Stellasaurus (meaning "star lizard"; both in reference to the shape of its head ornamentation and as an homage to the song "Starman" by David Bowie) is a genus of centrosaurine ceratopsid dinosaur that lived in Montana during the Late Cretaceous.

Its remains have been found in the Late Campanian age Two Medicine Formation, the same geological unit which its relatives Rubeosaurus (now seen as a synonym of Styracosaurus), Einiosaurus, and Achelousaurus were discovered in.

The describers saw it as a transitional form between Styracosaurus albertensis and Einiosaurus on a single evolutionary line that led to Achelousaurus and Pachyrhinosaurus.

Similar to Einiosaurus and Achelousaurus, no epiparietals or episquamosals, in the sense of separate "frill ossifications", have been found, indicating the three genera may have lacked them.

[1] Incidentally, they do not call the protuberances "epiparietalia" because this presupposes that they have grown together osteoderms, separate skin ossifications.

It is usually objected to this line of reasoning that given the existence of a second branch, the ancestor and the extinction are already implicit and thus do not make it more unlikely.

[1] Xenoceratops foremostensis Medusaceratops lokii Wendiceratops pinhornensis Sinoceratops zhuchengensis Machairoceratops cronusi Diabloceratops eatoni Albertaceratops nesmoi Nasutoceratops titusi Avaceratops lammersi Spinops sternbergorum Centrosaurus apertus Coronosaurus brinkmani Styracosaurus ovatus Styracosaurus albertensis Stellasaurus ancellae Einiosaurus procurvicornis Achelousaurus horneri Pachyrhinosaurus lakustai Pachyrhinosaurus perotorum Pachyrhinosaurus canadensis Dinosaurs that lived alongside Stellasaurus include the basal ornithopod Orodromeus, hadrosaurids (such as Hypacrosaurus, Maiasaura, and Prosaurolophus), the centrosaurines Brachyceratops and Einiosaurus, the leptoceratopsid Cerasinops, the ankylosaurs Edmontonia and Euoplocephalus, the tyrannosaurid Daspletosaurus (which appears to have been a specialist of preying on ceratopsians), as well as the smaller theropods Bambiraptor, Chirostenotes, Troodon, and Avisaurus.