| Dryosaurus | |
|---|---|
 | |
| D. altus,Beneski Museum of Natural History | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Ornithopoda |
| Family: | †Dryosauridae |
| Genus: | †Dryosaurus Marsh,1894 |
| Type species | |
| †Dryosaurus altus (Marsh, 1878) [originallyLaosaurus altus] | |
| Other species | |
Dryosaurus (/ˌdraɪəˈsɔːrəs/DRY-ə-SOR-əs, meaning 'tree lizard',Greekδρῦς (drys) meaning 'tree, oak' andσαυρος (sauros) meaning 'lizard'; the name reflects the forested habitat, not a vague oak-leaf shape of its cheek teeth as is sometimes assumed) is agenus of anornithopoddinosaur that lived in theLate Jurassic period. It was aniguanodont (formerly classified as ahypsilophodont). Fossils have been found in the westernUnited States and were first discovered in the late 19th century.Valdosaurus canaliculatus andDysalotosaurus lettowvorbecki were both formerly considered to represent species ofDryosaurus.[1][2][3]
Based on known specimens, they had been estimated to have reached up to 3 metres (9.8 ft) long and to have weighed up to 100 kilograms (220 lb).[4] However, as no known adult specimens of the genus have been found, the adult size remains unknown.[5] In 2018, the largest specimen (CM 1949) was concluded to be from another species; revising the identity of this specimen put the previous research on size and growth into question.[6]
Dryosaurus had a long neck, long, slender legs and a long, stiff tail. Their arms, however, with five fingers on each hand, were short. They had a horny beak andcheek teeth.[citation needed] Some scientists suggest that it had cheek-like structures to prevent the loss of food while the animal processed it in the mouth.[who?] The teeth ofDryosaurus were characterized by a strong median ridge on the lateral surface.[7]
In 1876,Samuel Wendell Williston inAlbany County,Wyoming discovered the remains of smalleuornithopods. In 1878, ProfessorOthniel Charles Marsh named these as a new species ofLaosaurus,Laosaurus altus. Thespecific namealtus, meaning "tall" inLatin, refers to it being larger thanLaosaurus celer.[8] In 1894, Marsh made the taxon a separate genus,Dryosaurus.[9] The generic name is derived from the Greek δρῦς,drys, "tree, oak", referring to a presumed forest-dwelling life mode. Later it was often assumed to have been named after an oak-leaf shape of its cheek teeth, which, however, is absent. Thetype species remainsLaosaurus altus, thecombinatio nova isDryosaurus altus.[9]
The holotype, YPM 1876, was found in a layer of the Upper Brushy Basin Member of theMorrison Formation, dating from theTithonian. It consists of a partial skeleton including a rather complete skull and lower jaws. Several other fossils from Wyoming have been referred toDryosaurus altus. They include specimens YPM 1884: the rear half of a skeleton; AMNH 834: a partial skeleton lacking the skull from theBone Cabin Quarry; and CM 1949: a rear half of a skeleton dug up in 1905 by William H. Utterback. From 1922 onwards inUtah,Earl Douglass discoveredDryosaurus remains at theDinosaur National Monument. These include CM 11340: the front half of a skeleton of a very young individual; CM 3392: a skeleton with skull but lacking the tail; CM 11337: a fragmentary skeleton of a juvenile; and DNM 1016: a left ilium dug up by technician Jim Adams.[10] Other fossils were found inColorado. InLily Park,Moffat County,James Leroy Kay and Albert C. Lloyd in 1955 recovered CM 21786, a skeleton lacking skull and neck. From 'Scheetz’ Quarry 1, atUravan,Montrose County, in 1973Peter Malcolm Galton andJames Alvin Jensen described specimen BYU ESM-171R found byRodney Dwayne Scheetz and consisting of some vertebrae, a left lower jaw, a left forelimb and two hindlimbs.[11]
Rodney D. Scheetz and his family discovered a fossil locality around five miles from Uravan, Colorado in the spring of 1972. This site, unintentionally exposed by a bulldozer, was found to contain fossil fragments, said to be in such condition they looked like unfossilized bone.[12] The site was noted in a 1973 paper,[13] and Scheetz continued to dig at the site annually until publishing a short note in 1991. By then around 2500 fragments had been excavated, almost all of which specimens are thought to have belonged toDryosaurus. At least eight individuals are represented, with ages ranging from juvenile to embryonic; finding specimens of embryonic age is exceptionally rare for dinosaur fossils. Eggshells were also represented in the sample. Scheetz voiced his intention to continue work at the site following the publishing of the note.[12]
