Pa-le-0n-tol-ogy (Paleontology)
The study of the forms of life existing in prehistoric or geologic times,
as represented by the fossils of plants, animals, and other organisms.
A historical science
Paleontology is one of the historical sciences, along with archaeology, geology, biology, astronomy, cosmology, philology and history itself.[3] This means
that it aims to describe phenomena of the past and reconstruct their causes.[4] Hence it
has three main elements: description of the phenomena; developing a general
theory about the causes of various types of change; and applying those theories
to specific facts.[3]
When trying to explain past phenomena, paleontologists and other historical
scientists often construct a set of hypotheses about the causes and then look
for a "smoking
gun", a piece of evidence which indicates that one of the hypotheses
is a better explanation than the others. Sometimes the "smoking gun" is
discovered by a fortunate accident during other research, for example the
discovery by Luis Alvarez and Walter Alvarez of an iridium-rich layer at the Cretaceous–Tertiary boundary made asteroid impact and volcanism the most favored explanations for the Cretaceous–Tertiary extinction event.[4]
The other main type of science is experimental science, which is often said
to work by conducting experiments to disprove hypotheses about the
workings and causes of natural phenomena – note that this approach cannot prove
a hypothesis is correct, since some later experiment may disprove it. However,
when confronted with totally unexpected phenomena, such as the first evidence
for invisible radiation, experimental scientists often use the same
approach as historical scientists: construct a set of hypotheses about the
causes and then look for a "smoking gun".[
Types of
preservation
Permineralization
A permineralized trilobite,
Asaphus
kowalewskii
Permineralization
occurs after burial, as the empty spaces within an organism (spaces filled with
liquid or gas during life) become filled with mineral-rich groundwater and the
minerals precipitate from the groundwater, thus occupying the empty spaces. This
process can occur in very small spaces, such as within the cell
wall of a plant cell. Small scale permineralization can produce
very detailed fossils. For permineralization to occur, the organism must become
covered by sediment soon after death or soon after the initial decaying process.
The degree to which the remains are decayed when covered determines the later
details of the fossil. Some fossils consist only of skeletal remains or teeth;
other fossils contain traces of skin, feathers or even soft tissues. This is a form of diagenesis.
Recrystallized scleractinian
coral (aragonite to calcite) from the Jurassic
of southern Israel
Casts and molds
In some cases the original remains of the organism have been completely
dissolved or otherwise destroyed. When all that is left is an organism-shaped
hole in the rock, it is called an external mold. If this hole is later
filled with other minerals, it is a cast. An endocast or internal mold is formed when
sediments or minerals fill the internal cavity of an organism, such as the
inside of a bivalve or snail or the hollow of a skull.
Authigenic
mineralisation
This is a special form of cast and mold formation. If the chemistry is right,
the organism (or fragment of organism) can act as a nucleus for the
precipitation of minerals such as siderite, resulting in a nodule forming around
it. If this happens rapidly before significant decay to the organic tissue, very
fine three-dimensional morphological detail can be preserved. Nodules from the
Carboniferous Mazon Creek fossil beds of Illinois, USA, are among
the best documented examples of authigenic mineralisation.
Replacement
and recrystallization
Replacement occurs when the shell, bone or other tissue is replaced
with another mineral. In some cases mineral replacement of the original shell
occurs so gradually and at such fine scales that microstructural features are
preserved despite the total loss of original material. A shell is said to be
recrystallized when the original skeletal compounds are still present but
in a different crystal form, as from aragonite to calcite.
Adpression
(compression-impression) fossils
Compression fossils, such as those of fossil ferns,
are the result of chemical reduction of the complex organic molecules composing
the organism's tissues. In this case the fossil consists of original material,
albeit in a geochemically altered state. This chemical change is an expression
of diagenesis. Often what remains is a carbonaceous film known as a phytoleim, in which case
the fossil is known as a compression. Often, however, the phytoleim is lost and
all that remains is an impression of the organism in the rock—an impression
fossil. In many cases, however, compressions and impressions occur together. For
instance, when the rock is broken open, the phytoleim will often be attached to
one part (compression), whereas the counterpart will just be an impression. For
this reason, it has proved to convenient to have a combined term for both modes
of preservation: adpression.[19]
Bioimmuration
The star-shaped holes (Catellocaula
vallata) in this Upper Ordovician bryozoan represent a soft-bodied organism
preserved by bioimmuration in the bryozoan skeleton.[20]
Bioimmuration is a type of preservation in which a skeletal organism
overgrows or otherwise subsumes another organism, preserving the latter, or an
impression of it, within the skeleton.[21] Usually
it is a sessile skeletal organism, such as a bryozoan or an
oyster, which grows along a substrate, covering other sessile encrusters. Sometimes the bioimmured organism is
soft-bodied and is then preserved in negative relief as a kind of external mold.
There are also cases where an organism settles on top of a living skeletal
organism and grows upwards, preserving the settler in its skeleton.
Bioimmuration is known in the fossil record from the Ordovician[22] to the
Recent.[21]
To sum up, fossilization processes proceed differently for different kinds of
tissues and under different kinds of conditions.
