What are the Xenarthra

The continents of North and South America have been isolated for hundreds of millions of years at a time and this has allowed the evolution of some very strange and unique plants and animals. Among the strangest are the animals grouped in the Mammalia Order Xenarthra: the armadillos, sloths and anteaters. All but the Armadillo are found only in South America. Along with the Opossum, the Armadillo moved north when the South American plate collided with the North American plate to form the land bridge of Central America. Sloths stayed in the rainforests of the Amazon while anteaters ranged across both the grasslands and forests of South America. Xenarthrids suffered greatly when the plates collided because this allowed carnivores to invade South America. Many species became extinct, especially the largest and slowest forms, so today this order is much reduced in number of species.

These three groups of animals do not look that similar but this is due to different lifestyles. Underneath they show their common ancestry, which has caused scientists to put them together as the Xenarthrids. Xenarthra means strange joints and this is the characteristic that armadillos, sloths and anteaters share. In the lumbar vertebrae there is an extra articulation that is found in no other group of mammals. This gives their backbones extra strength and explains the characteristic defense pose that an anteater takes. He stands on his hind legs, braces himself on his tail and can then use his powerful clawed front legs to defend himself.

Anteaters are specialised for eating termites and ants. Their powerful claws are used to dig out their prey. Their long thin snouts probe into the colonies and an even longer sticky tongue penetrates into the smallest openings to capture their food. Anteaters have lost their teeth which aren’t needed and which would just get in the way. In the Caenozoic Era there were many more species of anteaters than exist today and some were much larger as well. Anteaters have elongated heads as well and the brain is quite small. This is also true of their cousins the sloths. There are three living species of Anteaters: the giant anteater which lives on the pampas and grasslands of South America, the much smaller silky anteater, which is only the size of a squirrel and lives in the rainforests of Central and South America. It is also nocturnal so rarely seen. The third species is the tree anteater or tamandua which is larger but also lives in forests.

There are two families of sloths. The first are the so-called two-toed sloths, family Megalonychidae. The name two-toed is slightly misleading as they have two toes on the front feet, but all sloths have three toes on the hind legs. Perhaps they should be called the two-fingered sloths instead. Three-toed sloths are slghtly smaller than the two-toed species and are classed in the family Bradypodidae. All sloths are tree dwellers and move very slowly from tree to tree in search of their food. They are herbivores. There are only five species of sloths left, but in the past there were many more, including the giant ground sloth, Megatherium, which was as large as an elephant. Sloths used to be classified as Edentata, which means toothless, but as well as being quite different from true edentates, the sloths do have teeth. They have molars at the backs of their mouths for crushing the leaves and other plant matter in their diet.

The last members of this strange order are the armadillos, family Dasypodidae. There are about twenty species of armadillos, mostly living in South and Central America but a few have moved north to Texas and beyond. Since they have a low body temperature, they do not survive well in cold climates, so that will be the factor that will stop their northern migration. Although with global warming who knows? They may someday even make it to Canada.

Armadillos are characterised by bony plates that protect them from predators. Only the three banded armadillos are able to roll up in a ball. The rest have more plates and are too inflexible for this trick. Fossil armadillos were much larger than modern species: Glyptodon was the size of a Volkswagon beetle!

There are basically nine types of living armadillos. The largest is the Giant armadillo, genus Priodontes. The smallest are the pink fairy armadillo (Chlamyphorus), the dwarf armadillo or pichi (Zaedyus) and the greater fairy armadillo (Calyptophractus). There are six species of nine banded or long nosed armadillos, the types that are found in the southern US. There are three species of hairy armadillos (Chaetopractus), one species of the six banded armadillo (Euphractus) and two species of three banded armadillos (Tolypeutes). There are also four species of naked tailed armadillos (Cabassous).

All armadillos are insectivorous, ground-dwelling, diurnal animals. They have poor eyesight but good hearing and their best sense is smell, which helps them find termites, ants and other invertebrates. They have long, sticky tongues and sharp digging claws, like their cousins, the anteaters. The nine-banded armadillo is unusual in that the female has four identical young that develop from the same egg and share the same placenta. Female armadillos can also delay implantation of eggs which allows them to wait until environmental conditions are suitable for rearing young. Baby armadillos have soft shells that harden as they mature.

Some people find armadillos annoying because they dig holes in their lawns and gardens, but they are unique animals with just as much right to survival as we do. A little tolerance goes a long way and maybe a realisation that a few holes in the garden are a small price to pay for having such unusual and interesting neighbors.

for more information: www.xenarthra.org