Achelousaurus. Achelous' Lizard. Strangely named, considering that Achelous is a minor deity of the Greek pantheon and the remains of Achelousaurus were discovered in Montana in the Two Medicine Formation and described in 1995 by Scott Sampson. Achelousaurus was approximately 20 feet (6m) long and weighed an estimated 3 tonnes. Achelousaurus was a Centrosaurine dinosaur and as such was smaller than a Chasmosaurine, like Triceratops, but larger than Protoceratopsians, like Protoceratops. Being a Centrosaurine, Achelousaurus possessed an adorned parietal frill with two large spines forming from the parietal and dense bones over the eye where earlier Centrosaurines possessed supraorbital horncores and another area of dense bone over the nasal bone in place of the extremely large nasal horncores of other earlier Centrosaurines. Achelosaurus was one of the last of the Centrosaurines and marks the near pinnacle of the family's evolutionary tree. It is closely related, perhaps not directly, to Einiosaurus and Pachyrhinosaurus, older and younger Centrosaurines respectively. So far three skulls have been unearthed from the Two Medicine Formation, which makes study of Achelousaurus a little more comprehensive, as multiple samples of any skeleton will do, and means that our understanding of Achelousaurus is that much more in depth.