Achelousaurus

*Achelousaurus horneri*

***Achelousaurus*** (typically pronounced /əˌkiːlɵˈsɔrəs/, though /ˌækɨˌloʊ.əˈsɔrəs/ might be a better reflection of the butchered etymology;"Achelous's lizard") is a genus of centrosaurine ceratopsid dinosaur from the Late Cretaceous Period of what is now North America 74 million years ago. 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.

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 genera 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).

## Description

They were quadrupedal herbivores with parrot-like beaks, rough bosses (raised bony areas) on their snouts and one pair behind their eyes, and a pair of horns on the end of their long bony frills. With body lengths of up to six meters (20 feet) and a weight of three tonnes, *Achelousaurus* were medium-sized ceratopsians.

## History of discovery

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, who in 1987 headed the team that excavated the holotype skull of *Achelousaurus*, MOR 485. 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 have horns, giving them the appearance of having had the horns ripped off. Achelous was also celebrated for his shapeshifting ability, just as *Achelousaurus* appear to combine features of other ceratopsian dinosaurs.

*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, 74 mya.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.

## Classification

In 1995, Sampson formally placed Achelousaurus in the Ceratopsidae, more precisely the Centrosaurinae. In all analyses, Einiosaurus and Achelousaurus are part of the clade Pachyrhinosaurini. By definition, Achelousaurus is a member of the clade Pachyrostra (or "thick-snouts"), in which it is united with Pachyrhinosaurus. In 2010, Gregory S. Paul assigned A. horneri to the genus Centrosaurus, as C. horneri. This has found no acceptance among other researchers, with subsequent taxonomic assessments invariably keeping the generic name Achelousaurus.

## JPInstitute.com Description

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 Triceratops, 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.

## Links

https://web.archive.org/web/20040606160352fw\_/http://www.jpinstitute.com/dinopedia/dinocards/dc\_achelousaurus.html