skulls as indicative for a taxon in between Stegoceras and Pachycephalosaurus; these have not been consistently referred to a new genus. Finally, Horner thought there was a taxon present that was transitional between Daspletosaurus and Tyrannosaurus. In 2017, tyrannosaurid remains from the Two Medicine Formation were named as a new species of Daspletosaurus: Daspletosaurus horneri. The 2017 study considered it plausible that D. horneri was a direct descendant of D. torosus in a process of anagenesis, but rejected the possibility that D. horneri was the ancestor of Tyrannosaurus. Other ceratopsians from the Two Medicine Formation include Einiosaurus and Stellasaurus. In addition, remains of other indeterminate and dubious centrosaurines, including Brachyceratops, are known from the formation and though they may represent younger stages of the three valid genera, this is not possible to demonstrate. Whereas Horner assumed that Einiosaurus and Achelousaurus were separate in time, in 2010 Donald M. Henderson considered it possible that at least their descendants or ancestors were overlapping or sympatric and thus would have competed for food sources unless there had been niche partitioning. The skull of Achelousaurus was more than twice as strong than that of Einiosaurus in its bending strength and torsion resistance. This might have indicated a difference in diet to avoid competition. The bite strength of Achelousaurus, measured as an ultimate tensile strength, was 30.5 newtons per square millimeter (N/mm2) at the maxillary tooth row and 18 N/mm2 at the beak. Wilson and colleagues found that since the Two Medicine centrosaurines were separated stratigraphically, they were therefore possibly not contemporaneous. However, in 2021 a study by Wilson and Scannella pointed out that specimen MOR 591 was of a younger individual age than the Einiosaurus skull MOR 456 8-8-87-1, but of the same size. If MOR 591 could indeed be referred to Achelousaurus, this might indicate this genus reached its adult size more quickly. The indeterminate specimen TMP 2002.76.1 is from the Dinosaur Park Formation and, if it belongs to Achelousaurus, the genus would be the stratigraphically oldest known pachyrhinosaurine taxon. Achelousaurus would then also be the only Campanian ceratopsid known from more than one formation. Both animals occur right below the marine shales of the Bearpaw Formation, but due to longitudinal differences, TMP 2002.76.1 is about 500,000 years older than the Achelousaurus fossils from the Two Medicine Formation. See also Timeline of ceratopsian research References Bibliography Campanian genus extinctions Campanian genus first appearances