Encyclopedia
Fossils are the
mineralized or otherwise preserved remains or traces of animals, plants, and other organisms. The totality of fossils and their placement in
fossiliferous rock formations and
sedimentary layers is known as the fossil record. The study of fossils is called
paleontology.
Fossils usually consist of the disarticulated or partially preserved remains of the organism itself. However, fossils may also consist of the marks left behind by the organism while it was alive, such as the footprint or feces of a
dinosaur or
reptile. These types of fossil are called
trace fossils as opposed to body fossils. Finally, past life leaves some markers that cannot be seen but can be detected in the form of chemical signals; these are known as
chemical fossils or
biomarkers.
Fossil sites with exceptional preservation are known as Lagerstätten. These formations may have resulted from carcass burial in an anoxic environment with minimal bacteria, thus delaying decomposition. Lagerstätten span
geological time from the
Cambrian. Examples are the Cambrian Maotianshan shales and
Burgess Shale, the
Devonian Hunsrück Slates, the Jurassic
Solnhofen limestone, and the Carboniferous
Mazon Creek localities.
The oldest known structured fossils are most likely
stromatolites. Now understood to be formed by the entrapment of sediment by mucous-like sheets of cyanobacteria, the oldest of these formations dates from 3.5 billion years ago. Even older deposits of heavy
carbon that are indicative of even earlier life are currently proposed as the remains of the earliest known life on
Earth.
Developments in interpretation of the fossil record
Ever since recorded history began, and probably before, people have found fossils, pieces of rock and
minerals which have replaced the remains of biologic organisms or preserved their external form. These fossils, and the totality of their occurrence within the sequence of Earth's rock
strata is referred to as the
fossil record.
The fossil record was one of the early sources of data relevant to the study of
evolution and continues to be relevant to the
history of life on Earth.
Paleontologists examine the fossil record in order to understand the process of evolution and the way particular species have evolved.
Various explanations have been put forth throughout history to explain what fossils are and how they came to be where they were found. Many of these explanations relied on folktales or mythologies. In China the fossil bones of ancient mammals including
Homo erectus is an extinct species of genus
Homo [i]. ...
were often mistaken for “dragon bones” and used as medicine and aphrodisiacs. In the West the presence of fossilized sea creatures high up on mountainsides was proof of the biblical deluge. During the Renaissance more scientific views of fossils began to emerge.
Leonardo Da Vinci noticed some discrepancies with the biblical account:
"If the Deluge had carried the shells for distances of three and four hundred miles from the sea it would have carried them mixed with various other natural objects all heaped up together; but even at such distances from the sea we see the oysters all together and also the shellfish and the cuttlefish and all the other shells which congregate together, found all together dead; and the solitary shells are found apart from one another as we see them every day on the sea-shores.
And we find oysters together in very large families, among which some may be seen with their shells still joined together, indicating that they were left there by the sea and that they were still living when the strait of Gibraltar was cut through. In the mountains of Parma and Piacenza multitudes of shells and corals with holes may be seen still sticking to the rocks..."
William Smith , an English canal engineer, observed that rocks of different ages preserved different assemblages of fossils, and that these assemblages succeeded one another in a regular and determinable order. He observed that rocks from distant locations could be correlated based on the fossils they contained. He termed this the principle of faunal succession.
Smith, who preceded
Charles Darwin, was unaware of biological evolution and did not know why faunal succession occurred. Biological evolution explains why faunal succession exists: as different organisms evolve, change and go extinct, they leave behind fossils. Faunal succession was one of the chief pieces of evidence cited by Darwin that biological evolution had occurred.
Early
naturalists well understood the similarities and differences of living species leading
Linnaeus to develop a hierarchical classification system still in use today. It was Darwin and his contemporaries who first linked the hierarchical structure of the great tree of life in living organisms with the then very sparse fossil record. Darwin eloquently described a process of descent with modification, or evolution, whereby organisms either adapt to natural and changing environmental pressures, or they perish.
When Charles Darwin wrote
On the Origin of Species by English [i] naturalist [i] ...
, the oldest animal fossils were those from the
Cambrian Period, now known to be about 540 million years old. The absence of older fossils worried Darwin about the implications for the validity of his theories, but he expressed hope that such fossils would be found, noting that: "only a small portion of the world is known with accuracy." Darwin also pondered the sudden appearance of many groups in the oldest known Cambrian fossiliferous strata.
Since Darwin's time, the fossil record has been pushed back to 3.5 billion years before the present. Most of these fossils are microscopic bacteria or microfossils. However, macroscopic fossils are now known from the late Proterozoic. The
Ediacaran biota dating from 575 million years ago collectively constitutes a richly diverse assembly of early multicellular
eukaryotes.
The fossil record and faunal succession form the basis of the science of biostratigraphy or determining the age of rocks based on the fossils they contain. For the first 150 years of geology, biostratigraphy and superposition were the only means for determining the relative age of rocks. The
geologic time scale was developed based on the relative ages of rock strata as determined by the early paleontologists and stratigraphers.
Since the early years of the twentieth century, absolute dating methods, such as radiometric dating have been used to verify the relative ages obtained by fossils and to provide absolute ages for many fossils. Radiometric dating has shown that the earliest known fossils are over 3.5 billion years old. Various dating methods have been used and are used today depending on local geology and context, and while there is some variance in the results from these dating methods, nearly all of them provide evidence for a very old Earth, approximately 4.6 billion years.
