GE 70B: Evolution of the Cosmos and Life
LABORATORY/DISCUSSION 2 WORKSHEET
FOSSILS, FOSSILIZATION, AND EVIDENCE OF
The purpose of today’s lab is to show you how fossils are preserved, as well as some of
the evidence for early life.
You should look closely at all of the fossils, trying to discern as much
as you can about the organism whose remnants you are observing.
Consult your lecture notes,
pay attention to the clues in this write-up, and ask your TA for help in answering questions.
There are three main types of fossils: body, trace, and chemical.
most common, and contain information about the physical form (i.e., morphology) of the ancient
are preserved marks left by an organism on the sediment (e.g., tracks,
trails) or on the remains of another organism (e.g., tooth marks on bone, drill holes in shells).
are the chemical traces of life.
Life changes the chemistry of the environment
around it, and these changes may be left in rocks.
Sometimes body fossils are preserved as imprints where the flattened external shape of
the organism compresses the sediment that it is buried within.
In these cases, the organism
decays, but the sediment surrounding it conforms to its shape and begins to harden prior to
Occasionally, some original organic matter can be seen on the fossil itself.
This typically occurs in lower oxygen environments because this means less bacterial activity
(aerobic bacteria are responsible for most decay).
In these cases, the organic material has usually
been degraded (via heat and pressurization) to a thin sheet of carbon, preserved as a black or
brown colored film covering the imprint of the fossil.
If the imprint fossil lacks any remnant
organic matter, it is called an
; if it contains organic matter, it is called a
In other cases of preservation, a shell can be filled with sediment as it is buried, and then
become dissolved away by acid in the groundwater (CO
becomes incorporated in rainwater as
carbonic acid, which can dissolve away calcium carbonate shells–this reaction is also responsible
for the formation of caverns in limestone).
The resulting sediment “
” or “
” can then be
preserved in the fossil record.
are produced when the shell dissolves away and all that
remains is the petrified sediment that was deposited
the shell, thereby preserving only the
internal shape of a shell.
is produced when sediment later fills in the cavity produced by
the dissolved shell, reproducing its external shape.
In still other preservational cases, the original
mineral of the shell can be replaced by a new mineral (
); often this occurs when
pore waters precipitate minerals into a cavity produced by a dissolved shell.
In other cases, the
pore spaces within invertebrate shells, or in the cells of wood, can be infiltrated with a mineral