 The function of this structure is unclear, but it may have been involved in the support of a dermal caudal fin.Prior to their description, the specimens that would later be assigned to "Umoonasaurus" were thought to represent a new species of "Leptocleidus".In 2006, Kear and colleagues found "Umoonasaurus" to belong to the superfamily Pliosauroidea and be the most basal member of the family Rhomaleosauridae.