Bringing up baby: Developmental simulation of the adult cranial morphology of Rungwecebus kipunji

Rungwecebus kipunji is a recently discovered, critically endangered primate endemic to southern Tanzania. Although phenetically similar to mangabeys, molecular analyses suggest it is more closely related to Papio or possibly descended from an ancient population of baboon-mangabey hybrids. At present, only a single kipunji specimen, an M1-stage juvenile male, is available for study; thus, the cranial morphology of the adult kipunji is unknown. In this study, we used developmental simulation to estimate the adult kipunji's 3D cranial morphology. We examined variation in cercopithecine developmental vectors, applied selected vectors to the juvenile cranium, and compared the resulting simulated adults to actual adult male papionins. Differences between papionin developmental vectors were small and statistically insignificant. This uniformity suggests conservation of an ancestral papionin developmental program. Simulated kipunji adults were likewise extremely similar. As a group, the simulated adults were morphometrically distinct from other papionins, corroborating the kipunji's generic status. Simulated adults were phenetically most similar to Lophocebus aterrimus but were distinguished from all adult papionins by the same unique traits that characterize the kipunji juvenile: a tall neurocranium, broad face, short nasal bones, concave anteorbital profile, and dorsally rotated palate. This concordance between juvenile and estimated-adult morphologies confirms that papionin cranial shape is largely established before M1 eruption. The estimated kipunji adult's neurocranium strongly resembles that of Papio, providing the first cranial evidence supporting their phylogenetic relationship. If the kipunji does indeed have a hybrid origin, then its phenetic affinity to L. aterrimus favors Lophocebus as the proto-kipunji's paternal lineage.