The extinct ocean-going plesiosaurs were unique within vertebrates because they used
two flipper pairs identical in morphology for propulsion. Although fossils of these
Mesozoic marine reptiles have been known for more than two centuries, the function
and dynamics of their tandem-flipper propulsion system has always been unclear and
controversial. We address this question quantitatively for the first time in this
study, reporting a series of precisely controlled water tank experiments that use
reconstructed plesiosaur flippers scaled from well-preserved fossils. Our aim was
to determine which limb movements would have resulted in the most efficient and effective
propulsion. We show that plesiosaur hind flippers generated up to 60% more thrust
and 40% higher efficiency when operating in harmony with their forward counterparts,
when compared with operating alone, and the spacing and relative motion between the
flippers was critical in governing these increases. The results of our analyses show
that this phenomenon was probably present across the whole range of plesiosaur flipper
motion and resolves the centuries-old debate about the propulsion style of these marine
reptiles, as well as indicating why they retained two pairs of flippers for more than
100 million years.
