FIGURE 1. Specimens representative of two taxa that provide fossil calibrations within the waterbird assemblage. 1, Limnofregata azygosternon (USNM 22753) from the early Eocene Green River Formation represents the sister taxon (with its congener Limnofregata hasegawai ) to Fregata, and calibrates the node Fregatidae + Suloidea. 2, Rhynchaeites messelensis (SMF 218) from the middle Eocene of Messel. Together with other Messel Rhynchaeites messelensis specimens and Rhynchaeites sp. (MGUH 20288) from the early Eocene Fur Formation, these represent the sister taxon to Threskiornithidae, and MGUH 20288 calibrates the node uniting Threskiornithidae and its extant sister taxon. Scale bars equal 50 mm.

FIGURE 2. Preferred phylogenetic tree based on the phylogenomic analysis of Hackett et al. (2008) showing the family-level relationships (with silhouettes of representatives within each family) within the waterbird assemblage, with five fossil calibrations (Table 1) included. Groups color-coded by membership in traditional avian orders: Pelecaniformes (green), Ciconiiformes (orange), Procellariiformes (blue), Sphenisciformes (brown), Podicipediformes (gold), Gaviiformes (purple). Stratigraphy and associated ages from Cohen et al. (2013; see also Walker et al., 2013). Branches (internodes) outside of fossil calibrations are set to a unit length to make topological relationships clear, and are not intended to represent actual divergence times. Double hash marks indicate that the clade containing Phaethon, Podiceps, and Phoenicopterus is actually recovered as distantly related to the waterbird clade (i.e., it is not its sister-taxon). Nodes that are numbered correspond to fossil calibrations described in the text. Asterisk next to Waimanu manneringi references a vetted calibration from Ksepka and Clarke (2015). Abbreviations: E, Early. M, Middle. L, Late.

FIGURE 3. Alternative phylogenetic tree based on the osteological analysis of Smith (2010) showing the family-level relationships (with silhouettes of representatives within each family) within the waterbird assemblage, with five fossil calibrations (Table 1) included. Groups color-coded by membership in traditional avian orders: Pelecaniformes (green), Ciconiiformes (orange), Procellariiformes (blue), Sphenisciformes (brown), Podicipediformes (gold), Gaviiformes (purple). Stratigraphy and associated ages from Cohen et al. (2013; see also Walker et al., 2013). Branches (internodes) outside of fossil calibrations are set to a unit length to make topological relationships clear, and are not intended to represent actual divergence times. Nodes that are numbered correspond to fossil calibrations described in the text. Asterisk next to Waimanu manneringi references a vetted calibration from Ksepka and Clarke (2015). Abbreviations: E, Early. M, Middle. L, Late.