"Adopting the Phylogenetic Species Concept, Gippoliti (2017) suggested that baumstarki Matschie, 1905, and pyrrhonotus (Hemprich and Erhenberg, 1829) should be treated as full species, with pyrrhonotus having two subspecies, the nominate form and also the white-nosed formosus Elliot, 1909, with a narrow horizontal black band from the eye to the ear; this latter from southwest Ethiopia, Uganda, and presumably the south east of South Sudan. Erythrocebus baumstarki and E. p. patas in this arrangement have black faces and noses, whereas E. pyrrhonotus has a white nose. Yvonne de Jong and Thomas M. Butynski (in litt. to Russell A. Mittermeier, 17 January 2020) also argued that baumstarki should be "a full species based on its geographic isolation (>180 km to the nearest other population [pyrrhonotus in Laikipia to the north ... separated by a forest barrier]) and its distinct phenotype (e.g., black nose, lack of white moustache, and dark grey upper limbs, whereas pyrrhonotus has a white nose, white moustache, and lacks dark grey on the upper hind legs)."" (Anthony Rylands, pers. comm., 20 December 2020)