#slothmonday Nº 2

The vertebrae of xenathrans

Sloths are classified in the superorder Xenarthra, a group of placental mammals that also includes armadillos and anteaters. Among their varied anatomical peculiarities, their vertebra have extra intervertebral joints that give name to the whole group (Xenarthra means strange joints). Those strange joints are found specifically in the lumbar and last thoracic vertebrae.

In mammals, the vertebrae articulate to each other with two pair (one anterior and one posterior) joints in the processes called zygapophyses, which help in the stability of the backbone. However, xenarthrans have at least two pairs of those intervertebral joints.

It is likely that the first xenarthrans were powerful diggers, such as armadillos are today, and those joints must have evolved to give the backbone even more stability.