Application of
synchrotron X-ray tomographic techniques to early Cambrian bilaterian
embryonic microfossils has recently yielded new insights of
metazoan evolution at its earliest stages. The tomography technique provides previously unattainable three-dimensional resolution at the limits of fossilization. Fossils of two enigmatic bilaterians, the worm-like
Markuelia and a putative, primitive
protostome,
Pseudooides, provide a peek at
germ layer embryonic development. These 543 Ma old embryos support the emergence of some aspects of
arthropod development earlier than previously thought in the late Proterozoic. The preserved embryos from
China and
Siberia underwent rapid diagenetic phosphatization resulting in exquisite preservation, including cell structures. This research is a notable example of how knowledge encoded by the fossil record continues to contribute otherwise unattainable information on the emergence and development of life on Earth. For example, the research suggests
Markuelia has closest affinity to priapulid worms, and is adjacent to the evolutionary branching of Priapulida,
Nematoda and
Arthropoda.
Even with the wealth of information now known about fossils, some groups maintain non-scientific beliefs based on the earlier views of the fossil record.
Rarity of fossils
Fossilization is actually a rare occurrence because most components of formerly-living things tend to decompose relatively quickly following death. In order for an organism to be fossilized, the remains normally need to be covered by
sediment as soon as possible. However there are exceptions to this, such as if an organism becomes frozen, desiccated, or comes to rest in an anoxic environment such as at the bottom of a lake. There are several different types of fossils and fossilization processes.
Due to the combined effect of taphonomic processes and simple mathematical chance, fossilization tends to favor organisms with hard body parts, those that were widespread, and those that lived for a long time. On the other hand, it is very unusual to find fossils of small, soft bodied, geographically restricted and geologically ephemeral organisms, because of their relative rarity and low likelihood of preservation.
Larger specimens are more often observed, dug up and displayed, although microscopic remains are actually far more common in the fossil record.
Some casual observers have been perplexed by the rarity of
transitional species within the fossil record. The conventional explanation for this rarity was given by
Darwin, who stated that "the extreme imperfection of the geological record," combined with the short duration and narrow geographical range of transitional species, made it unlikely that many such fossils would be found. Simply put, the conditions under which fossilization takes place are quite rare; and it is highly unlikely that any given organism will leave behind a fossil. Niles Eldredge and
Stephen J. Gould developed their theory of punctuated equilibrium in part to explain the pattern of stasis and sudden appearance in the fossil record.
Permineralization
Permineralization occurs after burial, as the empty spaces within an organism 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.
Replacement and compression fossils
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 a
mould fossil or typolite. If this hole is later filled with other minerals, it is called a
cast fossil and is considered a replacement fossil since the original materials have been completely replaced by new, unrelated ones. In some cases replacement occurs so gradually and at such fine scales that no "hole" in the rock can ever be discerned and microstructural features are preserved despite the total loss of original material.
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.
To sum up, fossilization processes proceed differently for different kinds of tissues and under different kinds of conditions.
Trace fossils
Trace fossils are the remains of trackways, burrows, footprints, eggs and eggshells, nests, droppings and other types of impressions. Fossilized droppings, called coprolites, can give insight into the feeding behavior of animals and can therefore be of great importance.
Microfossils
'Microfossil' is a descriptive term applied to fossilized plants and animals whose size is just at or below the level at which the fossil can be analyzed by the naked eye. A commonly applied cut-off point between "micro" and "macro" fossils is 1 mm, although this is only an approximate guide. Microfossils may either be complete organisms in themselves or component parts of larger animals or plants. Microfossils are of critical importance as a reservoir of
paleoclimate information, and are also commonly used by biostratigraphers to assist in the correlation of rock units.
Resin fossils
Fossil resin is a natural
polymer found in many types of strata throughout the world, even the
Arctic. The oldest fossil resin dates to the Triassic, though most dates to the Tertiary. The excretion of the resin by certain plants is thought to be an evolutionary adaptation for protection from insects and to seal wounds caused by damage elements. Fossil resin often contains other fossils called inclusions that were captured by the sticky resin. These include bacteria, fungi, other plants, and animals. Animal inclusions are usually small invertebrates, predominately
arthropods such as insects and spiders, and only extremely rarely a
vertebrate such as a small lizard. Preservation of inclusions can be exquisite, including small fragments of
DNA.
Pseudofossils
Pseudofossils are visual patterns in rocks that are produced by naturally occurring geologic processes rather than biologic processes. They can easily be mistaken for real fossils. Some pseudofossils, such as
dendrites, are formed by naturally occurring fissures in the rock that get filled up by percolating minerals. Other types of pseudofossils are kidney ore and
moss agates, which look like moss or plant leaves.
Concretions, round or oval-shaped nodules found in some sedimentary strata, were once thought to be
dinosaur eggs, and are often mistaken for fossils as well.
Living fossils
Living fossil is a term used for any living species which closely resembles a species known from fossils, i.e., as if the fossil had "come to life". This can be a species known only from fossils until living representatives were discovered, such as the
coelacanth and the
ginkgo tree, or a single living species with no close relatives, or a small group of closely related species with no other close relatives, such as the
horseshoe crabs or the
nautilus, that are the sole survivors of a once large and widespread group in the fossil record.
See also
The
geological time scale is used by geologist [i]s and other scientists to describe the timing and ...
References
External links
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- - Fossil collecting locations in Europe and the rest of the World
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