Interesting story over at ScienceDaily.  The oldest known horned dinosaur has been found in Montana.

The limited fossil record for neoceratopsian–or horned dinosaurs–from the Early Cretaceous in North America restricts scientists’ ability to reconstruct the early evolution of this group. The authors of this study have discovered a dinosaur skull in Montana that represents the first horned dinosaur from the North American Early Cretaceous that they can identify to the species level. The authors named the dinosaur Aquilops americanus, which exhibits definitive neoceratopsian features and is closely related to similar species in Asia. The skull is comparatively small, measuring 84 mm long, and is distinguished by several features, including a strongly hooked rostral bone, or beak-like structure, and an elongated and sharply pointed cavity over the cheek region. When alive, the authors estimate it was about the size of a crow.

The size of a crow!  Wow, not the size you might expect for a horned dinosaur!  The actual paper is at http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0112055.