Osteological correlates of the respiratory and vascular systems in the neural canals of Mesozoic ornithurines <i>Ichthyornis</i> and <i>Janavis</i>

In birds, the neural canal houses a variety of anatomical structures including the spinal cord, meninges, spinal vasculature, and respiratory diverticula. Among these, paramedullary diverticula and the extradural dorsal spinal vein may leave behind osteological correlates in the form of pneumatic foramina and fossae, and a bilobed geometry of the neural canal, respectively. While recent studies have cast light on the evolution of avian skeletal pneumaticity, evidence for these respiratory and vascular structures has never been reported in Mesozoic ornithurines, raising questions about the evolutionary origins of these modern components of the avian respiratory and vascular systems. Here, we investigated the neural canals of Ichthyornis and Janavis, which provide the first evidence of paramedullary diverticula and spinal vasculature in Mesozoic ornithurine birds. In both taxa, numerous pneumatic foramina are present inside the vertebral canals, primarily in the cervical and thoracic vertebrae. Ichthyornis and Janavis also both exhibit evidence of a large, extradural dorsal spinal vein in the cervical and thoracic regions, as indicated by a bilobed geometry of the neural canal. Some vertebrae of Ichthyornis also preserve paired ventrolateral channels suggesting the presence of additional spinal vessels, although a lack of information on spinal vasculature in extant birds hinders identification of specific vascular structures. These results cast new light on the detailed soft tissue anatomy of Ichthyornis and Janavis, and affirm the utility of these osteological correlates which can be applied to other fossil avialans.