Fossil Xenarthra


Order: Xenarthra

Xenarthrans are thought to be the most primitive of the placentals. Xenarthra means 'extraneous joints', a name referring to the supplementary articulations that all xenarthrans exhibit between some of the trunk and the tail vertebrae. Also, unusually their ischium and ilium are both fused to their anterior caudal vertebrae. Furthermore they have very reduced teeth and in the past, they were referred to as the Edentata, meaning 'toothless ones'. The modern anteater does not have any teeth at all.

Extinct sloths range as far back as the Oligocene. Early species were about the size of the extant species alive today, such as the Miocene Hapalops. Over time gigantic species evolved, called giant ground sloths, such as Megatherium who reached up to 6 metres in length and 3 tonnes in weight. These species were as big as modern African elephants and thus could not live in trees the way modern sloths do. Instead they used their powerful forelimbs and huge claws to pull down branches of trees upon which they fed. Fossilised trackways show that Megatherium could walk on its hind legs. When stationary in this position, it is thought that they would have rested on their stout powerful tails and bowel shaped pelvis. Giant ground sloths like Megatherium lived through the Pliocene and died out only 11'000 years ago. They ranged throughout South, Central and North America and were thought to have been preyed upon by early man. The sloths that remained small adapted to life in the trees and gave rise to the extant species of tree sloth that are still around today.


Example species:

Ground sloth

Length: 6 m
Region: South America
Time: Pliocene and Pleistocene (5.3-0.01 MYA)
Status: Extinct


Giant ground sloth (Megatherium)
Tring Zoological Museum, UK

(Click on image for a larger picture)


Extinct ancestors of the armadillos were the glyptodonts. These were very large, armoured animals that could weigh up to 2 tonnes and measure 3 m in length. Their armour comprised interlocking hexagonal scutes that fused to form a domed shell around the abdomen. The armour could weigh as much as 400 kg on a 2 tonne glyptodon such as Panochthus. Rings of dermal armour have also been found placed over the head and on the tail. Fractured shells suggest that glyptodonts probably used their tails as weapons in intraspecific fights and calculations have revealed that a large glyptodon with a 40 kg tail could swing the tip up to 12 m/s.


Example species:


Length: 3 m
Region: South America
Time: Pliocene and Pleistocene (5.3-0.01 MYA)
Status: Extinct



Author: Emma-Louise Nicholls
Last updated: 20th November 2005
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Websites produced by students on the MSc Palaeobiology programme in the Department of Earth Sciences at the University of Bristol for academic year 2005-6