Evolution of Whale

Sinonyx (60 million years ago)

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Indohyus was about a 12 to15 pound mammal that looked like a miniature deer. The Indohyus had heavy bones making it slow on land and easier prey for predators.In the water the same structural feature would help it stay on the bottom, where it could forage and hide.It has an unusual bone structure around the animal’s ear. It was a little bowl-shaped structure. The inside of that bone is very thick, the outside of the bone is very thin.Whales are the only mammal with this unique bone structure.

Indohyus (49 million years ago)

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it was about a 12 to15 pound mammal that looked like a miniature deer. Had heavy bones making it slow on land and easier prey for predators. In the water the same structural feature would help it stay on the bottom, where it could forage and hide.It has an unusual bone structure around the animal’s ear. It was a little bowl-shaped structure. The inside of that bone is very thick, the outside of the bone is very thin. Whales are the only mammal with this unique bone structure.

Ambulocetus( 47 million years ago)

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Is one of the best examples of a transitional form between whales and terrestrial animals. It has lacked a blowhole, but otherwise had a skull and tooth structure very similar to modern whales. Its spine moved up and down for swimming the way that modern whales do, but its four legs were still functional enough that it was probably able to walk on land in the same way that sea lions do today. It is best described as being an amphibious, it swam, ate fish but still connected to land.

Rodhocetus(46 million years ago)

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It shows a further progression toward the traits of modern whales. It had developed a powerful tail for swimming and a more flexible spine for tail movements.This is thus the earliest known mammal to swim predominantly using its tail for propulsion rather than its legs. It still appears to have legs strong enough to move on land, but they were shorter and probably even less useful than those of Ambulocetus before it and is thus the first example of blowhole evolution.

Basilosaurus(between 35 and 45 million years ago)

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Another transitional fossil organism. This was a very large (~15 meters or 50 ft long), serpentine animal that was originally though to be the remains of a sea serpent. Had a very small pelvic girdle and hind limb bones that were far too small to bear the animal’s weight on land. Its vertebral column shares characteristics of whales with tail flukes (fins), but flukes contain no bone and are therefore unlikely to fossilize. The nostrils had fused into a large single blowhole.

Dorudon(40 million years ago)

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Finally, the Dorudon lived about 40 millions years ago, at the same time as Basilosaurus, and is the most likely ancestor to modern whales. Like Basilosaurus, Dorudon was fully aquatic and lacked large enough limbs to move around on land. It was much smaller (around 4-5 meters or 15 ft) than Basilosaurus. Despite its similarity to modern whales, Dorudon lacked the capability to use underwater sounds to located objects (echolocation) as modern whales can.

Adaptations(From Sinonyx to Humpback Whale)

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Adaptations(Streamlined Body)

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Have you ever noticed how humpback whales do not have ears that stick out of the sides of their heads? Humpback whales, like all cetaceans, also have internalized reproductive organs. This evolution has helped whales and dolphins reduce drag while swimming through the water.

Adaptations(Forelimbs Modified Into Flippers)

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The forelimbs that the early ancestors of whales had have been modified into flippers for easy steering through the water. Whales also slap their flippers on the surface of the water as a means of communication. The sound from “pec slapping” can carry several miles across the water.

Adaptations(Tail Broadened Horizontally)

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Cetaceans (whales, dolphins, porpoises) have tails that are flattened horizontally. This is unlike fish, who have tails that are flattened vertically. The strong fluke of a cetacean gives them strong propulsion through the water column.

Adaptations(Hind Limbs Disappeared Completely)

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Fossils of ancient whales found in the middle-East include hind limbs. These ancient animals walked on all four legs like a dog until they evolved into their current aquatic form. Whales today do not have hind limbs; however, they have vestigial (left-over) pelvis bones that are part of their skeleton.

Adaptations(Blubber Instead of Hair)

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Terrestrial mammals have hair to protect their skin from the elements of nature and keep them warm. Whales, however, have lost most of their hair throughout the process of evolution. Instead, whales such as humpbacks, have thick layers of fat called blubber, which acts as insulation to keep them warm. Blubber also helps provide buoyancy.

Adaptations(Nostrils on Top of Head)

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Over millions of years, the nostrils of whales evolved from the front of their head (like a dog’s nostrils) to the top of their head. The evolution took place so whales may rise to the surface of the water and breathe without sticking their entire head out of the water.

Adaptations(Highly Developed Hearing)

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Humpbacks whales and other cetaceans have hearing that is highly developed. Sounds travel much better in water than in the air, so whales have developed an auditory sense that is stronger than their visual sense.

The End

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Indohyus image courtesy of Carl Buell
https://coast.noaa.gov/data/SEAMedia/Presentations/PDFs/Grade%204%20Unit%204%20Lesson%201%20Whale%20Evolution.pdf