What are the ancestors and nearest land-based relatives of seals, walruses, and other non-cetacean marine mammals?
I have heard theories about the cetaceans (whales, dolphins, etc.). Although they are totally aquatic now, they have clear vestigals signs of having decended from land-dwellers.
The seals and walruses do not seem closely related to the cetaceans. Seals and walruses spend time out of the water. Seals still have thick fur coats. I would assume that these animals also have land-dwelling ancestors. Are seals and walruses more recent converts to an aquatic lifestyle than cetaceans? Or are they successful enough the way they are and have remained unchanged for a long period? What land-dwelling mammals share the nearest common ancestors with seals and walruses? Are these similar at all to the ceteceans’ ancestors and relatives?
And what the heck is a manatee? Are they a stop on the way from land dwelling herbivor to cetacean? Or an example of convergence forms for similar environmental niches?
1.) Order Cetacea ( whales and dolphins ): Possibly diverged from an extinct order of terrestrial carnivores, the Creodonta, at the end of the Cretaceous ( ~ 100 mya ) and entered the sea in the Paleocene ( ~65-55 mya ). They certainly appear to have been established by the mid-Eocene ( ~50-40 mya ).
2.) Order Pinnipedia( eared seals, sea lions, walrus ):A little controversial here. One theory is that they are biphyletic, with the eared seals and walruses arising from the same line that originated the bears and sea lions arising from a common line with the ancestor of the otters. Now, however, the burden of evidence appears to be switching back to them being monophyletic. The earliest seal-like fossil dates from the late Oligocene/early Miocene( ~28-26 mya ) and is found in North America ( California actually ) which appears to be the area where they first evolved. From there they spread to South America and Europe, then parts more distant. Some systematists think instead of being in their own separate order, they should be subsumed under the Carnivora.
3.) Order Sirenia ( dugongs, sea cows, and manatees ): Appear to be most closely related to the Proboscidea ( elephants ) and Hyracoidea ( hyraxes ) with whom they are sometimes grouped as the Superorder Paenungulata. They seem to have originated roughly as the same time as the whales, i.e. the early Eocene ( ~55-50 mya ).
Lego: Good point. I assume the source I used for that post ( Walker’s Mammals of the World ) were pushing things wayyy back into the realm of abstract possibilities ( and think, reding a little more carefully that they may have been claiming that the Creodonts may have arose at the end of the Cretaceous, not that the Cetacea began diverging at that point ). Their cite was: Gaskin, D. E. 1976. The evolution, zoogeography, and ecology of Cetacea. Oceanogr. Mar. Biol. Ann. Rev. 14:247-346.
Generally it was been accepted that based on paleontological data the whales are descended from Mesonychids ( which definitively appeared around 65 mya ), which some people consider Creodonts and some don’t. The Cetacea per se, are generally dated to about 50 mya. Now recent molecular evidence indicates a closest modern relationship with the Artiodactyls, specifically hippos. The two camps are somewhat at odds. But considering there isn’t any Mesonychid DNA to test and the fact that hippos are a much younger lineage than the Cetacea, it is somewhat in flux. Though I imagine one doesn’t necessarily contradict the other. I’m no expert but I think the exact relationship of the Mesonychids to other groups might be a little hazy - At least I get that impression. And the Artiodactyls are a notoriously complicated group.
err… that should be - “Generally it has been accepted…” and “…and I think, reading a little more carefully…”
Also, checking a little more thoroughly ( there’s always so much contradictory stuff to plow through ), it appears that it is pretty unlikely that Artiodactyls could share a common answer with Mesonychids ( or Mesonychians depending on your classification scheme ).
And you all thought taxonomy was just dusty columns of Latin and Greek! Pshaw! We have controversy, rivalry, competing claims, confusion, soap-operaesque behind-the-curtains relatives…this is not a science for the faint of heart. :0)
Just for completeness sake, one more order is represented in oceanic mammals
Sea Otters are from the order Carnivora (specifically, Mustelidae family, which is one of my favorite, also including wolverines, badgers, skunks and minks).
), it appears that it is pretty unlikely that Artiodactyls could share a common answer with Mesonychids ( or Mesonychians depending on your classification scheme 
).