SLOTHS, ARMADILLOS AND ANTEATERS

Sloths, armadillos and anteaters belong to one of the oldest groups of mammals and are now placed together in their own clade – the Xenarthra. Living xenarthrans are the relicts of a spectacular radiation that produced some very strange mammals many million of years ago in South America, when this region was an enormous island isolated from other land masses. Some huge xenarthans, like the six-metre high Giant Ground Sloths and glyptodont, survived until historical times. Today only smaller tree sloths, anteaters and armadillos remain occupying very specialised ecological niches.

Although outwardly very dissimilar, sloths, armadillos and New World anteaters share a number of distinctive features:

• Their lumbar vertebrae have special articulations, xenarthrales, which reinforce the lumbar region;

• They have the most simple skulls of all mammals; teeth are peg-like or absent;

• The posterior vena cava, which returns blood to the heart from the hind quarters, is double; it is single in other mammals;

• Females have a primitive divided womb, like marsupials, and a common urinary and genital tract;

• Most occur in South and Central America; a few occur in the southern U.S.A.

The plaster model in the bottom of the case represents Megatherium, the giant ground sloth.