Period: Late Cretaceous
Order, Suborder, Family: Ornithischia, Ornithopoda, Hypsilophodontidae
Location: North America (United States)
Length: 6 1/2 feet (2 meters)
Orodromeus is a recently discovered dinosaur and one of the most spectacular. Orodromeus (the name means "mountain runner") was only about 6 1/2 feet long as an adult. What makes Orodromeus special is that it is known from adult skulls and skeletons, younger animals, juveniles, hatchlings, and even an articulated (the bones are attached) embryo in a whole egg. From all these fossils, we have a rich picture of the life and evolution of Orodromeus.
The mother Orodromeus laid about 12 eggs in a tight spiral, with the first egg in the center of the spiral. These eggs were not large, almost 6 inches high and 2 3/4 inches wide. When the embryos were ready to come out of their eggs, they pecked through the top of the shell and climbed out of the nest. These hatchlings were nearly as fully developed as adults and could leave the nest and feed themselves. Possibly for protection, these hatchlings stayed together after leaving the nest.
Hatchlings had long legs, with short front limbs and large heads. As they grew, their back limbs grew longer in proportion to the rest of their body and their heads got smaller. As an adult, Orodromeus was one of the longest-legged hypsilophodontids for its size. It must have been a very fast runner. It had a very long, straight tail, supported by many interwoven bony tendons. The tail balanced the front of the body when it ran.
Orodromeus had simple, primitive teeth, much like the early ornithischian Lesothosaurus. From the shape of these teeth, Orodromeus probably fed on fleshy fruits and possibly insects, especially when the animal was young.
Orodromeus comes from the Late Cretaceous Two Medicine Formation of western Montana. It was found with the hadrosaur Maiasaura and the meat-eating Albertosaurus and Troodon. Orodromeus may also have lived in southern Alberta. It is closely related to other hypsilophodontids, including Yandusaurus from the Late Jurassic of the People's Republic of China, Zephyrosaurus from the Early Cretaceous of the United States, and Hypsilophodon from the Early Cretaceous of England. Parksosaurus and Thescelosaurus from the Late Cretaceous of Canada and the United States were two other related hypsilophodontids.