Fossils are ___1___
remains or impressions of plants and animals. Their
appearance in exposed rock has fascinated humans
throughout history. Fossils and their placement in rock
are studied by scientists known as paleontologists. They
___2___ fossils in
order to construct the history of life on Earth.
Fossils are generally found in sedimentary rock. This
___3___ because the
remains of plants and animals tend to accumulate with
the inorganic debris that create such rocks. In rare
instances, fossils can form in igneous rock, such as
when volcanic ash or lava congeals around an insect
wing, or fills a footprint. Fossils can also be found in
metamorphic rock that has not been deformed enough to
destroy the fossil.
Only a tiny fraction of the organisms in the world
end ___4___
preserved as fossils. Most of the time, their soft parts
are quickly decayed or eaten. Even bones and shells
usually crumble ___5___
a few years. A range of special processes and
circumstances must occur for a life-form to be
preserved, or fossilized, for thousands or millions of
years. |