5.7D Fossils: What Happened Before
Key Concept 1: Fossils are traces, or preserved parts, of organisms that
lived in the past.
The remains or traces of animals and plants that lived thousands or even
millions of years ago are called fossils. In fact, the term “fossil” means
that the organism is older than some minimum age. Generally, they lived
before the end of the last glacial period. For an organism to become a
fossil, conditions have to be just right. Because fossil formation is rare, the
fossil record represents only a tiny, tiny fraction of all of the organisms
that once lived. When fossils are found, the preserved remains, and the
geology of the surrounding rock layer, are clues to the nature of the past
environment.
Trace fossils are footprints, burrows, or impressions that were made by
an animal or a plant while it was living and that have hardened into stone.
For example, at Dinosaur Valley State Park, southwest of Fort Worth, huge
dinosaurs left their footprints preserved in the soft mud of a shallow sea
that covered Central Texas 113 million years ago. The fossilized tracks
provide clues to the habits of the dinosaurs, such as travelling in herds, or
which species became predator or prey. The soft limestone mud in which
the tracks were found, the fossils of palm trees, and fossil shells provide
clues that a shallow sea once covered the area.
Fossil formation is not common. When most animals or plants die, they
simply decay or are eaten by another animal. However, on rare occasion,
an organism dies and is quickly covered by sediment at the bottom of a
body of water. After layers of sediment accumulate, pressure causes
minerals to replace the organism’s cells, or fill in a mold where the plant
or animal had dissolved. Continued pressure causes the minerals to
harden and a fossil is formed, if the rock layers are not disturbed.
Sometimes, such as in the case of complete dinosaur skeletons, entire
organisms are preserved. Another example is the Petrified Forest in
Arizona where trunks of dead trees were washed up onto the sides of the
rivers during big floods, and buried in sediment (dirt, rocks, sand, volcanic
ash). Over time, minerals replaced the wood, creating entire petrified
(fossilized) tree trunks. Fossils also formed in ancient times when insects
were preserved in amber (hardened tree sap), or when mammoths died
and were permanently frozen. However, most fossils found are simply
fragments of the original organism.
Key Concept 2: Fossils can be used to interpret past events and
environments.
When fossils are found, the preserved remains and the geology of the
surrounding rock layer are clues to the nature of the past environment. At
Dinosaur Valley, the fossilized tracks provide clues to the habits, such as
travelling in herds or which species became predator or prey, of the
dinosaurs. The soft limestone mud in which the tracks were found, the
fossils of palm trees, and fossil shells provide clues that a shallow sea
once covered the area. Marine fossils, such as plankton, that are found on
the top of a mountain are clues that the limestone rock layers were once
a beach, shallow sea, or reef and were later uplifted. When ferns and
amphibian fossils are found in layers of shale or coal, those are clues that
the area was once a swamp or lakebed. Scientists have learned, from the
oldest fossils in the Earth’s oldest rock layers, that bacteria and marine
creatures without backbones existed first. Fossils of plants, fish,
amphibians, reptiles, birds, and mammals show us that life and
environments on Earth developed and changed very slowly over hundreds
of millions of years. Fossils also show that certain life forms became
extinct, while others flourished.
Key Concept 3: Models can be used to represent the passage of time
and past organisms and environments.
In the lab activity, students will investigate models of laminated paper
plant and animal fossil images found at different depths in surrounding
“rock layers.” This experience models, to a small extent, how
paleontologists excavate, sort, and identify the fossil organism and its
environment. Each fossil image has a background rock layer that is a clue
to the type of environment in which the organism lived, such as an ocean
or a swamp. Students record their findings, determine the order in which
these fossils were deposited, the name of the organism, and the
environment in which they existed. Remember that models are limited
representations of the real world.