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Abstract New Guinea and surrounding islands are home to some of the richest assemblages of insular biodiversity in the world. The key geological drivers of species richness in this region are largely considered to be mountain uplift and development of offshore archipelagos—some of which have accreted onto New Guinea—with the role of mountain uplift and elevational gradients receiving more attention than the role of isolation on islands. Here, we examine the distribution of lineage richness and body-size diversity in a radiation of Melanesian lizards that is almost entirely absent from montane habitats but closely associated with islands—the geckos of the genusNactus. Our data indicate that eastern New Guinea—centred on the East Papuan Composite Terrane (EPCT)—shows particularly high levels of endemism and body-size diversity and is also inferred to be a source area for multiple independent colonisations elsewhere in New Guinea, the Pacific and Australia. TwoNactuslineages in Australia have closest relatives occurring to the north of New Guinea’s Central Cordillera, suggesting dispersal through this area in the mid-Miocene, possibly via seaways that would have isolated the islands to the east and west of the proto-Papuan region. Syntopic species tend to differ in body length; however, at a phylogenetic scale, this trait appears to be conservative, with small-sized and large-sized species clustered into separate lineages. These data suggest that species richness in MelanesianNactusis in part explained by morphological diversification enabling the presence of sympatric communities to exist, but to a greater extent by multiple instances of dispersal and extensive allopatric and parapatric speciation, especially in and around the islands of the EPCT. 

ContextSkinks comprise the dominant component of the terrestrial vertebrate fauna in Oceania, New Guinea, and Eastern Wallacea (ONGEW). However, knowledge of their diversity is incomplete, and their conservation needs are poorly understood. AimsTo explore the diversity and threat status of the skinks of ONGEW and identify knowledge gaps and conservation needs. MethodsWe compiled a list of all skink species occurring in the region and their threat categories designated by the International Union for Conservation of Nature. We used available genetic sequences deposited in the National Center for Biotechnology Information’s GenBank to generate a phylogeny of the region’s skinks. We then assessed their diversity within geographical sub-divisions and compared to other reptile taxa in the region. Key resultsApproximately 300 species of skinks occur in ONGEW, making it the second largest global hotspot of skink diversity following Australia. Many phylogenetic relationships remain unresolved, and many species and genera are in need of taxonomic revision. One in five species are threatened with extinction, a higher proportion than almost all reptile families in the region. ConclusionsONGEW contain a large proportion of global skink diversity on <1% of the Earth’s landmass. Many are endemic and face risks such as habitat loss and invasive predators. Yet, little is known about them, and many species require taxonomic revision and threat level re-assessment. ImplicationsThe skinks of ONGEW are a diverse yet underexplored group of terrestrial vertebrates, with many species likely facing extreme risks in the near future. Further research is needed to understand the threats they face and how to protect them.