BISC 102 Chapter Notes - Chapter 27: Phanerozoic, Radiometric Dating, Petrified Wood
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16 Jan 2017
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BISC 102 Chapter 27 – History of Life
Phylogenies and the History of Life
Tools for Studying History: The Fossil Record:
Fossil record provides direct evidence about what organism that lived in the past looked like,
where they lived, and when they existed.
o A fossil is a piece of physical evidence from an organism that lived in the past.
o The fossil record is the total collection of fossils that have been found throughout the
world.
How do Fossils Form?
Most of the processes that form fossils begin when part or all of an organism is buried in ash, sand, mud
or some other type of sediment.
How fossilization occurs
1. A tree lives in a swampy habitat. The tree drops leaves, pollen, and seeds into the
mud where decomposition is slow.
2. The tree falls. The trunk and branches break up as they rot.
3. Flooding brings in sand and mud, burying the remains of the trees.
4. Over millions of years, the mountains erode and the swamp is filled with sediment.
The habitat dries.
Preservation after Burial: Once burial occurs, several things can happen
If decomposition does not occur, the organic remains an be preserved intact - like the fossil
pollen
If sediments accumulate on top of the material and become cemented into rocks such as
udstoe, or shale, the sediets’ eight a opress the organic material below into a thin
carbonaceous film.
If the remain decomposes after they are buried, the hole that remains can fill with dissolved
minerals and faithfully create a cast of the remains.
If the remains rot extremely slowly, dissolved minerals can
gradually infiltrate the interior of the cells and harden into stone, forming a permineralized
fossil, such as petrified wood.
Fossilization is a Rare Event:
In most habitats, decompositions rapid and burial is slow. In reality, fossilization is an extremely
rare event.
Limitations of the Fossil Record
Habitat Bias:
Organisms that live in areas where sediments are actively being deposited – including beaches
mudflats, and swamps – are much more likely to form fossils than are organisms that live in
other habitats.
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BISC 102 Chapter 27 – History of Life
Within these habitats, burrowing organisms are already underground at death and are therefore
more likely to fossilize.
Organisms that live above ground in dry forests, grasslands, and deserts are much more likely to
fossilize.
Taxonomic and Tissue Bias:
Slow decay is almost always essential to fossilization, so organism with hard arts such as bone or
shells are most likely to leave fossil evidence. Organisms with hard parts have a much higher
tendency to be preserved than do worms.
A similar bias exists for tissues within organisms. Pollen grains are encased in a tough outer
coast that resists decay, so they fossilize much more readily than do flowers.
Temporal Bias:
Recent fossils are much more common than ancient fossils. This causes a temporal bias in the
fossil record.
Fossil-bearing rocks on land are constantly being broken apart and destroyed by the wind and
water erosion. The older a fossil is, the more likely it is to be demolished.
Abundance Bias: It is weighted towards common species
Organisms that are abundant widespread and present on Earth for long periods of time leave
evidence much more often than do species that are rare, local, or ephemeral.
Paleontologists – Scientists who study fossils.
Life’s Tie Lie
Originally, geologists used distinctive rock formations or fossilized organism to identify the boundaries
between names time intervals. Later researchers were able to use radiometric dating to assign absolute
dates – expressed as years before the present – to events in the fossil record.
Radiometric dating is based on decay rates of certain radioactive isotopes. By dating rocks near fossils,
researchers can also assign an absolute age to many of the species in the fossil record.
Precambrian:
Interval between the formation of Earth about 4.6 billion years ago, and the appearance of most
animal groups about 542 million years ago.
Divided into the Haden, Archaean, and Proterozoic eons.
Important things: Life as elusiel uiellular for ost of Earth’s histor, ad ogen was
virtually absent from the oceans and atmosphere for almost two billion years after the origin of
life.
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