Revision of the Egernia stokesii species-group (Reptilia: Scincidae), including the description of a new saxicoline species from Western Australia

The large spiny skinks of the Egernia stokesii species-group are a charismatic element of the Australian herpetofauna, including some of the largest skink species and the most advanced levels of sociality documented in reptiles. Despite their familiarity, however, there are several taxonomic uncertainties within the group. Herein we review the systematics of the E. stokesii species-group, with particular emphasis on Western Australian populations, through an integrated assessment of molecular genetic and morphological variation in museum specimens. The molecular genetic analyses reveal four genetically diagnosable lineages, with a primary split between an eastern Australian population and three Western Australian populations. Morphological examination of specimens across Australia disclosed greater variation than expected, with four groups recognised: two from Western Australia that largely correspond to the two subspecies (E. stokesii stokesii (Gray, 1845) and E. s. badia Storr, 1978), a previously undescribed isolated taxon to the north-east of E. s. badia in Western Australia, and a widespread taxon from south-eastern Australia. These morphological forms show a high degree of concordance with the genetic lineages identified through molecular analyses. The combined molecular, morphological and biogeographic evidence supports the recognition of four independently evolving taxa within the Egernia stokesii species-group. For Western Australian populations, owing to introgression in the Shark Bay area, we maintain the current subspecific taxonomy and redescribe E. s. stokesii from the Houtman Abrolhos Islands and the Shark Bay region and E. s. badia from the semiarid to arid western interior of the central and northern Avon Wheatbelt, the adjacent Yalgoo and southern Carnarvon bioregions, including Shark Bay. We also confirm the synonymy of the insular E. s. aethiops Storr, 1978 from Baudin Island in Shark Bay with E. s. stokesii. We describe the distinctive dark saxicoline populations from the Upper Murchison and adjacent central Yalgoo bioregions as a new species, E. umbra sp. nov. We also elevate E. zellingi (De Vis, 1884) to full species for the eastern populations from Queensland, New South Wales, South Australia and the Northern Territory, noting that further taxonomic resolution is required. Two of the Western Australian taxa (E. s. badia and E. umbra sp. nov.) are of elevated conservation concern owing to their large body size, small and fragmented distributions, as well as historical and contemporary threats from introduced herbivores and predators, clearing and modification of vegetation, damage to rocky habitats, and illegal collections and trade.