Ca10(PO4) 6(OH) 2
Ca2+ can be replaced
with Na+, Sr2+, Mg2+,
and rare-earth elements
PO43– can be replaced
with CO32–, SiO42–,
OH– can be replaced
with F–, Cl–, and CO32–
Fossils are the preserved
copies of the remains of liv-
ing organisms. The chance
that any living organism will
become fossils is quite low.
The fossils of sea creatures
are more common than those
of land creatures, because
most fossils start forming
after the organism is covered
in sand or mud—conditions
more common in the ocean.
On land, fossil deposition can
normal decomposition process is slowed under sand or mud, some-
times allowing the process of fossilization to take place.
So, how did SUE’s fossils form? First, its body started decomposing,
just as all organisms do when they die. Only a few hours after its death,
red blood cells started releasing iron, cells along the digestive tract
started leaking digestive enzymes, and muscle cells leaked calcium,
causing the muscles to contract and the body to stiffen. All of these
changes resulted in a soup of various types of molecules that fed microorganisms—mostly bacteria—already present in SUE’s guts. These
bacteria started multiplying while producing putrescine and cadaverine, two foul-smelling compounds that attract insects
and larger scavengers.
But even though SUE’s soft tissues were
soon lost to continuing decomposition,
the rest of its body was probably buried by
sand, silt, and mud from the nearby river,
allowing most of its bones to become
Bone is not as dry and lifeless as it may
seem. It is actually a porous structure
(photo below) that consists of collagen
protein embedded with a bone mineral
and permeated with living cells, blood
vessels, and nerves.
Collagen provides the bone with strength and flexibility. The bone mineral makes up 70% of bone and is responsible for its characteristic hardness and rigidity. It belongs to a group of minerals known as phosphate
minerals. While this bone mineral is maintained during life, after death, it
starts to react with chemicals in its environment, and the rest of the bone
material becomes mineralized.
How this happens is not completely understood, but scientists agree
that the pores and spaces left by the loss of organic material in the
bone must be rapidly filled by minerals, or the original bone will simply
dissolve. After the spaces in and around the bone are filled, the composition of the bone mineral may change, too, because various ions in it
are replaced with other ions from the environment (Fig. 1).
Fossils can form on land, in river
valleys, or at the bottom of the ocean,
but the best preserved fossils are those
that form at the bottom of river valleys.
Types of Fossils
There are three main types of fossils: impression fossils, trace fossils, and replacement fossils. This is how each type of fossils forms:
Impression fossils: When the living organism dies, either a cast or a mold
is formed. In the case of a mold, mud or sediment covers the body and forms an
impression of the organism. When the organism decays, the impression of the
body remains. A cast works much the same way, except that the cavity
of the impression is filled in by minerals, forming a “replica” image of
Trace fossils: Trace fossils include any impression
or other preserved sign of activity rather than the
preserved remains of the body of the actual organ-
ism itself. They include footprints, dung, nests, and
tooth marks, and they give hints about the behavior of
Replacement fossils: These fossils are replicas of living
beings—such as trees, land animals, and sea creatures—which
were trapped in an ocean, lake, or pond, where the once-living
beings were covered with silt and where oxygen was mostly excluded,
leaving the body intact long enough for fossilization to occur. As they
rotted, the organic parts were replaced by hard mineral deposits. The
minerals filled in the spaces and created a replacement fossil of the
The inside of a bone is porous, so when
animals die, minerals may fill in these
pores, creating a fossil.
Figure 1. After an animal dies, the bone mineral, which has a molecular
structure similar to hydroxyapatite [Ca10(PO4) 6(OH) 2], starts to react with
chemicals in its environment, and the calcium (Ca2+), phosphate (PO43–), and
hydroxide (OH–) ions that make up hydroxyapatite are replaced with other
ions, which are shown here. The chemical formula given for hydroxyapatite
is the one used to indicate the composition in one unit of the crystal structure.