Thyreophoroidea was first named by Nopcsa in 1928 and defined by Sereno in 1986, as "Scelidosaurus, Ankylosaurus, their most recent common ancestor and all of its descendants".

According to Madzia et al., Thyreophora is defined as the largest clade containing Ankylosaurus magniventris and Stegosaurus stenops but not Iguanodon bernissartensis and Triceratops horridus.

[2] They also defined the less inclusive Eurypoda as "the smallest clade containing Ankylosaurus magniventris and Stegosaurus stenops" to include the ankylosaurs and stegosaurs to the exclusion of basal thyreophorans.

A later study conducted by André Fonseca and colleagues in 2024 gave a formal definition for Thyreophoroidea in the PhyloCode as "the smallest clade containing Ankylosaurus magniventris, Scelidosaurus harrisonii, and Stegosaurus stenops".

[8] In their description of Jakapil the following year, Riguetti et al modified the same matrix and found it to occupy a position as the sister taxon to the Eurypoda.