What is the difference between fossil remains and fossil imprints

In cases where the original shell or bone is dissolved away , it may leave behind a space in the shape of the original material called a mould. At some point in the future, sediments may fill the space to form a matching cast. Soft-bodied sea creatures such as snails are commonly found as moulds and casts because their shells dissolve easily. A cast is a positive impression of the original material formed by contact with the mould. More about gastropods. This image is a mould of an ancient snail or slug called Bellerophon , a gastropod.

Fossils can form when mould of the interior of the shell is made by water-borne minerals percolating through it, but later the shell material dissolves away. The rarest form of fossilisation is the preservation of original skeletons and soft body parts.

Insects that have been trapped and preserved perfectly in amber fossilised tree resin are examples of preserved remains. A fly and spider trapped in amber. Many different animals have been fossilised in amber, from flies and mosquitoes to spiders and snails, and, in very rare instances, ostracods. Discovering Geology introduces a range of geoscience topics to school-age students and learners of all ages.

Often at this point only the bones and teeth remain. Many more layers of sediment build up on top. This puts a lot of weight and pressure onto the layers below, squashing them. Eventually, they turn into sedimentary rock. While this is happening, water seeps into the bones and teeth, turning them to stone as it leaves behind minerals. David adds, 'The water leaves mineral crystals behind in spaces in the bones. This is why dinosaur fossils often have a sponge- or honeycomb-like texture: the internal bone structure has been preserved.

Tree fossils, also known as petrified wood, form in the same way. This is why it's possible to count the growth rings of some fossil trees. Sometimes ground water dissolves the buried bone or shell, leaving behind a bone- or shell-shaped hole or imprint in the sediment. This is a natural mould. If water rich in minerals fills this space, crystals can form and create a fossil in the shape of the original bone or shell, known as a cast fossil.

Or sediment can fill the mould and form a cast fossil. These are the most common ways that marine animals with shells fossilise. This includes ammonites that went extinct at the same time as dinosaurs, as well as shellfish that are more like the limpets, oysters and mussels we can still find living on the beach today.

Trace fossils such as footprints form in a similar way. The footprint forms a natural mould and sediment then fills it forming a cast. Fossilised bivalve preserved as an internal and an external mould. The shell itself has dissolved away. How do we find fossils when they've been buried under millions of years' worth of rock?

It's down to a combination of uplift, weathering and erosion plus luck. Earth's surface is broken up into huge, irregularly shaped pieces - tectonic plates - that fit together like a jigsaw. These plates drift around very slowly, driven by heat from within Earth.

In certain parts of the world, these plates will collide. This can force areas of rock together and push them upwards. In the most dramatic instances, such uplift can form mountain ranges. This is why fossils of marine animals can be found at the top of Mount Everest. In places that were once covered by huge, heavy ice sheets that have now melted, rocks also undergo uplift. An ammonite fossil collected from more than 5, metres above sea-level in the Himalayas in Asia. Uplift is only part of the story.

Carbon dating uses the decay of carbon to estimate the age of organic materials, such as wood and leather. Fossils provide evidence that organisms from the past are not the same as those found today, and demonstrate a progression of evolution. Scientists date and categorize fossils to determine when the organisms lived relative to each other.

The resulting fossil record tells the story of the past and shows the evolution of forms over millions of years. Highly detailed fossil records have been recovered for sequences in the evolution of modern horses. The fossil record of horses in North America is especially rich and contains transition fossils: fossils that show intermediate stages between earlier and later forms. The fossil record extends back to a dog-like ancestor some 55 million years ago, which gave rise to the first horse-like species 55 to 42 million years ago in the genus Eohippus.

The first equid fossil was found in the gypsum quarries in Montmartre, Paris in the s. The tooth was sent to the Paris Conservatory, where Georges Cuvier identified it as a browsing equine related to the tapir.

