Abstract
Ichthyosaurs are an extinct group of fully marine tetrapods that were well adapted to aquatic locomotion. During their approximately 160 Myr existence, they evolved from elongate and serpentine forms into stockier, fish-like animals, convergent with sharks and dolphins. Here, we use computational fluid dynamics (CFD) to quantify the impact of this transition on the energy demands of ichthyosaur swimming for the first time. We run computational simulations of water flow using three-dimensional digital models of nine ichthyosaurs and an extant functional analogue, a bottlenose dolphin, providing the first quantitative evaluation of ichthyosaur hydrodynamics across phylogeny. Our results show that morphology did not have a major effect on the drag coefficient or the energy cost of steady swimming through geological time. We show that even the early ichthyosaurs produced low levels of drag for a given volume, comparable to those of a modern dolphin, and that deep ‘torpedo-shaped’ bodies did not reduce the cost of locomotion. Our analysis also provides important insight into the choice of scaling parameters for CFD applied to swimming mechanics, and underlines the great influence of body size evolution on ichthyosaur locomotion. A combination of large bodies and efficient swimming modes lowered the cost of steady swimming as ichthyosaurs became increasingly adapted to a pelagic existence.
Highlights
Ichthyosaurs were an iconic group of marine reptiles that lived from the Early Triassic to the early Late Cretaceous [1,2,3,4,5]
Validation experiments demonstrate that the computational fluid dynamics (CFD) simulations can replicate the experimental drag coefficients of various torpedo-like forms within less than 5% error, accurately capturing small variations in drag owing to the different fineness ratios (FR: the ratio between total length and maximum diameter)
Our computed drag coefficients for the bottlenose dolphin are consistent with those reported in the literature for gliding dolphins, rigid models and static CFD simulations of dolphins [35,38], as expected, they fall below estimates obtained from thrust-based methods [28,29] because the simulations used do not account for the dynamic effects of drag [12,13,29]
Summary
Ichthyosaurs were an iconic group of marine reptiles that lived from the Early Triassic to the early Late Cretaceous (ca 248–93.9 Ma) [1,2,3,4,5]. We divided the computed drag by the volume of each model at various hypothetical combinations of body length (1, 2 and 10 m) and velocity (from 1 to 5 m s21) for the models scaled to total length (electronic supplementary material, table S2), encompassing sizes observed in ichthyosaurs and velocities that are likely to occur in living aquatic and semiaquatic animals of those dimensions [9]. The intermediate forms included the anguilliform swimmers based on their high presacral vertebral counts, which point towards flexible backbones, and their caudal morphology, showing less conspicuous tailbends or absence thereof [15 – 17] These simple assumptions, adopted by previous studies [6], allow us to incorporate the potential effects of kinematics on the energy requirements of steady swimming in our models. Efficiency estimates from dynamic flow simulations show differences between anguilliform and carangiform of a similar order of magnitude [32,33]
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