Since its discovery in the 1824, scientists have scratched their heads at the odd body plan of the plesiosaur. The aquatic reptiles, which grew up to 15 meters long and lived from about 200 million years ago up to about 66 million years ago, are unique among animals for their four-flippered bodies. Thus, researchers have only been able to speculate at how they navigated ancient waters. Did the fore- and hindlimbs paddle in unison or separately? Did the animal employ a rowing motion or a flapping motion? To help resolve the mystery, computer scientists used preserved plesiosaur skeletons to construct a 3D model with realistic ranges of joint motion, densities, and musculature. They then placed the virtual reptiles in virtual water, and the computer ran several thousand different simulations with different joint motions, searching for strategies that produced good swimming. The results, published today in PLOS Computational Biology, reveal that almost all of the plesiosaur’s thrust was generated by a penguinlike flapping of the front limbs. As seen in the video above, in simulations where the back limbs were immobilized, the reptile still swam fairly well, but without the front limbs, it became very difficult to generate productive motion. Because of this disparity, the question of whether the animals’ fore- and hindlimbs moved in phase or out of phase doesn’t matter: The rear limbs were likely used to steer and stabilize the plesiosaur rather than generate thrust.