. Adventures with animals and plants. Biology. V38 The Earth. Fig. 489 hnprint of a strange birdlike miinial (Archaeornis) which lived in the Mesozoic era. (SMITHSONIAN INSTITUTION) pounds, its brain must have weighed lit- tle more than a pound. So poorly devel- oped a nervous system might be reason enough for its becoming extinct when it was obliged to compete with more intel- ligent animals, but no doubt there were many other factors which contributed to its dying out. The "King Tyrant" reptile (Tyran- nosaunis rex) with a monstrous body and enormous mouth armed with great teeth wa
. Adventures with animals and plants. Biology. V38 The Earth. Fig. 489 hnprint of a strange birdlike miinial (Archaeornis) which lived in the Mesozoic era. (SMITHSONIAN INSTITUTION) pounds, its brain must have weighed lit- tle more than a pound. So poorly devel- oped a nervous system might be reason enough for its becoming extinct when it was obliged to compete with more intel- ligent animals, but no doubt there were many other factors which contributed to its dying out. The "King Tyrant" reptile (Tyran- nosaunis rex) with a monstrous body and enormous mouth armed with great teeth was probably a flesh eater and a powerful fighter. Many of the flesh eaters were apparently heavily armored, and many were provided with long, strong tails. Marks in the fossil bones in- dicate that many bone fractures had oc- curred while the animal was alive; this leads us to the belief that these dinosaurs must have been mighty fighters. You will find Exercise 3 interesting. and Its Inhabitants Change unit x The dinosaurs died out completely in what seems to us like a sudden catastro- phe. There is not a single dinosaur fossil in the rocks of the next era. But, if you recall the length of a geologic era, you will realize that the dinosaurs may have died out gradually over a period of thou- sands and thousands of years. The Age of Mammals. This is the sixth (Cenozoic) era. It started about 60 mil- lion years ago and lasted to within a mil- lion years of the present. The dinosaurs had become extinct at the close of the last era but there were many other kinds of reptiles. There were many species of amphibians, thousands of species of fish, and countless thousands of species of in- vertebrates. But the earth in the sixth era came to look very different from the earth in the earlier eras. There were now many kinds of flowering plants growing in large numbers; a few of these had ap- peared in the fifth era. Birds became com- mon; and the mammals, of which there had been only a few
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