Five well-supported fossil calibrations within the "Waterbird" assemblage (Tetrapoda, Aves)

The "waterbird" assemblage is a group of aquatic and semi-aquatic birds that are characterized by extremely diverse morphologies, ecologies, and life histories. The group also includes fossil representatives that constitute some of the oldest records of Neoaves and are critical to calibrating the temporal diversification of modern birds. Herein, we provide a set of five well-supported fossil calibrations from the waterbird clade that will serve to provide robust temporal calibrations for the origins of: stem Phaethontes (tropicbirds); stem Threskiornithidae (ibises and spoonbills); stem Pelecanidae (pelicans); stem Fregatidae (frigatebirds); and stem Phalacrocoracidae (cormorants). We apply stringent criteria to justify both the phylogenetic placement and geochronologic context of these specimens, and discuss potentially older records to help focus future research and collection. The fossils described here affirm previous studies in recognizing that most major cladogenetic splits within the waterbird assemblage occurred by the Eocene, supporting interpretations of both rapid lineage diversification of waterbirds in the early Paleogene, and also the rapid establishment of body plans and possibly ecologically relevant morphologies during this time period. Nathan D. Smith. Department of Biology, Howard University, 415 College Street NW, Washington, DC 20059, USA and Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20013 USA. nathan.smith@howard.edu Daniel T. Ksepka. Bruce Museum, 1 Museum Drive, Greenwich, Connecticut 06830, USA. dksepka@brucemuseum.org