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Achelousaurus was a large, robust dinosaur, estimated to be around 6 meters (20 feet) long and weighing approximately 3 tons. Its skull, especially the holotype specimen MOR 485, measured about 1.62 meters (5.3 feet) in length, showing distinctive features like a large hooked beak and a bony frill adorned with a pair of curved spikes. Unlike many centrosaurines, Achelousaurus had nasal bones with prominent bosses instead of true horns, which may have had an evolutionary role in displays or fighting. This morphology suggests a dynamic evolutionary history, possibly attracting attention for sexual selection and combat among its species. (References: Scott D. Sampson, 1994; Paul, 2010)

The first fossils of Achelousaurus were found during Jack Horner's expeditions in Montana in 1987, with significant discoveries continuing into 1989. The initial discovery occurred after Horner was forced to seek new sites for excavation, leading to the investigation of Landslide Butte. A key specimen, the holotype MOR 485, was located in 1987 and eventually described by Scott D. Sampson in 1994. The Museum of the Rockies, where these fossils are housed, played an essential role in this discovery. The fossils provided vital insights into the physical characteristics and evolutionary implications of ceratopsians. (References: Jack Horner and associates, 1987-1989; Scott D. Sampson, 1994)

Achelousaurus is significant for understanding the evolution of ceratopsid dinosaurs, particularly within the Centrosaurinae subfamily. Scott D. Sampson proposed that Achelousaurus represents a transitional form between earlier species like Einiosaurus and later forms such as Pachyrhinosaurus. This lineage is theorized to demonstrate anagenesis, a type of evolution where species transform over time without branching into separate species. The fossil record indicates that Achelousaurus had traits that reflect adaptations, possibly linked to environmental pressures on the Laramidia landmass it inhabited. (References: Sampson et al., 1992; Scott D. Sampson, 1994)

Achelousaurus is a genus of centrosaurine ceratopsid dinosaur that lived during the Late Cretaceous period, approximately 74.2 million years ago. It is primarily known from fossil discoveries made in Montana, USA, particularly from the Two Medicine Formation. Described by paleontologist Scott D. Sampson in 1994, Achelousaurus was identified from a variety of skull material representing different ages of individuals, from juveniles to adults. The name 'Achelousaurus' combines 'Achelous', referencing the Greek river deity, and 'saurus', meaning lizard, indicating its mythical significance and dinosaur classification. (References: Scott D. Sampson, 1994; Two Medicine Formation, Montana)

Achelousaurus, as a ceratopsian, was likely a herbivore with adaptations suited for a diet of low-growing vegetation. Its functional skull ornamentation suggests that the nasal and supraorbital bosses may have served purposes in sociosexual display or combat. Research indicates that these dinosaurs exhibited social behaviors resembling modern herd dynamics, potentially living in groups that provided advantages in survival and reproductive success. The analysis of fossilized bone beds linked to Achelousaurus also points to drought-related mortality events in their ecosystems, demonstrating the impact of environmental changes. (References: Horner & Dobb, 1997; McDonald & Farke, 2011)