page_title: Achelousaurus
section_title: Sampson names Achelousaurus
hierarchical_section_title: Achelousaurus / History of discovery / Sampson names Achelousaurus
caption_attribution_description: English: Herakles fighting the river-god Achelous for Deianeira. Side A from an Attic red-figure column-krater, ca. 450 BC. From Agrigento. Français&#160;: Héraclès combattant Achéloös pour Déjanire. Face A d'un cratère à colonnettes attique à figures rouges, v. 450 av. J.-C. Provenance&#160;: Agrigente.
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 had continued his studies of the material since 1989. In 1994, in a talk during the annual meeting of the Society of Vertebrate Paleontology, he named "Taxon C" as a new genus and species, Achelousaurus horneri. Although an abstract was published containing a sufficient description, it did not identify a holotype, a name-bearing specimen. In 1995, in a subsequent article, Sampson indicated specimen MOR 485 as the holotype specimen of Achelousaurus horneri. The generic name consists of the words Achelous, the name of a Greek mythological figure, and saurus, which is Latinized Greek for lizard. Achelous (Ἀχελῷος) is a Greek river deity and a shapeshifter who was able to transform himself into anything. During a fight with Hercules, the mythical hero, Achelous took the form of a bull, but lost the battle when one of his horns was removed. This allusion is a reference to the supposedly transitional traits of the dinosaur and the characteristic loss of horns through ontogenetic and phylogenetic development, and thus through individual change and evolution. Dodson, in 1996, praised the generic name for being original and intelligent. The specific name honors Jack Horner, for his research on the dinosaurs of the Two Medicine Formation in Montana. Sampson also named "Taxon B" as the genus Einiosaurus in the same article wherein Achelousaurus was described. He said paleontologists needed to be cautious when naming new ceratopsian genera because their intraspecific variation (i.e., variation within a species) might be mistaken for interspecific differences (between species). Until 1995, only one new genus of centrosaurine dinosaur had been named since Pachyrhinosaurus in 1950, namely Avaceratops in 1986. Achelousaurus thus holds particular importance for being one of the few ceratopsid genera named in the late twentieth century.
  The holotype specimen MOR 485 was collected by Hostetter and Ray Rogers from the Landslide Butte Field Area about 40 km (25 mi) northwest of Cut Bank. In 1995 Sampson described it as the partial skull of an adult animal including the nasal and supraorbital (region above the eye socket) bosses (roundish protuberances instead of horns), and the parietal bones. Additionally, MOR 485 preserves some bones of the skull rear and sides, which in 2009 were listed by Tracy L. Ford as a right squamosal bone, the left squamosal, both maxillae, both lacrimal bones, both quadrate bones, both palatine bones, the braincase and the basioccipital bone. In 2015, Leonardo Maiorino reported that as part of the same specimen a fragmentary lower jaw has been catalogued as MOR 485-7-12-87-4. A right squamosal bone from another adult individual was recovered from the same Canyon Bone Bed site as MOR 485 (and catalogued under the same number), but only reported in 2010. Two other specimens were collected on the Blacktail Creek, 35 km (22 mi) to the south of Cut Bank and referred to Achelousaurus by Sampson in 1995. Specimen MOR 591 is a partial skull and an about 60% complete skeleton of a sub-adult specimen that includes the vertebral column, pelvis, sacrum and a femur. It also includes lower jaws, catalogued as MOR 591-7-15-89-1. Both skull and lower jaws are nearly complete, lacking only the braincase and occipital region. MOR 591 is smaller than the holotype with a skull base length of about 60 cm (24 in). Specimen MOR 571 includes a partial skull and lower jaws with associated ribs and vertebrae of an adult. The skull consists of only the parietals, and the lower jaws are limited to their upper rear bones, the surangulars and articulars. A fifth specimen is MOR 456.1, a subadult. None of the specimens were of an advanced individual age. According to Andrew McDonald and colleagues, the Achelousaurus finds represented single individuals, not bone beds.
caption: Achelous loses his horn to Hercules on an Attic krater
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