Phylogenetic Relationships and Monophyly of Loons, Grebes, and Hesperornithiform Birds, with Comments on the Early History of Birds

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Abstract
A cladistic analysis of the skeletons of loons (Gaviidae), grebes (Podicipedidae), and the Cretaceous diving birds, Hesperornis and Baptornis, supports the hypothesis that they form a monophyletic group (here called the Gaviomorphae) within the class Aves. Two lineages within the gaviomorphs can be delineated: (1) loons + grebes, and (2) Hesperornis + Baptornis. The Early Cretaceous Enaliornis and the Late Cretaceous Neogaeornis are related to the second lineage; the Late Cretaceous Lonchodytes, often placed near loons, is apparently not a gaviomorph but perhaps a charadriiform. Skeletal evidence also suggests that penguins (Spheniscidae) are the sister-group of the Gaviomorphae. Arguments that similarities of gaviomorph taxa represent convergence are not well founded. First, morphological differences among taxa do not constitute valid evidence against their monophyly, as many previous workers have argued. Second, in no case has anyone supporting convergence presented a corroborated alternative phylogenetic hypothesis. A close relationship among gaviomorphs, penguins, and apparently also the Procellariiformes and Pelecaniformes implies that a lineage of aquatic birds was established very early in avian history, presumably in the Early Cretaceous or Late Jurassic.

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