According to a recent NSF news release, a robot named Madeleine has been used to test theories of locomotion in existing and extinct animals. Madeleine swims underwater using four flippers like many extinct animals or with two flippers like modern animals. The scientsts learned that having four flippers doesn't improve the top speed but does increase energy use. This may explain why natural selection favored two flipper animals over four flipper animals like the plesiosaurs. More detailed information on Madeliene and the research results can be found in the paper, Four flippers or two? Tetrapodal swimming with an aquatic robot (PDF format).