Microfossils, on the other hand, are extremely common. The microfossils include very early bacteria and algae; the unicellular organisms called foraminiferans, which were common in the Tertiary Periods; and fossil pollen. The study of micro fossils is a specialized field called micropaleontology.
Fossils of single-celled organisms have been recovered from rocks as old as 3.5 billion years. Animal fossils first appear in late Precambrian rocks dating back about a billion years. The occurrence of fossils in unusual places, such as dinosaur fossils in Antarctica and fish fossils on the Siberian steppes, reflects both shifting of continental position by plate tectonics and environmental changes over time. The breakup of the supercontinent Pangaea in the Triassic Period pulled apart areas that were once contiguous and shared the same flora and fauna. In particular, the plates carrying the southern hemisphere continents—South America, southern Africa, the Indian subcontinent, Australia, and Antarctica—moved in different directions, isolating these areas. Terrestrial vertebrates were effectively marooned on large islands. Thus, the best explanation for dinosaurs on Antarctica is not that they evolved there, but that Antarctica was once part of a much larger land mass with which it shared many life forms.
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