Home |
Xenarthrans | Sloths | Rainforests |
Miscellaneous | Email

Quick Links

Sloths

Anteaters

Armadillos

Links

Search

Dictionary

Print-Friendly

Home > Xenarthans

Xenarthrans

Ancient
Xenarthrans

Xenarthrans of
Today

Learn More!

The origin of the xenarthrans takes us back around 75 to 80
million years ago, near the end of the Age of Dinosaurs. During this period, the
distinct anscestral lines that would result in sloths, armadillos, and anteaters
emerged. Following the end of the Mesozoic Era (around 65 million years ago),
these ancient lineages would develop into some of the most specialized - and
bizarre - creatures found in the New World.

The oldest xenarthran fossils found in South America date
back to around 60 to 65 million years ago. At this time, the Earth was a very
different place. Tropical rainforests covered much of the world. The climate,
compared to present-day, was much warmer. Shifts in the continental plates
resulted in a complete isolation of South America from other land masses.
Between 70 million years ago to as recently as five million years ago,
xenarthrans and other mammals developed separately from the rest of the world.

The Earth has changed in the past 5 million years. We are
currently in an ice age: a time period when polar caps of ice exist on Earth. At
present, this glacial ice is confined to the Arctic and Antarctic regions. But
two million years ago, glacial sheaths were much larger and covered one-third of
our planet's surface. With much of the planet's water turned to ice, ocean
levels dropped. This action exposed land that had previously been underwater -
including a land bridge between North and South America. After 60 million years,
xenarthrans and other mammals were free to migrate to other areas of the world.

Ancient Xenarthrans

Scientists have determined that early xenarthrans split into
three distinct evolutionary lineages, each with its own distinct
characteristics. First was a group of land-dwelling armored plant-eaters, whose
descendants would develop into modern-day armadillos. The second consisted of
insectivores specializing in ants and termites (armadillos). The third group,
represented by modern-day sloths, were a tree-dwelling group of animals lacking
armor.

Some of these ancient xenarthrans were massive and powerful.
Glyptodon, for example, was an armored armadillo-like creature with a
five to ten foot (1.5 to 3 meter) long shell on its back. Perhaps more
well-known is Megatherium americanum, known popularly as a giant ground
sloth. This huge bear-like creature grew to the size of an elephant and
possessed massive claws twelve inches (30 centimeters) long.

The number of xenarthran genera throughout the Tertiary
period (65 to 1.8 million years ago) was ten times the amount still in existence
today. Fossilized remains of some of these ancient ancestors date to as recently
as 10,000 years ago. Scientists are still working on theories as to why
creatures such as Glyptodon and ground sloths became extinct.
Modern-day xenarthrans, conversely, generally remain plentiful in their natural
habitat, using a combination of specialized diets and unique physical
adaptations. They are found in South America, Central America, and the southern
United States.

The Xenarthrans of Today

In the 18th century, some scientists grouped these mammals
into an order called Edentata, or "toothless". Also included in this order were
pangolins, which are found in Africa and southeast Asia. Pangolins are indeed
very similar to the xenarthrans: they possess armored plates (like armadillos),
lack teeth, and possess a long, sticky tongue for extracting ants and termites
(like anteaters). However, pangolins lack several of the distinct
specializations shared by the xenarthrans, and thus are now considered to be a
part of the order Pholidota. The term "xenarthra", from the Greek "xeno"
("strange") and "arthron" ("joint"), is now more popularly used to describe
sloths, anteaters, and armadillos.

Today, xenarthrans are grouped into two orders: Cingulata
(armadillos) and Pilosa (anteaters and sloths). These mammals possess several
unique traits which make them different from other mammals on Earth. The first
is the existence of additional moving parts, or articulations, between the
vertebrae just above the hips. The flexibility of these articulations, known as
"xenarthrales", is important especially to armadillos for digging or burrowing.
The pelvic bones of xenarthrans are also unique. Instead of being separate
bones, as in most mammals, they are fused together into one solid bone. But the
most intriguing adaptation of xenarthrans is probably their abnormally low
metabolic rate and variable body temperatures.

One common trait in mammals is that they are "warm-blooded" -
in other words, mammals self-regulate their body temperature and keep it within
a specific range. For example, human body temperature is usually around 98 to 99
degrees Fahrenheit (37 Celsius). Keeping the body at a specific temperature
requires the use of energy - energy that comes from food. A lower and variable
body temperature allows xenarthrans to conserve that energy, and use it only
when most needed.

To further aid in the conservation of energy, xenarthrans
possess special blood vessel structures called retes, or retia
mirabilia (meaning "marvelous nets"). Within these structures, arteries and
veins are located beside each other as they pass to and from the body's
extremities. Instead of circulating heat throughout the body, the retes return
it back to the center of the body, ensuring that vital organs do not become too
cold. Another example of this thermal efficiency is the existence of a double
posterior vena cava vein, which returns heat from the posterior portion of the
body back to the heart.

These special adaptations have allowed sloths to avoid
extinction and flourish within their natural habitat. Unfortunately, the
specific ecological niche which has sheltered sloths for tens of thousands of
years is quickly disappearing - largely due to human intervention.

Learn More!

Discover more about the amazing sloth.

Put ancient history into perspective with our Xenarthran
Timeline. (The free Acrobat Reader is needed to view this file.)

Explore the Amazonian tropical rainforest, home of
the sloth and other xenarthrans.

For websites containing images of xenarthran skulls and more, check out
our Links Page.

Top
of Page

Xenarthra.Org written and designed by SlothWerks. Site Hosting by
Tiger
Technologies. Coded with web standard XHTML and CSS. For a complete list of sources used, please see our References page. Xenarthra.Org is
a cookie-free, ad-free website.