His sketch of the entire animal matched later skeletons found at the site. During the H. Beagle survey expedition, Charles Darwin had remarkable success with fossil hunting in Patagonia. The original sequence of species believed to have evolved into the horse was based on fossils discovered in North America in the s by paleontologist Othniel Charles Marsh.

The sequence, from Eohippus to the modern horse Equus , was popularized by Thomas Huxley and became one of the most widely known examples of a clear evolutionary progression. The species depicted are only four from a very diverse lineage that contains many branches, dead ends, and adaptive radiations.

One of the trends, depicted here, is the evolutionary tracking of a drying climate and increase in prairie versus forest habitat reflected in forms that are more adapted to grazing and predator escape through running. Since then, as the number of equid fossils has increased, the actual evolutionary progression from Eohippus to Equus has been discovered to be much more complex and multibranched than was initially supposed.

Detailed fossil information on the rate and distribution of new equid species has also revealed that the progression between species was not as smooth and consistent as was once believed.

Although some transitions were indeed gradual progressions, a number of others were relatively abrupt in geologic time, taking place over only a few million years. The series of fossils tracks the change in anatomy resulting from a gradual drying trend that changed the landscape from a forested habitat to a prairie habitat. Early horse ancestors were originally specialized for tropical forests, while modern horses are now adapted to life on drier land.

Successive fossils show the evolution of teeth shapes and foot and leg anatomy to a grazing habit with adaptations for escaping predators. The horse belongs to the order Perissodactyla odd-toed ungulates , the members of which all share hoofed feet and an odd number of toes on each foot, as well as mobile upper lips and a similar tooth structure. This means that horses share a common ancestry with tapirs and rhinoceroses.

Later species showed gains in size, such as those of Hipparion , which existed from about 23 to 2 million years ago. The fossil record shows several adaptive radiations in the horse lineage, which is now much reduced to only one genus, Equus , with several species.

Homology is the relationship between structures or DNA derived from the most recent common ancestor. A common example of homologous structures in evolutionary biology are the wings of bats and the arms of primates. Although these two structures do not look similar or have the same function, genetically, they come from the same structure of the last common ancestor.

Homologous traits of organisms are therefore explained by descent from a common ancestor. If we go all the way back to the beginning of life, all structures are homologous!

Homology in the forelimbs of vertebrates : The principle of homology illustrated by the adaptive radiation of the forelimb of mammals. All conform to the basic pentadactyl pattern but are modified for different usages. The third metacarpal is shaded throughout; the shoulder is crossed-hatched. In genetics, homology is measured by comparing protein or DNA sequences.

Homologous gene sequences share a high similarity, supporting the hypothesis that they share a common ancestor. Homology can also be partial: new structures can evolve through the combination of developmental pathways or parts of them. As a result, hybrid or mosaic structures can evolve that exhibit partial homologies. For example, certain compound leaves of flowering plants are partially homologous both to leaves and shoots because they combine some traits of leaves and some of shoots.

Homologous sequences are considered paralogous if they were separated by a gene duplication event; if a gene in an organism is duplicated to occupy two different positions in the same genome, then the two copies are paralogous. A set of sequences that are paralogous are called paralogs of each other.

Paralogs typically have the same or similar function, but sometimes do not. It is considered that due to lack of the original selective pressure upon one copy of the duplicated gene, this copy is free to mutate and acquire new functions. Homology vs. This is because they are similar characteristically and even functionally, but evolved from different ancestral roots.

Paralogous genes often belong to the same species, but not always. For example, the hemoglobin gene of humans and the myoglobin gene of chimpanzees are considered paralogs. This is a common problem in bioinformatics; when genomes of different species have been sequenced and homologous genes have been found, one can not immediately conclude that these genes have the same or similar function, as they could be paralogs whose function has diverged.

The opposite of homologous structures are analogous structures, which are physically similar structures between two taxa that evolved separately rather than being present in the last common ancestor.

