted in ulnar specimens from the members C and E (e.g., L 373-3 and OMO 57/4-1972-
164) while climbing and leaping are characterized in specimens from Lower G (e.g., OMO
222-1973-2751) and Member L (e.g., OMO 342-10019), respectively. This analysis confirms
the arboreal substrate preferences of *Rhinocolobus* and add new insights regarding the mixed
substrate preferences of *P. mutiwa* and its climbing abilities. By documenting morphologically
distinct specimens in presumed sympatry in members C, E and Lower G, the present work
also appears as a first step towards a better understanding of the niche partitioning of the
early colobines. This last point is of tremendous value given the diverse Plio-Pleistocene
primate paleocommunity hitherto documented within African paleoecosystems.
## Acknowledgements

We are grateful to the National Museum of Ethiopia/Ethiopian Heritage Authority (ex-ARCCH, Ministry of Tourism) for allowing to access collections and perform research. We deeply thank the NME staff (T. Getachew, S. Selassie, and G. Tekle Yemanebirihan) for guidance and support with collection study. We are thankful to the curators of various institutions who allowed us to study extant specimens under their care (E. Gillisen, RMCA; J. Cuisin, MNHN; L. Costeur and F. Dammeyer, NMB; D. Berthet MHNL; C. Zollikofer and M. Ponce de León UZH-MA; J. Kibii, NMK; A. van Heteren and M. Hiermeier, ZSM). We thank William Kimbel and Kaye Reed for providing access to the Hadar cercopithecid fossil collection. L. Pallas is grateful to the doctoral school Theodore Monod (Faculty of Fundamental and Applied Sciences, University of Poitiers) and Groupement de Recherche (GDR) Rift for financial support. We thank the CFEE and PALEVOPRIM staff for administrative support (M. Bridonneau, A.-L. Goujon, F. Guy, C. Ménard, L. Painault, G. Reynaud, H. Seyoum, A. Solomon). We are grateful to L. J. Hlusko, M. Nakatsukasa, J. Braga, L. Shapiro, A. Houssaye, G. Thiery, and C. Grohé for insightful comments on an earlier version of this manuscript. We are also grateful to the Editor S. Frost, and to M. Anderson and one anonymous reviewer for thoughtful comments on the manuscript. We are deeply indebted to the hundreds of people who participated to the fieldwork missions of the IORE, of the OGRE and of other research programs, who managed collections and databases, who prepared specimens, who contributed to their study, who provided financial support, who helped with administrative processes, and who provided advice and moral support. The OGRE is a joint program of PALEVOPRIM, the CFEE and the EHA principally funded by the Ministry of Europe and Foreign Affairs, the National Research Agency, the Région Nouvelle-Aquitaine, CNRS INEE, PALEVOPRIM, and the Fyssen Foundation. The OGRE is extremely grateful to the EHA, the SNNPR, the South Omo Zone, the Nyangatom and Dassanetch Weredas and their people for their help and reception.

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