Reconstruction of the Musculotendinous System of the Necks of Sauropods and the Dodo Based on Anatomical Comparison with Modern Birds and Implications for Biomechanical Function and Ecological Roles
Authors:   , Li Liguo , Dodson Peter , Spotila James Robert 
Published: July 2021

Herein I redescribe the giant titanosaurian sauropod Daxiatitan binglingi, which has previously only received a preliminary description. The skeleton was incompletely preserved. The articulation of the vertebral column of Daxiatitan suggests that strong connective tissue prevented disarticulation, but parallel dorsal ribs and disarticulated limb bones indicate that these elements were disarticulated by paleo-disturbance prior to burial. Examination of the vertebral column and appendicular skeleton of Daxiatitan brings several anatomical insights to the reconstruction of the soft-tissue attachments. The complicated "N" & "M"-pattern laminae on the neural arches, plus multiple curved ridges on the lateral surfaces of the dorsal vertebrae likely indicate tendons and soft tissue attachment sites. Huge pneumatic fossae on the lateral surfaces of the centra indicate the presence of pneumatic diverticula from cervical air sacs. A system of three air sacs is suggested to have filled the pneumatic space. A reconstruction of the ventral musculotendinous system of the neck of the sauropod Daxiatitan is based on comparisons with the distinctly different osteological and myological patterns in the great blue heron (Ardea herodias) and the tundra swan (Cygnus columbianus): the neck of the heron possesses long, light muscles (LL configuration), whereas the swan possesses short, strong muscles (SS configuration). The LL overlapping-tendon pattern of the great blue heron represents a long tendon-light muscle system supported osteologically by deep pulley-grooves on the carotid processes of the cervical vertebrae. Trunk-shifted cervical muscles, nerves, and blood vessels operate biomechanically as a "remote command center" to control the whole neck. The dominant long tendon, trunk-centered muscle belly and kinked, S-shaped neck are controlled by one muscle group, the m. Longus colli ventralis and three key vertebrae. In contrast, the SS chevron pattern of the short cervical tendons of the swan, and the absence of grooves on the carotid processes, indicate cervical flexibility to control individual cervical vertebra rather than the neck as a whole unit. The uniformly muscled swan maintains more of its body mass in the neck. In the SS, chevron pattern, the musculotendinous, vascular, and nervous systems act in concert as an integrated network, which controls the positions of individual cervical vertebrae for flexibility. The contrast of the two patterns suggests that the hyperelongate cervical ribs of Daxiatitan and other sauropods had a feeding or self-defense function. Long-necked sauropods, including Daxiatian and Mamenchisaurus, better fit the LL tendon-overlapping pattern in locomotion and body mass balance, while short-ribbed sauropods, including Diplodocus and Apatosaurus, better fit the SS pattern. A reconstruction of the cervical myology of the dodo, Raphus cucullatus, is based on anatomical comparison with the Nicobar pigeon (Caloenas nicobarica) and reveals robust cranio-cervical osteological features that supported robust neck muscles. Bifurcate cervical spinous processes in the dodo and its kin-the extinct Rodrigues solitaire (Pezophaps solitaria) as well as the gigantic birds such as Dinornis robustus (South Island giant moa)-provide large surface area for the attachment of the elastic interspinal ligament, which functioned for dorsal flexion of the neck. The elongation of the cervical vertebrae, the bifurcation of their spinous processes, and the orientation of their zygapophyses in the dodo suggest it, as well as the Rodrigues solitaire, maintained an elevated, elongate, laterally flexible, S-shaped neck pose. This novel, vertical neck pose epitomized the success of the giant flightless pigeons both in their cranio-cervical biomechanical functions and ecological roles. The adaptation contributed to the acquisition of suitable higher, wider bush-fruit-browsing ecological niches for the flightless dodo and its kin in lowland and highland ecosystems. These adapted traits helped the dodo expand its ecological niche to survive on Mauritius before humans arrived.