Argentinopachycephalosaurus

Argentinopachycephalosaurus is an extinct species of dinosaur part of the extinct group of bird-hipped dinosaurs, Pachycephalosaurids. It has the longest name of any named dinosaur, its name means Argentine Pachycephalosaurus. It resembled a North American Pachycephalosaurid, Stygimoloch (which may be a juvenile Pachycephalosaurus instead), but unlike Stygimoloch, Argentinopachycephalosaurus was indeed a fully-grown adult as the inside of their bones confirmed.