End Of An

About 75 million years ago there were more kinds of dinosaur than ever; yet 10 million years later all but the birds had vanished. Indeed, no land animal heavier than a large dog survived. Also gone were the pterosaurs and many sea creatures. At least 80 theories have tried to explain how so much life was wiped off the face of the Earth. Most are absurd - no one still thinks that dinosaurs became too large to breed, for instance. But experts argue to this day about what must have happened. Although it is hard to tell from fossil evidence how quickly the mass extinction took place, many scientists suspect it was caused by a sudden catastrophe, such as a massive comet or asteroid collision.

Last of the dinosaurs

This remarkable fossil of the duck-billed dinosaur Edmontosaurus shows it apparently curled up as it was when it died in the late Cretaceous. Edmontosaurus was one of the species that survived right up to the end of the Cretaceous; then it mysteriously vanished. Studies of the fossil record reveal that, just after Edmontosaurus and the other dinosaurs disappeared, ferns became suddenly common. Perhaps these plants were spreading to recolonize a devastated landscape.

Prehistoric Craters Earth
The asteroid or comet that made the Chicxulub crater hit Earth with a force 10,000 times greater than all the world's nuclear bombs put together.

Deep impact

In the early 1990s, geologists discovered a 112 mile (180 km) wide crater in Mexico. It seems to have formed when a comet or asteroid smashed into Earth 65 million years ago - exactly when the dinosaurs disappeared. The impact would have been phenomenal. Vast clouds of rock and dust would have filled the atmosphere, hiding the Sun. Maybe the dinosaurs died out during the dark, freezing months that resulted from this catastrophe.

end of an era

Death in the oceans

Whatever catastrophe destroyed the dinosaurs 65 million years ago also caused mass extinction in the seas. The casualties included plesiosaurs, ammonites and belemnites (both relatives of octopuses), certain fish, and tiny chalk-forming single-celled organisms. Ichthyosaurs and sea crocodiles had vanished already, perhaps outcompeted by sharks.

This slab of_

limestone is made entirely of ammonite shells. Ammonites were octopuslike animals that lived in coiled shells, their tentacles waving out of the open end to capture prev.

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Ammonite

Volcanic eruptions can launch huge amounts of ash, dust, and rock fragments into the sky.

Ammonite

Volcanoes

When the Chicxulub impact happened, dinosaurs were probably already suffering the effects of huge volcanic eruptions in India. For thousands of years, cracks in the ground oozed lava that piled up miles thick across an area as large as Alaska and Texas combined. Dust and ash thrown into the sky might have screened out the Sun and changed the planet's climate.

Siltckwaves after impact might have given the crater two rims.,

Siltckwaves after impact might have given the crater two rims.,

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DINOBIRDS

In 1861 an astonishing fossil turned up in a German quarry. It was a beautifully preserved skeleton of a creature almost identical to the midget dinosaur Compsognathus, except for one shocking difference: it had feathers. This animal, called Archaeopteryx, is now thought to have been a halfway stage in the evolution of birds from small predatory dinosaurs. So perhaps dinosaurs were not wiped out after all, and now live all around us. Some scientists disagree with this theory, but paleontologists have recently found more feathered "dinobirds," making the line between dinosaurs and birds ever more blurred.

Dinoturkey

Turkey-sized Caudipteryx (right), whose discovery was announced in 1998, seems to have been both a bird and a dinosaur. Downy feathers covered its body, and long feathers sprouted from its arms and fan-shaped tail; yet its "wings" were too small for flight. Its skull, hips, and feet were like those of a predatory dinosaur and, unlike modern birds, it had teeth and clawed hands. Caudipteryx was less birdlike than Archaeopteryx, but it lived much later. This suggests it was a flightless descendant of early dinobirds, rather than a dinosaur that was turning into a bird.

Archaeopteryx

¡Like a modern bird, Archaeopteryx had ^ backward-pointing big toes for perching.

