The Fossil Record
 Just what is the fossil record and why do we study it?
 The fossil record is the history of life on earth. Fossils can
tell us about the environment present when the organisms
were alive. They mark sometimes rather specific periods
of geologic time. They can tell us about changes in
climate, tectonic activity and patterns in evolution.
 To start we need to have a basic understanding of
g e o l o g i c ti m e
Geologic Time Scale
Quaternary
Cenozoic
Mesozoic
Paleozoic
Tertiary
Pleistocene
Pliocene
Miocene
Oligocene
Eocene
Paleocene
Cretaceous
Jurassic
Triassic
Permian
Carboniferous Pens ylvanian
Mississippian
Devonian
Silurian
Ordovician
Cambrian
PreCambrian
Proterozoic
Archaen
Vendian/Ediacaran
Geologic Time Scale
Quaternary
Cenozoic
Mesozoic
Paleozoic
PreCambrian
Pleistocene
Pliocene
Miocene
Tertiary
Oligocene
Eocene
Paleocene
Cretaceous – “creta” meaning chalk
Jurassic – From the Jura Mtns
Triassic – 3 obvious divisions in Germany
Permian – From the Perm Mtns in Russia
Carboniferous Pens ylvanian – Coal bearing
Mississippian
Devonian – For Devon
Silurian - Named for old welsh tribe
Ordovician – Named for old welsh tribe
Cambrian - Latin “cambria” for W ales
Vendian/Ediacaran – named for a fossil fauna
 Zones – defined by fossil assemblages and named for one
of the primary members
 These are index fossils
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Distinctive (easily identified)
Geographically widespread
Plentiful
Facies independent
Rapidly evolving
Short lived (geologically)
 Common index fossils:
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Cambrian – Trilobites
Ordovician/Silurian – Graptolites
Devonian through Cretaceous – Ammonites
Messozoic to recent - Microfauna
Locally others are well known and used such as Echinoderms,
specifically crinoids in the Silurian – Pennsylvanian of North
America
Samples of index fossils:
Pentrimites sp. A blastoid from the Permian
 Crinoid with
arms and column
 Crinoid calyx showing
Good plate detail
 Other fossil organisms while widespread extend throughout too
much of geologic time to serve as good indexes
 Brachiopods
 Gastropod
 Bivalves - oyster
What is a Fossil?
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Recognizable evidence of past life
Generally only hard parts are preserved
The faster the burial, the better the chance for preservation
High energy environments yield fewer fossils
Trace fossils are indirect evidence of life
 Throughout all the early study of fossils four questions
were asked:
 Are fossils really organic remains?
 How did they get into the rocks?
 When did they get there – while rocks were formed or sometime
after?
 How did they become petrified?
“Being a paleontologist is like being a coroner except that all
the witnesses are dead and all the evidence has been left
out in the rain for 65 million years. “
Mike Bret-Surman, 1994
 Taphonomy (ta·fon’·omy) The study of how living
organisms become fossils.
 This involves:
 Studying the processes that may have biased the sample. Or, in
other words….reconstructing what has happened to the
organism from the moment it died until now.
Step one - what type of fossilization has happened?
Types of fossilization
 Unaltered remains
 The remaining material is unaltered from the original state.
Examples would be Mammoths flash frozen so that their flesh
was essentially freeze dried and still edible 30,000 years later;
bones from animals that were entrapped in the La Brea tar pits
over 40,000 years ago; original shell material from Cretaceous
molluscs and ammonites preserved in moist clays. Insects
preserved in amber.
 Permineralization
 CaCO3 or Silica dissolved in ground water precipitates out in the
pore spaces in the buried “hard parts” after the soft material
decays. None of the original material is removed and in some
cases the preservation is so detailed that cell structure can be
studied.
 Recrystallization
 Some shells are made of aragonite. When the organism dies,
the relatively unstable aragonite changes to the more stable
calcite. The original shape of the fossil is preserved, but the
larger calcite crystals distort the finer detail.
 Dissolution and Replacement
 When buried material dissolves and leaves a cavity in the
sediment, that cavity can be filled with other sediment or mineral
crystals. The cavity is the mold and the filling is the steinkern or
cast. If the original material is replaced without leaving a void,
the fossil is present, but made of non-original material – silica or
pyrite perhaps.
 Carbonization
 The volatile organic materials disperse when the organism dies
and a carbon film remains. This black residue preserves the
outline and sometimes the internal detail of the organism.
Plants, fish and graptolites may be examples of this type of
preservation
Will it become a fossil?
 What determines if an organism will become fossilized?
 Probably “pure, dumb luck”
 Biological agents – predators, bacteria, algae, vegetation all
work to break down the remains…. Rapid Burial is a must!
 Mechanical agents – wind, wave action, currents (both air and
water), the energy of the environment…the most durable parts
will survive the longest before burial
 Diagenesis – action of groundwater can remove shell and bone
before they have a chance to be preserved. Original
composition and groundwater chemistry are critical factors.
 Discovery – the fossils have to be in a place where they will be
available to the paleontologist for study
 Considering all the factors, the odds of an organism
becoming a fossil and then being discovered are extremely
s m al l .
 Of the nine phyla of marine invertebrates that fossilize well,
it is estimated that 85-97% of all the species that ever lived
have never been fossilized.
 Much easier to study the higher taxonomic levels than the
lower ones…..
Lagerstätten
 Extraordinary fossil localities around the world. The ones
any paleontologist would give anything to find and publish.
 Burgess Shale – Middle Cambrian, British Columbia
 Solnhofen Limestone – Upper Jurassic, Southern Germany
 Mazon Creek – Pennsylvanian, NE Illinois