Introduction

During the Mesozoic Era, the climate of the world was generally warm and moist. Although there were arid times and regions, much of the land surface was in many ways similar to that of the Amazon valley of today, with vast swamps and rainforests. At higher altitudes and away from the swamps there were more open areas with less dense vegetation. The swamp fauna during most of the period consisted of tree ferns and horsetails, that of the open regions of giant conifers, cycads and ferns. As shown in Table 3, of the five major extinctions in the fossil record, one took place at the end of the Permian period, another at the end of the Triassic, and the most recent one at the end of the Cretaceous. Minor extinctions also occurred during the Permian period, and throughout the Mesozoic Era (Fig. 1).

At the end of the Triassic, there was an outburst of volcanic activity in what is now North America, but during the succeeding Jurassic period the land was generally low-lying, with lakes, pools, meandering rivers, and swamps, all liable to invasion by the sea. There was a continuation of mild, subtropical climates and, consequently, the vegetation was similar to but more luxuriant than it had been in the Trias. The early Cretaceous saw a continuation of the Jurassic flora but, by the Middle Cretaceous, a rich and almost modern flora had become widespread. This included beech, poplar, magnolia, laurel, willow, and fig trees. Flowers gave colour to the scene and insects thrived. The climate continued at first to be mild,but later, as continental drift increased, seasonal changes

■ Table 3. Dates and intensities of the five major extinctions in the fossil record. Based on Jablonski(1989)

Millions of years

Observed

Calculated

before the present

loss of families9 (%)

species loss (%)

Ordovician

440

26 (12)

84

Devonian

350

22 (14)

79

Permian

250

51 (52)

95

Triassic

205

22 (12)

79

Cretaceous

45

16 (11)

70

a Percentage loss of families of marine invertebrates is shown in brackets a Percentage loss of families of marine invertebrates is shown in brackets set in. Finally, with the great mountain-building movements of the Laramide revolution (which produced the Rocky Mountains and the Andes), much of the old swamp and lakeland was drained and the 'Age of Reptiles' came to a close. But what had the world been like before it began?

The earliest known fossils of animals that were probably reptiles are among the stem-reptiles (parareptiles). There may well have been a small extinction about this time - between the Mississippian and Pennsylvanian periods (into which the Carboniferous is frequently divided). Only a number of ammonoids and some brachiopods seem to have disappeared, however,which is surprising because a major phase of glaciation began then. As the ice caps grew on Gond-wanaland (the southern of the two continents into which the land was then separated), the sea level fell. But there was no mass extinction, either in the sea or on land.

Was this article helpful?

0 0
Pregnancy And Childbirth

Pregnancy And Childbirth

If Pregnancy Is Something That Frightens You, It's Time To Convert Your Fear Into Joy. Ready To Give Birth To A Child? Is The New Status Hitting Your State Of Mind? Are You Still Scared To Undergo All The Pain That Your Best Friend Underwent Just A Few Days Back? Not Convinced With The Answers Given By The Experts?

Get My Free Ebook


Post a comment