The study of the forms of life existing in prehistoric or geologic times,
as represented by the fossils of plants, animals, and other organisms.
A historical science
Paleontology is one of the historical sciences, along with archaeology, geology, biology, astronomy, cosmology, philology and history itself.[3] This means
that it aims to describe phenomena of the past and reconstruct their causes.[4] Hence it
has three main elements: description of the phenomena; developing a general
theory about the causes of various types of change; and applying those theories
to specific facts.[3]
When trying to explain past phenomena, paleontologists and other historical
scientists often construct a set of hypotheses about the causes and then look
for a "smoking
gun", a piece of evidence which indicates that one of the hypotheses
is a better explanation than the others. Sometimes the "smoking gun" is
discovered by a fortunate accident during other research, for example the
discovery by Luis Alvarez and Walter Alvarez of an iridium-rich layer at the Cretaceous–Tertiary boundary made asteroid impact and volcanism the most favored explanations for the Cretaceous–Tertiary extinction event.[4]
The other main type of science is experimental science, which is often said
to work by conducting experiments to disprove hypotheses about the
workings and causes of natural phenomena – note that this approach cannot prove
a hypothesis is correct, since some later experiment may disprove it. However,
when confronted with totally unexpected phenomena, such as the first evidence
for invisible radiation, experimental scientists often use the same
approach as historical scientists: construct a set of hypotheses about the
causes and then look for a "smoking gun".[
Types of
preservation
Permineralization
A permineralized trilobite,
Asaphus
kowalewskii
Permineralization
occurs after burial, as the empty spaces within an organism (spaces filled with
liquid or gas during life) become filled with mineral-rich groundwater and the
minerals precipitate from the groundwater, thus occupying the empty spaces. This
process can occur in very small spaces, such as within the cell
wall of a plant cell. Small scale permineralization can produce
very detailed fossils. For permineralization to occur, the organism must become
covered by sediment soon after death or soon after the initial decaying process.
The degree to which the remains are decayed when covered determines the later
details of the fossil. Some fossils consist only of skeletal remains or teeth;
other fossils contain traces of skin, feathers or even soft tissues. This is a form of diagenesis.
Recrystallized scleractinian
coral (aragonite to calcite) from the Jurassic
of southern Israel
Casts and molds
In some cases the original remains of the organism have been completely
dissolved or otherwise destroyed. When all that is left is an organism-shaped
hole in the rock, it is called an external mold. If this hole is later
filled with other minerals, it is a cast. An endocast or internal mold is formed when
sediments or minerals fill the internal cavity of an organism, such as the
inside of a bivalve or snail or the hollow of a skull.
Authigenic
mineralisation
This is a special form of cast and mold formation. If the chemistry is right,
the organism (or fragment of organism) can act as a nucleus for the
precipitation of minerals such as siderite, resulting in a nodule forming around
it. If this happens rapidly before significant decay to the organic tissue, very
fine three-dimensional morphological detail can be preserved. Nodules from the
Carboniferous Mazon Creek fossil beds of Illinois, USA, are among
the best documented examples of authigenic mineralisation.
Replacement
and recrystallization
Replacement occurs when the shell, bone or other tissue is replaced
with another mineral. In some cases mineral replacement of the original shell
occurs so gradually and at such fine scales that microstructural features are
preserved despite the total loss of original material. A shell is said to be
recrystallized when the original skeletal compounds are still present but
in a different crystal form, as from aragonite to calcite.
Adpression
(compression-impression) fossils
Compression fossils, such as those of fossil ferns,
are the result of chemical reduction of the complex organic molecules composing
the organism's tissues. In this case the fossil consists of original material,
albeit in a geochemically altered state. This chemical change is an expression
of diagenesis. Often what remains is a carbonaceous film known as a phytoleim, in which case
the fossil is known as a compression. Often, however, the phytoleim is lost and
all that remains is an impression of the organism in the rock—an impression
fossil. In many cases, however, compressions and impressions occur together. For
instance, when the rock is broken open, the phytoleim will often be attached to
one part (compression), whereas the counterpart will just be an impression. For
this reason, it has proved to convenient to have a combined term for both modes
of preservation: adpression.[19]
Bioimmuration
The star-shaped holes (Catellocaula
vallata) in this Upper Ordovician bryozoan represent a soft-bodied organism
preserved by bioimmuration in the bryozoan skeleton.[20]
Bioimmuration is a type of preservation in which a skeletal organism
overgrows or otherwise subsumes another organism, preserving the latter, or an
impression of it, within the skeleton.[21] Usually
it is a sessile skeletal organism, such as a bryozoan or an
oyster, which grows along a substrate, covering other sessile encrusters. Sometimes the bioimmured organism is
soft-bodied and is then preserved in negative relief as a kind of external mold.
There are also cases where an organism settles on top of a living skeletal
organism and grows upwards, preserving the settler in its skeleton.
Bioimmuration is known in the fossil record from the Ordovician[22] to the
Recent.[21]
To sum up, fossilization processes proceed differently for different kinds of
tissues and under different kinds of conditions.