Archaeopteryx

Archaeopteryx

Despite its teeth and bony tail, Archaeopteryx was clearly a bird, as it had wings fringed with long flight feathers exactly like those of birds today. The shafts of its feathers were off center, a feature that helps to generate lift during flight. But its shallow breastbone indicates that Archaeopteryx's flapping muscles were weak. It could probably take only short, low gliding flights around the desert islands it inhabited.

DINOBIRDS

In a flap

Although it could not fly, Caudipteryx probably had other uses for its wings. Perhaps they helped it swoop to the ground from trees. Or perhaps it flapped them and fanned its tail to intimidate rivals or attract mates, as birds do today. Caudipteryx probably pecked up plant foods of various kinds and ground them down between stones in its stomach. Its long legs would have made it a very fast runner.

Hoatzins live in the i forests of South America.

Ground up or tree down?

Hoatzins are unusual birds that have claws on their wings when young. Hatchlings use these to clamber about in trees. Some people think the first birds clawed their way up tree trunks like this, then fluttered down. Others believe that flight first began as they ran after prey, flapping their feathered arms to gain speed.

Million years ago

Velociraptor

Was velociraptor a dinobird?

Velociraptor ' -rv had many characteristics in common with Archaeopteryx. Its fingers and long arms were very similar, and a crescent-shaped bone in its wrist - also found in birds today - meant Velociraptor may have folded its arms sideways like wings. Perhaps it even had feathers, as in this model. We may never know. Unfortunately, feathers survive as fossils only in rocks made of the very finest particles.

Fossil finds

Archaeopteryx

\ caudipteryx

Archaeopteryx

\ caudipteryx

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Million years ago

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FOSSILS

Baryonyx

e know so much about dinosaurs thanks to fossil remains of their bodies, footprints, and droppings in rocks that were once sand or mud. Fossils include teeth, mineral-hardened bones, and hollows (molds) left by footprints or bones that dissolved. Perhaps only one dinosaur in a million was fossilized, and far fewer left whole skeletons. The rest vanished completely eaten, rotted away, or eroded by weather. But some finds are truly spectacular. They include fossils of dinosaurs fighting when smothered by sandstorms, and whole herds drowned by floods.

compsognathus

These two dinosaurs lived near a river 150 million years ago.

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One has died, its flesh has rotted away, and its skeleton lies on a dried river bed.

The skeleton is how fossils form buried by mud and river sediment, and

In order for a skeleton to be ^ bones become „s fossilized before it decomposes, hard as rock over it must be buried quickly, for millions of years. instance by windblown sand or mud washed into a river. Over millions of years the sand or mud turn into rock. Water trickling through the ground deposits minerals inside pores in the bones, making them harden. But if water or scavengers scattered the bones before burial, experts will find it difficult to put them back together.

Baryonyx

Today the rock that contains the fossilized dinosaur has come to the surface and is being eroded. Scientists have found the animal's remains.

Engraved in stone

This chicken-sized Compsognathus was found in fine-grained limestone rock that preserved tiny details, including its last meal, a lizard. The small dinosaur lived on a tropical island where southern Germany now stands. It probably drowned in a shallow lagoon, and fibers in its neck shrank and pulled back its head. Tides or gentle currents might explain why some bones have drifted apart.

. Neck curled back

Eye socket

Compsognathus

Compsognathus

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Euoplocephalus

Hadrosaur

Saltasaurus Euoplocephalus

Impressive skin

Traces of dinosaur skin are a rare and special find. Skin does not fossilize, but impressions in rock show that most dinosaurs had pebbly scales. Large scales covered the plant-eating hadrosaurs' skins, bony plates shielded armored dinosaurs such as Euoplocephalus, and bony studs protected the sauropod Saltasaurus. Some theropods, such as Caudipteryx and the bird-dinosaur Archaeopteryx, sprouted feathers.

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