Brontotholus harmoni was named by Woodruff et al. (2025) [The relative size of the frontoparietal indicates that Brontotholus harmoni was a large-bodied pachycephalosaurid (Fig. 1). While lacking frontoparietal autapomorphies, Brontotholus is easily diagnosable via a unique combination of characters including (Fig. 2): (i) a frontoparietal dome that is anteroposteriorly longer than laterally wide (as in Acrotholus audeti, Amtocephale gobiensis, Colepiocephale lambei, Foraminacephale brevis, Pachycephalosaurus wyomingensis, Sphaerotholus edmontonensis, and Stegoceras validum; throughout ontogeny this ratio decreases); (ii) in dorsal view the dome is sub-triangular being laterally wider posteriorly and strongly tapering anteriorly (as in C. lambei and Pa. wyomingensis); (iii) in anterior and posterior view, the dome has a low dorsal profile, being over 1.5 times laterally wider than dorsoventrally thick (in the largest specimens; MOR 480 and TMP 1989.69.21; as in F. brevis, Platytholus clemensi, while quite variable in Pa. wyomingensis and Sp. buchholtzae); (iv) many of the peripheral ele�ment sutural contacts—such as the posterior supraorbital and postorbital—have a vaulted, or triangularly peaked dorsal margin (as in Ac. audeti and F. brevis, and a few of the dorsal margins of C. lambei are somewhat triangular); (v) the posterior supraorbital contacts have a prominent dorsoventral concavity (the posterior supraorbital contacts have a slight c. mid sulcus in F. brevis); (vi) several projecting dorsoventral ridges and grooves present on the sutural contacts of the posterior supraorbital and postorbital (as in Ac. audeti); (vii) the posterior parietal process is greatly antero-posteriorly reduced (as in Ac. audeti, C. lambei, F. brevis, Pa. wyomingensis, Pr. prenes, and Sp. buchholtzae), and is strongly medially tapered and triangular in posterior view (as in Ac. audeti and Sp. buchholtzae, and somewhat in F. brevis).]. Its type specimen is MOR 480, a partial skull (frontoparietal dome), and it is a 3D body fossil. Its type locality is Achelousaurus type, MOR TM-060, which is in a Campanian terrestrial siltstone in the Two Medicine Formation of Montana. It is the type species of Brontotholus.