Sunday, November 18, 2012

The Superorder Xenarthra: More Than Meets The Eye

Here's a joke for you: what does the sloth, the armadillo, and the anteater have in common?  Unfortunately it's a pretty terrible joke and not very funny at all, so you might want to keep it to yourself next time you are at a party.  The answer is that they are all in the superorder Xenarthra.  See?  I told you it was bad.

The Xenarthrans are a large group with the lowest metabolic rate of all of the therian mammals (essentially all mammals except for the egg-laying monotremes).  In our Animal Spotlight on the sloth, we discussed the Top 10 sleepiest animals: the sloth was number 2, and the armadillo was right behind it in third place!  But enough about sleep!  There are many very interesting types of Xenarthrans, both living and dead, so let's take a look, shall we?

Before we look at any Xenarthrans in detail, let's just touch upon their spread across the world.  All of the Xenarthrans evolved in the millions of years of isolation experienced by South America prior to the fairly recent formation of the Isthmus of Panama around 3 MYA during the Pliocene Epoch.  During this event, known as the Great American Interchange, many Xenarthrans went north into Central and North America, while many other animals headed south.  We will look at some individual cases of this throughout the post! 

First off, we have the order Cingulata, which includes the extant armadillos, as well as the extinct glyptodonts and pampatheres.  These guys all have something called "dermal armor," meaning "skin armor," which is composed of many epidermal (skin) scales that overlap.  These scales are typically referred to as "scutes," and are made up of bone, surrounded by a covering layer of horn.  Scutes have evolved in many different animals over the years, a fascinating example of convergent evolution.  Below are a few pictures of non-Xenarthran animals that have, or had, scutes!

When an armadillo rolls into a ball, it is protected on all sides by its dermal armor!  Interestingly, the pangolin, a creature once thought to be a Xenarthran but now known not to be, does the same thing!  This is probably at least part of the reason why many people believed them to be related.

Besides the armadillos, the order Cingulata includes the similarly-armored pampatheres, and the much more interesting glyptodonts, both extinct.  The glyptodonts look like a cross between an ankylosaur and an armadillo, and were pretty big, especially compared to the armadillos!  Some glyptodonts went north during the Great American Interchange but, unlike the armadillos, were not able to survive to the present day. 

Next up, we have the family Folivora, or the family of sloths.  Now, we have ALREADY TALKED ABOUT THE EXTANT TREE SLOTHS IN ANOTHER EXCELLENT POST, so we won't really discuss them today.  We will, instead, take a brief look at the giant ground sloths!

At least five ground sloths were successful in their transition from South to North America.  One interesting fact about the ground sloths is that many cryptid hunters (people who believe in Bigfoot, the Yeti, the Loch Ness Monster, etc.) believe the mapinguari, a a mythological creature of Bolivia and Brazil, to be either a cultural memory of a ground sloth from thousands of years ago, or actually a surviving ground sloth or two that managed to survive until very recently, and perhaps is still alive today.  Many parts of the description of the animal match up to what we know, or think we know about these giant ground sloths, including size, the sloping back, long claws, and caiman-like skin.  For those of you who don't know, the caiman is a crocodilian, related to crocodiles and alligators.  This might look weird on a giant sloth creature, but preserved skin of a giant ground sloth shows a type of dermal armor similar to the armadillo and the crocs.  So who knows!

Actually, there is one more sloth thing!  The other day, my friend Kristie Chua sent me something funny.  It read, "If you ever feel uncoordinated, just remember that sometimes a sloth will mistake its own arm for a tree branch, grab it, and fall to its death."  I'm not sure if it's true or not, but it most certainly seems plausible!  Either way, it definitely makes you feel better about yourself!

Finally, we have the anteaters!  Below are a few pictures that I took at the Denver Zoo A FEW MONTHS AGO, and below that we have a picture of a giant anteater skull.  Notice how the lower and upper jaws have been fused together!  Then, below that, is a picture of the tongue of the giant anteater, which can actually be longer than the skull itself!  Enjoy!

This birthday post goes out to Sam Lippincott, happy birthday Sam!  If you have a birthday coming up, just email me the date at with the date and your favorite animal, and I will do my best to get a post in!

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