The distinctive four-flippered body-shape has caused considerable speculation about what kind of stroke plesiosaurs used. The only modern group with four flippers are the sea turtles, which only use the front pair for propulsion. Conybeare and Buckland had already compared the flippers with bird wings. However, such a comparison was not very informative, as the mechanics of bird flight in this period were poorly understood. By the middle of the nineteenth century, it was typically assumed that plesiosaurs employed a rowing movement. The flippers would have been moved forward in a horizontal position, to minimise friction, and then axially rotated to a vertical position in order to be pulled to the rear, causing the largest possible reactive force. In fact, such a method would be very inefficient: the recovery stroke in this case generates no thrust and the rear stroke generates an enormous turbulence. In the early twentieth century, the newly discovered principles of bird flight suggested to several researchers that plesiosaurs, like turtles and penguins, made a flying movement while swimming. This was e.g. proposed by Eberhard Fraas in 1905, and in 1908 by Othenio Lothar Franz Anton Louis Abel. When flying, the flipper movement is more vertical, its point describing an oval or "8". Ideally, the flipper is first moved obliquely to the front and downwards and then, after a slight retraction and rotation, crosses this path from below to be pulled to the front and upwards. During both strokes, down and up, according to Bernoulli's principle, forward and upward thrust is generated by the convexly curved upper profile of the flipper, the front edge slightly inclined relative to the water flow, while turbulence is minimal. However, despite the evident advantages of such a swimming method, in 1924 the first systematic study on the musculature of plesiosaurs by David Meredith Seares Watson concluded they nevertheless performed a rowing movement.