The Earth was formed about 4.6 billion years ago. Internally, the Earth can be divided into concentric layers: solid inner core; liquid outer core; mantle-mostly pliable, but brittle near the top. Covering the mantle is a relatively, thin, brittle rocky crust.
The crust is broken into a mosaic of rigid plates. The plates, with their continental "rafts," drift across the mantle driven by underlying convection currents. Two hundred and fifty million years ago the continents were clustered together forming a super continent known as Pangea. Over the years crustal movement has separated the continents, slowly moving them to their present position. In addition, plate movements are responsible for changes in the size and position of oceans and the creation of mountains and other landforms. All of this movement is explained by the theory of plate tectonics.
Over the course of time, as the land moves and changes, so does the climate. For instance, oceans tend to moderate climate along coastal strips while interior areas tend toward more severe climates. Mountains often act as moisture barriers with one side of the mountain capturing rainfall while the opposite side, receiving little rain, is very dry. Weather, in its turn affects the landscape-wearing down mountains, flooding lowlands, heating and cooling.
Life forms are affected as changes occur in landforms and climate. Depending on their ability to adapt to change, plants and animals will either survive or become extinct.
Geologists have developed a timescale dividing the history of the Earth into eons, eras and shorter intervals of time. One of these, the Mesozoic ("middle life") Era, began approximately 248 million years ago and ended approximately 65 million years ago. The Mesozoic Era, which is divided into three parts, is often called the Age of Reptiles. Dinosaurs appeared about 228 million years ago during a part of the Mesozoic Era known as the Triassic Period.
The Triassic World, 248-206 million years ago
During most of the Triassic Period all of the continents were clustered together to form a super continent called Pangea. The global climate was mostly warm and wet. However, inland areas far from the sea were dry and desert-like. There were no ice caps.
Ferns, horsetails and cycads thrived in moist places, while ginkgoes and conifers grew in the drier areas.
The early dinosaurs were mostly small predators. By the end of the Triassic Period species of dinosaur had increased in size and plant-eating dinosaurs had made their appearance. Since Pangea was still mostly together these early dinosaurs were able to migrate and disperse throughout the world.
It was during the Triassic that the last of the synapsids (mammal-like reptiles) disappeared, leaving in their place the first, small, true mammals. In addition, the Triassic world was populated by assorted invertebrates, amphibians, fish and other non-dinosaur reptiles such as crocodilians, turtles, and the sea-going ichthyosaurs.
The Jurassic World, 206-144 million years ago
During the Jurassic Period, Pangea split into two large masses-Gondwana in the south and Laurasia in the north. In the early Jurassic the topography was generally low with many shallow inland seas. Still there were no ice caps.
The global Jurassic climate was warm and humid-ideal conditions for a lush, green world. Conifers, cycads, palm-like cycadeoids, ferns and ginkgos continued to thrive.
During the Jurassic Period dinosaurs became the ruling land animals. Some dinosaurs grew to be gigantic. There were plant-eaters six stories high and meat-eaters that stretched longer than a school bus. There were also many smaller dinosaurs.
Jurassic dinosaurs shared the Earth with various invertebrates, salamanders, fish, ichthyosaurs, plesiosaurs, small mammals, and archaeopteryxan early bird.
The Cretaceous World, 144 to 65 million years ago
By the end of the Cretaceous, the Earth's continents had moved to positions similar to today's. A circum-equatorial ocean-The Tethys Sea-created generally tropical conditions. The climate was mostly warm, but ranged from cool to hot with clear-cut seasons. As the Cretaceous Period progressed, the climate tended toward cooler and drier.
There was a trend toward less heavily forested areas, perhaps the result of heavy browsing by earlier dinosaurs. Flowers and broad-leafed trees such as magnolia appeared. Still, there were no grasses.
Isolation of the continents contributed to the evolution of a wide variety of dinosaur types. Dinosaurs lived in deserts and polar regions. They grew both huge and small. Efficient plant-chewers and the most powerful meat-eaters appeared near the end of the Cretaceous time.
