Asymmetric fin shape changes swimming dynamics of ancient marine reptiles' soft robophysical models.

Animals have evolved highly effective locomotion capabilities in terrestrial, aerial, and aquatic
environments. Over life's history, mass extinctions have wiped out unique animal species with specialized
adaptations, leaving paleontologists to reconstruct their locomotion through fossil analysis. Despite
advancements, little is known about how extinct megafauna, such as the Ichthyosauria one of the most
successful lineages of marine reptiles, utilized their varied morphologies for swimming. Traditional
robotics struggle to mimic extinct locomotion effectively, but the emerging soft robotics field offers
a promising alternative to overcome this challenge. This paper aims to bridge this gap by studying
Mixosaurus locomotion with soft robotics, combining material modeling and biomechanics in physical
experimental validation. Combining a soft body with soft pneumatic actuators, the soft robotic platform
described in this study investigates the correlation between asymmetrical fins and buoyancy by recreating
the pitch torque generated by extinct swimming animals. We performed a comparative analysis of
thrust and torque generated by Carthorhyncus, Utatsusaurus, Mixosaurus, Guizhouichthyosaurus, and
Ophthalmosaurus tail fins in a flow tank. Experimental results suggest that the pitch torque on the torso
generated by hypocercal fin shapes such as found in model systems of Guizhouichthyosaurus, Mixosaurus
and Utatsusaurus produce distinct ventral body pitch effects able to mitigate the animal's non-neutral
buoyancy. This body pitch control effect is particularly pronounced in Guizhouichthyosaurus, which
results suggest would have been able to generate high ventral pitch torque on the torso to compensate
for its positive buoyancy. By contrast, homocercal fin shapes may not have been conducive for such
buoyancy compensation, leaving torso pitch control to pectoral fins, for example. Across the range of the
actuation frequencies of the caudal fins tested, resulted in oscillatory modes arising, which in turn can
affect the for-aft thrust generated.

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