Gregory S. Paul in his 2010 field guide to dinosaurs (2nd edition published in 2016) suggested that the Utah material represented a separate species,[14] which was confirmed by Carpenter and Galton (2018), who described the Dinosaur National MonumentDryosaurus as a new species,D. elderae.[6]
In 2003 remains from Portugal were tentatively referred toDryosaurus, possibly meaning it was more widespread.[clarification needed][15]
Dryosaurus subsisted primarily on low growing vegetation in ancient floodplains.[7] A quick and agile runner with strong legs,Dryosaurus used its stiff tail as a counterbalance.[16] It probably relied on its speed as a main defense againstcarnivorous dinosaurs.[citation needed]
ADryosaurus hatchling found at Dinosaur National Monument in Utah confirmed thatDryosaurus followed similar patterns of craniofacial development to other vertebrates; the eyes were proportionally large while young and the muzzle proportionally short.[7] As the animal grew, its eyes became proportionally smaller and its snout proportionally longer.[7]
TheDryosaurusholotype specimen YPM 1876 was discovered in Reed's YPM Quarry 5, in the Upper Brushy Basin Member, of theMorrison Formation. In the Late Jurassic Morrison formation of Western North America,Dryosaurus remains have been recovered from stratigraphic zones 2–6.[17] This formation is a sequence of shallow marine and alluvial sediments which, according toradiometric dating, ranges between 156.3 million years old (Ma) at its base,[18] to 146.8 million years old at the top,[19] which places it in the lateOxfordian,Kimmeridgian, and earlyTithonianstages of the Late Jurassic period. In 1877 this formation became the center of theBone Wars, a fossil-collecting rivalry between early paleontologistsOthniel Charles Marsh andEdward Drinker Cope. The Morrison Formation is interpreted as asemiarid environment with distinctwet anddry seasons. The Morrison Basin where dinosaurs lived, stretched from New Mexico to Alberta and Saskatchewan, and was formed when the precursors to theFront Range of the Rocky Mountains started pushing up to the west. The deposits from their east-facingdrainage basins were carried by streams andrivers and deposited inswampy lowlands, lakes, river channels andfloodplains.[20] This formation is similar in age to theSolnhofen Limestone Formation inGermany and theTendaguru Formation inTanzania.[citation needed]
The Morrison Formation records an environment and time dominated by gigantic sauropod dinosaurs such asBrontosaurus,Camarasaurus,Barosaurus,Diplodocus,Apatosaurus andBrachiosaurus. Dinosaurs that lived alongsideDryosaurus included the herbivorousornithischiansCamptosaurus,StegosaurusandNanosaurus. Predators in this paleoenvironment included thetheropodsTorvosaurus,Ceratosaurus,Marshosaurus,Stokesosaurus,Ornitholestes andAllosaurus.[21]Allosaurus accounted for 70 to 75% of theropod specimens and was at the toptrophic level of the Morrisonfood web.[22] Other animal taxa that shared this paleoenvironment includedbivalves,snails,ray-finned fishes,frogs,salamanders,turtles,sphenodonts,lizards, terrestrial and aquaticcrocodylomorphs, and several species ofpterosaur. Early mammals were present in this region, such asdocodonts,multituberculates, symmetrodonts, andtriconodonts. The flora of the period has been revealed by fossils ofgreen algae,fungi,mosses,horsetails,cycads,ginkgoes, and several families ofconifers. Vegetation varied fromriver-lining forests oftree ferns, andferns (gallery forests), to fernsavannas with occasional trees such as theAraucaria-like coniferBrachyphyllum.[23]
Other sites that have producedDryosaurus material include Bone Cabin Quarry, the Red Fork of the Powder River in Wyoming andLily Park in Colorado.[7]
