Cryptic diversity and non-adaptive radiation of montane New Guinea skinks (Papuascincus; Scincidae)

New Guinea, the world's largest and highest tropical island, has a rich but poorly known biota. Papuascincus is a genus of skinks endemic to New Guinea's mountain regions, comprising two wide-ranging species and two species known only from their type series. The phylogeny of the genus has never been examined and the relationships among its species – as well as between it and closely related taxa – are hitherto unknown. We performed the first large-scale molecular-phylogenetic study of Papuascincus, including sampling across the genus' range in Papua New Guinea. We sequenced three mitochondrial and two nuclear markers from 65 specimens of Papuascincus and reconstructed their phylogenetic relationships. We also performed species-delimitation analyses, estimated divergence times and ancestral biogeography, and examined body-size evolution within the genus. Papuascincus was strongly supported as monophyletic. It began radiating during the mid-Miocene in the area now comprising the Central Cordillera of New Guinea, then dispersed eastward colonising the Papuan Peninsula. We found evidence of extensive cryptic diversity within the genus, with between nine and 20 supported genetic lineages. These were estimated using three methods of species delimitation and predominantly occur in allopatry. Distribution and body-size divergence patterns indicated that character displacement in size took place during the evolutionary history of Papuascincus. We conclude that the genus requires comprehensive taxonomic revision and likely represents a species-rich lineage of montane skinks.