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For the extinct crocodilian, see Dyrosaurus.
Temporal range: Late Jurassic, 155–145Ma
Ceratosaurus & Dryosaurus.jpg
D. altus skeleton (front) with Ceratosaurus
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Order: Ornithischia
Clade: Ornithopoda
Superfamily: Dryosauroidea
Family: Dryosauridae
Genus: Dryosaurus
Marsh, 1894
Species: † D. altus
Binomial name
Dryosaurus altus
(Marsh, 1878 [originally Laosaurus altus])

Dryosaurus (/ˌdr.ɵˈsɔrəs/ DRY-o-SAWR-əs; meaning 'tree lizard', Greek δρυς/drys meaning 'tree, oak' and σαυρος/sauros meaning 'lizard'; the name reflects the forested habitat, not a vague oak-leaf shape of its cheek teeth as is sometimes assumed) is a genus of an ornithopod dinosaur that lived in the Late Jurassic period. It was an iguanodont (formerly classified as a hypsilophodont). Fossils have been found in the western United States, and were first discovered in the late 19th century. Valdosaurus canaliculatus and Dysalotosaurus lettowvorbecki were both formerly considered to represent species of Dryosaurus.[1][2][3]


Model in Poland

Dryosaurus had a long neck, long, slender legs and a long, stiff tail. Its arms, however, with five fingers on each hand, were short. Known specimens were about 8 to 14 feet (2.4 to 4.3 m) long and weighed 170 to 200 pounds (77 to 91 kg). However, the adult size is unknown, as no known adult specimens of the genus have been found.[4]

Dryosaurus had a horny beak and cheek teeth and, like other ornithopods, was a herbivore. Some scientists suggest that it had cheek-like structures to prevent the loss of food while the animal processed it in the mouth.

A quick and agile runner with strong legs, Dryosaurus used its stiff tail as a counterbalance.[5] It probably relied on its speed as a main defense against carnivorous dinosaurs.

Diet and dentition[edit]

The teeth of Dryosaurus were, according to museum curator John Foster, characterized by "a strong median ridge on the lateral surface."[6]Dryosaurus subsisted primarily on low growing vegetation in ancient floodplains.[6]

Growth and development[edit]

A Dryosaurus hatchling found at Dinosaur National Monument in Utah confirmed that Dryosaurus followed similar patterns of craniofacial development to other vertebrates; the eyes were proportionally large while young and the muzzle proportionally short.[6] As the animal grew, its eyes became proportionally smaller and its snout proportionally longer.[6]

Paleobiogeography and fossil distribution[edit]

Morrison Formation[edit]

Pelvis, leg, and tooth

The Dryosaurus holotype specimen YPM 1876 was discovered in Reed’s YPM Quarry 5, in the Upper Brushy Basin Member, of the Morrison Formation. In the Late Jurassic Morrison formation of Western North America, Dryosaurus remains have been recovered from stratigraphic zones 2-6.[7] A spectacular digsite near Uravan, Colorado held hundreds of D. altus fossils which represented multiple stages of the animal's life cycle. This formation is a sequence of shallow marine and alluvial sediments which, according to radiometric dating, ranges between 156.3 million years old (Ma) at its base,[8] to 146.8 million years old at the top,[9] which places it in the late Oxfordian, Kimmeridgian, and early Tithonian stages of the Late Jurassic period. In 1877 this formation became the center of the Bone Wars, a fossil-collecting rivalry between early paleontologists Othniel Charles Marsh and Edward Drinker Cope. The Morrison Formation is interpreted as a semiarid environment with distinct wet and dry seasons. The Morrison Basin where dinosaurs lived, stretched from New Mexico to Alberta and Saskatchewan, and was formed when the precursors to the Front Range of the Rocky Mountains started pushing up to the west. The deposits from their east-facing drainage basins were carried by streams and rivers and deposited in swampy lowlands, lakes, river channels and floodplains.[10] This formation is similar in age to the Solnhofen Limestone Formation in Germany and the Tendaguru Formation in Tanzania.

The Morrison Formation records an environment and time dominated by gigantic sauropod dinosaurs such as Camarasaurus, Barosaurus, Diplodocus, Apatosaurus and Brachiosaurus. Dinosaurs that lived alongside Dryosaurus included the herbivorous ornithischians Camptosaurus, Stegosaurus and Othnielosaurus. Predators in this paleoenvironment included the theropods Saurophaganax, Torvosaurus, Ceratosaurus, Marshosaurus, Stokesosaurus, Ornitholestes[11] and Allosaurus, which accounting for 70 to 75% of theropod specimens and was at the top trophic level of the Morrison food web.[12] Other vertebrates that shared this paleoenvironment included bivalves, snails, ray-finned fishes, frogs, salamanders, turtles, sphenodonts, lizards, terrestrial and aquatic crocodylomorphans, and several species of pterosaur. Early mammals were present in this region, such as docodonts, multituberculates, symmetrodonts, and triconodonts. The flora of the period has been revealed by fossils of green algae, fungi, mosses, horsetails, cycads, ginkgoes, and several families of conifers. Vegetation varied from river-lining forests of tree ferns, and ferns (gallery forests), to fern savannas with occasional trees such as the Araucaria-like conifer Brachyphyllum.[13]

Other sites that have produced Dryosaurus material include Bone Cabin Quarry, the Red Fork of the Powder River in Wyoming and Lily Park in Colorado.[6]

See also[edit]


  1. ^ Tom R. Hübner and Oliver W. M. Rauhut (2010). "A juvenile skull of Dysalotosaurus lettowvorbecki (Ornithischia: Iguanodontia), and implications for cranial ontogeny, phylogeny, and taxonomy in ornithopod dinosaurs". Zoological Journal of the Linnean Society 160 (2): 366–396. doi:10.1111/j.1096-3642.2010.00620.x. 
  2. ^ McDonald AT, Kirkland JI, DeBlieux DD, Madsen SK, Cavin J et al. (2010). Farke, Andrew Allen, ed. "New Basal Iguanodonts from the Cedar Mountain Formation of Utah and the Evolution of Thumb-Spiked Dinosaurs". PLoS ONE 5 (11): e14075. doi:10.1371/journal.pone.0014075. PMC 2989904. PMID 21124919. 
  3. ^ Galton, P.M., 1977. "The Upper Jurassic dinosaur Dryosaurus and a Laurasia-Gondwana connection in the Upper Jurassic", Nature 268(5617): 230-232
  4. ^ Horner, John R.; de Ricqlés, Armand; Padian, Kevin; and Scheetz, Rodney D. (2009). "Comparative long bone histology and growth of the "hysilophodontid" dinosaurs Orodromeus makelai, Dryosaurus altus, and Tenontosaurus tillettii (Ornithischia: Euornithopoda)". Journal of Vertebrate Paleontology 29 (3): 734–747. doi:10.1671/039.029.0312. 
  5. ^ Marshall (1999) pp. 138-139
  6. ^ a b c d e "Dryosaurus altus," Foster (2007) pp. 218-219.
  7. ^ "Appendix," Foster (2007) pp. 327-329.


  • Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. pp. 138–139. ISBN 1-84028-152-9. 

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