"Castorocauda" likely had 14 thoracic, 7 lumbar, 3 sacral and 25 tail vertebrae. Like some mammals, it had plated ribs, and the ribs extended into the lumbar vertebrae. Plating occurred on the proximal margins (the part of the rib closest to the vertebra), and, in "Castorocauda", they may have served to increase the insertion area (the part of a muscle which moves while contracting) of the iliocostalis muscle on the back, which would interlock nearby ribs and better support the torso of the animal. Plated ribs are present in arboreal (tree-dwelling) and fossorial (burrowing) xenarthrans (sloths, anteaters, armadillos and relatives). The tail vertebrae are flattened dorsoventrally (shortened vertically and widened more horizontally); and each centrum has two pairs of transverse processes (which jut out diagonally from the centrum) on the headward side and another on the tailward side, making the centrum appear somewhat like the letter H from the top-view looking down. This tail anatomy is similar to beavers and otters, which use their tails for paddling and propulsion.