Cervical ligament systems in sauropod dinosaurs: what support is there?

Sauropod dinosaurs, such as Diplodocus and Dicraeosaurus, have been the subject of numeroushypotheses about what ligamentous structures could have aided in lifting and supporting their long necks. Because the supportive tissues rarely fossilize, palaeontologists historically have relied on assumptions and on presumed osteological correlates, such as rugosities, on cervical vertebrae to infer their nature. Based on comparisons with extant animals, options for supportive cervical ligaments in sauropods include mammal-style nuchal ligaments, avian-style interlaminar elastic ligaments, reptilian-style supraspinal ligaments, or some combination thereof. This study tested for the presence of a mammal-style nuchal ligament using histology and micro-computed tomography of cervical hemispinous processes of Apatosaurus and Diplodocus, as well as the examination of gross morphological features of the cervical vertebrae of Apatosaurus, Diplodocus, and Camarasaurus. A non-uniform bone orientation in the thin sections from a hemispinous processes of Diplodocus and from micro-CT imaging of a hemispinous process of Apatosaurus suggests that a dorsally or dorsolaterally positioned supraspinal ligament, rather than a mammal-like nuchal ligament, attached to the distal tips of the spines. Additionally, initial observations of pseudospinous tubercula in Diplodocus and Apatosaurus suggests that they also possessed interlaminar elastic ligaments in at least portions of their necks. Whether or not such ligaments were the sole means of cervical support and whether or not such ligaments were capable of providing entirely passive (non- or minimally muscle aided) neck support remain unclear. These findings could be incorporated into future models of sauropod neck mobility and further the understanding of sauropod feeding styles and mechanics.