Not much was known about Early Cretaceous dinosaurs of North America until paleontologists discovered new kinds of dinosaurs in Lower Cretaceous rock strata in Utah and Arizona. The North American dinosaurs are more similar to Early Cretaceous dinosaurs of Europe and Asia than they are to those from Africa or South America.
The recent discoveries in Utah and Arizona included two armored dinosaurs: Gastonia and Animantarx. The first was about 12 to 15 feet long and resembled its close relative, the British dinosaur Polacanthus. The second was about half that size and lived several million years later. It was the first dinosaur discovered by tracing the radioactivity of its fossilized bones; they were not visible at the surface where they were buried.
From the same general location where Gastonia was found came the scattered bones of the large meat-eater Utahraptor. About six feet long and built like a giant Deinonychus, Utahraptor was armed with "killer claws" more than a foot long on its feet. It may have used them to tear open its prey. Being a dromaeosaurid, it may have been covered with feathers like an ostrich. Nedcolbertia was a much smaller meat-eating dinosaur, about six feet long, that lived at the same time as Utahraptor.
Sauropods are scarce in the Early Cretaceous of North America, but their partial skeletons are found from time to time. Cedarosaurus and the Sonorasaurus were smaller relatives, about 50 feet long, of the gigantic Late Jurassic sauropod Brachiosaurus. Other Early Cretaceous plant-eating dinosaurs discovered in North America included Zuniceratops, the world's earliest known horned dinosaur with well-developed brow horns, and Eolambia, an ornithopod dinosaur very close to the ancestry of the later duckbilled dinosaurs.