- Name: Achelousaurus horneri
- Name Meaning: Achelou's Lizard (lit. Akheloos Lizard)
- Diet: Herbivore
- Length: 6 meters (20 feet)
- Time Period: Late Cretaceous
- Classification: Ceratopsids --> Centrosauridae --> Pachirhinosaurini
- Place Found: Montana, USA
- Describer: Sampson, 1995
Achelousaurus (meaning “Achelous’s lizard”) is a genus of centrosaurine ceratopsid dinosaur from the Late Cretaceous Period of what is now North America. It was a quadrupedal herbivore with a parrot-like beak, a rough boss (raised bony area) on the snout and two more behind the eyes, and two horns on the end of its long bony neck frill. With a total body length of 6 meters (20 feet), Achelousaurus was a medium-sized ceratopsian.
The genus and the one named species (A. horneri) were both named by paleontologist Scott Sampson in 1995. The specific name honors Jack Horner, an influential American paleontologist famous for his Montana dinosaur discoveries. The generic name Achelousaurus is a complex reference to Greek mythology. Achelous, an important Greek river deity, had one of his horns torn off by Hercules, in a mythological fight with the legendary hero. All three known skulls of Achelousaurus have rough bosses in the same places where other ceratopsians had horns, giving it the appearance of having had its horns ripped off. Achelous was also celebrated for his shapeshifting ability, just as Achelousaurus appears to combine features of other ceratopsian dinosaurs.
Early reports suggested that Achelousaurus represented a transitional form between ceratopsians with modified horns like Einiosaurus (with which A. horneri shares two horns on the end of the frill), and the derived, hornless Pachyrhinosaurus (Horner et al., 1992). While they may or may not form a direct line of descent, all three of these species are at least closely related, and are often united in the tribe Pachyrhinosaurini, inside the subfamily Centrosaurinae and the family Ceratopsidae (Sampson, 1995; Dodson et al., 2004).
Achelousaurus is known from the U.S. state of Montana, in the Two Medicine Formation, which preserves sediments dated from the Campanian stage of the Late Cretaceous Period, between 83 and 74 million years ago. Achelousaurus was found in the highest levels of the formation, so it is probably closer to the end of that timeframe. Other dinosaurs found in this formation include Daspletosaurus, Bambiraptor, Euoplocephalus, Maiasaura, and Einiosaurus.
Scientists have so far recovered three skulls and some postcranial material from the Two Medicine, all housed at the Museum of the Rockies in Bozeman, Montana. The skull of a full-grown Achelousaurus (including the frill horns) is over 5 feet (1.6 meters) long.
Achelousaurus is a relative of another frilled ceratopsian dinosaur, Pachyrhinosaurus. They shared the same strange lump of bone on their nose, called a boss. This bony lump was positioned where other frilled dinosaurs, like , had their nose horns.
Achelousaurus was a fairly large plant-eating dinosaur that looked similar in body type to the other larger members of its North American family. There is ongoing discussion as to whether this dinosaur warrants its own genus. There seems to be ongoing speculation that it is a species of Pachyrhinosaurus, perhaps showing gender differentiation. Most likely, it will take a more complete specimen to settle the debate. It is at present considered by many to be somewhere between the Pachyrhinosaurus and Einiosaurus.