Tertiary water striders (Hemiptera, Gerromorpha, Gerridae) from the central Tibetan Plateau and their palaeobiogeographic implications

The semi-aquatic hemipteran family Gerridae (water striders), similar to other gerromorphan bugs, is a distinct group noted for their ability to walk on the surface of water, especially with respect to their locomotion, feeding, and behaviour. The three principal Holarctic gerrine genera, Aquarius Schellenberg, Gerris Fabricius and Limnoporus Stal, are important organisms for studies of biogeography, evolutionary trends and ecological adaptations of the Gerridae. Fossil gerrids are comparatively sparse, and direct fossil evidence elucidating the early diversification and biogeography of the extant widespread genus Aquarius remains elusive. Here we re-describe and reinterpret the extinct species Aquarius lunpolaensis (Lin, 1981) based on new well-preserved conspecific specimens from contemporaneous localities in the Lunpola and Nima basins of central Tibet, China. The new fossils confirm the placement of these fossils in Aquarius as evidenced by their large body size, first antennal segment longer than the following two segments combined, and well-developed abdominal connexival spines. The new discovery of a series of larval instars at different developmental stages confirms that "Halobates bagonensis" should be interpreted as an exuvium of the dominant species Aquarius lunpolaensis and demonstrates that the extant western Palaearctic najas-group had a much wider distribution in the late Oligocene (Chattian). The occurrence of an Aquarius water skater supports the hypothesis that the elevation of central Tibet was comparatively low, some 25 million years ago.