The origin of the world's smallest flightless bird, the Inaccessible Island Rail Atlantisia rogersi (Aves: Rallidae)

Rails (Aves: Rallidae) are renowned for their extreme dispersal capability, which has given rise to numerous island lineages. Many insular species lost the ability to fly as a response to release from predator pressure—a feature causing rapid extinction when humans subsequently introduced mammals. The world's smallest extant flightless bird, the Inaccessible Island Rail Atlantisia rogersi, is endemic to Inaccessible Island, Tristan da Cunha archipelago, in the central South Atlantic Ocean. It is placed in a monotypic genus, but its taxonomic affinity, as well as geographic origin, are disputed. Contrary to its suggested Old World origin, we demonstrate that the Inaccessible Island Rail is nested within the mainly South American ‘Laterallus clade’ and that it colonized ≥3 million-year-old Inaccessible Island from South America c. 1.5 million years ago. The taxonomy of rails has traditionally been based on morphology, and convergent evolution has caused many cases of misclassification. We suggest a re-classification within the ‘Laterallus clade’ and call for extended coverage of taxon sampling for DNA sequencing.