In general, it is hard to determine the maximum speed of extinct sea creatures. For plesiosaurs, this is made more difficult by the lack of consensus about their flipper stroke and gait. There are no exact calculations of their Reynolds Number. Fossil impressions show that the skin was relatively smooth, not scaled, and this may have reduced form drag. Small wrinkles are present in the skin that may have prevented separation of the laminar flow in the boundary layer and thereby reduced skin friction.