Question

10. The age of the reptiles, the Mesozoic era, is marked by the rise of the phytoplankton (including diatoms & dinoflagellates), the great radiation of reptiles, and the dominance of the seed plants. The first mammals appear in the Triassic, but they are not a dominant species. Note the “small” mass extinction at the end of the Triassic. Flowering plants also appear and diversify in the Jurassic, but like the mammals, the do not dominate the biosphere. It isn’t until the mass extinction at the end of the Cretaceous (which wipes out all of which groups?) that mammals and flowering plants will begin to take on the most important roles.

11. The Cenozoic is marked by the dominance of those two groups, and the emergence of the nitrogen-fixation symbiosis in which group of plants? When did the hominin radiation occur?

12. What is meant by “background extinction rate” and how does our present period’s extinction rate compare with that?

Answer 1

11. The Cenozoic era is the time period during which the modern flora and fauna emerged which are currently present on the Earth.

Nitrogen fixation phenomenon is seen in the legumenous plants where the bacteria present in the soil surrounding the root of the plants fix the nitrogens in the form which can be taken up hy the plants.

The adaptive radiation is the process where a species changes it's niche, behaviour and other related aspects which helps them to survive according to the rapidly changing environmental conditions and nature. It may be in different directions. The hominin includes all the stages in the human evolution. It has taken a long period which includes the Cenozoic.

12. The background extinction is the normal or natural extinction which has no effect of humans. It was normally found when there were not much human interference. But at present the extinction rate is affected by human activities. The rate of extinction is increased in this period.

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