iii, 33 p. : 26 cm

A new subfamily, Limnofregatinae, is erected for the first known Tertiary specimens of the pelecaniform family Fregatidae. These are described as a new genus and species, Limnofregata azygosternon. The holotype is a nearly complete skeleton with feather impressions. Two additional specimens, consisting of a partial skeleton and a fragment of ulna, are referred to the same species. All are from freshwater lake deposits in the Lower Eocene Green River Formation of Wyoming, which are roughly 50 million years old. The principal differences between Limnofregata and modern frigatebirds (Fregata) are seen in its shorter and less hooked rostrum, proportionately shorter wings and longer hindlimb, in the lack of fusion in the pectoral girdle, and in the lack of extensive pneumatization of the skeleton. The fossilform is, nevertheless, well advanced along the lines of modern frigatebirds and nothing in its morphology seems to preclude its being ancestral to Fregata. The fossil provides additional support for placing the suborder Fregatae between the more primitive tropicbirds (suborder Phaethontes) and the members of the suborder Pelecani. Frigatebirds in the early Tertiary evidently occupied different habitats or a wider range of habitats than the modern forms and have only subsequently been restricted to a purely oceanic environment

Bibliography: p. 33

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