Bat wings and bird wings evolved independently and are considered analogous structures. Genetically, a bat wing and a bird wing have very little in common; the last common ancestor of bats and birds did not have wings like either bats or birds.

Wings evolved independently in each lineage after diverging from ancestors with forelimbs that were not used as wings terrestrial mammals and theropod dinosaurs, respectively.

It is important to distinguish between different hierarchical levels of homology in order to make informative biological comparisons. In the above example, the bird and bat wings are analogous as wings, but homologous as forelimbs because the organ served as a forearm not a wing in the last common ancestor of tetrapods.

Analogy is different than homology. Although analogous characteristics are superficially similar, they are not homologous because they are phylogenetically independent. Analogy is commonly also referred to as homoplasy. Convergent evolution occurs in different species that have evolved similar traits independently of each other. Sometimes, similar phenotypes evolve independently in distantly related species.

For example, flight has evolved in both bats and insects, and they both have wings, which are adaptations to flight. However, the wings of bats and insects have evolved from very different original structures. This phenomenon is called convergent evolution, where similar traits evolve independently in species that do not share a recent common ancestry.

Convergent evolution describes the independent evolution of similar features in species of different lineages. The two species came to the same function, flying, but did so separately from each other. Both sharks and dolphins have similar body forms, yet are only distantly related: sharks are fish and dolphins are mammals.

Such similarities are a result of both populations being exposed to the same selective pressures. Within both groups, changes that aid swimming have been favored. Thus, over time, they developed similar appearances morphology , even though they are not closely related.

One of the most well-known examples of convergent evolution is the camera eye of cephalopods e. Their last common ancestor had at most a very simple photoreceptive spot, but a range of processes led to the progressive refinement of this structure to the advanced camera eye.

Eye evolution : Vertebrates and octopi developed the camera eye independently. In the vertebrate version the nerve fibers pass in front of the retina, and there is a blind spot 4 where the nerves pass through the retina.

This means that octopi do not have a blind spot. These fossils, called index fossils , are widespread but only existed for a relatively brief period of time.

When a particular index fossil is found, the relative age of the bed is immediately known. Many fossils may qualify as index fossils. Ammonites, trilobites, and graptolites are often used as index fossils, as are various microfossils , or fossils of microscopic organisms. Fossils of animals that drifted in the upper layers of the ocean are particularly useful as index fossils, as they may be distributed all over the world.

In contrast to index fossils, living fossils are organisms that have existed for a tremendously long period of time without changing very much at all. For example, the Lingulata brachiopods have existed from the Cambrian period to the present, a time span of over million years! Modern specimens of Lingulata are almost indistinguishable from their fossil counterparts Figure Fossils are our best form of evidence about the history of life on Earth.

In addition, fossils can give us clues about past climates, the motions of plates, and other major geological events. The first clue that fossils can give is whether an environment was marine underwater or terrestrial on land. Along with the rock characteristics, fossils can indicate whether the water was shallow or deep, and whether the rate of sedimentation was slow or rapid.

The amount of wear and fragmentation of a fossil can allow scientists to estimate the amount of wave action or the frequency of storms. Often fossils of marine organisms are found on or near tall mountains. For example, the Himalayas, the tallest mountains in the world, contain trilobites, brachiopods, and other marine fossils. This indicates that rocks on the seabed have been uplifted to form huge mountains. In the case of the Himalayas, this happened when the Indian Subcontinent began to ram into Asia about 40 million years ago.

Fossils can also reveal clues about past climate. For example, fossils of plants and coal beds have been found in Antarctica. Although Antarctica is frozen today, in the past it must have been much warmer.

One of the most fascinating patterns revealed by the fossil record is a number of mass extinctions , times when many species died off. Although the mass extinction that killed the dinosaurs is most famous, the largest mass extinction in Earth history occurred at the end of the Permian period, about million years ago.

The cause of these mass extinctions is not definitely known, but most scientists believe that collisions with comets or asteroids were the cause of at least a few of these disasters. Skip to main content. Geologic History.