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Giant of the skies - The Early Cretaceous Earth  

2010-08-10 21:15:19|  分类: 英文起步 |  标签: |举报 |字号 订阅

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http://www.bbc.co.uk/sn/prehistoric_life/dinosaurs/chronology/127mya1.shtml

Giant of the skies - The Early Cretaceous Earth


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The Early Cretaceous(白垩纪) Earth was home to the largest animals ever to fly. But on the ground, life was changing too - and a species of dinosaur was about to undergo a very important change. Dr Jo Wright explains why.

Giant gliders(滑翔机)

The largest animals ever to fly were pterosaurs(翼手龙). With wingspans(翼展) of up to 12m, they could be as large as a small modern glider. Pterosaurs were a peculiar mixture of bird and bat. Their long beaks remind us of birds, but they had membranous(膜状的) wings like bats.

These wings were attached to their legs and they walked on all fours, with an erect(竖直的), rather than sprawling(匍匐) posture. Although not necessarily very speedy on land, they walked competently(胜任地,适用地). Their tracks have been found in rocks around the world.

There were two kinds of pterosaurs, ones with and others without tails. The tailed pterosaurs lived from the Late Triassic period to the Late Jurassic period, when they were replaced by the tailless ones. It is this second group, including Ornithocheirus(鸟掌龙), which grew to great sizes.


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Iguanodon(禽龙) - Chewing gives it an evolutionary advantage.

Early Cretaceous Earth

The early Cretaceous period was also a time of great change for animal life on the land. New types of dinosaurs - the ornithopods(鸟掌类) - were appearing, and they would later be among the most numerous on land.

Small versions of these dinosaurs had actually been around since the Early Jurassic period, but only now did they come into their own. Iguanodon was one of the earliest of the ornithopods. There were several species in Europe, Asia and North America.

Cheeky chewers

Although smaller than the sauropods, the ornithopods were more numerous. They were able to chew their food and hold it in their cheeks while doing so. Chewing broke their food into smaller pieces, greatly reducing digestion time. Sauropods didn't chew their food at all. They needed huge bodies so their guts could break down food over several days. As they didn't have cheeks, they had the big reptilian smile we see in modern lizards and crocodiles.

Bird-like dinosaurs


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Utahraptor(犹他盗龙) - Its sickle(镰刀) claw makes it a deadly fighter.

New kinds of theropods also appeared in the Early Cretaceous period. These were the kinds popularly known as 'raptors'(猛禽龙), such as Utahraptor and Velociraptor(速龙). These theropods were some of the most bird-like dinosaurs that ever existed. They had fairly long arms and a more bird-like pelvis(骨盆). Birds are descended from theropod dinosaurs like these - over 100 characters in the skeleton link these groups. Fossil evidence from the USA indicates that these types of dinosaurs may have hunted in packs(成群).

The power of flight

The earliest known bird – Archaeopteryx(始祖鸟) - comes from the Late Jurassic period in Germany. By the Early Cretaceous period, birds had radiated and diversified. Fossils have been found in Spain and China. Most of these early birds look very primitive and some still have teeth. But by the Late Cretaceous period they looked look far more modern and had diversified into different niches(生态位,栖所). Some wading(涉水) or diving birds had even lost their power of flight again.
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By the late Cretaceous, birds looked very like they do today.

A world of colour

In the plant kingdom there were great changes too. Flowering plants appeared in the Late Jurassic period, and in the Early Cretaceous period the first flowers appeared. Soon flowers and flowering plants were to become the most common plants on Earth.

The earliest known fossil flower is very small. It was found in the Wealden Formation sediments(沉积物) of southern England. During the Early Cretaceous period, southern England was a low-lying, richly vegetated(植物生长) river floodplain. The climate was humid(潮湿的) and sub-tropical(亚热带), although there may have been wet and dry seasons, as there is evidence of forest fires. By the Late Cretaceous period the world would have looked rather more familiar to us, although there were still no grasses.

This article was written to accompany episode four(第四集) of Walking with Dinosaurs.

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