The Jamaican monkey Xenothrix mcgregori is one of several extinct endemic platyrrhines known from the late Quaternary of the Greater Antilles. Until recently, the hypodigm of Xenothrix was limited to the holotype partial mandible and a handful of tentatively referred postcranial elements. Here we describe several additional fossils attributable to Xenothrix, including the first cranial remains, all of which were recovered in cave deposits in the Jackson's Bay region of southern Jamaica. In addition to a partial face from Lloyd's Cave and a maxillary fragment of a different individual from the same site, the craniodental collection includes two incomplete mandibles with poorly preserved cheekteeth from nearby Skeleton Cave. The new specimens confirm a distinctive derived feature of Xenothrix, i.e., reduced dental formula in both jaws (2/2 1/1 3/3 2/2). Although no examples of the maxillary canine are yet known, its alveolus is notably small. Similarly, although the upper face of Xenothrix is also unknown, it is clear that the maxillary sinuses were large enough to encroach significantly on the bases of the zygomatic processes. The nasal fossa is also very large and wider than the palate at the latter's widest point. A similar condition is seen in the extinct Cuban monkey Paralouatta varonai.
Xenothrix continues to generate disputes among platyrrhine specialists because its unusual combination of apomorphies complicates its systematic placement. Rosenberger's recent “Aotus hypothesis” stipulates that Xenothrix is a close relative of the living owl monkey (Aotus) and is not a pitheciid sensu stricto. Two fundamental characters used to support this hypothesis—hypertrophied orbits and enlarged central incisors—can be shown to be inapplicable or uninterpretable on the basis of the existing hypodigm of Xenothrix. The new craniodental evidence confirms our earlier cladistic results showing that the Antillean monkeys (Xenothrix mcgregori, Paralouatta varonai, and Antillothrix bernensis) are closely related and that Callicebus is their closest joint extant mainland relative. This may be expressed systematically by placing the three Antillean taxa in Xenotrichini, new tribe, adjacent to Callicebini, their sister-group within subfamily Callicebinae (Pitheciidae).