Dryosaurus

*Dryosaurus altus*

* Timothy Bradley Artwork
* Robert F Walters Artwork

***Dryosaurus*** (/ˌdraɪəˈsɔːrəs/ DRY-ə-SAWR-əs, meaning 'tree lizard', Greek δρῦς (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*.

## Description

Based on known specimens, they had been estimated to have reached at least 8 to 14 feet (2.4 to 4.3 m) long,[citation needed] and to have weighed up to 170 to 200 pounds (77 to 91 kg).[citation needed] However, as no known adult specimens of the genus have been found, the adult size remains unknown. In 2018, when 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.

*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 and cheek 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 of *Dryosaurus* were characterized by a strong median ridge on the lateral surface.

## Discovery and naming

In 1876, Samuel Wendell Williston in Albany County, Wyoming discovered the remains of small euornithopods. In 1878, Professor Othniel Charles Marsh named these as a new species of *Laosaurus*, *Laosaurus altus*. The specific name altus, meaning "tall" in Latin, refers to it being larger than Laosaurus celer. In 1894, Marsh made the taxon a separate genus, *Dryosaurus*. 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. The type species remains Laosaurus altus, the combinatio nova is *Dryosaurus altus*.

The holotype, YPM 1876, was found in a layer of the Upper Brushy Basin Member of the Morrison Formation, dating from the Tithonian. It consists of a partial skeleton including a rather complete skull and lower jaws. Several other fossils from Wyoming have been referred to *Dryosaurus altus*. They include specimens YPM 1884: the rear half of a skeleton; AMNH 834: a partial skeleton lacking the skull from the Bone Cabin Quarry; and CM 1949: a rear half of a skeleton dug up in 1905 by William H. Utterback. From 1922 onwards in Utah, Earl Douglass discovered *Dryosaurus* remains at the Dinosaur 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. Other fossils were found in Colorado. In Lily 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, at Uravan, Montrose County, in 1973 Peter Malcolm Galton and James Alvin Jensen described specimen BYU ESM-171R found by Rodney Dwayne Scheetz and consisting of some vertebrae, a left lower jaw, a left forelimb and two hindlimbs.

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. The site was noted in a 1973 paper, 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 to *Dryosaurus*. 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.

Gregory S. Paul in his 2010 field guide to dinosaurs (2nd edition published in 2016) suggested that the Utah material represented a separate species, which was confirmed by Carpenter and Galton (2018), who described the Dinosaur National Monument *Dryosaurus* as a new species, *D. elderae*.

## Paleobiology

*Dryosaurus* subsisted primarily on low growing vegetation in ancient floodplains. A quick and agile runner with strong legs, *Dryosaurus* used its stiff tail as a counterbalance. It probably relied on its speed as a main defense against carnivorous dinosaurs.

### Growth and development

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. As the animal grew, its eyes became proportionally smaller and its snout proportionally longer.

## JPInstitute.com Description

Far different from the common view of dinosaurs, Dryosaurus had excellent eyesight and was small, fast, and agile. A graceful animal, its only means of defense was greater speed than that of predators. It also may have lived in herds, which would have provided it with early warning signs when danger approached. It is quite likely to have had nesting behavior and that it took care of the hatchlings.

Dryosaurus had large eyes, a hard horny beak, long thin legs, and a stiffened tail to provide balance. It appears to have been closely related to the hypsilophodontids - small agile dinosaurs. The fact that Dryosaurus has some iguanodontid features lead to confusion in placing it in the family tree.

## Dinosaur Field Guide Description

ryosaurus
("oak-tree lizard") literally lived in the
shadows of most Late Jurassic plants. It
was a small to medium-sized dinosaur
and was very lightly built (as compared
to its closest relative, the bulkier Camp-
tosaurus). Dryosaurus was one of the
ornithopods, or beaked dinosaurs. Its
pelvis is small compared to the rest of
the body. The arms are very short and
the long legs not powerfully built. The
skull has a short face and relatively big
eyes.
A dinosaur like Dryosaurus had to
rely on running from predators (rather
than standing and fighting), so it had to
grow as fast as possible. The longer its
legs, the faster it would run. Not all
dinosaurs grew as fast as Dryosaurus.
Modern lizards, for example, grow in
spurts, depending on the season. But
Dryosaurus's bones grew fast at all times,
which was a great benefit to its survival.

### Fun Facts

Dryosaurus
could outrun a human.

### Trivia

The African species
of Dryasaurus is named
after General Lettow-
Vorbeck a German hero of
World War I.

## Links

http://web.archive.org/web/20040806045343fw\_/http://www.jpinstitute.com/dinopedia/dinocards/dc\_dryos.html#