Mobility and hydrodynamic implications of the long neck in plesiosaurs
Plesiosaurs are extinct marine reptiles that lived during the Mesozoic, well-known for their unique body plan with two pairs of flippers and an elongated neck. What this long neck was used for is unclear. Various hypotheses have been proposed regarding its use, however biomechanical implications of these scenarios remain untested. Even the effects on locomotion are poorly understood. This thesis sets out to explore biomechanical implications of long-necked plesiosaurs. Interpreting results on range of motion (RoM) in extant taxa can help suggest biomechanical and ecological restrictions of the neck in extinct animals, such as plesiosaurs. RoM in digitized plesiosaur necks was calculated by digital manipulation using a method validated against physical and digital manipulation of an ostrich neck. Neck vertebrae from the posterior-most half of an ostrich neck were manipulated to determine dorsoventral and lateral RoM at successive stages of dissection. Neck vertebrae from four plesiosaur specimens were digitised to study RoM, and showed plesiosaurs potentially had large ranges of motion in the neck, highly dependent on the number of cervical vertebrae. Extreme examples need only small rotation between each bone to produce large overall rotation, even accounting for a reduction in flexibility due to soft tissue. Results showed the largest motions occurred laterally, rather than dorsoventrally. Computational fluid dynamics was used to investigate the hydrodynamics of plesiosaurs. Sensitivity analyses and validation of the software were carried out in a virtual flume using a sphere and cylinder with known flow responses. The virtual flume was then used for idealized plesiosaur models (three neck lengths and two neck thicknesses). The long-necked plesiosaur model did not meaningfully increase drag, implying no disadvantage to the hydrodynamic performance during forward swimming speeds of 1-10 m/s. Additionally, at low speeds, the feeding envelope would have increased greatly in long-necked forms. Neck thickness did alter drag, as the thick-necked plesiosaur model experienced lower drag compared with the thin-necked model at swimming speeds from 1-10 m/s. This could indicate thick-necked plesiosaurs reduced overall drag, and spend less energy on straight, forward swimming compared with thin-necked forms. In conjunction with soft tissue evidence, this would provide support for a more sea lion-like morphology. Bending the neck evenly in lateral flexion showed greatest turning radius and drag in long-necked plesiosaurs and least in short-necked forms, demonstrating short-necked plesiosaurs were more likely to make faster neck turns and have shorter turning radius, compared with long-necked forms.
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