Sea Cow

August 17, 2011 - 3:30 pm
Irradiated by LabRat
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In a post about the affability of manatees, Peter asks:

. Fascinating creatures . . . I’d love to know how they evolved, and what particular ecological niche they came to occupy.

OK!

Manatees and dugongs are Sirenians, an old order of placental mammals that has its first roots in Paenungulata, one of the earliest groups of placental mammals to diversify away. It’s a quirky clade in that there are only three groups alive today that descend from this group- the Sirenians, the Proboscideans (elephants), and the Hyracoideans, or hyraxes. The Sirenians and the Proboscideans are the more closely related of the living groups, and are grouped together along with another extinct group in the Tethytheria, which evolved along the shores of the Tethys ocean that existed between the Gondwana and Laurasian supercontinents, around where the Indian ocean and India itself are now.

What would became Sirenia started wandering into the water at around the same time some other famous aquatic mammals did, during the early Eocene, the period about ten million years after the extinction of the dinosaurs that can also be described as “1001 Nifty Things You Can Do With A Mammal”, evolutionarily speaking.

The Eocene was a period somewhat warmer than now, but more crucially, also featured a much more moderate temperature gradient from pole to pole- it might not have been dramatically warmer overall, but there was far less truly cold water, almost no ice, and there were far more shallow seas between the continents, which had not yet drifted all that far apart. The Sirenians were, effectively speaking, simply grazing animals that found good fodder in rivers and these shallow seas; like the cetaceans, they started off as robust four-legged animals that spent most of their time in the water (much like modern hippos, ecologically if not genetically speaking) and slowly lost their ability to function on land, as well as hind limbs, over time. Between their choice of habitat and their dense, heavy skeletons, the Sirenian fossil record is one of the smoother examples of major anatomical transition out there.

Global cooling was not kind to order Sirenia, and with the exception of Steller’s sea cow (which seems to have become extinct in short order after its official discovery, because mariners treated it exactly as the name implies) the extant Sirenians were reduced to a small slice of their original diversity, eating seagrasses in tropical oceans and river grasses in rivers and estuaries.

So, there you have it: they’re grazing animals that evolved out of a process of following the fodder, during a period when “into the water” seemed like a really good idea to more than one mammal group, and the niche they fill is, essentially, that of legless, amiable hippos. Were it not for the invention of the extended ocean journey for hominids, and subsequently the powerboat, it would be a pretty idyllic life.

No Responses to “Sea Cow”

  1. Old NFO Says:

    Well done, thanks for the education :-) Saw a lot of them in Florida, and they ARE very placid mammals.

  2. Mad Rocket Scientist Says:

    It’s not “OK!”, it’s “Challenge Accepted!”

  3. perlhaqr Says:

    *moo glub*

  4. Peter Says:

    What Perlhaqr said. No, wait . . .

    :-)

    Thanks, Labrat. Interesting to know how they evolved. Having spent so much time with elephants, hippos and the like in Africa, that may have something to do with why I find manatees and dugongs so interesting – an instinctive association, perhaps?

  5. ozymandias Says:

    “1001 Nifty Things You Can Do With A Mammal” <– epic win. :)

  6. LabRat Says:

    I’m surprised you didn’t mention hyraxes… :)

  7. Peter Says:

    Hyraxes? Oh, you mean dassies!

    http://en.wikipedia.org/wiki/Rock_Hyrax

    Yes, I grew up around them. They’re all over Table Mountain in Cape Town, and are a well-known tourist attraction at the cable station at the summit (and real pests, begging for food!). When I used to hike in the Cedarberg mountains, I’d occasionally find them trying to eat the foot of my sleeping-bag at three in the morning . . . although they were probably trying to steal the filling for use as nesting material.

    :-)

  8. perlhaqr Says:

    It was my sea cow impression. :)

    More seriously, though, I never would have guessed that manatees, elephants, and hyrax…es? Hyraxii? Hyraces? were all related.

  9. Ted N Says:

    Cool stuff, thanks for the read!

  10. LabRat Says:

    Perl- not all that closely, it’s mostly worth mentioning at all because they’re the sole survivors of that particular lineage, and the results were… diverse.

  11. Justthisguy Says:

    Mrs. Adamson of “Born Free” was right fond of her pet rock hyrax. Said it was easy to toilet-train. Oh, and let yer kitteh ride on the top of the car as did Elsa. My old guy used to use the luggage rack on top of the Chevy Malibu wagon. Kitties want to be able to jump clear if the human driving does something weird.

  12. Steve Bodio Says:

    Don’t forget elephant “shrews” and golden “moles” (;-)– “Afrotheria”.

    I once stroked a wild manatee from a canoe in a backwater of the St John’s river in Florida– must be the most placid wild megafauna alive. Most domestics are warier with strangers!

  13. Matt G Says:

    Of all the experiences that I had on a trip to Belize in the summer of 2009, my favorite was snorkeling out of a large sailboat off the reef, within easy sight of a manatee feeding along the leeward side of the reef. It was surreal; she was suddenly there, close enough for me to easily see without my glasses. Our group all hung in a crescent beneath the water as still as possible, watching as this large mammal showed enormous grace as it turned to look at us. Then she was simply gone. No jerky movements. Nothing to make me believe that she was upset– she just Was No Longer There.

    I suddenly realized that the manatee was more adept than I had been giving it credit for, at moving with easy grace in the shallow reef water. Awesome.

  14. perlhaqr Says:

    Heh, diverse is a good way to put that. I dunno. I guess I just don’t really have a good gut feel for the lengths of time involved and the branching nature of evolution. I mean, I understand in my head, but there a part of me that thinks in terms of human history, (“Wow, three thousand years ago was a really long time!”) and, as an American, badly at that, even, (“Wow, two hundred years ago was a really long time!”).

    But I guess there was some common ancestor that turned into pygmy marmosets and us, though I’m still not sure there isn’t more physiological difference between manatees and elephants.

  15. DirtCrashr Says:

    Oh to live on Table Mountain,
    with the Hyraxes begging for food…
    You can’t be twenty-thousand on Table Mountain
    Though you’re thinking that
    the Eocene’s too soon…

  16. Steve Bodio Says:

    I forgot– aardvarks are also distant (Afrotheria) relatives. The new Discover Evolution special issue has a glancing mention (on early placentals and successful marsupials actually).