Geologic Time & Fossils
(Interlude D, & Chapter 12)
• People originally thought that fossils Early Fossil Sketches by Nicolaus Steno (1638–1686)
were placed in rocks by supernatural
beings.
• Later (~450 B.C.) a Greek historian
suggested that fossils were evidence of
past life.
• Soon after (~300 B.C.) Aristotle
abandoned this idea and suggested that
life-like shapes grew within the rocks,
but were inorganic.
• During the Renaissance, the idea that
fossils represent remnants of past life
was once again believed.
• Nicholaus Steno started the modern
field of paleontology by suggesting that
fossil containing rocks started as loose
sediment with fragments of organisms
that later hardened into rock.
, Types of Fossils
• Fossils mainly occur in sedimentary rocks and come in a variety of
fossil types
• (Body) Fossils – Whole bodies or pieces of bodies of past organisms
– E.g. bones, insects preserved in amber/tar, mammoths frozen in ice, etc…
• Trace Fossils – Features left behind that are evidence of past life.
– E.g. footprints, burrows, etc…
Shells in limestone Dinosaur footprint in mudstone Burrows in siltstone
, Fossil Preservation
Not all organisms that die get preserved in rocks. In fact, only a small percentage
do, because it takes special circumstances for a fossil to be preserved.
• Death in an anoxic (oxygen-poor) environment
– Oxygen reacts with organisms and breaks them down, and scavengers may eat the
carcass.
• Rapid Burial
– If an organism dies in an environment that has a high sedimentation rate, it may get
buried before it rots or is scavenged.
• The Presence of Hard Parts
– Organisms with hard parts (bones, shells, etc…) take longer to break down. Soft shelled
organisms (e.g. jellyfish) decay very quickly.
• Lack of Diagenesis or Metamorphism
– Diagenetic and Metamorphic processes (e.g. recrystallization) can destroy fossils
• In a typical shallow marine environment, ~30% of organisms have a high
preservation potential (hard, sturdy shells), ~40% have a low preservation potential
(fragile shells), and ~30% have no sturdy parts and will only be preserved in very
rare circumstances. Of the 30% with a high preservation potential, only a few will
die in environments that will preserve them.
, Extraordinary Fossils
• Although organisms with soft parts are rarely preserved, there are a number of
locations throughout the world that at one time had conditions that were just right
for preserving some delicate life forms.
• La Brea Tar Pits, Los Angeles – mammals that got trapped in tar seeps were
preserved, e.g. saber tooth tigers and mammoths
• Solenhofen Limestone, Germany – 150Ma, many rare and well exposed fossils
including the oldest bird fossil, Archaeopteryx.
• Burgess Shale, British Columbia, Canada – many soft shelled creatures, such as
sponges. Researchers have been able to determine the contents of the stomachs of
ancient creatures here.
Archaeopteryx in the Solenhofen Ls.
A trilobite in the Burgess Shale
(Interlude D, & Chapter 12)
• People originally thought that fossils Early Fossil Sketches by Nicolaus Steno (1638–1686)
were placed in rocks by supernatural
beings.
• Later (~450 B.C.) a Greek historian
suggested that fossils were evidence of
past life.
• Soon after (~300 B.C.) Aristotle
abandoned this idea and suggested that
life-like shapes grew within the rocks,
but were inorganic.
• During the Renaissance, the idea that
fossils represent remnants of past life
was once again believed.
• Nicholaus Steno started the modern
field of paleontology by suggesting that
fossil containing rocks started as loose
sediment with fragments of organisms
that later hardened into rock.
, Types of Fossils
• Fossils mainly occur in sedimentary rocks and come in a variety of
fossil types
• (Body) Fossils – Whole bodies or pieces of bodies of past organisms
– E.g. bones, insects preserved in amber/tar, mammoths frozen in ice, etc…
• Trace Fossils – Features left behind that are evidence of past life.
– E.g. footprints, burrows, etc…
Shells in limestone Dinosaur footprint in mudstone Burrows in siltstone
, Fossil Preservation
Not all organisms that die get preserved in rocks. In fact, only a small percentage
do, because it takes special circumstances for a fossil to be preserved.
• Death in an anoxic (oxygen-poor) environment
– Oxygen reacts with organisms and breaks them down, and scavengers may eat the
carcass.
• Rapid Burial
– If an organism dies in an environment that has a high sedimentation rate, it may get
buried before it rots or is scavenged.
• The Presence of Hard Parts
– Organisms with hard parts (bones, shells, etc…) take longer to break down. Soft shelled
organisms (e.g. jellyfish) decay very quickly.
• Lack of Diagenesis or Metamorphism
– Diagenetic and Metamorphic processes (e.g. recrystallization) can destroy fossils
• In a typical shallow marine environment, ~30% of organisms have a high
preservation potential (hard, sturdy shells), ~40% have a low preservation potential
(fragile shells), and ~30% have no sturdy parts and will only be preserved in very
rare circumstances. Of the 30% with a high preservation potential, only a few will
die in environments that will preserve them.
, Extraordinary Fossils
• Although organisms with soft parts are rarely preserved, there are a number of
locations throughout the world that at one time had conditions that were just right
for preserving some delicate life forms.
• La Brea Tar Pits, Los Angeles – mammals that got trapped in tar seeps were
preserved, e.g. saber tooth tigers and mammoths
• Solenhofen Limestone, Germany – 150Ma, many rare and well exposed fossils
including the oldest bird fossil, Archaeopteryx.
• Burgess Shale, British Columbia, Canada – many soft shelled creatures, such as
sponges. Researchers have been able to determine the contents of the stomachs of
ancient creatures here.
Archaeopteryx in the Solenhofen Ls.
A trilobite in the Burgess Shale
