Acorn barnacles (Crustacea, Thoracica, Balanomorpha) from the Eocene and Oligocene of the Isle of Wight and Hampshire (United Kingdom) and Manche, northern France

Balanomorph cirripedes from the Eocene–Oligocene of the Hampshire Basin (United Kingdom) and the Middle Eocene of the Cotentin Peninsula, Manche (France) are described. A new genus, Vectibalanus , is founded, with the type species Balanus unguiformis J. de C. Sowerby, 1846; assigned to this are also Balanus erisma J. de C. Sowerby, 1846 and Vectibalanus mortoni sp. nov. In addition, a new species of Lophobalanus Zullo, 1984, L. fresvillensis sp. nov., is described. This is the first record of that genus from outside the eastern USA and the oldest species known to date. Cladistic analysis of 24 morphological characters suggests that Vectibalanus unguiformis is sister taxon to a group comprising the most derived balanomorph taxa, and thus represents an important transition in the evolution of the group, with the initiation of development of a complex parietal wall structure. Vectibalanus unguiformis was evidently adapted to low salinity habitats (10–30 ppt), and is the oldest known brackish water barnacle. The other species ( V. erisma, V. mortoni sp. nov.) occupied more clearly marine environments (>30 ppt). Balanomorph barnacles appeared simultaneously in the Priabonian (Upper Eocene) of the Gulf and Atlantic seaboards of the USA and northwest Europe, which probably represents a northerly migration from Tethys.