From 220 to 66 million years ago, a strange order of marine reptiles called plesiosaurs roamed Earth's oceans. Though they were a diverse bunch in many respects, every known species did share one major trait: They all had four broad, paddle-like flippers. And in most cases, the hind pair almost looked like a duplicate of the front set, both in size and shape.
That makes plesiosaurs, which were not dinosaurs but lived at the same time, unique among ocean-going vertebrates. That's because no other marine reptile or mammal — extinct or extant — sports four flippers of similar size.
So how did this unique body plan work? Researchers might've just found the answer. On Aug. 30, 2017, an international team of scientists published a new study on plesiosaur locomotion. Headed by paleontologist Luke Muscutt, the group used measurements from a pair of British species to construct two replica flippers, one forelimb and one hindlimb, with 3-D printing technology.
These were affixed to a custom-built robot, which moved the replicas around in a tank of water. To see exactly how the paddles would churn things up, Muscutt and company released colorful dyes into the tank as well, and tracked their motion through water.
Afterward, the scientists put the robotic legs through numerous simulations designed to test out different swimming styles. They found that the most energy-efficient stroke called for both flippers to work in concert. When the front limb flapped in this test, it created two vortices in the water. As the current pushed these backward, the hind flipper weaved in between them. By capitalizing on the wake generated by its frontal counterpart, the rear limb made its own flapping motion 60 percent more powerful.
This finding contradicts a 2013 study, which argued that plesiosaurs swam like sea turtles, using their forelimbs to push themselves forward while the rear paddles acted as rudders. Using the robot, Muscutt's team also tested this existing hypothesis. They found that the rear paddles tended to produce drag if kept in a stationary position. But by using all four paddles to actively generate thrust, a plesiosaur could move more efficiently.
However, the co-authors acknowledge that, like most aquatic animals, plesiosaurs probably adjusted their swimming style as the situation called for it. Furthermore, the study is complicated by the fact that several plesiosaur species had forelimbs that were noticeably longer than their rear ones, or vice versa. Follow-up research will have to take species-specific data into account.
This video from lead author Muscutt explains the unique hydrodynamics, and shows the experiment in action: