Earth Dynamics | Earth and Time | Extinction | What are Dinosaurs? | Where do they come from? | Dinosaur Classification : Saurischia | Ornithischia
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; and mantle, which is 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 supercontinent known as Pangea. Over the years the crust’s movements have 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. The theory of plate tectonics explains all this movement.
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 time scale 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.
INTERACTIVE SCALE MODEL TIMELINE
The Triassic World, 248-206 million years ago
During most of the Triassic Period all of the continents were clustered together to form a supercontinent called Pangea. The global climate was mostly warm and inland areas far from the sea were often arid. There were no ice caps.
Ferns, horsetails and cycad-like trees, ginkgoes and primitive cone-bearing trees thrived in moist climates.
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 primitive crocodilians, turtles, flying reptiles, 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 ginkgoes continued to thrive under a greenhouse atmosphere with high levels of carbon dioxide.
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, lizards, turtles, flying reptiles, small mammals, and Archaeopteryx—an 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 became generally cooler, but evidence of polar ice is meager. 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 ongoing processes throughout this period. The 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 debated, but among the more interesting theories is that an asteroid or comet struck the earth, producing a natural pollution event of global magnitude. Another theory invokes widespread volcanic eruptions. 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, such as pterodactyls. These distinct groups of reptiles were not dinosaurs.
Dinosaurs are a specialized group of prehistoric, 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 many, later 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).
Dinosaurs lived all over the Earth, but their fossils are not found everywhere. Seas, plants, buildings, and layers of rock now cover over the rock layers that contain dinosaur fossils. But some places, where dinosaur-aged rock shows on the surface of the Earth, produce many dinosaur skeletons. Different places expose different dinosaurs and times in dinosaur life. Around the world, new kinds of dinosaurs are now being discovered at a record rate-a new kind every six weeks!
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.
Theropoda
Generally, theropods are further divided into two groups: the carnosaurs and coelurosaurs.
The carnosaur group includes the large flesh-eaters such as Tyrannosaurus and Allosaurus. Carnosaur characteristics include a large head, short-S-curved neck, sharp blade-like teeth, short trunk, bird-like hind feet with three forward-pointing toes, and large tail.
Coelurosaurs are mostly small, speedy dinosaurs. Other characteristics include a low narrow head, sharp teeth, long neck, arms, legs, and tail. Each hand has three fingers with strong, curved claws; each foot three main toes. The bones are thin-walled and fragile. Included in the coelurosaur group are the famous small raptor dinosaurs of Jurassic Park, which appear to be closely related to Archaeopteryx, the oldest known bird.
Sauropodomorpha
The sauropodomorphs are also divided into two groups: prosauropods, and sauropods.
Prosauropods appeared in the late Triassic and were extinct by the early Jurassic. They were small to medium-sized quadrupeds that may have been able to walk upright on their hind feet. Plateosaurus belongs to this group.
Sauropods are some of the best-known dinosaurs. Their characteristics include small head, teeth shaped for biting but not chewing, long neck, huge body, pillar-like limbs, five-toed feet, sharp claws on each "thumb," and a long tail. These herbivorous dinosaurs are mostly very large such as Brachiosaurus and Apatosaurus.
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 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 (predentary) 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, scientists recognize four subgroups (suborders) of "bird-hipped" dinosaurs:
Stegosauria
These quadrupedal dinosaurs have hind legs longer than the front, a small head, and are well-known for plates running down the back. Stegosaurus is the best known member of this group.
Ankylosauria
These quadrupedal armored dinosaurs have back, head, and tail covered with bony plates. The skull was broad; the neck short and the tail relatively long. Ankylosaurus and Nodosaurus are well known members of this group.
Ornithopoda
Bipedal, small to medium size, these dinosaurs often have a flat duck-like beak or bony head crest. The feet have three or four toes with hoof-like claws. The hands have four or five fingers. Maiasaura and Iguanodon are ornithopods.
Ceratopsia
Commonly described as resembling a rhinoceros, these quadrupedal dinosaurs have a bony frill behind the head and usually have horns above their eyes or on their nose, a parrot-like beak, and cheek teeth designed for shearing tough plants. Triceratops is the best known of the ceratopsians.
The thick-headed Pachycephalosauria are considered to be a fifth group.
 Teacher's Guide Intro |
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 Dinosaurs in the Exhibit |
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.
Teachers Guide materials developed by San Diego Natural History Museum and used by permission. Some activities adapted or developed by the North Carolina Museum of Natural Sciences.
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