Xenarthra: The Unique Mammals of South America

Key Characteristics of Xenarthra

Xenarthrans share several evolutionary, anatomical, and physiological traits that set them apart from other placental mammals.

Evolutionary Traits

• South American origins – Xenarthrans evolved in isolation on South America, while their sister group, Afrotheria, diversified in Africa.
• Unique vertebral structure – The name “Xenarthra” means “strange joints,” referring to their extra articulations in the spine, which provide increased flexibility and strength.
• Ancient lineage – Some extinct xenarthrans, like giant ground sloths and glyptodonts, were among the largest land mammals of the Ice Age.

Anatomical and Physiological Features

• Tooth reduction or absence – Xenarthrans either have simple peg-like teeth (sloths, armadillos) or no teeth at all (anteaters).
• Strong claws – Used for digging (armadillos), climbing (sloths), or tearing into insect nests (anteaters).
• Slow metabolisms – Most xenarthrans have low body temperatures and slow digestion, an adaptation to nutrient-poor diets.

Reproductive and Developmental Aspects

• Unusual reproductive traits – Some armadillos, like the nine-banded armadillo, give birth to identical quadruplets.
• Long maternal care – Sloths and anteaters carry and nurse their young for extended periods.

Major Groups of Xenarthra

Xenarthra consists of two main orders, each with specialized adaptations.

Pilosa (Sloths and Anteaters)

This order includes both slow-moving arboreal sloths and fast-striking anteaters.

Sloths (Suborder: Folivora)

• Tree-dwelling herbivores – Sloths live in tropical rainforests, feeding on leaves and fruits.
• Slow metabolism – Their low-energy diet has led to an extremely slow lifestyle.
• Symbiotic algae – Their fur hosts algae, providing camouflage and additional nutrients.

Anteaters (Suborder: Vermilingua)

• Specialized insectivores – Anteaters use their long tongues and sticky saliva to consume ants and termites.
• Powerful forelimbs – They have strong claws to tear open insect nests and defend themselves from predators.
• Highly developed sense of smell – Anteaters rely on olfaction rather than sight to locate food.

Cingulata (Armadillos)

Armadillos are the only living mammals with a bony shell for protection.

• Armor-plated bodies – Their bony plates provide defense against predators.
• Burrowing specialists – Most species dig extensive burrows and prefer dry environments.
• Insectivorous tendencies – While some eat plants, most armadillos feed on insects and small invertebrates.

Xenarthra vs. Afrotheria: What’s the Difference?

Xenarthra and Afrotheria are the two major groups of Atlantogenata. While Xenarthrans evolved in South America, Afrotherians diversified in Africa and include elephants, manatees, and aardvarks.

Key takeaway: Xenarthrans evolved in South America, while Afrotherians thrived in Africa, leading to distinct evolutionary adaptations despite their shared Gondwanan origins.

Summary

• Xenarthra includes some of the most specialized mammals, from sloths and anteaters to armadillos with bony armor.
• It consists of two major orders: Pilosa (sloths & anteaters) and Cingulata (armadillos).
• Xenarthrans evolved in South America, adapting to forest canopies, dry plains, and underground burrows.
• Despite their differences, all xenarthrans share unique spinal structures, slow metabolisms, and specialized feeding strategies.
• Xenarthrans are evolutionary oddities, shaped by South America’s isolation into some of the planet’s most unique mammals.

Animals in the superorder Xenarthra

Linnaeus’s Two-toed Sloth

Hoffmann’s Two-toed Sloth