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J How Old Is the Earth?
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acritarchs algae ammonoids amphibians angiosperms areas became extinct bones bony brachiopods bryozoans called Cambrian carbon carnivores cells Cenozoic cephalopods changes climate coelom common conodonts conspicuous continents corals Courtesy of Field Courtesy of National Cretaceous crust Devonian dinosaurs diverse early earth echinoderms environment Eocene evolution evolutionary evolved fauna Field Museum Figure filter feeders fishes floras flowering plants formed fossil record gametophyte genetic geologic graptolites gymnosperms habitats herbivores important invertebrates Jurassic kinds known land plants layers limestone living lophophore major groups mammals marine communities Mesozoic microscopic migration million Miocene Museum of Natural National Museum Natural History North America oceans Ordovician organisms paleontologists Paleozoic pelecypods Pennsylvanian Permian photosynthetic phyla phylum populations Precambrian predators preserved primitive protists reefs reptiles sea floor sediment sedimentary rocks shale shell Silurian skeleton skull species specimen spores sporophyte structure teeth terrestrial tion tissues Triassic trilobites vascular plants vertebrate