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The evolution of Earth’s first mammals eventually brought placental birth

Posted in Animals, Dinosaurs, Nature, Prehistory on Thursday, 7 June 2012

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This edited article about prehistoric mammals originally appeared in Look and Learn issue number 720 published on 1 November 1975.

Protoceratops eggs, picture, image, illustration

Reptiles, like the dinosaurs before them , lay their eggs and abandon them; here we see the Protoceratops young hatching from their unattended dinosaur eggs

All mammals are thought to have descended from the carnivorous reptile, therapsid which lived in Permian and Triassic times. This had upright legs instead of splayed-out reptilian ones and a wide variety of teeth. In the Jurassic the first true mammal appeared, the tiny, shrew-like triconodon.

About 70 million years ago the Cenozoic (or New Life) era began with the Palaeocene; the climate was mild and the vegetation tropical. The small mammals which had lived in the shadow of the dinosaurs were ready to replace them as the dominant form of life.

Mammals were better equipped for survival than reptiles. They were warm-blooded and could cope with changing climatic conditions. They developed larger brains than reptiles. Thirdly, although early mammals were egg-layers, later marsupials (pouched mammals) and then placental mammals, which gave birth to fully developed live babies, evolved. Marsupials and placental mammals care for their young until they can fend for themselves, unlike reptiles which lay their eggs on the ground and then abandon them.

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