Check the listing for details. Achelousaurus Extinct Ceratops Dinosaur Bone Fossil in Display Case. Condition: Used. Listed at 17.99 USD. First fossils from Achelousaurus were discovered within Montana in 1987 by a group led by Jack Horner, with more discoveries being made in the year 1989. A huge centrosaurine, known as Achelousaurus is believed to have been 20 feet long and had an estimated weight of 3.3 tons.