| Charles Darwin commented that Ichthyornis, as one of the “toothed birds” from the Late Cretaceous of Kansas, offered some of “the best support to the theory of evolution” (in litt., C. Darwin to O. C. Marsh, August 31, 1880). Ichthyornis figures no less prominently today. It is one of the closest outgroups to crown clade Aves, and remains one of the only Mesozoic avialans known from more than a handful of specimens. As such, Ichthyornis is an essential taxon for analyses of deep divergences within Aves because of its influence in determining the morphologies ancestral to the crown clade.; Ichthyornis, however, has languished in need of new anatomical description and taxonomic revision. Many of the best Ichthyornis specimens were inaccessible, plastered in to Yale Peabody Museum (YPM) exhibit mounts for nearly a century. The focus of this dissertation was the YPM Ichthyornis collection, the largest such collection at any institution.; The elements removed from the mounts were identified to the specimens with which they were originally associated. Study of all 81 YPM specimens yielded the following results: (1) there is evidence for only one species of Ichthyornis, rather than the eight previously proposed; (2) 78 specimens are part of this species, Ichthyornis dispar , (3) two previously-identified species are not part of Ichthyornis; and (4) one new clade is identified. This analysis also provided a case study in the application of phylogenetic nomenclature at the species level. The morphology of Ichthyornis dispar is described in detail from the holotype and referred specimens.; Phylogenetic analyses of 202 morphological characters, scored for 33 terminal taxa, evaluated the relationships among Mesozoic ornithurines including Ichthyornis dispar and the newly identified taxa. Previously proposed, and several new, morphological characters were scored for all Mesozoic ornithurines represented by more than a single bone. Analysis of 23 core taxa produced 2 MPTs (L: 384, CI: 0.66). The methodological problems posed by extensive missing data for the remaining ten taxa were investigated, and the relationships of these taxa were then analyzed. The phylogenetic analyses have implications for previous hypotheses of the timing and pattern of the origin of Aves. |
