Use of three-dimensional geometric morphometrics for the identification of closely related species of Caucasian rock lizards (Lacertidae: Darevskia)

Geometric morphometrics is a better tool to evaluate the variation of shape than ‘traditional’ morphometrics. In reptiles, it outperforms morphometrics based on linear measurements and scalation. In an earlier study, two-dimensional outline-based geometric morphometrics in six species of rock lizards (Darevskia) showed that their shapes reflected the species divergence pattern. This allowed us to separate species from different clades, but we could not distinguish closely related species within a clade. We hypothesized that three-dimensional (3D) head shape data would be sufficiently discriminative to identify closely related species. To test this hypothesis, we compared head shapes of three closely related species of the ‘rudis’ clade, Darevskia portschinskii, Darevskia valentini and Darevskia rudis, with the last species treated as two distinct groups (D. r. obscura vs. D. r. rudis), using 3D landmark data. The 3D analysis isolated the species of the ‘rudis’ clade from each other and even separated individuals from populations that showed a genetic introgression pattern. The analysis showed that D. r. obscura is morphologically as distinct from D. r. rudis as from the other nominal species. For this reason, we suggest elevating the status of D. r. obscura to species level, i.e. Darevskia obscura Lantz & Cyrén, 1936.