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(pronounced OHM-den-oh-SAWR-us) Ohmdenosaurus (meaning: "Ohmden, Germany lizard") was a long-necked, plant-eating dinosaur from the early Jurassic Period, about 191 million years ago. This sauropod was about 13 ft (4 m) long. Ohmdenosaurus was named by paleontologist Wild in 1978. The type species is O. liasicus. Few fossil bones (only a tibia and tarsus) have been found in Germany.

The olfactory bulbs are the parts of the brain that sense smell.

The olfactory cavity is the part of the inside of an animal's skull that once housed the olfactory bulbs. The relative size of the olfactory cavity indicates how well the animal could sense smells. For example, the Tyrannosaurus rex nicknamed Sue had a large olfactory cavity, suggesting that Tyrannosaurus rex had an acute sense of smell.

(pronounced OL-ig-oh-SAWR-us) Oligosaurus (meaning: "small lizard") is an invalid name for Rhabdodon, a small, plant-eating dinosaur from the late Cretaceous period. Oligosaurus was named by Seeley in 1881.

(pronounced OH-may-SAWR-us) Omeisaurus (meaning: "Mount Emei (China) lizard") was a very long-necked plant-eating dinosaur from the late Jurassic Period, about156-145 million years ago. This cetiosaurid sauropod was about 68 ft (20 m) long; it had a tiny head and relatively short tail. Omeisaurus was named by paleontologist Young in 1939. Many Omeisaurus fossils (perhaps a herd) were found in Sichuan, China. The type species is O. junghsienensis.

Animals that eat both plants and animals.

Ontogeny is the course of development of an organism.

(pronounced o-fee-ah-ko-don) Ophiacodon was a genus of pelycosaurs that did not have a sail (they were not dinosaurs - and lived before the dinosaurs appeared). Ophiacodon was a quadruped with four short, sprawling, clawed legs, a long tail, and large jaws in a narrow but large skull. It was up to about 12 feet (3.6 m) long and weighed from 65 to 110 pounds (30-50 Kg). These meat-eaters lived near water and probably ate fish and amphibians. They lived and may have been semi-aquatic. They lived during from the early Permian period (roughly 270 million years ago). Fossils have been found in Texas, USA. Classification: Synapsida (synapsids), Pelycosauria (pelycosaurs like Edaphosaurus and Dimetrodon), Suborder Eupelycosauria, Family Ophaicodontidae (very primitive pelycosaurs), Genus Ophaicodon.

This ichthyosaur was a medium-sized marine carnivore which lived in the Late Jurassic period. It inhabited the wide oceans and may only have approached the shore to give live birth to its young. It was probably a powerful swimmer that chased squid and fish with its long toothless snout. It grew up to approximately 5 metres long.

(pronounced oh-PIS-tho-SEEL-ih-CAWD-ee-ah) Opisthocoelicaudia (meaning: "hollow-backed tail (vertebrae)") was a sauropod dinosaur (perhaps a camarasaurid) that lived during the late Cretaceous Period. This large quadruped was about 34 ft (10.5 m) long and weighed roughly 23700 kg; its femur (thigh bone) was 139.5 cm long. Fossils (almost complete, but missing the neck vertebrae) have been found in Mongolia. Opisthocoelicaudia was named by Borsuk-Bialynicka in 1977.

Opisthocoelus vertebrae are vertebrae in which the main body is convex in the front (anterior) and concave in the back (posterior). Compare with amphicoelus vertebrae.

The orbit is the hole in the skull for the eye.

(pronounced or-coh-MEEM-us) Orcomimus was meat-eating, bipedal dinosaur, an advanced theropod that lived during the late Cretaceous Period, roughly 84-65 million years ago. Fossils of this Ornithomimosaur, including a pelvis and a hindlimb, were found in South Dakota, USA. Orcomimus was named by Triebold in 1997.

In classification, an order is a group of related or similar organisms. An order contains one or more families. A group of similar orders forms a class.

Ordovician Period
The Ordovician is a period of geological time between 505 and 438 million years ago in the Palaeozoic era. It is separated into three epochs - the Early Ordovician (505 to 478 MYA), the Middle Ordovician ( (478 to 453 MYA) and the Late Ordovician (453 to 438 MYA).

(pronounced OR-ee-oh-DONT) Oreodonts (meaning: "mountain tooth") were the most common hoofed herbivore (plant-eating) mammals in North America during the Oligocene (from about 35-5 million years ago). Oreodonts were quadrupedal ruminants closely related to camels, pigs, and sheep; most oreodonts were about the size of a dog or a pig. The skull was elongated and the upper canines were chisel shaped; the cheek teeth were used for grinding. Many fossils have been found in North America. Some oreodonts include Mesoreodon, Brachycrus (3.25 ft long), Promerycocherus (3.25 ft long), Sespia, and Merycoidodon (4.5 ft long, pictured above). Classification: Class Mammalia (mammals), Order Artiodactyla (even-toed ungulates), Suborder Tylopoda (padded feet - oreodonts, camels), Family Merycoidodontidae, many genera and species. Joseph Leidy coined the term oreodont in 1853.

