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This was the first evolution of a dolphin called Pakicetidae. They were first discovered in 1983, 50 million years ago. This animal has vestigial structural changes because at this time, this animals had legs and now dolphins don't have legs. The main adaptation of a Pakicetidae is the structure of ears because they are adapted to an underwater hearing. They also have dense bones to walk under the water. Its eyes were positioned close together on top of the skull, and would have allowed Pakicetidae to see above water even when submerged.

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The second evolution of the dolphin was called that Ambulocetidae. This species was discovered on 1993 and was described on 1994. This species has analogous structures It had an adaptation in the nose that enabled it to swallow underwater, and its periodic bones had a structure like those of whales, enabling it to hear well underwater.

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The 3rd evolution of the dolphin is called a Remingtonocetidae. This specie was discovered in 1973. The structure that this specie has are analogous structures. Early middle Remingtonocetidae have skeletons indicating that they were long bodied, with a long cranial rostrum, short limbs, fused sacral vertebrae, and a powerful tail. The nostrils were placed close to the forehead, they had small eyes and the inner ear were more adapted to underwater life.

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The next species is called the protocetidae. It was discovered about 40 million years ago. The protocetidae has analogous structures. This animal had  large, powerful tail and feet, which would have allowed them to be excellent swimmers. Just like that remingtonocetidae, they had the nostril placement the same, close to the forehead.

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The 5th specie is called a Basilosauridae. This species was discovered 41.3 to 33.9 million years ago and was first described in 1834. I would say that the basilosauridae has analogous structures. The basilosauridae's front legs were transformed into flippers, and their back legs shrunk in size. Their tails became bulkier to support their swimming, while their nostrils shifted higher and higher on their heads to assist breathing at the surface of the water.

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The next species is called a Dorudontidae. The Dorudontidae was first discover about 40.4 to 33.9 million years ago in Eocene. This animals has analogous structural changes. The dorudontidaes were built for underwater survival. Its incisor and canine teeth were sharp and pointed, and placed along its entire mouth. This is different from the incisor and canine teeth of a dog, which sit close together at the front of their mouths.

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The 7th evolution is called Mysticeti, which is also called Baleen Whales. Molecular phylogeny suggests that Mysticeti's were found 26 to 17 million years ago. But the earliest fossils found was 34 million years ago. Mysticeti's have homologous structural changes. A Mysticeti's, or baleen whale's, circulatory system adjusts to conserve or dissipate body heat and maintain body temperature. Some arteries of the flippers, flukes, and dorsal fin are surrounded by veins. Because of this, some heat from the blood traveling through arteries is transferred to venous blood rather than the environment. They also lack teeth but use baleen to filter food from the water.

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The final evolution of dolphins are Odontoceti. This species was first discovered around 30 million yeas ago. The Odontoceti has vestigial structural changes. Odontocetes have conical teeth designed for catching fish or squid. They have well developed hearing that is well adapted for both air and water, so much that some can survive even if they are blind. Some species are well adapted for diving to great depths.