Dinosaurs

There are thousands of photos of REAL dinosaurs on the web. Just search for any type of Bird. Birds are theropod dinosaurs.

If you're referring to mesozoic dinosaurs like Tyrannosaurus, Triceratops etc, then the reason we don't have photos of those is because they went extinct long before humans had evolved.

See Measuring Past Biodiversity, by Jeremy B. C. Jackson and Kenneth G. Johnson. Science 2001 September 28; 293: 2401-2404. [Summary] [Full Text] [Supplemental Data]

Multicellular animals first appeared about 600 million years ago in the early Paleozoic and there was a rapid rise in number of families during the Cambrian and Ordovician. Diversity remained relatively constant (perhaps even declining) up until about 200 million years ago and then it rose again to its current all-time high of close to 800 families.

Four eras are recognized, and are characterized by typical life forms:

1. Precambrian: The origin of life.

2. Paleozoic ("Ancient Life"): The origin of plants, most invertebrate types, the first vertebrates (back-boned animals, including fishes, amphibians, and reptiles).

3. Mesozoic (the "Age of Reptiles"): The origin of flowering plants, dinosaurs, birds, and mammals.

4. Cenozoic (the "Age of Mammals"): The diversification of flowering plants, insects, birds and mammals, and the appearance of humans.

The eras are divided into periods. Biological diversity was dramatically depleted by five mass extinction episodes at the ends of the Ordovician, Devonian, Permian, Triassic and Cretaceous Periods. At each of these times a large fraction of existing species was wiped out, leaving the survivors to repopulate the biological world. The most famous of these was the extinction at the end of the Cretaceous because this ended the age of dinosaurs and made possible the evolution and dominance of mammals. But it was not the most devastating of the mass extinctions. Now we will review the history of life in more detail.

In addition to prokaryotes and eukaryotes, a third major group of organisms, called Archaea, consisting of about 500 species but making up about 30% of the biomass on Earth, was not discovered until 1977. They live in the most extreme environments on Earth - the hottest, coldest, and highest-pressure environments, so they are sometimes called "extremophiles". Most of their known biomass is in the Antarctic.

Many of the important fossils telling the story of the Cambrian explosion were found in a fossil bed in the Canadian Rockies called Burgess Shale, where both soft- and hard-bodied animals were buried in an underwater mudslide and preserved in water so deep and oxygen-free that they did not decompose. They are described in a book by Stephen J. Gould called "Wonderful Life". View pictures of the actual fossils.

Two different views on the role of chance and selection in evolution:

Wonderful Life: The Burgess Shale and the Nature of History by Stephen Jay Gould (1990). W.W. Norton & Company.

The Crucible of Creation: The Burgess Shale and the Rise of Animals by Simon Conway Morris (1998). Oxford UniversityPress, 272 pp.

The Cambrian explosion probably generated over 100 major animal groups with fundamentally different body plans, but only about 30 survived to this day. Examples of those that survived are arthropods (insects, spiders, crustaceans), echinoderms (sea urchins and starfish), mollusks (snails, clams), and chordates, the latter represented in the Cambrian by worm-like animals with a rod-like backbone but eventually giving rise to the vertebrates. Although large numbers of new families evolved in the later periods of earth history, these later innovations did not include major reorganizations of the body plan, but rather variations on themes that were established in the Cambrian.

We will look at the kinds of animals and plants that were present on the earth at different periods, starting with a jump to the Carboniferous because at that time, both plants and animals had moved out on to the land and were beginning to diversify.

Unlike modern fish that breathe through gills, some of the Carboniferous fish breathed air through two lungs, and they had developed lobe-like fins. The fascinating search for a surviving relative, the Coelacanth, has been recounted in Samantha' Weinberg's "A Fish Caught in Time": click on the picture. These fish live in the deep ocean, and since their discovery in 1938 more than 200 more of them have been captured.� The other surviving relative of these Carboniferous fish is the lungfish.

The Carboniferous fish gave rise to modern bony fishes.� In these the fins have lost their lobes, and one lung has been lost and the other converted to a swim bladder. They also gave rise to the amphibians, in which both lungs have been retained and the limbs strengthened and specialized for walking. These amphibians, like modern frogs and salamanders, hatched from eggs and spent their larval period in the water as tadpoles. They then went through a change of body organization called metamorphosis, in which they lost their tail and developed legs, and moved out on to the land as adults. They were the first land-dwelling vertebrates, and some of them were massive animals two or three feet long. In addition to the primitive amphibians there were some very primitive reptiles that were totally terrestrial, like today's lizards.� In these animals the egg gave rise directly to a miniature version of the adult, which could survive on the land.

