Desmostylus hesperus. Illustration and photo-reconstruction of a Desmostylus hesperus raising dust from the sea floor in search of food. Desmostylus hesperus is an extinct mammal that lived on the coast of the North Pacific in the middle Miocene period (15.8 - 12 Mya). Desmostyluses reached 2.5 metres in length and weighed 400 kilograms. In water they are mainly paddled with their front feet, as polar bears do. The length of the skull of the largest species, (Desmostylus hesperus) reached 90 centimetres.