Hypotheses on rostral shield evolution in fossorial lizards derived from the phylogenetic position of a new species of Paracontias (Squamata, Scincidae)
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- Miralles, A., Köhler, J., Vieites, D.R. et al. Org Divers Evol (2011) 11: 135. doi:10.1007/s13127-011-0042-6
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In squamate reptiles the rostral shield constitutes one of the most advanced cases of reduction in the number of scales in the rostral region, an evolutionary trend clearly associated with a burrowing lifestyle. This structure is characterized by the fusion of the rostral scale with all adjacent scales into a large, smooth and conical plate covering the snout, totally encompassing the nostrils, with a horizontal groove running posteriorly from either nostril. In lizards this structure evolved several times independently, in various lineages of limbless skinks and in the family Dibamidae. We performed a multilocus phylogenetic analysis of combined mitochondrial and nuclear DNA sequences from the fossorial genus Paracontias, including P. vermisaurus, a new species described herein under an integrative taxonomic approach. The resulting phylogeny supports monophyly of Paracontias, with the following internal topology: [P. kankana (P. vermisaurus sp. n. (((P. minimus + P. brocchii) (P. manify + P. hildebrandti)) (P. rothschildi + P. fasika)))]. The molecular data, coupled with a comparative morphological study, allows us to investigate the evolution of the snout scales into a single large rostral shield in Paracontias. We discuss the evolutionary processes through which the rostral shield may have originated (e.g. fusion of scales, number and order of steps involved), and conclude that intuitive and apparently obvious hypotheses for scale homologies based on position and size only (as usually formulated in squamate taxonomy) may be highly misleading, even in closely related species. We develop the hypothesis that the rostral shield may provide several functional advantages for fossorial species in facilitating burrowing and protecting the head from strong physical stress, e.g. smoother surface reducing friction between the tegument and the substrate, reduction in the number of flexible sutures resulting in strengthened tegument, and the rostral tip likely playing a role as a shock-absorbing buffer.