The Jurassic and Cretaceous Periods

In contrast to the Triassic, the Jurassic period (205-145 mya) was a time of moist climates and luxurious forests. The first flowering of giant dinosaurs began towards the end of the Early Jurassic, and a wealth of dinosaurian fossils has been found in Late Jurassic strata. Indeed, the Late Jurassic was the time during which the largest of the dinosaurs existed.

In many ways, the climates of the Cretaceous (145-65 mya) resembled those of the Jurassic, but towards the end of the period the world became drier and, as already mentioned, the climate seasonal. During the Early Cretaceous, the sea level was some 25 m higher than it is today. About 100 mya, however, it rose dramatically to about 200 m above the present level. All the above-mentioned factors may have contributed to the extinctions that occurred at the end of the Mesozoic Era (Chap. 12). Unlike the Triassic and Jurassic periods, which are both divided into three epochs, the Cretaceous is separated into two.

The predominant elements of the Mesozoic flora were gymnosperms (Fig. 15). Among them were various Cycadoidea - low forms with short, globular or barrel-shaped trunks sometimes bearing large and brightly coloured flowers. There were also tree-like species with slender stems that branched only occasionally.

■ Fig. 15. The Mesozoic landscape

Both types bore rich crowns of long, palm-like leaves. When the conifers appeared they spread rapidly and soon became abundant. Alongside these were various types of ginkgo, pine-like taxodiums, and huge sequoias. Cryptograms lost their former leading position among the vegetation. Ferns and horsetails still grew in damp places along the banks of rivers or lakes, but they were much smaller than they had been previously. In the absence of grass, the ground was either bare or covered by algae, bryophytes, and pteridophytes. Finally, during the Cretaceous period, as the seasons set in, deciduous plants appeared. These shed their leaves during the dry season or in winter, when climatic conditions became unfavourable, and thus the vegetation came to resemble that of the world today (Spinar 1995).

The vegetation during the Mesozoic Era was not only different from that of earlier times, but was also more diverse. Consequently the fauna diversified also to fill the numerous ecological niches that appeared. This diversity is reflected among the wonderful reptiles discussed in the following chapters.

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