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Xenarthrans
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The origin of the xenarthrans takes us back around = 75 to 80=20 million years ago, near the end of the Age of Dinosaurs. During this = period, the=20 distinct anscestral lines that would result in sloths, armadillos, and = anteaters=20 emerged. Following the end of the Mesozoic Era (around 65 million years = ago),=20 these ancient lineages would develop into some of the most specialized - = and=20 bizarre - creatures found in the New World.
The oldest xenarthran fossils found in South = America date=20 back to around 60 to 65 million years ago. At this time, the Earth was a = very=20 different place. Tropical rainforests covered much of the world. The = climate,=20 compared to present-day, was much warmer. Shifts in the continental = plates=20 resulted in a complete isolation of South America from other land = masses.=20 Between 70 million years ago to as recently as five million years ago,=20 xenarthrans and other mammals developed separately from the rest of the = world.=20
The Earth has changed in the past 5 million years. = We are=20 currently in an ice age: a time period when polar caps of ice exist on = Earth. At=20 present, this glacial ice is confined to the Arctic and Antarctic = regions. But=20 two million years ago, glacial sheaths were much larger and covered = one-third of=20 our planet's surface. With much of the planet's water turned to ice, = ocean=20 levels dropped. This action exposed land that had previously been = underwater -=20 including a land bridge between North and South America. After 60 = million years,=20 xenarthrans and other mammals were free to migrate to other areas of the = world.=20
Ancient Xenarthrans
Scientists have determined that early xenarthrans = split into=20 three distinct evolutionary lineages, each with its own distinct=20 characteristics. First was a group of land-dwelling armored = plant-eaters, whose=20 descendants would develop into modern-day armadillos. The second = consisted of=20 insectivores specializing in ants and termites (armadillos). The third = group,=20 represented by modern-day sloths, were a tree-dwelling group of animals = lacking=20 armor.
Some of these ancient xenarthrans were massive and = powerful.=20 Glyptodon, for example, was an armored armadillo-like creature = with a=20 five to ten foot (1.5 to 3 meter) long shell on its back. Perhaps more=20 well-known is Megatherium americanum, known popularly as a = giant ground=20 sloth. This huge bear-like creature grew to the size of an elephant and=20 possessed massive claws twelve inches (30 centimeters) long.
The number of xenarthran genera throughout the = Tertiary=20 period (65 to 1.8 million years ago) was ten times the amount still in = existence=20 today. Fossilized remains of some of these ancient ancestors date to as = recently=20 as 10,000 years ago. Scientists are still working on theories as to why=20 creatures such as Glyptodon and ground sloths became extinct.=20 Modern-day xenarthrans, conversely, generally remain plentiful in their = natural=20 habitat, using a combination of specialized diets and unique physical=20 adaptations. They are found in South America, Central America, and the = southern=20 United States.
The Xenarthrans of Today
In the 18th century, some scientists grouped these = mammals=20 into an order called Edentata, or "toothless". Also included in this = order were=20 pangolins, which are found in Africa and southeast Asia. Pangolins are = indeed=20 very similar to the xenarthrans: they possess armored plates (like = armadillos),=20 lack teeth, and possess a long, sticky tongue for extracting ants and = termites=20 (like anteaters). However, pangolins lack several of the distinct=20 specializations shared by the xenarthrans, and thus are now considered = to be a=20 part of the order Pholidota. The term "xenarthra", from the Greek "xeno" = ("strange") and "arthron" ("joint"), is now more popularly used to = describe=20 sloths, anteaters, and armadillos.
Today, xenarthrans are grouped into two orders: = Cingulata=20 (armadillos) and Pilosa (anteaters and sloths). These mammals possess = several=20 unique traits which make them different from other mammals on Earth. The = first=20 is the existence of additional moving parts, or articulations, between = the=20 vertebrae just above the hips. The flexibility of these articulations, = known as=20 "xenarthrales", is important especially to armadillos for digging or = burrowing.=20 The pelvic bones of xenarthrans are also unique. Instead of being = separate=20 bones, as in most mammals, they are fused together into one solid bone. = But the=20 most intriguing adaptation of xenarthrans is probably their abnormally = low=20 metabolic rate and variable body temperatures.
One common trait in mammals is that they are = "warm-blooded" -=20 in other words, mammals self-regulate their body temperature and keep it = within=20 a specific range. For example, human body temperature is usually around = 98 to 99=20 degrees Fahrenheit (37 Celsius). Keeping the body at a specific = temperature=20 requires the use of energy - energy that comes from food. A lower and = variable=20 body temperature allows xenarthrans to conserve that energy, and use it = only=20 when most needed.
To further aid in the conservation of energy, = xenarthrans=20 possess special blood vessel structures called retes, or retia=20 mirabilia (meaning "marvelous nets"). Within these structures, = arteries and=20 veins are located beside each other as they pass to and from the body's=20 extremities. Instead of circulating heat throughout the body, the retes = return=20 it back to the center of the body, ensuring that vital organs do not = become too=20 cold. Another example of this thermal efficiency is the existence of a = double=20 posterior vena cava vein, which returns heat from the posterior portion = of the=20 body back to the heart.
These special adaptations have allowed sloths to = avoid=20 extinction and flourish within their natural habitat. Unfortunately, the = specific ecological niche which has sheltered sloths for tens of = thousands of=20 years is quickly disappearing - largely due to human intervention. =
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