(pronounced or-nat-oh-THO-us) Ornatotholus was plant-eating, thick-skulled bipedal dinosaur that lived during the late Cretaceous Period, roughly 84-71 million years ago. It may have been about 3 m long. Skulls of this Pachycephalosaurid were found in Alberta, Canada, and Montana, USA. Ornatotholus was named by Galton and Sues in 1983. The type species is O. browni.

The ornithschia, or bird-hipped dinosaurs are one of the two main sub divisions of the dinosaurs. They were all herbivores. They evolved in Triassic period and the group died out with the mass extinction at the end of the Cretaceous period. The name derives from the distinctive arrangement of the pelvis and hip structure.

(Pronounced or-NITH-oh-KIE-rus) Ornithocheirus (meaning: "bird hand") was a large pterodactyloid pterosaur, a flying reptile that lived during the Cretaceous Period. It had a long, tapering, toothed snout with a bony crest. The skull was roughly 4.8 ft (1.5 m) long; the body was 11.5 ft (3.5 m) long. Ornithocheirus had a wing span of about 38 ft (12 m) and had a very short tail. It was not a dinosaur, but was closely related to them. Fragmentary fossils of 36 different species of this flying fish-eater (including about 100 hollow, light-weight bones) have been found in England, Germany, and the Czech Republic (and perhaps Australia and South America). Ornithocheirus was named by Seeley in 1869.

Ornithodira (meaning: "bird necks") is the clade which includes dinosaurs (including their early ancestors the lagosuchians) and pterosaurs. They have s-shaped necks.

(pronounced or-NITH-oh-LES-teez) Ornitholestes (meaning: "bird robber") was a fast theropod from the late Jurassic Period. It had long legs, clawed hands, and a small bony crest on its snout.

(pronounced or-NITH-oh-MIME-us) Ornithomimus (meaning: "bird mimic") was an ostrich-like Ornithomimidae, a theropod with a toothless beak, long legs, and hollow bones. This meat-eating dinosaur was about 15-20 feet (4.5-6 m) long and lived during the late Cretaceous Period, roughly 76-65 million years ago.

A group of herbivore, bird-hipped dinosaurs which ranged from the Middle Triassic period to the end of the Cretaceous period. Many were bipedal or at least able to rear up on their hind legs. Generally small to medium in size, one of the largest was Iguanodon of the Early Cretaceous period.

This great flying carnivorous pterosaur lived in the Early Cretaceous period. Its huge wingspan meant it could migrate great distances. It had a wingspan of 12 metres, its body length was 3 1/2 metres and its weight was about 100 kg.

This small, two legged carnivore lived in the Late Jurassic period. It had small conical teeth and a small head with long balancing tail. It grew up to two metres long and weighed up to 12 kg. It probably lived off lizards and small mammals.

(pronounced or-nith-oh-SOOK-us) Ornithosuchus (meaning: "bird crocodile") was a mid to late Triassic period thecodont, (the group of reptiles from which the arcosaurs, including the dinosaurs, evolved). It was not a dinosaur, but was closely related to dinosaurs and pterosaurs. It was a bipedal (walked on two legs) carnivore (meat-eater) that had long legs, short arms, five-fingered hands, a long snout, a long tail and sharp teeth. It was about 6 feet long and weighed about 100+ pounds. Fossil have been found in England (and a few other locations). Ornithosuchus was named by Huxley in 1877. Ornithosuchus used to be thought to be a dinosaur.

(pronounced or-nith-EUR-an) Ornithurans are a clade that includes modern birds (neornitheans) and their close ancestors (which included hesperornithiformes, patagopterygiforms, and ambiortimorphs, toothed bird groups from the Mesozoic Era).

(pronounced OR-oh-DROHM-ee-us) Orodromeus (meaning: "mountain runner") was a small, lightly-built, fast-running, plant-eating dinosaur about 8 feet (2.5 m) long. This ornithischian ornithopod lived during the late Cretaceous Period about 77 to 73 million years ago. Fossils have been found at Egg Mountain, Montana, USA. (Eggs were also found at the site, but they belong to Tröodon) It was named by paleontologists Horner and Weishampel in 1988. The type species is O. makelai.

(pronounced ORTH-oh-SEER-us) Orthoceras (meaning: "straight horn") were primtive types of nautiloids that looked like squid with a long, straight (or slightly curved), conical, chambered, and furrowed shell. The shell ranged from a few centimeters to over 9 feet long. The Orthoceras lived in the largest chamber in the shell. These invertebrates swam in shallow seas using jet-propulsion and breathed with gills. Orthoceras lived from the middle Ordovician Period to the Devonian Period, from about 470 to 360 million years ago. Fossils are common and have been found on many continents including the Americas, Africa, Europe, and Asia. Classification: Phylum Mollusca, Class Cephalopoda (Nautiloidea), Order Orthocerida, genus Orthoceras, many species.

