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I look at the variety. Whales, seals, orcas, manatees...then the semi-aquatics like the hippos, etc.

Can assume that at one time in earth's history, there was more water around, than there already is? Forcing these branches of mammals into the water? Or was it by choice, to get food, or safety from predators? 

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It's lots of factors combined, and each case is unique. 

In the case of otters, it seems to be pretty clear that it's about food. They are closely related to stoats and martens and mink etc, which don't seem to be under too much predator pressure. The otters ancestors were just adept at catching aquatic food, and the adaptations followed naturally. And it's easy to see seals and sea lions taking that a stage further. 

But of course, the predators factor is always there. The ancestors of the otters might also have got an aquatic dividend by having a rapid means of escape from predators. Looking at beavers, that looks more likely to be the dominant advantage from being semi-aquatic. 

Other animals, like moose, get a seasonal benefit from the water, finding a food source of aquatic plants that is a real benefit, and providing minerals that they struggle to get elsewhere (from memory).

I don't think the amount of water varies enough to have an effect. There were lots of swamps, at certain times and places, which is where the coal comes from, but other areas had vast deserts, and lots of in-betweens. 

The dinosaurs and other reptiles did the aquatic thing, just like the mammals did so it's not unique to mammals. Even birds have done it, as in penguins and flightless cormorants etc. With them it was definitely food availability. Where the food could be reached without flying, for the lifetime of the bird, the wings became flippers. 

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Thanks. Would you say that they all came from a common ancestor, or, by coincidence, different branches of these mammals took to the seas?

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Definitely different. 

Otters are of the weasel family, related to stoats, mink, martens and ferrets etc. Seals and Sea lions are more closely related to bears. Whales are most closely related to Hippos, even the fish and meat eating Dolphins and Killer Whales. 

And way out on it's own you have the Duck-Billed Platypus which lays eggs, has poison spurs, and has electrical senses, and is as far apart from you and me as a mammal can get.

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Would be interesting to know what evolutionary forces led to the platypus.
Any thoughts ?

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27 minutes ago, MigL said:

Would be interesting to know what evolutionary forces led to the platypus.
Any thoughts ?

Having a complete guess, I imagine an animal that searched the leaf litter for food, using it's electrical sensing apparatus in it's nose at night, rather than it's eyes or sense of smell. It's not much of a step, to start including shallow water trickles in the search for invertebrates, and gradually evolving more aquatic features and moving up to shallow streams and rivers.  

It's closest relatives, the Echidnas, actually evolved back from a Platypus-like aquatic animal to a dry land ant and termite eater. Evolution has no sense of direction. It just flows like water, in whatever direction is more favoured at the time.

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I just watched a "Rise of the Continents" episode on Curiositystream? The Afrika episode? These episodes surprisingly veer off from geological events into paleontology and evolution :). 

According to the host, 100M years ago, as Pangea was splitting up, volcanic mountains rose up underwater. This displaced the water, resulting in a 300 meter rise in sea levels. 

There was mass flooding, and the host credits this with leading to the evolution of marine mammals. The Whale, at least. 

 

Edited by OdinSon2k11

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