A New Hesperornithid and the Relationships of the Mesozoic Birds

One of the most important avian fossils is a nearly complete skeleton of a hesperornithid from the late Cretaceous Niobrara Chalk Formation of western Kansas found by H. T. Martin in 1894. Williston assigned it to Hesperornis gracilis, a form named by Marsh but never really diagnosed or illustrated. Because of the size and apparent completeness of Marsh's monumental work on the Mesozoic birds of North America, these birds have been largely ignored since its publication. This has largely been the fate of H. T. Martin's exceptional specimen, parts of which have been figured under both Hesperornis gracilis, and H. regalis. This specimen can now be shown to be a previously undescribed genus of hesperornithiform bird, that provides the basis for a re-evaluation of the relationships of the Hesperornithiformes to other Mesozoic birds. Archaeopteryx is a member of a side-branch of avian evolution, the subclass Sauriurae, which became extinct at the end of the Mesozoic. The hesperornithiform Enaliornis is the earliest bird which can be shown to belong to the subclass Omithurae to which all modem birds belong.