The arthropods of the Carboniferous had also moved onto the land and had in fact taken to the air. Many of the ancient insects were gigantic compared to present day ones, among them mayflies with a 14-inch wing span, and giant six-winged insects, in which the first pair of wings had been already reduced to nubs but not eliminated as they are in all present-day insects. These insects had long sucking mouthparts with which they sucked the juices out of large primitive pinecones. Feeding on these insects were dragonflies with a 30-inch wingspan!

Reptiles such as the large sail-backed carnivore Dimetrodon, were common at this time. Some of these early reptiles had elaborate sails on the back. The purpose of these structures is not known although they are thought to have been involved in temperature regulation; that is, to radiate heat when the animal was too hot and to absorb it when the animal was too cold.

The most successful land animals of the middle Permian were many kinds of mammal-like reptiles, which were quite varied and included both herbivores and carnivores. Most of them were large, heavy and slow moving. None of them survived, so we can't tell if they were warm-blooded or if they had mammary glands, like true mammals, but they did have a jaw articulation like that found in mammals and they had several types of teeth, which is a mammalian feature.

At the end of the Permian, 245 million years ago, huge numbers of animals became extinct. About 96% of all marine animal species and 52% of the families disappeared in the greatest mass extinction the world has ever known. Recent data shows that 8 of 27 orders of insects, 21 of 27 families of reptiles, 6 of 9 families of amphibians and most of the terrestrial plant life disappeared at this time. This happened over a remarkably short period - about one million years. The most popular theory is that it was caused by flood volcanism in Siberia - huge continent-sized floods of scorching hot magma, about a mile deep, poured out from fissures in the Earth's crust and spread over the land. The eruption lasted for about a million years, generating about 2 million times as much lava as was involved in the eruption of Mount St. Helens in 1980. This might have generated enough debris to block out sunlight and trigger an ice age, enough sulfuric acid to acidify the oceans, and/or enough carbon dioxide to cause global warming by a greenhouse effect. However, studies reported in 2001 suggest an alternative explanation - that the extinction might have been caused, like the later one that finished off the dinosaurs, by a giant meteorite hitting the Earth. It took about 100 million years for the species diversity to recover from this extinction event.

The Jurassic and Cretaceous together were the age of giant reptiles. For 150 million years, the dominant vertebrates on the land were the dinosaurs. (By comparison, humans have been on the earth for only about 3 million years).

The dinosaurs are composed of two distinct orders:

The Ornithischians (bird-hip dinosaurs) included the duck-billed dinosaurs or Hadrosaurs, Stegosaurs with their plate-like armor along the back, Ankylosaurs which were heavily armored and flattened, and the rhinoceros-like horned dinosaurs or Ceratopsians (example - Triceratops).

The Saurischians (lizard-hips) included the Sauropods - gigantic herbivorous dinosaurs with extremely long necks and tails, such as Brontosaurus and Brachiosaurus - the largest terrestrial animals that ever lived; and Theropods - carnivorous dinosaurs with enormous skulls, powerful teeth and ridiculously small front legs, Tyrannosaurus being the best known example but Giganotosaurus was bigger. Fossils of a new species of theropod, about the size of a dog, were recently discovered on Madagascar.

The dominant animals of the oceans were the ichthyosaurs, some of which were as large as medium sized whales, the long-necked plesiosaurs and some marine crocodiles. The dinosaurs also took to the air and evolved some spectacular and huge flying reptiles called pterosaurs. One of them, Quetzalcoatlus, was the largest flying vertebrate the world has ever known, with a 40-foot wingspan!

Some cretaceous dinosaur fossils discovered in China had a downy covering resembling primitive feathers, which probably functioned in keeping the animal warm rather than for flight. But a recently reported fossil showsfeathers on all four legs, and these may have been used in gliding. These small dinosaurs are the only ones to show early wing development, so they may have been early ancestors of birds. The next stage of bird evolution, where the front limbs are wing-like but the hind limbs still leg-like, are seen in the Jurassic fossils of Archaeopteryx and its relatives. Archaeopteryx has been the focal point of many arguments between creationists and evolutionists, mainly concerned with whether it was a true intermediate between dinosaurs and birds, or just an unusual type of early bird. It had claws on its wings, once suggested to be leftovers of the dinosaur anatomy - but at least three modern species of birds also have wing claws (hoatzin and touraco, only in young stages; ostrich, even in the adult). Recent studies of the front limbs show that the dinosaur digits correspond to the human thumb, index and middle finger, whereas those of early and present-day birds (and alligators and turtles) correspond to the index, middle and ring fingers. The feathers are considered the defining characteristic of birds, and were apparently fully developed in Archaeopteryx. All of these features show that Archaeopteryx was an early bird, without any distinct reptilian features.

