102 CHAPTER 5 | Field Methods in Archaeology and PaleoanthropologyFossilization is most apt to occur among marine ani-
mals and other creatures living near water. Concentrations
of shells and other parts of organisms are covered and
completely enclosed by the soft waterborne sediments that
eventually harden into shale and limestone in the follow-
ing fashion: As the remains of organisms accumulate on
shallow sea, river, or lake bottoms, they become covered
by sediment and silt, or sand. These materials gradually
harden, forming a protective shell around the skeleton of
the organism. The internal cavities of bones or teeth and
other parts of the skeleton fill in with mineral deposits
from the sediment immediately surrounding the speci-
men. Then the external walls of the bone decay and are
replaced by calcium carbonate or silica.
Unless protected in some way, the bones of a land
dweller are generally scattered and exposed to the dete-
riorating influence of the elements, predators, and scav-
engers. Occasionally, terrestrial animals living near lakes
or rivers become fossilized if they die next to or in the
water. A land dweller may also become fossilized if itAn organism may be preserved in the bottoms of
lakes and sea basins, where the body or body part may be
quickly covered with sediment. An entire organism may
also be mummified or preserved in tar pits, peat, oil, or
asphalt bogs, in which the chemical environment prevents
the growth of decay-producing bacteria.
It is especially rare to find an entire organism fossil-
ized, let alone a human one. Fossils generally consist of
scattered teeth and fragments of bones found embedded
in rock deposits. Most have been altered in some way in
the process of becoming fossilized. Taphonomy (from
the Greek for “tomb”), the study of the biologic and geo-
logic processes by which dead organisms become fossils,
provides systematic understanding of the fossilization
process vital for the scientific interpretations of the fossils
themselves.To excavate the ancient Stone Age site Sima de los Huesos or “Pit of Bones,” Spanish paleoanthropologist
Juan Luis Arsuaga and his team spend nearly an hour each day traveling underground through a narrow
passage to a small enclosed space, rich with human remains. Here, fossils are excavated with great care and
transported back to the laboratory, where the long process of interpretation and analysis begins.© Javier Trueba/Madrid Scientific Films
taphonomy The study of how bones and other materials
come to be preserved in the earth as fossils.©^
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