Name: _____________________________ Date: __________________________
What are Fossils?
The only direct way we have of learning about dinosaurs is by studying
fossils. Fossils are the remains of ancient animals and plants, the traces or impressions
of living things from past geologic ages, or the traces of their activities. Fossils have
been found on every continent on Earth, maybe even near where you live.
The word fossil comes from the Latin word fossilis, which means "dug up." Most
fossils are excavated from sedimentary rock layers . Sedimentary rock is rock that
has formed from sediment, like sand, mud, small pieces of rocks. Over long periods
of time, these small pieces of debris are compressed (squeezed) as they are buried
under more and more layers of sediment that piles up on top of it. Eventually, they are
compressed into sedimentary rock. The layers that are farther down in the Earth are
older than the top layers.
The fossil of a bone doesn't have any bone in it! A fossilized object has the same
shape as the original object, but is chemically more like a rock.
Paleontology is the branch of biology that studies the forms of life that existed in
former geologic periods, chiefly by studying fossils.
HOW FOSSILS FORM
Fossils of hard mineral parts (like bones and teeth) were formed as follows:
• Some animals were quickly buried after their death (by sinking in mud, being
buried in a sand storm, etc.).
• Over time, more and more sediment covered the remains.
• The parts of the animals that didn't rot (usually the harder parts likes bones and
teeth) were encased in the newly-formed sediment.
• In the right circumstances (no scavengers, quick burial, not much weathering), parts
of the animal turned into fossils over time.
• After a long time, the chemicals in the buried animals' bodies underwent a series of
changes. As the bone slowly decayed, water infused with minerals seeped into
the bone and replaced the chemicals in the bone with rock-like minerals. The
process of fossilization involves the dissolving and replacement of the original
minerals in the object with other minerals (and/or permineralization, the filling
up of spaces in fossils with minerals, and/or recrystallization in which a
mineral crystal changes its form).
This process results in a heavy, rock-like copy of the original object - a fossil. The
fossil has the same shape as the original object, but is chemically more like a rock!
Some of the original hydroxy-apatite (a major bone consitiuent) remains, although it
is saturated with silica (rock).
Types of fossils
Fossils can be divided into two categories, fossilized body parts (bones, claws, teeth,
skin, embryos, etc.) and fossilized traces, called ichnofossils (which are footprints,
nests, dung, toothmarks, etc.), that record the movements and behaviors of the
dinosaurs.
The four types of fossils are:
• mold fossils (a fossilized impression made in the substrate - a negative image of the
organism)
• cast fossils (formed when a mold is filled in)
• trace fossils = ichnofossils (fossilized nests, gastroliths, burrows, footprints, etc.)
• true form fossils (fossils of the actual animal or animal part).
There are six ways that organisms can turn into fossils, including:
• unaltered preservation (like insects or plant parts trapped in amber, a hardened
form of tree sap)
• permineralization=petrification (in which rock-like minerals seep in slowly and
replace the original organic tissues with silica, calcite or pyrite, forming a
rock-like fossil - can preserve hard and soft parts - most bone and wood fossils
are permineralized)
• replacement (An organism's hard parts dissolve and are replaced by other
minerals, like calcite, silica, pyrite, or iron)
• carbonization=coalification (in which only the carbon remains in the specimen other elements, like hydrogen, oxygen, and nitrogen are removed)
• recrystalization (hard parts either revert to more stable minerals or small crystals
turn into larger crystals)
authigenic preservation (molds and casts of organisms that have been destroyed or
dissolved).
Name: __________________________________________ Date: __________________________________
6th grade
Lab # ___ - Fossil
Fossil number
Length
Weight
Draw
#1
#2
#3
#4
Conclusion: ______________________________________________________________________________
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