Henry F. Osborn (1857-1935) was a US paleontologist who found and named many dinosaurs in Mongolia and the US in the early 1900's. He was the curator of the American Museum of Natural History starting in 1891. He did extensive research on brontotheres ( early, rhino-like mammals). Osborn also named and described the following dinosaurs: Albertosaurus (1905), Asiatosaurus (1924), Ornitholestes (1903), Oviraptor (1924), Pentaceratops (1923), Prodeinodon (1924), Psittacosaurus (1923), Saurornithoides (1924), Struthiomimus (1916), Tyrannosaurus (1905), and Velociraptor (1924).

(pronounced oh-san-oh-SAWR-us) Oshanosaurus was a sauropod dinosaur from the early Jurassic Period in what is now China. The type species is O. youngi. It was named by paleontologist Zhao in 1983.

(pronounced OS-tra-KOD) Ostracod (meaning: "shell like") are also called seed shrimp or mussel shrimp. These tiny freshwater and marine crustaceans belong to the subclass Ostracoda. They are scavenger that have a shrimp-like body plus two hard shells connected by a hinge; they have one or two appendages. Ostracods range in size from micoscopic to about an inch (2.5 cm) long. There are about 20,000 species of living ostracods and many more extinct species. These very common animals are used as index fossils, helping to date rock layers. The oldest-known ostracods are from the Cambrian period; they became widespread during the Ordovician and remain so.

Ostracoderms (also called Agnaths) are extinct, primitive, armored, jawless fish that lived during from the Ordovician period to the Devonian period. These vertebrates had bodies that were protected by bony plates and scales. Fossils have been found in North America and Europe. Some ostracoderms include the Pteraspids.

John H. Ostrom is a US paleontologist and author who found (1964) and named (1969) Deinonychus with Grant E. Meyer et alia. He named Microvenator (1970), Sauropelta (1970), and Tenontosaurus (1970). Ostrom has championed the theory that birds arose from theropod dinosaurs. Ostrom also argued that the dinosaur's must have high body temperatures given their erect posture (1969). Ostrom's books include "The Strange World of Dinosaurs" (Putnam, 1964), "Dinosaurs" (Carolina Biological Supply Co., 1984), "Marsh's Dinosaurs : The Collections from Como Bluff" (with McIntosh and Dodson, Yale Univ Press, 2000), and others. The Cretaceous period bird/dinosaur Rahonavis ostromi was named to honor Ostrom.

(pronounced OTH-ni-EE-lee-ah) Othnielia was a ornithopod dinosaur from the late Jurassic Period, about 156 to 145 million years ago. It was a bipedal plant-eater with a horny beak and strong jaws. It had long legs with very long shins; this made it a very fast runner. It had short arms, a long, stiff tail, and was about 4.6 feet (1.4 m) long. It had five-fingered hands and four-toed feet, all clawed. Its fossils (two partial skeletons and teeth) were found in Colorado, Utah, and Colorado in the western USA. It was named by Galton in 1977 in honor of Othniel Marsh, the fossil hunter.

Otogornis was an early, toothless bird from the early Cretaceous Period. Otogornis, belonging to the group Enantiornithes (toothless birds), was found in China and named by Hou in 1994.

Otway Range
A region near Melbourne, Australia which has produced numerous vertebrate remains from the Early Cretaceous period. The area is particularly important because at this time it was located much closer to South Pole and any animals it contains would have had to cope with long periods of darkness and cold during the winter months.

(pronounced oo-RAHN-oh-SAWR-us) Ouranosaurus (meaning: "brave monitor lizard") was a sail-backed, plant-eating, ornithischian dinosaur. It was an iguanodontid from the early Cretaceous Period, about 115 million years ago.

Oviparous animals hatch from eggs. Fossilized eggs from some prehistoric animals have been found.

These evolved in Jurassic times. They formed reefs around islands on the seabed and rocks of warm southerly oceans.

Sir Richard Owen (1804-1892) was a British comparative anatomist who coined the term dinosauria (meaning: "fearfully great lizard") as a suborder of large, extinct reptiles in 1841. He also named and described the following dinosaurs: Bothriospondylus (1875), Cetiosaurus (1841), Chondrosteosaurus (1876), Dacentrurus (1875), Dinodocus (1884), Echinodon (1861), Massospondylus (1854), Nuthetes (1854), Polacanthus (1867), and Scelidosaurus (1859).

(pronounced oz-RAP-tor) Ozraptor (meaning: "Australian plunderer") was a 6.5 to 10 feet (2-3 m) long bipedal carnivore (meat-eater) from the mid-Jurassic Period. It was a Tetanurine (advanced theropod dinosaurs that had three-fingered hands, maxillary fenestra, long legs, and a stiff tail). Ozraptor was named by Long and Molnar in 1998 and is known from a part of a tibia (shin bone) found in Australia.