What Are Xenarthrans?

Question: What Are Xenarthrans?

Answer: The strangest name I've ever encountered for a group of animals is quite possibly 'xenarthrans'. It evokes images of extra terrestrials that have beamed down from some distant planet. But xenarthrans are far less alien than that. Xenarthrans (the common name for members of the Order Xenarthra) form a group of mammals that include creatures such as armadillos, sloths, and anteaters.

Members of the Order Xenarthra share the following characteristics:

• unique joints in their backbone that provide extra strength and support when digging and burrowing
• few or no teeth
• small brain

Some species that belong to this group of mammals include:

• Giant anteater (Myrmecophaga tridactyla)
• Nine-banded armadillo (Dasypus novemcinctus)
• Southern tamandua (Tamandua tetradactyla)

Xenarthrans comprise an ancient group of placental mammals that once roamed across Gondwanaland before the continents of the Southern Hemisphere separated into their present day configuration.

When Gondwanaland divided, it formed South America, Africa, India, Arabia, New Zealand, and Australia. At that time, xenarthrans were isolated on the continent of South America. Since then, they have been gradually spreading and now can be found in areas of Central America and southern parts of North America.

Though xenarthran populations were absent from Africa, Asia, and Australia, these regions contain unrelated species that evolved to resemble xenarthrans. Similar environmental conditions in these distant parts of the world resulted in species that, although unrelated, adapted in a similar manner and as a result resemble each other in some ways. This evolutionary dynamic is known as convergent evolution.

Examples of species that display convergent evolution with the xenarthrans include the aardvark (Africa), the pangolin (Africa and SE Asia), and the spiny anteater (Australia). These animals all have genetically different ancestors than the xenarthrans and consequently belong to different orders than the xenartrhans, yet they have evolved similar characteristics.