Evolution of Mammals to Sea


Whales, dolphins, manatees, seals, sea lions, sea otters and walrus are all ocean living mammals. So why are they considered mammals and not fish?

Unlike fish and much like mammals, all of these creatures give birth to live young, the mothers produce milk and the young drink it. As well as that they all have hair and breath oxygen with lungs. They're warm blooded. Their tails move top to bottom whereas most fish move side to side. Many fish including sharks don't have bones, whales do.

Why are there mammals in the ocean?

Despite being big creatures, or in the case of the blue whale, the biggest to ever live. The oldest baleen whale (filter feeding) ever to be found was 15-20 million years old. On the other hand, the oldest evidence of sharks is 400 million years. Meaning sharks have been around for roughly 20 times as long as baleen whales.

DNA testing has been done and it found that whales share the most DNA with hippos. Similarities can be observed as they both give birth to live young underwater and they communicate with underwater sounds.

Convergent Lines

It's important to remember that whales didn't come from hippos but are cousins. That is to say, they have a common ancestor. Before hippos were hippos and whales were whales, there was a separate species of animal that would eventually split off and evolve into both. These transitional forms have since gone extinct and remains are hard to come across. Nevertheless, basilosaurs, pakicetus and Ambulacitus fossils all appear to represent transitional forms.

Within the world today a second mammal class have evolved to life in the ocean. Pipicides include seals, sea lions and walruses. These mammals all share a lot in common with the otter.

A third class would also include Manatee, Dugong and sirens. Manatees are actually closer related to elephants and therefore not directly related to seals or whales. 

Locomotion

Fish are characterized by their side to side locomotion. However, mammals swim up to down.

From current animals morphology and the fossil record, it appears that cetceans evolved propulsion by means of the tail. Penipeds on the other hand seem to have evolved propulsion by means of fused rear limbs. 

Starting with a weasel


https://youtu.be/sjQUhJcVsTM

Moving on to otters, a close relative of weasels. Notice the awkward walk. They're more specialized to water than woodland.


https://youtu.be/J7f6s2g8C0I

Sea Otter


Now, Otters fall into a different taxonomic class along with weasels and skunk but represent a very possible precursor to sea lions. Sea otters are the biggest member of the weasel family. 

Unlike most marine mammals, the sea otter's primary form of insulation is an exceptionally thick coat of fur, the densest in the animal kingdom. The giant Amazonian otter has short felt like fur.

Although it can walk on land, the sea otter is capable of living exclusively in the ocean while most otters are semi aquatic. Sea Otters can hold their breath for 5 minutes.

https://youtu.be/gIoQGs8lvxk

Sea Lion


Sea Lions share a lot of features with otters but belong to a different branch of animals. They can hold their breath from 8-20 mins. They can also move well on land.

https://youtu.be/5-TkUOvuuNE

Seals


Seals move very badly on land. They have upper and lower arms and legs concealed within their skin. Only their hands and feet extend outside the body envelope.

Seals hold their breath for up to 30 mins. 


https://youtu.be/kq4ghiiSm0Q

https://youtu.be/l6mGixhCcYY

Manatee


Manatees are genetically closest to elephants. However, they demonstrate a possible similarity to a transitional form.

They have lost their rear legs but still have fingernails. They still have whiskers. Unlike seals and sea lions, manatee are herbivores.

They only hold their breath for 30 mins also.

https://youtu.be/_AvjXSEWBXk

Whales

Most whales can hold their breath for about an hour. Beluga Whales look more like dolphin and killer whales. Baleen whales on the other had such as humpback and blue whales only eat krill.

Whales are genetically much closer to humans than whale sharks. 

Whales give birth to live young, the mothers produce milk and the young drink it. As well as that they all have hair and breath oxygen with lungs. They're warm blooded. Their tails move top to bottom whereas most fish move side to side. Many fish including sharks don't have bones, whales do.


Conclusion

This is essentially evolution in practice. From a weasel to whales. Again, whales didn't come from any of these but instead their own similar ancestors.

https://youtu.be/8cn0kf8mhS4

https://youtu.be/UwlWKQJxEmE

https://youtu.be/aAOsf004FqQ

https://youtu.be/notqMQ5cfYc



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