1. Memories of Past Life: Fossils and Evolution
Interlude E

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Figure E.1a,c; EARTH5 Figure E.1a,b
Fossils
Fossil shells in 400-Ma sandstone. (Ma means “million-year old.”)
Fossil leaves in 300-Ma shale.
Fossils are the remnants or traces of ancient living organisms.
They are created by burial with rock material and preserved after lithification.
Paleontology is the study of fossils.
Fossil skeleton in 200-Ma sandstone
A drawer of fossil specimens in a museum.
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Figure E.2
Fossilization
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4. Frozen or Dried Fossils
Figure E.3a
Frozen or Dried Fossils
This 1-m long baby mammoth, found in Siberia, died 37,000 years ago.
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5. Fossils Preserved in Tar or Amber
Figure E.3b,c
Fossils Preserved in Tar or Amber
A fossil skeleton of a 2-m-high giant ground sloth from the La Brea Tar Pits in California.
This insect became embedded in amber about 200 million years ago.
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6. Preserved Hard Parts Fossil shells, the “hard parts” of invertebrates.
Figure E.3d
Preserved Hard Parts
This might also be a good time to introduce students to molds and casts.
Fossil shells, the “hard parts” of invertebrates.
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7. Carbonized Impressions
Figure E.3d
Carbonized Impressions
Flattened molds or casts created when soft organisms are pressed between layers of sediment.
Thin film of carbon remains on surface of impression.
The carbonized impressions of fern fronds in a shale.
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Figure E.3f
Permineralization
Minerals precipitate in porous material; wood or bone are the most common.
Petrified wood from Arizona. It is so hard that it remains after the rock that surrounded it has eroded away.
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Figure E.4a; EARTH5 E.5b,c
Trace Fossils
Trace fossils are formed by the action of an organism—footprints, burrows, feeding marks, coprolites.
This would also be a good time to discuss chemical fossils, like biomarkers and isotope ratios.
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Figure E.4b
Fossil Size
Macrofossils: large enough to see with naked eye
Microfossils: can only be seen with a microscope
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11. Extraordinary Fossils
Figure E.6a
Extraordinary Fossils
Fossils that preserve soft parts.
Very few organisms are fossilized.
Favorable conditions for fossilization: rapid burial, low-energy depositional setting, presence of hard body parts, oxygen-poor (anoxic) environment.
In exceptional environments, soft parts and soft-bodied organisms can be preserved.
A 50-million-year-old mammal fossil was chiseled from oil shale near Messel, Germany. It still contains the remains of skin.
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12. Extraordinary Fossils
Figure E.5b
Extraordinary Fossils
Archaeopteryx from the 150-million-year-old Solnhofen Limestone of Germany. The imprints of feathers are clearly visible.
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13. Taxonomy: The Study of How to Classify Organisms
EARTH5 Figure E.9
Taxonomy: The Study of How to Classify Organisms
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14. Fossil Identification
Figure E.6a
Fossil Identification
Fossil identification can be done in the field by looking at the morphology of the fossil.
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Figure E.6b
Cambrian Explosion
Started about 542 Ma.
Lasted about 20 Ma.
Life diversified into orders.
This is a good time to explain to students that although we have many fossils, the fossil record is incomplete because:
There’s a lot of rock paleontologists haven’t searched.
Not all organisms are preserved.
Erosion.
Non-deposition.
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Figure E.7; EARTH5 Figure E.7b
Evolution
Phylogenetic trees show the evolutionary relationships among organisms.
Evolution is the change in a population over a succession of generations due to the transfer of inheritable characteristics.
Occurs due to natural selection.
Rate of evolutionary change is controlled by catastrophism, gradualism, punctuated equilibrium.
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Figure E.8
Extinction
Occurs when the last members of a species die.
Mass extinction: worldwide abrupt decrease in number of fossil genera
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Think–Pair–Share
Think about the geology where you live.
Where is the closest location where fossils are likely to be found?
What sort of fossils?
Why are they preserved there?
What type of preservation would you expect?
The answer will depend on where the school is located. Students should be thinking about the fact that most fossils are found in sedimentary rocks and should also think about depositional environments of sedimentary rock and what that means for the preservation potential of fossils.
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19. Interlude E Photo Credits
1 Stephen Marshak
2 (bottom right): Photo by William L. Jones from the Stones & Bones Collection, (3): Stephen Marshak
4 Sovfoto/UIG via Getty Images
5 (left): Stephen Marshak; (right): Dirk Wiersma/ Science Source
6 Stephen Marshak
7 Kevin Schafer/Corbis
8 Stephen Marshak
9 (all): Stephen Marshak
10 Hannes Grobe, Alfred Wegener Institute
11 Courtesy of Senckenberg, Messel Research Department
12 Humboldt-Universität zu Berlin Museum für Naturkunde Photo by W. Harre
15 Illustration by Karen Carr and Karen Carr Studio, Inc. © Smithsonian Institution
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