The type species for the genus is Planicoxa venenica, first described by Tony DiCroce and Kenneth Carpenter in 2001. Its discovery was made in the Poison Strip Sandstone Member of the Cedar Mountain Formation in Grand County, Utah, United States. Planicoxa refers to the flat appearance of the ilium formed by horizontal folding over of the postacetabular process (rear portion of the ilium), the defining characteristic of the genus; venenica, the species name, is Latin for “poison” in reference to the Poison Strip Sandstone Member of the Cedar Mountain Formation where the discovery was made. This new taxon is represented by a well-preserved ilium, femora, tibiae, and vertebrae, as well as other material. The femora are classical ornithopod, but the ilium has a short, horizontal postacetabula