The predominant land plants in the Jurassic were the Cycads, primitive palm-like seed plants. A few species of cycads still exist in tropical and sub-tropical regions today, and although they are used as ornamental plants some of them are facing possible extinction. Early Cretaceous saw the evolution of flowering plants (angiosperms). The flowers provided a new food source for pollinators, mainly insects, with profound effects on the evolution of those forms.

The end of the Cretaceous saw the culmination of dinosaur evolution. Dinosaurs were more varied and adapted for more different modes of life than any other group, before or since. There were at least 100 species of dinosaurs. Some of them even lived in the Arctic, which was probably a lot warmer than it is now. They had achieved a level of evolutionary success that guaranteed them a permanent place in history, and yet they were soon to disappear entirely from the face of the earth. Several different groups of bird-like dinosaurs had evolved, and all except one - Archaeopteryx, the ancestor of all modern birds - died out at about the same time as the rest of the dinosaurs.

Further reading:

Species, Speciation, and the Environment by Niles Eldredge

The Sixth Extinction by Niles Eldredge

There have been numerous theories to account for the extinction of dinosaurs. But during the 1980's strong evidence was obtained to support the idea, originally proposed by Luis Alvarez, that a global catastrophe, caused by the impact of an asteroid, comet or meteorite, was responsible. Enormous amounts of debris would have been thrown into the atmosphere, making the Earth so cold and dark that cold-blooded animals like dinosaurs were unable to survive.

Supporting the impact theory, about 150 Impact Craters have now been discovered on the earth. One of the most spectacular is the Barringer Crater in the Arizonadesert. (picture by NASA). The Barringer crater was formed about 30,000 years ago (much too young to have anything to do with dinosaur extinction). It is 7/10 mile across and 560 feet deep, and is thought to have been caused by an iron meteorite 200 feet in diameter, weighing one million tons, that hit the earth at a speed of 30,000 mph and released an amount of energy equivalent to the most powerful nuclear devices yet made. But it is tiny compared to some older craters.

The impact responsible for the extinction of the dinosaurs would have produced a crater at least 100 miles across. The entire planet shows a 3 mm-thick layer of rock at the appropriate level (i.e. exactly at the boundary between Cretaceous and Tertiary), containing several kinds of evidence for the impact:

a high concentration of the element iridium, which is rare in terrestrial rocks but common in meteorites

"shocked" quartz grains showing parallel, colored markings which are an indicator of violent impact

unusual amino acids, thought to be extraterrestrial in origin

In North America the 3 mm layer is underlain by a similar layer 2 cm thick, and it is thickest (46 cm) at sites near to Haiti and Cuba, suggesting an impact site in the Caribbean Sea. This layer contains many glass-like beads, mm to cm in diameter, that were probably formed by melting of rocks during the impact. The molten rock would have been ejected, and then would have fallen back to earth. The composition of these layers suggests an ocean impact.

Over the past few years geologists have discovered a 200 mile-diameter submarine crater (the Chicxulub crater), of exactly the right age but buried under 2 km of sediment, off the coast of the Yucatan peninsula in Central America. There are actually two layers, at least in western North America, suggesting that there were at least two impacts, possibly from different parts of a comet that broke up under the influence of the sun's gravitational field. It has been estimated that after the collision there was so much dust in the atmosphere that the entire planet was completely dark for 1-3 months. The lack of sunlight would have caused the extinction of many plants and animals. Among the animals, cold-blooded forms like dinosaurs would have been affected more than the warm-blooded mammals.

Geologists have found that the rock in the Chicxulub crater is unusually rich in sulfur. They conclude that the impact could have produced a global sulfur dioxide fog that could have caused darkness and acid rain for over a decade.

An impact crater in northwestern Australia has been dated to the end of the Permian period, suggesting a possible connection to the mass extinctions that occurred at that time.

For a long time people found it hard to believe in such spectacular collisions. But impact craters are very common on many planets and the moon. And in 1994 a comet that had already broken into 13 pieces crashed into the planet Jupiter. Although the crash was not visible from earth, we were able to see the effects of the impact soon after it happened. This made it much easier to believe that comets can crash into planets. However, not everybody believes that this kind of catastrophe finished off the dinosaurs.

The carnivorous dinosaurs were the B group in this question:

1. raptors
2. tyrannosaurs
3. ornithomimids
4. large theropods
   
5. small theropods

Meat-eaters outnumbered plant eaters, which are listed in the A group in this question:

1. sauropods
   
2. ankylosaurs
3. stegosaurs
4. hadrosaurs
5. pachycephalosaurs
6. ornithopods
7. ceratopsians
8. titanosaurs

The 4 prepositional phrases are:

1. about the extinction
2. of dinosaurs
3. with such force
4. in the air