Illustration of Sea Serpents from Gesner's 'Historia Animalium', volume 4, 1558. Later hand colouring. Italic marginalia by contemporary scholar. Gessner's accounts included creatures which exist, and many creatures which we now know do not. There were small sea snakes in the tropics, and there seemed every chance to scholars in the 16th Century that leviathan Sea Serpents roamed the deep. They may hardly have seemed stranger than the great whales. Gessner's accounts were based on first hand reports as well as the 'authority' of earlier authors such as Aristotle and Pliny.