PT - JOURNAL ARTICLE AU - Juan Benito AU - Albert Chen AU - Laura E. Wilson AU - Bhart-Anjan S. Bhullar AU - David Burnham AU - Daniel J. Field TI - 40 new specimens of <em>Ichthyornis</em> provide unprecedented insight into the postcranial morphology of crownward stem group birds AID - 10.1101/2022.01.11.475364 DP - 2022 Jan 01 TA - bioRxiv PG - 2022.01.11.475364 4099 - http://biorxiv.org/content/early/2022/01/12/2022.01.11.475364.short 4100 - http://biorxiv.org/content/early/2022/01/12/2022.01.11.475364.full AB - Ichthyornis has long been recognized as a pivotally important fossil taxon for understanding the latest stages of the dinosaur–bird transition, but little significant new postcranial material has been brought to light since initial descriptions of partial skeletons in the 19th Century. Here, we present new information on the postcranial morphology of Ichthyornis from 40 previously undescribed specimens, providing the most detailed morphological assessment of Ichthyornis to date. The new material includes four partially complete skeletons and numerous well-preserved isolated elements, enabling new anatomical observations such as muscle attachments previously undescribed for Mesozoic euornitheans. Among the elements that were previously unknown or poorly represented for Ichthyornis, the new specimens include an almost-complete axial series, a hypocleideum-bearing furcula, radial carpal bones, fibulae, a complete tarsometatarsus bearing a rudimentary hypotarsus, and one of the first-known nearly complete three-dimensional sterna from a Mesozoic avialan. Several pedal phalanges are preserved, revealing a remarkably enlarged pes presumably related to foot-propelled swimming. Although diagnosable as Ichthyornis, the new specimens exhibit a substantial degree of morphological variation, some of which may relate to ontogenetic changes. Phylogenetic analyses incorporating our new data and employing alternative morphological datasets recover Ichthyornis stemward of Hesperornithes and Iaceornis, in line with some recent hypotheses regarding the topology of the crownward-most portion of the avian stem group, and we establish phylogenetically-defined clade names for relevant avialan subclades to help facilitate consistent discourse in future work. The new information provided by these specimens improves our understanding of morphological evolution among the crownward-most non-neornithine avialans immediately preceding the origin of crown group birds.Competing Interest StatementThe authors have declared no competing interest.