were smaller, two-legged carnivores, too, such as Ornithomimus and the Dromaeosaurus, which had specialized feet and their unique, slashing, raptorial claws. The herbivorous saurischians that are best known are the large, four-legged sauropods, the largest dinosaurs to have walked Earth. These dinosaurs had long necks and tails, with relatively tiny heads. Included in this group are Brachiosaurus, Camarasaurus, Diplodocus, Mamenchisaurus, and Seismosaurus.
What kinds of dinosaurs were in the ornithischian group?
The ornithischians were all herbivores and had two-and four-legged types. There were the four-legged armored dinosaurs such as Ankylosaurus and Stegosaurus. There were large, horned dinosaurs such as Eucentrosaurus and Triceratops. Two-legged types included Iguanodon, and many of the duck-billed dinosaurs (hadrosaurs), such as Corythosaurus, Lambeosaurus, and Maiasaura.
What events led to the dominance of dinosaurs in the Mesozoic era?
Approximately 250 million years ago, at the end of the Permian period, or the beginning of the Triassic period (and thus, the end of the Paleozoic era and the beginning of the Mesozoic era), there was a mass extinction. This extinction eliminated close to 90 percent of all the species present on our planet (extraordinarily close to total extinction on Earth). The extinction was not selective; it eliminated organisms in the oceans and on land, including many invertebrates, armored fish, and reptiles.
The true reasons for the extinction are unknown, although there are several theories. One is that a collision with an asteroid or comet caused dust and debris to fly into the upper atmosphere, cutting off sunlight and radically changing the global climate. Another idea is that the moving continents changed the climate, sea levels, and thus, habitats, causing some species to change and adapt, while others died out. Still another theory focuses on Siberian flood basalts, in which tons of volcanic material erupted over a huge area in Asia toward the end of the Permian period, changing the climate and certain habitats.
Whatever the scenario, species that survived did so by adapting to the ecological niches that became vacant, allowing them to further evolve. After the Permian extinction, and throughout the Mesozoic era, it was reptiles in general, and the dinosaurs specifically, that diversified the most and became the dominate species on the planet.
How long were dinosaurs dominant on Earth?
The dinosaurs were the dominate species on Earth for approximately 160 million years. The Mesozoic era, often referred to as the “age of the reptiles,” lasted from approximately 250 to 65 million years ago. It includes the Triassic, Jurassic, and Cretaceous periods.
Were there dinosaurs at the beginning of the Mesozoic?
Swedish naturalist Carolus Linnaeus devised the classification system for plants and animals that is still used today, both for living species and for species of the past such as dinosaurs (iStock).
It is interesting to note that at the beginning of the Mesozoic, there were no true dinosaurs; other reptiles dominated the landscape. But by the end of the Triassic period, the dinosaurs became dominant, and they stayed that way for around 160 million years. Dinosaurs were not the only form of life that existed during this time. For example, there were smaller, lizard-like reptiles, small early mammals, insects, amphibians, invertebrates, and a wide variety of plants. In fact, these organisms helped the dinosaurs to stay in charge because many of the dinosaurs used this abundance of life for their sustenance and growth.
Within what percent of the geologic time scale did dinosaurs live?
Scientists estimate that dinosaurs existed on Earth for about 160 million years. Thus, dinosaurs were only around 3.1 percent of the time that has passed since the formation of Earth.
When did the age of dinosaurs end?
The age of the dinosaurs came to an end approximately 65 million years ago. From this point in time, there are currently no known dinosaur fossils. The time of the great dinosaur (and other species) extinction is used by scientists to delineate the end of the Cretaceous period as well as the end of the Mesozoic era. After this point, the Cenozoic era begins, starting with the Tertiary period.
What is the Triassic period and how did it get its name?
The Triassic period follows the Permian period on the geological time scale. During this time, dinosaurs first began to evolve from the early reptiles, the first primitive mammals appeared, and the armored amphibians and mammal-like reptiles died out. The Triassic was one of the first labeled divisions on the geologic time scale, and it is the first of three periods (the others are the Jurassic and Cretaceous) making up the Mesozoic era.
The Triassic was first named in 1834 by German geologist Friedrich August von Alberti (1795–1878) to describe a three-part division of rock types in Germany. It was originally called the Trias, and is still called this by many European geologists. It is named after three, or “tri,” layers of sedimentary rocks representative of the time period: from bottom to top, a sandstone, limestone, and copper-bearing shale. The three distinct rock formations are, from the bottom up, the Bunter (mostly Early Triassic), the Muschelkalk (Middle Triassic), and the Keuper (mostly Late Triassic).
How long did the Triassic period last?
The geologic time scale is not exact, and depending on the country or scientist, the dates of the Triassic period can vary by about 5 to 10 million years. On the average, the Triassic period is said to have lasted from about 250 to 205 million years, for a total of about 45 million years in length.
What are the divisions of the Triassic period?
In general, the informal way to define parts of the Triassic period is to use the terms early, middle, and late Triassic. More formally, they are capitalized (Early, Middle, and Late), or sometimes (Lower, Middle, and Upper) and include subdivisions within those groupings. The following lists a general interpretation of the Triassic epochs (although note that many researchers use slightly different notations; for example, many do not list the Rhaetian Age):
Triassic Period
| Epoch | Age | Millions of Years Ago (approximate) |
| Early | Olenekian | 245–242 |
| Induan | 250–245 | |
| Middle | Ladinian | 234–227 |
| Anisian |