Along with dinosaurs other animal life diversified. There were many modern kinds of insects and other invertebrates, frogs, fish, turtles, lizards, snakes, small mammals, and very large pterosaurs.
Throughout the Mesozoic Era, during a period of about 150 million years, there were many different kinds of dinosaurs. Evolution and extinction were on-going processes throughout this period. The periodic extinction of certain dinosaur species was matched by the evolution of new species. Thus, the dinosaur faunas of the Triassic, Jurassic, and Cretaceous were quite different from each other. The extinctions at and near the end of the Cretaceous were especially significant because of their finality. The exact causes of these extinctions are not known, but there are many theories. Some of these include: eggshell thinning, egg-stealing mammals, disease, asteroid impact, volcanic eruptions, and gradual climatic change.
The Age of Reptiles had ended. A new era began-the Cenozoic. Birds and mammals became the dominant back-boned animals filling niches left by the once ruling reptiles.
Not all of the reptiles living during the Mesozoic were dinosaurs. There were reptiles living in the sea with paddle-like feet such as the plesiosaurs, mosasaurs, and ichthyosaurs and there were reptiles with wings like pterodactyls. These distinct groups of reptiles were not dinosaurs.
Dinosaurs are a specialized group of pre-historic, land-dwelling reptiles that lived during the Mesozoic Era. They are characterized by such things as walking upright with limbs that support the body from beneath instead of sprawled to the side, and by certain features of the skull and foot. While most reptiles are "cold-blooded" and generally do not care for their young, it is thought that at least some, if not all, dinosaurs were "warm-blooded" and fossil evidence suggests that some of them cared for their young.
Some dinosaurs walked on four feet (quadrupedal) while others walked on two feet (bipedal). Some dinosaurs were carnivores, while many others were herbivores and some may have been omnivores. There were small dinosaurs (chicken-sized) and huge dinosaurs (90 feet or more long, 50 feet tall and weighing 80 or more tons).
Biological classification is a tool used to sort living and extinct organisms into groups that have shared characteristics. Ideally, such groups share a common ancestry and are closely related genealogically. While some organisms easily fit into a group others do not. Sometimes there is disagreement among scientists on what constitutes a group and who or what belongs in it. The following is a simplified summary of dinosaur classification. For more details see the list of references.
Several hundred types of dinosaurs have been described. However, they are generally divided into two major groups and several sub-groups. The two major groups are based on differences in hip bone structure. Saurischia are the so-called "lizard-hipped" dinosaurs and Ornithischia the "bird-hipped." This is an unfortunate terminology, since birds are actually descended from saurischian dinosaurs.
Additional characteristics of the above groups include the following.
The Saurischia or lizard-hipped dinosaurs are so named because the three main bones of the hip girdle are arranged, like most reptiles, with the pubis (front bone) pointing forward.
The Saurischia are further divided into two groups: Theropods, the predatory dinosaurs; and Sauropodomorphs, which include the very large, long-necked, herbivorous dinosaurs.
The Ornithischia or "bird-hipped" dinosaurs are so named because the three main bones of the hip girdle are arranged with the pubis pointing backwards and parallel with the ischium. This configuration is superficially similar to that of modern, but not fossil, birds. In addition a unique characteristic of the ornithischians is the structure of the lower jaw. The lower jaw bones (dentaries) are capped by a small (predentory) bone forming the point of the chin. Ornithischians also have rows of long bony rods along the sides of the spine. Other characteristics include horny beaks, a lack of front teeth, cheek teeth, and cheek pouches (for holding food). All of the ornithischians were herbivores.
Generally, scientist recognize four subgroups (suborders) of "bird-hipped" dinosaurs:
The Dinosaurs of Jurassic Park: The Lost World was produced by Dinosaur Exhibitions, LLC, under the direction of "Dino" Don Lessem, and in partnership with Universal Studios and Amblin Entertainment. The exhibition was designed by Museum Design Associates. Jurassic Park and Lost World are trademarks of Universal City Studios and Amblin Entertainment.