Posted by: Rob Viens | August 27, 2013

Meet the Xenarthra

Over the next week or so Darwin’s rather short entries pertain to two things – (1) reuniting with the Beagle and (2) excavating bones in Punta Alta. Since many of the ground sloth fossils that he discovered came from this region, it seemed like a good time to spend a few days exploring the world of the mighty sloth (and it’s kin). Hence, it is time for:

Sloth Week logo

To start us off, I thought I would share a little about the sloths extended family (technically superorder) – the Xenarthra. The Xenarthra (literally “strange joints”) are a group of mammals that evolved in South America in the Paleogene (about 60 million years ago), and were isolated there until a few million years ago when North and South America were joined (see Bridging the Continents to see Darwin’s speculations about this event). They consist of three distinct groups of animals – armadillos, anteaters and sloths. (Of these, the later two have been determined to be more closely related and placed in the same order – Pilosa). The taxonomic relationships can be seen below:

Xenarthra taxonomy

Armadillos (Order Cingulata)

When we think of armadillos, we tend to picture their hard, yet flexible, outer “shell”. You know, the one that allows them to roll up in a ball like a giant “pill bug”.  This “shell” is actually made up of osteoderms – “plates of dermal bone covered in relatively small, overlapping keratinized epidermal scales called “scutes”” (Wikipedia). These plates are arranged in bands (running perpendicular to the spine) that are connected by skin tissue.  This gives the armadillo a certain degree of flexibility, unlike a turtle shell which is completely rigid (though also made of bony plates). Osteoderms are found throughout the animal kingdom, most notably in some reptiles (such as crocodiles) and dinosaurs (like the iconic stegosaurus). They are much rarer in mammals.

Nine-banded Armadillo from North America (from Wikipedia Commons)

Nine-banded Armadillo

There are currently about 20 living species of armadillos – all of which can be found in South America.  At least two species can also be found further north in Central and North America.

Darwin (and others before him) found evidence for at least two other types of prehistoric cingulata – the glyptodons and pampatheres.  More on these “large armored tanks” later this week.

Anteaters (Order Pilosa, Suborder Vermilingua)

The name says it all – what makes anteaters special is that they have adapted to eating ants.  This is actually not as easy as it sounds, because ants (which are closely related to wasps and bees) sting.  Luckily, evlution has kicked in and made the anteater an ant eating machine.

Anteaters start by digging into ant nests using their powerful claws, while their long fur protects them from external stings.  Then they use their long tongue to quickly lap up as many ants as possible. There distinctive long head contains a tongue that (1) can flick in and out of the mouth 150 times a minute and (2) is covered in salva and small barbs that help catch the ants.  Since the anteater quickly ingests and starts to digest the ants, they never really have a chance to fight back.

There are four living species of anteater – all found in South America including the giant anteater shown below (from Wikipedia Commons):

Giant Eater

It should be noted that convergent evolution has given us several other species that appear and behave similarly to the South American anteaters.  However these species, which include pangolins and aardvarks (found in the Old World), are not biologically related to anteaters at all.

Sloths (Order Pilosa, Suborder Tardigrada)

The poor sloth – forever associated with one of the seven deadly sins.  In reality, their “slothiness” is a result of their diet and very low metabolism, which cause them to move very slowly and stay relatively still for long periods of time in order to conserve energy.  Their slow lifestyle has actually mad it possible for them to  host a relatively complex ecosystems (consisting of plants and insects) on their body. (The cyanobacteria that grow on the sloth’s fur also help protect them from predators by helping them blend in with the surrounding forest.)

Sloths primarily eat leaves (making them folivores). This results in an arboreal lifestyle, and many sloths rarely travel to the forest floor (usually only about once a week to “use the facilities”). The problem with eating leaves, however, is that they are hard to digest and don’t contain a lot of energy. Hence the sloth’s low metabolic rate and low body temperature (about 10°F lower than humans on average).  This also results in sloths having large stomachs that contain symbiotic bacteria to help break down the leaves (digestion can take over a month in some cases). Like other Xenarthra, sloths have long and powerful claws, which in their case, are used to help them hand upside down in the forest canopy.

There are currently 6 species of sloth living in South and Central America – all of them are tree sloths. They are divided into the two-toed sloths (Megalonychidae family) and the three-toed sloths (Bradypodidae family). Interestingly, these living families are actually more closely related to the extinct giant ground sloths then they are to one another.

Three-toed sloth in Costa Rica (from Wikipedia Commons):

Three toed sloth

Much more in the coming days on the now-extinct mega-sloths that roamed South America as recently as about 10,000 years ago.  Their bones were one of Darwin’s really unique and important discoveries on the voyage.

Speaking of our friend Darwin – on this day in 1833, he was reunited with the Beagle:

“A boat with Mr Chaffers arrived from the ship, we waited till the evening for a cow to be killed, to take fresh meat on board. We did not start till late, but the night was beautiful & calm. — The ship had moved her berth, & we had a long hunt after her, at last arrived on board at ½ after one oclock.” (Aug 26)

Stay tuned all week for more sloths and friends! (RJV)

PS – Much of the research on prehistoric Xenarthra this week comes from the recent book I noted in an earlier post – Megafauna: Giant Beasts of Pleistocene South America by Fariña, Vizcaíno, and de Iuliis. I highly recommend it if you want to learn more about these amazing animals.


  1. In honor of sloth week, I plan to move more slowly – but will nevertheless still have to eat more than leaves!

    • Excellent idea! I like the idea of a week of sloth 🙂

  2. “Endless Forms Most Beautiful” does not readily come to mind when viewing this entry, but what the heck! Evolution is not about beauty but what works and evidently sloths and the like are still in the game! I wonder how many ants (or leaves) constitute a fulfilling meal?

    • Oh, I don’t know. I have a new respect for the wonder and beauty of sloths after a couple of days of reading about them. One never ceases to be amazed by the variety of life!

  3. […] made of calcium carbonate in their skin. Much like the bony plates found in the Xenthera (see Meet the Xenarthra), these little plates are called ossicles. It is these plates that make the “skin” of a […]

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