Ankylosaurus magniventris is the largest of all known ankylosaurids (length approximately 8-9 m; skull length to over 750 mm; width over 770 mm). The 1908 type material was missing both the coossified overlapping caudal vertebrae and the massive club of fused dermal plates at the tip of the tail, however. Brown characterized the family Ankylosauridae as having the "backbone stiff," and noted the wide, bowed shape of the animal's ribs and its supposed "strongly arched" back (an error based on a presumed resemblance to stegosaurs and glyptodonts--ankylosaurs had rather flat backs); thus, the additional meanings "stiff lizard" and "curved lizard" probably should be read in the name Ankylosaurus as well.