Extinction was not believed possible. By the late s, however, arguments that fossils are in fact the hardened remnants of past life began to win the day. Devout Christians such as John Ray and William Smith played an important role in describing and understanding the true nature and distribution of fossils. These early Christian geologists saw God as creator of these life forms that lived in distant ages past. Fossilization is an extremely rare event, but fossils are plentiful.
How can this be? The likelihood of fossilization depends greatly on local conditions and the makeup of the organism. After they die, most organisms are eaten or decompose quickly because they are exposed to the air.
In order to become fossilized, organisms must be preserved in low-oxygen conditions oxygen feeds the bacteria that break down dead organisms. The oldest fossils, microbial mats in Greenland , are 3. The majority of fossils come from creatures with hard body parts like teeth and bone, but there are many other kinds of fossils, too.
A mammoth frozen in ice can be preserved with remarkably little degradation for tens of thousands to millions of years. Scientists have found pristine, individual leaves pressed in ancient lakebeds, sometimes with the original pigment intact until exposed to air. Scientists have even discovered soft tissue preserved inside fossilized dinosaur bones! The processes by which these different types of fossils form, called taphonomy , is an exciting area of study within paleontology.
Fossils are most often found in sedimentary rocks, rarely found in igneous rocks those formed from magma , and almost never found in metamorphic rocks those altered by heat and pressure. Sedimentary rock forms when mineral and organic material is laid down and eventually cemented in layers called strata. Sometimes this can occur rapidly, but it typically takes millions of years for appreciable layers to form.
Geologists call this the principle of superposition.
Dating Fossils – How Are Fossils Dated?
Over time, beds of rock may be tilted, folded, broken, or otherwise disturbed. Yet scientists can often determine the likely sequence of events that led to these results. The fossils in each layer of rock are thus a sampling of the kinds of organisms that lived in those environments when the layers formed. Fossil distribution is strikingly ordered in several ways. That is, some kinds of creatures were found with each other and not with others. Trilobites are found in lower, older layers; giant insects and ferns higher up in younger layers; dinosaurs higher still in even younger layers; and so on.
The order of fossil groups in different layers is highly consistent from place to place—even across continents. This pattern of one group of fossils seemingly being replaced by another is so consistent that it became known as the principle of faunal succession. While both ideas were surprising in the s, they are widely accepted today, even among those who contest the theory of evolution.
- GEOL Lecture 4: Deep Time: Dating the fossil record?
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If this were the case, we would expect fossils of all types to be mixed together. But never do we find trilobites and flowering plants in the same layer, nor a single dinosaur in the Grand Canyon though they are found, as expected by the age of the rocks, in higher layers in the adjacent Grand Staircase formation. The fossil record clearly testifies along with other geological evidence that there has never been a catastrophic, global flood event.
Yet that does not mean there is no historical basis for the Flood story. The biblical account may be called theological history: Many Bible scholars and scientists who are committed to the inerrancy of Scripture reject Flood geology for both biblical and scientific reasons. Fossils also display order in other ways. Not only are certain layers of rock identifiable the world over by the fossils they contain, but moving up a column of rock, one can see a trend toward larger organisms within lineages. The size trend only works within lineages, not between them: Furthermore, small organisms are present in every layer, but no large animals are found at the bottom.
These trends are difficult to square with Young-Earth Flood geology but are exactly what one would expect if rock layers took millions of years to form and if life forms are related by common ancestry. In one sense they are right: An analogy helps to show why: If you picked a grave at random from a family cemetery, it is unlikely that you would find the burial place of your great-grandfather your direct ancestor , but very likely that you would find one of a distant cousin your family, but not your direct ancestor.
True transitional forms are thus expected to be extremely rare.
Fossil Record ( Read ) | Biology | CK Foundation
Archaeopteryx is a classic example—it represents the transition from non-avian dinosaurs to birds, but is not the direct ancestor of any birds alive today. Scientists have found many, many possible transitional fossils. Sometimes entire sequences of transitional fossils are known. These major transitions include one of many lineages of fish adapting to land, a lineage of early theropod dinosaurs evolving flight, land mammals adapting to a marine habitat, and one lineage of primates into humans.
We now turn to one example: Fossils of land animals, or tetrapods, first appear in rocks that are about million years old. In older rocks, only sea creatures are found.
But in , scientists found a fossilized fin , million years old, with eight digits similar to the five fingers humans have on their hands. However, the fin was undoubtedly that of a fish, which means this fossil is strong evidence of a transitional form. One of the great success stories in the examination of the fossil record was the finding of a near-perfect fossilized transition between a vertebrate adapted for water and one adapted for land.
Evolutionary biologist Neil Shubin set out to find a more complete transitional specimen than the fin. These skeptics do not provide scientific evidence for their views. Current understanding of the history of life is probably close to the truth because it is based on repeated and careful testing and consideration of data. The rejection of the validity of fossils and of dating by religious fundamentalists creates a problem for them:. Fossil sequences were recognized and established in their broad outlines long before Charles Darwin had even thought of evolution.
Early geologists, in the s and s, noticed how fossils seemed to occur in sequences: The first work was done in England and France.
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Then, geologists began to build up the stratigraphic column, the familiar listing of divisions of geological time — Jurassic, Cretaceous, Tertiary, and so on. Each time unit was characterized by particular fossils.fgc-engineering.com/3475.php
Accuracy of Fossils and Dating Methods
The scheme worked all round the world, without fail. From the s onwards, geologists noted how fossils became more complex through time. The oldest rocks contained no fossils, then came simple sea creatures, then more complex ones like fishes, then came life on land, then reptiles, then mammals, and finally humans.
Since , paleontologists, or fossil experts, have searched the world for fossils. In the past years they have not found any fossils that Darwin would not have expected. Darwin and his contemporaries could never have imagined the improvements in resolution of stratigraphy that have come since , nor guessed what fossils were to be found in the southern continents, nor predicted the huge increase in the number of amateur and professional paleontologists worldwide.
All these labors have not led to a single unexpected finding such as a human fossil from the time of the dinosaurs, or a Jurassic dinosaur in the same rocks as Silurian trilobites. Paleontologists now apply sophisticated mathematical techniques to assess the relative quality of particular fossil successions, as well as the entire fossil record. These demonstrate that, of course, we do not know everything and clearly never will , but we know enough.
Today, innovative techniques provide further confirmation and understanding of the history of life. Biologists actually have at their disposal several independent ways of looking at the history of life - not only from the order of fossils in the rocks, but also through phylogenetic trees. Phylogenetic trees are the family trees of particular groups of plants or animals, showing how all the species relate to each other. Phylogenetic trees are drawn up mathematically, using lists of morphological external form or molecular gene sequence characters.
Modern phylogenetic trees have no input from stratigraphy, so they can be used in a broad way to make comparisons between tree shape and stratigraphy. The majority of test cases show good agreement, so the fossil record tells the same story as the molecules enclosed in living organisms. Dating in geology may be relative or absolute. Relative dating is done by observing fossils, as described above, and recording which fossil is younger, which is older.
The discovery of means for absolute dating in the early s was a huge advance. The methods are all based on radioactive decay:. The first radiometric dates, generated about , showed that the Earth was hundreds of millions, or billions, of years old. Since then, geologists have made many tens of thousands of radiometric age determinations, and they have refined the earlier estimates.