40 new specimens ofIchthyornisprovide unprecedented insight into the postcranial morphology of crownward stem group birds

Abstract

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.

The Evolution and Fossil Record of Palaeognathous Birds (Neornithes: Palaeognathae)

“…If this material indeed belongs to a lithornithid, it would provide compelling evidence that the clade survived across the boundary. However, it should be noted that several Mesozoic stem ornithurines also have a hooked acromion that approaches the condition seen in Lithornithidae [64,81,82]. Thus, the identity of this fossil remains uncertain, and more material needs to be recovered from both this formation and other contemporaneous localities to clarify which groups of total-clade palaeognaths persisted across the K-Pg boundary.…”

Topology-Based Three-Dimensional Reconstruction of Delicate Skeletal Fossil Remains and the Quantification of Their Taphonomic Deformation

“…The three-dimensional retrodeformation method presented herein has been developed and tested on two case studies. It was first developed for and applied to several cervical vertebrae (CV) of the diplodocine sauropod Galeamopus pabsti, based on a single individual (SMA 0011/NMZ 1000011; Tschopp and Mateus, 2017), and further refined to reconstruct the sternum of the crownward stem-bird Ichthyornis dispar (Marsh, 1880;Clarke, 2004;Field et al, 2018;Benito et al, 2022). This refinement combined information from multiple individuals preserving different aspects of each bone in 2D and 3D, allowing an idealised composite reconstruction.…”

“…The basis for the reconstruction and retrodeformation of the sternum of Ichthyornis were several 2D and 3D preserved specimens (Figure 2), described by Benito et al (2022), showing extensive overlap and thus allowing individual well preserved parts to be combined into an idealised composite reconstruction, hereafter referred to as a "prototype"; see Table 1. The terminology describing the preservation of the sterna follows Baumel and Witmer (1993) and Livezey and Zusi (2006).…”

“…The different specimens of Ichthyornis were compared, and individual well preserved regions, such as the crossed coracoid sulci in FHSM VP-18702 (Supplementary Figure 2A) or the craniolateral process and ventral lip of NHMUK PV A 905 (Supplementary Figure 2B), were detached from their original retopology meshes and combined into a new composite reconstruction. The well preserved coracoids KUVP 2281, ALMNH PV 1993.2.133, and YPM VP.001733 (Clarke, 2004;Benito et al, 2022) were used to assist in the shape reconstruction of the individual asymmetrical coracoid sulci while keeping their posterior margins symmetrical. The general shape and curvature of the lateral sternal plates were guided by sterna of comparable extant taxa, such as Anas discors, Ardea alba, Burhinus oedicnemus, and Sterna hirundo (Marsh, 1880;Clarke, 2004;Benito et al, 2022), while ensuring that the outline of the Ichthyornis sternum remained consistent with the preserved specimens (Supplementary Figure 2C).…”

“…The well preserved coracoids KUVP 2281, ALMNH PV 1993.2.133, and YPM VP.001733 (Clarke, 2004;Benito et al, 2022) were used to assist in the shape reconstruction of the individual asymmetrical coracoid sulci while keeping their posterior margins symmetrical. The general shape and curvature of the lateral sternal plates were guided by sterna of comparable extant taxa, such as Anas discors, Ardea alba, Burhinus oedicnemus, and Sterna hirundo (Marsh, 1880;Clarke, 2004;Benito et al, 2022), while ensuring that the outline of the Ichthyornis sternum remained consistent with the preserved specimens (Supplementary Figure 2C).…”