The earliest most complete skeleton of Theropithecus

Abstract

The middle Pliocene site of Woranso-Mille in the Afar Region of Ethiopia has yielded numerous significant early hominin fossils representing multiple, coexisting taxa. Here we report on another significant discovery, the oldest partial skeleton of the papionin, Theropithecus. The specimen was recovered from the Aralee Issie collection area over multiple field seasons from 2004 through 2019. The specimen was unearthed in situ from the fluvial facies of the Mesgid Dora Tuff dated to 3.66–3.57 Ma. The partial skeleton, ARI-VP-1/26, is that of a subadult male lacking a skull. In the absence of unambiguously associated craniodental remains, the male status of the specimen was established from the dimensions of the long bones in comparison to those of other Theropithecus from Woranso-Mille. ARI-VP-1/26 is noteworthy because it preserves partial hand and foot skeletons, including a complete set of metacarpals from the left side. The theropith status of ARI-VP-1/26 was established based on the detailed anatomy of the postcranial skeleton, especially the proximal and distal humerus, proximal radius, and proximal femur. The morphology of the postcranium of ARI-VP-1/26 is consistent with that of recognized Theropithecus from Woranso-Mille and, specifically, with specimens recognized as Theropithecus oswaldi cf. darti from other sites. The ratio of the lengths of the first metacarpal to metacarpals 2–5 in ARI-VP-1/26 is intermediate between that seen in extant Papio and Theropithecus. In Theropithecus gelada and Theropithecus brumpti, the pairing of pollical and indical metacarpals of near equal length contributes to the species' high opposability index and is associated with a ‘manual grazing’ feeding habit. Cercopithecids constitute 43% of the identified vertebrates at Aralee Issie, and T. oswaldi cf. darti is the most common mammalian species. The monkeys of Aralee Issie lived in an open shrubland habitat, but the specific reasons for their high prevalence at the site are unclear.