Their flight pattern is an example of undulatory locomotion. This poses a problem to animals, such as fishes and salamanders, which undergo undulatory movement. Fish, shark, and snakes are all examples of animals that locomote using undulatory locomotion. Stingray pectoral fin locomotion can be divided into two categories&colon; undulatory and oscillatory. The suture line is distinct, impressed and undulatory. However, while this pattern is characteristic of undulatory locomotion, it too can vary with environment. The skin of these animal that use undulatory motion to locomote have several distinct characteristics. Whittaker suggested that gravity possessed a wavelike "undulatory" character. A standing wave is an undulatory motion that stays in one place. Thus Einstein was contradicting the undulatory theory of light held by Planck. The undulatory theory of light also assumes the existence of a medium. Tupilakosaurs were aquatic, and used an undulatory mode of swimming much like that of eels. The undulatory pectoral fin motion diagnostic to this taxon is known as rajiform locomotion. This often works for solids, or (linearized) undulatory systems near equilibrium.