Manatee mom and calf

As of the writing of this post, there are four clades of marine mammals. Things could change if some crazy fossil is found that alters all current knowledge, but for now we only have to know about four clades. Now is a good time to explain what a clade is. According to the fabulous glossary in the Encyclopedia of Marine Mammals, a clade is “an evolutionary line; a monophyletic group containing all the descendants of the most recent common ancestor of that group.”

So basically, it’s a group of animals that all descended from the same animal. For marine mammals the four clades include: pinnipeds (sea lions, walruses and seals), cetaceans (whales, dolphins and porpoises), sirenians (manatees and dugongs) and the desmostylians. You mean you haven’t heard of the desmonstylians? Well, that’s because they are extinct. Apparently, the desmonstylians used to be classified with the sirenians, but then an entire skeleton was discovered proving that wasn’t the best choice and now they are classified in their own extinct clade.

And what about the polar bears and sea otters, where do they fit? They have been classified with land animals, based on their past. Can you imagine being associated with land animals? How embarrassing. If they only knew…