Darren Naish at Tetrapod Zoology has an excellent roundup of recent work on the group of monkeys named in the title, following the discovery of a new mangabey monkey called a kipunji. He notes that molecular data suggest a diphyletic (two separate evolutionary branches) origin of mangabeys, some of which are more closely related to baboons, and some of which are more closely related to mandrills. He also notes that it is an unanswered question whether the "dog-face" morphology of these animals is primitive, or evolved three times separately, and that fossil data don't resolve the matter either. It's a very good post, as so many of his are.
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Now, are baboons/drills older than mangabeys, or are mangabeys older than baboons/drills? If the latter it might be profitable to seek a common ancestor of the two mangabey groups.
Macaques date from the late Miocene-Early Pliocene, ditto for baboons. The earliest mangabey I am familiar with (Cercocebus) dates to the late Pliocene. My guess is that since macaques are considered to be the least specialized (and likely reflect the ancestral condition) it would be short faced...
Crud, I should think more before I hit post...I should also mention that Darren's post is excellent and I thoroughly enjoyed reading it.
It's of course always moot whether ancestral states for a diverse group need to be "least specialised" or not. Seems to me there need be no rule on this matter that would aid inference on its own.
Well yes, that was a clumsy way of saying it. The point was macaques have quite a few traits that are considered ancestral catarrhine traits - which is where I should have stopped...