page_title: Achelousaurus
section_title: Phylogeny
hierarchical_section_title: Achelousaurus / Evolution / Phylogeny
caption_attribution_description: Nasal horncore of Stellasaurus ancellae holotype MOR 492 in right lateral, left lateral and anterior views. NH, nasal horncore; NO, nasal overgrowth. Scale bar 10 cm.
context_page_description: Achelousaurus is a genus of centrosaurine ceratopsid dinosaur that lived during the Late Cretaceous Period of what is now North America, about 74.2 million years ago. The first fossils of Achelousaurus were collected in Montana in 1987, by a team led by Jack Horner, with more finds made in 1989. In 1994, Achelousaurus horneri was described and named by Scott D. Sampson; the generic name means "Achelous lizard", in reference to a Greek mythological figure, and the specific name refers to Horner. The genus is known from a few specimens consisting mainly of skull material from individuals, ranging from juveniles to adults.
  A large centrosaurine, Achelousaurus was about 6 m long, with a weight of about 3 t. As a ceratopsian, it walked on all fours, had a short tail and a large head with a hooked beak. It had a bony neck-frill at the rear of the skull, which sported a pair of long spikes, which curved towards the outside. Adult Achelousaurus had rough bosses above the eyes and on the snout where other centrosaurines often had horns in the same positions. These bosses were covered by a thick layer of keratin, but their exact shape in life is uncertain.
context_section_description: Sampson felt, in 1995, that there was not enough evidence to conclude that Achelousaurus was a direct descendant of Einiosaurus. Unlike Horner, he decided to perform a cladistic analysis to establish a phylogeny. This showed an evolutionary tree wherein Achelousaurus split off between Einiosaurus and Pachyrhinosaurus, as Horner had predicted. Contrary to Horner's claim, Styracosaurus albertensis could not have been a direct ancestor, as it was a sister species of Centrosaurus in Sampson's analysis.
  Subsequent studies have sought to determine the precise relationships within this part of the evolutionary tree, with conflicting results regarding the question whether Styracosaurus albertensis or Einiosaurus might have been in the direct line of ascent to Achelousaurus. In 2005, an analysis by Michael Ryan and Anthony Russell found Styracosaurus more closely related to Achelousaurus than to Centrosaurus. This was confirmed by analyses by Ryan in 2007, Nicholas Longrich in 2010, and Xu et al. in 2010. The same year Horner and Andrew T. McDonald moved Styracosaurus ovatus to its own genus, Rubeosaurus, finding it a sister species of Einiosaurus, while Styracosaurus albertensis was again located on the Centrosaurus branch. They also assigned specimen MOR 492, the basis of "Taxon A", to Rubeosaurus. In 2011, a subsequent study by Andrew T. McDonald in this respect replicated the outcome of his previous one, as did a publication by Andre Farke et al. In 2017, J.P. Wilson and Ryan further complicated the issue, concluding that MOR 492 ("Taxon A") was not referable to Rubeosaurus and announcing that yet another genus would be named for it. Wilson and colleagues moved MOR 492 to the new genus Stellasaurus in 2020, which therefore corresponds to "Taxon A". Their study found Rubeosaurus ovatus to be the sister species of Styracosaurus albertensis, and concluded Rubeosaurus to be synonymous with Styracosaurus.
  Before Achelousaurus was described, Pachyrhinosaurus canadensis had been considered a solitary aberrant form among centrosaurines, set apart from them by its unusual bosses. Achelousaurus gave evolutionary context to the Canadian species, while expanding the temporal and geographical range for what came to be seen as "pachyrhinosaurs." In all analyses, Achelousaurus and Pachyrhinosaurus were sister groups. In 2008, another closely related species was named, Pachyrhinosaurus lakustai. In that study, the term "Pachyrhinosaurs" was used for the clade consisting of Achelousaurus and Pachyrhinosaurus. When Pachyrhinosaurus perotorum was described in 2012, the clade name Pachyrostra was coined, uniting the two genera; Achelousaurus is the basalmost pachyrostran. Shared derived traits (or synapomorphies) of the group are an enlarged nasal ornamentation and a change of the nasal and brow horns into bosses. At the end of the Campanian, there seems to have been a trend of pachyrostrans replacing other centrosaurines. Also in 2012, the clade Pachyrhinosaurini was named, consisting of species more closely related to Pachyrhinosaurus or Achelousaurus than to Centrosaurus. Apart from Einiosaurus and Rubeosaurus, this included Sinoceratops and Xenoceratops, according to a 2013 study.
  Cladistic analyses develop gradually, reflecting new discoveries and insights. Their results can be shown in a cladogram, with the relationships found ordered in an evolutionary tree. The cladogram below shows the phylogenetic position of Achelousaurus in a cladogram from Wilson and colleagues, 2020.
caption: Nasal horn of Stellasaurus, the possible ancestor of Einiosaurus and Achelousaurus
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