Forty new specimens of Ichthyornis provide 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 complete morphological assessment of the postcranial skeleton 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.

Reconstructing locomotor ecology of extinct avialans: a case study of Ichthyornis comparing sternum morphology and skeletal proportions

“…B 290: 20222020 impennis) using the 'bind.tip' function in the R package phytools [43] in R 4.1.3 [44]. Ichthyornis dispar was appended to the tree using a custom function, with a minimum age estimate of 83.5 Ma from [18,22].…”

“…[22]. It should be noted that these specimens show a broad size distribution owing to well-documented intraspecific variation within Ichthyornis [18,22]. However, given that no individual specimen (including the most complete FHSM VP 18702) preserved all elements without breakage, computing mean values provides a sufficient estimate of the true proportions of Ichthyornis given the data available.…”

“…Ichthyornis dispar, a Late Cretaceous (95-83.5 Ma) ornithurine from North America [16][17][18][19][20][21][22] has long been thought of as a sophisticated powered flier, and its postcranial morphology is more similar to flying neornithines than that of any other known stem birds. Indeed, its skeleton has often been considered tern-or gull-like, based on its size, proportions and general body shape [17,18,[22][23][24]. Various studies have proposed aquatic habits for Ichthyornis based on its occurrence in hemipelagic marine chalk localities, interpreting it as a piscivore that may have been a non-swimming flier, a surface swimmer, or an underwater foot-propelled swimmer [13,17,18,22,23,25,26].…”

“…Indeed, its skeleton has often been considered tern-or gull-like, based on its size, proportions and general body shape [17,18,[22][23][24]. Various studies have proposed aquatic habits for Ichthyornis based on its occurrence in hemipelagic marine chalk localities, interpreting it as a piscivore that may have been a non-swimming flier, a surface swimmer, or an underwater foot-propelled swimmer [13,17,18,22,23,25,26]. However, detailed statistical analyses of Ichthyornis' flight capabilities have yet to be undertaken.…”

“…Until recently, the sternum of Ichthyornis was known only from a few poorly preserved fragments, which preserved notable features such as a deep, anteriorly projecting sternal keel, a broad manubrium, and asymmetrical coracoid sulci, the last of which is a poorly understood feature shared with a number of deeply diverging neornithine clades [17,18]. However, a wealth of new Ichthyornis specimens described by [22] reveals the specialized caudal margin morphology of the ichthyornithine sternum, with reduced lateral and intermediate trabeculae; features that, among crown birds, are characteristic of soaring flight [37].…”