of semi-aquatic Eocene cetaceans was probably the result of BMP antagonists counteracting interdigital apoptosis during embryonic limb development. Modifications to signals in these tissues likely contributed to the origin of an early form of hyperphalangy in fully aquatic cetaceans about 35 million years ago. The process continued over time, and a very derived form of hyperphalangy, with six or more phalanges per digit, evolved convergently in rorqual whales and oceanic dolphins, and was likely associated with another wave of signaling within the interdigital tissues. Although toothed cetaceans have five digits, most baleen whales have four digits and even lack a metacarpal. In the latter (mysticetes), the first digit ray may have been lost as late as 14 million years ago. Flipper evolution in turtles Sea turtles evolved in the Cretaceous. Their flippers developed gradually by a series of stepwise adaptations, with the most fundamental traits of flippers appearing in the deepest nodes (the earliest times) in their phylogeny. These initial traits evolved only once among chelonioids, and the bauplan was refined through a secondary process of specialization. Evers et al. identified characters related to the pectoral girdle and forelimb that are related to the modification of sea turtle arms and hands into flippers. Key biomechanical features of flippers flattening of elements lengthening of the humerus reduction of mobility between individual flipper elements Fundamental traits for flipper movement lateral position of the humeral process change in the angle of the internal scapula Foraging behavior Because of the specialization of flippers and their hydrodynamic constraints, it was thought that they were not used to significantly interact with the environment, unlike the legs of terrestrial tetrapods. However, the use of limbs for foraging is documented in marine tetrapods. Use of the flippers for foraging behavior is observed in marine mammals such as walruses, seals, and manatee, and even in reptiles such as sea turtles. Among turtles, observed behaviors include a green turtle holding a jellyfish, a loggerhead rolling a scallop on the sea floor, and a hawksbill turtle pushing against a reef for leverage to rip an anemone loose. Based on presumed limb use in ancestral turtles, these behaviors may have occurred as long ago as 70 million years. See also Fish fin Homology (biology) References Animal anatomy Evolution of animals