Rhynchocephalia is the order that contains two living species and commonly called as tuatara. Externally they look like lizards but internally they are very different. Tuataras have many unique morphological characteristics, such as their feeding apparatus, a cold-adapted physiology, and a particular skull structure. Tuatara males also lack an intromittent penis, and mating is achieved by cloacal apposition. Thus, the correct answer is option D.