The heavy armour, forming a veritable shell on the backs of ankylosaurids and their clubbed tails, makes them look superficially similar to the mammalian glyptodonts (and to a lesser degree to the giant meiolaniid turtles of Australia).
This grouping was ignored by most researchers until the late 1990s, when it was used as a subfamily (Polacanthinae) by Kirkland for a natural group recovered by his 1998 analysis suggesting that Polacanthus, Gastonia, and Mymoorapelta were closely related within the family Ankylosauridae.
Some authors have suggested that Tianzhenosaurus is actually a junior synonym of Saichania chulsanensis, but this matter remains unresolved, and Vickaryous et al. (2004) placed it within the Ankylosauridae, nested as the sister group to Pinacosaurus.