Hesperornithes

Hesperornithes is an extinct and highly specialised clade of Cretaceous toothed birds. Hesperornithean birds, apparently limited to former aquatic habitats in the Northern Hemisphere, include genera such as Hesperornis, Parahesperornis, Baptornis, Enaliornis, and Potamornis, all strong-swimming predatory waterbirds. Many, if not all, species were completely flightless. The largest known hesperornithine, described in 1999 and named Canadaga arctica, may have reached a maximum adult length of over 1.5 metres (5 ft).

Hesperornithean birds were the only Mesozoic birds to colonize the oceans. They became extinct at the Cretaceous–Paleogene boundary, along with enantiornithine birds, all non-avian dinosaurs, and many other mostly reptilian life forms.

Anatomy and ecology

Most of what is known about this group rests on analyses of single species, as few provide sufficiently diverse fossils for analysis. In most cases, what holds true for one hesperornithine also applies to the others, as these birds were quite stereotypical and extreme in their autapomorphies.