A. "Eoraptor", an early saurischian, B "Lesothosaurus", a primitive ornithischian, C A saurischian pelvis ("Staurikosaurus") D "Lesothosaurus" pelvis
The following is a simplified list of dinosaur groups based on their evolution. Groups with a dagger (†) next to them don't have any living members.
The Archosaurs evolved into two main clades: those related to crocodiles, and those related to dinosaurs.
The first known dinosaurs were bipedal predators that were one to two metres long.
The earliest confirmed dinosaur fossils include the saurischian ('lizard-hipped') dinosaurs "Herrerasaurus" 230–220 mya, "Staurikosaurus" possibly 230–225 mya, "Eoraptor" 231.4 mya, and "Alwalkeria" 230–220 mya. "Saturnalia", 232–225 mya, may be a basal saurischian or a prosauropod. The others are basal saurischians.
Among the earliest ornithischian ('bird-hipped') dinosaurs is "Pisanosaurus" 230–220 mya. Although "Lesothosaurus" comes from 199–189 mya, skeletal features suggest that it branched from the main Ornithischia line at least as early as "Pisanosaurus".
Early saurischians were similar to early ornithischians, but different from modern crocodiles. Saurischians differ from ornithischians by keeping the ancestral configuration of bones in the pelvis (shown in a diagram above). Another difference is in the skull: the upper skull of the Ornithischia is more solid, and the joint connecting the lower jaw is more flexible. These features are adaptations to herbivory; in other words, it helped them grind vegetable food.
Dinosaurs were a varied group of animals. Adaptive radiation let them live in many ecological niches. Paleontologists have identified over 500 different genera and 1,000 species of non-avian dinosaurs. Their descendants, the birds, number 9,000 living species, and are the most diverse group of land vertebrates.
The largest dinosaurs were herbivores (plant-eaters), such as "Apatosaurus" and "Brachiosaurus". They were the largest animals to ever walk on dry land. Other plant-eaters such as Iguanodon had special weapons, to help them fight off the meat-eaters. For example, "Triceratops" had three horns on its head shield, "Ankylosaurus" was covered in boney plates, and "Stegosaurus" had spikes on its tail.
The carnivores were bipedal (walked on their back legs), though not as we do. Their body was more towards the horizontal, balanced at the back by their tail. Some were very large, like "Tyrannosaurus", "Allosaurus" and "Spinosaurus", but some were small, like "Compsognathus". It was the smaller sized meat-eaters that may have evolved into birds. The first fossil bird, "Archaeopteryx", had a skeleton which looked much like that of a dinosaur.
Dinosaurs were primitively "bipedal": their probable ancestors were small bipedal Archosaurs. The date of the early dinosaur genus "Eoraptor" at 231.4 million years ago is important. "Eoraptor" probably resembles the common ancestor of all dinosaurs; its traits suggest that the first dinosaurs were small, bipedal predators. The discovery of primitive, pre-dinosaur, types in Middle Triassic strata supports this view. Analysis of their fossils suggests that the animals were indeed small, bipedal predators.
Those dinosaurs which returned to four-legged stance kept all four legs under their body. This is much more efficient than the sprawling legs of a lizard.
The big sauropods could never have reached so large a size without their pillar-like legs. A review surveys what we know about the mechanics of dinosaur movement.
A major change in outlook came in the 1960s, when it was realised that small theropods were probably warm-blooded. The question of whether all theropods or even all dinosaurs were warm blooded is still undecided.
It is now certain (from fossils discovered in China: see Jehol biota) that small theropods had feathers. This fits well with the idea that they were warm-blooded, and that the origin of birds can be traced to a line of small theropods.
Warm blooded animals have a high metabolic rate (use up food faster). They can be more active, and for longer, than animals who depend on the environment for heating. Therefore, the idea of warm-blooded dinosaurs insulated by feathers led to the idea that they were more active, intelligent and faster runners than previously thought.
Main-stream palaeontologists have followed this view for small theropods, but not for larger herbivores. Since we know that the size of a "Stegosaur"'s brain was about the size of a walnut, there is good reason to think its intelligence was limited.
Despite their great success over a long period, there were life-styles which the dinosaurs never evolved. None ever evolved to live entirely in water, as many mammals do, though "Spinosaurus" was semi-aquatic. They never dominated the small terrestrial niche. All through the Mesozoic most small vertebrates were mammals and lizards.
The extinctions at the end of the Cretaceous were caused by one or more catastrophic events, such as massive asteroid or meteorite impacts (like the Chicxulub impact), or increased volcanic activity.
Several impact craters and massive volcanic activity, such as that in the Deccan Traps in India, have been dated to the approximate time of the extinction event. These geological events may have reduced sunlight and hindered photosynthesis, leading to a massive disruption in Earth's ecology.
Did any terrestrial dinosaurs survive the great extinction event? Several fossils have been found in the Hell Creek Formation about 40,000 years later than the K/T extinction event. Many scientists dismiss the "Paleocene dinosaurs" as re-worked, that is, washed out of their original places and then re-buried in much later sediments. An associated skeleton (e.g. more than one bone from the same individual) found above the K/T boundary would be convincing, but no such finds have been reported.
Books about dinosaurs have been popular, especially with children, but adults have also enjoyed these kinds of books. In Edwardian times, Arthur Conan Doyle wrote a novel about a plateau filled with dinosaurs which he called "The Lost World".
"Jurassic Park" in 1990 started a new phase in dinosaur popular culture when it was followed by the movie of the same name in 1993.
Stone Age
The Stone Age was a prehistoric time when people made tools from stone . Wood, bones, and other materials were also used for tools, but those things don't last as long, so more stone tools are found. Stone (especially a hard kind of stone called flint) was used to cut things.
The period began with the first stone tools, about 2.7 million years ago. Some groups of people were still in the stone age into the 20th century. They also killed animals for food and clothing. They used animal skin for their shelters.
The time after the Stone Age is the Bronze Age, named after the metal bronze. The Stone Age ended when people discovered the art of smelting (making metals). The first metal used was copper, followed by bronze. People probably began using bronze instead of just stone in the Middle East sometime between 3000 and 2000 BC.
The Stone Age is divided by archaeologists (people who study relics) into three sections: Paleolithic ("old stone"), Mesolithic ("middle stone"), and Neolithic ("new stone"). Pottery was invented during the Neolithic Stone Age.
The Palaeolithic is a prehistoric era distinguished by the development of stone tools. It covers the greatest portion of humanity's time (roughly 99% of human history) on Earth, from about 2.7 million years ago to about 20,000 years ago. It was followed by the Mesolithic and Neolithic cultures.
Adolf Hitler
Adolf Hitler (20 April 1889 – 30 April 1945) was a German politician and the leader of Nazi Germany. He became Chancellor of Germany in 1933, after a democratic election in 1932. He became "Führer" (leader) of Nazi Germany in 1934.