A species-level phylogeny of the Cretaceous Hesperornithiformes (Aves: Ornithuromorpha): implications for body size evolution amongst the earliest diving birds

Despite extensive discoveries across the globe over the past two centuries, little phylogenetic work has been done on the Hesperornithiformes. Spanning the late Early to Late Cretaceous, hesperornithiforms are one of the most diverse groups of Mesozoic birds in terms of both their geographical distribution and the wide differences in body size and diving specializations. This study presents the first phylogenetic analysis of the Hesperornithiformes that includes a majority of the described taxa, enabling the first detailed look at evolutionary relationships within the clade. The results of this study support the monophyly of the Hesperornithiformes, which is recovered as the sister clade to the avian crown group, Neornithes. Within the Hesperornithiformes, the Brodavidae and Hesperornithidae are monophyletic while the Baptornithidae are polyphyletic. Little evidence of species-level taxonomic differentiation is found within <i>Hesperornis</i>, with many species indistinguishable from <i>Hesperornis regalis</i>. Evolution within the Hesperornithiformes provides a fascinating example of progressive development of specialized diving adaptations in birds. The acquisition of these diving specializations appears to be uncorrelated to the independent evolution of multiple large increases in body size.