Osteological characteristics of extant, quadrupedal, aquatic or amphibious mammals are compared and used to determine the presence and state of development, or absence, of aquatic adaptations in the order Desmostylia. The specimens compared include seventeen bones from in each species four higher taxa: Rodentia, Carnivora, Mustelidae and Artiodactyla. The aquatic form is compared with its terrestrial counterpart within each group. Quantitative evidence of convergence is used to sort out characters that are adaptive in relation to aquatic habits. Aquatic conformations or tendencies are identified as those characters, displayed by aquatic forms in different taxonomic groups, that have more in common with each other than with corresponding characters of land animals in the same taxa. About 80% of the 49 aquatic adaptive characters identified are also found in desmostylians. This analysis documents the gradual evolutionary transition of desmostylians to an aquatic mode of life. Furthermore, the results show that the desmostylian swam better using its hind-limbs at least than the hippopotamus.
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