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Xenarthrans

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.

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