Abstract
The sauropterygian clade Plesiosauria arose in the Late Triassic and survived to the very end of the Cretaceous. Plesiosauria evolved the greatest species diversity of any marine reptile clade, attaining a global distribution. Plesiosauria consist of two clades, Rhomaleosauridae and Neoplesiosauria. Basal Neoplesiosauria have long necks with at least 30 cervicals, but show qualitative osteological evidence for a stiff neck. Here we quantify neck mobility in lateral, ventral, and dorsal directions based on finite element modeling of neck vertebrae from the Middle Jurassic plesiosaur Cryptoclidus eurymerus. We model the mobility in a single motion segment, consisting of two adjacent cervical vertebrae and the joints connecting them. Based on the model with a maximum intervertebral spacing of 3 mm, we find that in Cryptoclidus, the maximum angle of lateral deflection in the motion segment was 2°. The maximum angle of ventral deflection was 5° and of dorsal deflection was 5°. When these values are multiplied by the number of cervical vertebrae, it becomes apparent that neck mobility was limited in all directions. The maximum angle of total lateral deflection in the neck was 67°. The maximum angle of total ventral deflection was 148° and of total dorsal deflection was 157°. This raises the question of the function of such a long, multi-segment but immobile neck. We posit that the long neck served in hydrodynamic and visual camouflage, hiding the bulk of the body from the small but abundant prey, such as schooling fish and squid. Neck immobility may have been advantageous in withstanding strong hydrodynamic forces acting on the neck during predatory strikes.
Highlights
Plesiosaurians are secondarily aquatic reptiles known from the Late Triassic to the Late Cretaceous (Storrs, 1993; Wintrich et al, 2017), which show a body design adapted extremely well to the aquatic environment
The range of motion (ROM) increased with increased intervertebral disc (IVD) length in all load cases, but it is generally small, ranging from at most 5◦ to less than one degree
In the load cases with the 3-mm IVD, dorsal and ventral ROM is above 4◦, decreasing to about 3◦ in the 2-mm IVD model
Summary
Plesiosaurians are secondarily aquatic reptiles known from the Late Triassic to the Late Cretaceous (Storrs, 1993; Wintrich et al, 2017), which show a body design adapted extremely well to the aquatic environment. This is evidenced most clearly by the modification of the limbs into flippers used in a unique locomotion style, fourwinged underwater flight. There are forms with an extremely small head and a long neck, the plesiosauromorph body plan. The third body plan, mainly seen in the Rhomaleosauridae, is a larger head and shorter neck than the small-headed and long-necked plesiosaurs, but a smaller head and a longer neck than the pliosaurs
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