The "polacanths" are an unusual group of armored dinosaurs. They share features of both ankylosaurids (4 spikes on back of head, highest point on skull in front of eyes) and nodosaurids (lateral spikes, narrow jaws, no tail club) as well as some unique features (fused armor plate over the hips). They could be either early nodosaurids, early ankylosaurids, a branch independent of the two, or an unnatural grouping comprising of early members of both groups. (As a family, they would be called "Polacanthidae"; as a subfamily, "Polacanthinae".) The most recent evidence suggests they are basal nodosaurids, and that they may or may not be a valid grouping.
Regardless of their exact phylogenetic classification, the polacanths, like all ankylosaurians, are Earth Dinosaurs.