Extinction Essay, Research Paper
Introduction
Earth probably began about 4.5 billion years ago. A history of this life on Earth is recorded in fossil remains and traces of past life. It is a record of evolution for both life and the planet. Life and the planet have co-evolved, with life causing changes to the planet which in turn drive changes in life.
The Fossil Record
A history of these changes is recorded in the layers or ’strata’ of rock laid down over the millions of years, and by changes in the groups of fossils found in these rocks. Scientists learn about extinctions by studying the fossil record and the chemical composition of rocks and chemicals around fossil excavation sites. Looking at the relative abundance of fossil families in a rock layer indicates whether or not plant and animal families in the world at that time were thriving and diversifying or in decline.
A mass extinction will appear in the rock layers as a dead zone (containing only a few fossil remains) between layers with evidence of extensive life above and below it. The dead zone represents the time of a mass extinction and its aftermath.
The study of extinctions is based on the fossil record, which is incomplete and skewed. This is due to the fact that not all fossils can be found or have been preserved over this long period of time. There are probably many organisms that we don’t even know about because we didn’t find any of their remains. Fossils are any preserved remnant or impression left by an organism that lived in the past. Sedimentary rocks are the richest sources of fossils. The fossil record documents macroevolution. Macroevolution is the story of the major events in the history of life that are revealed by the fossil record.
The fossil record is a primary source of information for evaluating the history of life. While the fossil record is unique in that it documents the presence of life for over 80% of the earth’s history, it provides a biased record of past.
There are two forms of dating rocks. As sediments are laid down the rock strata reformed. Layers of rock that are deeper were laid down at an earlier period than were layers closer to the surface. This type of dating is called relative dating. The second way of dating rock is through a process known as absolute dating. Age is given in years instead of relative terms like, before and after, early and late. Radiometric dating is the method most often used to determine the ages of rocks and fossils on a scale of absolute time.
Radiometric dating is based on the fact that living organisms contain certain atomic isotopes in certain ratios. Each radioactive isotope has a fixed rate of decay, known as its half-life. Knowing both the half-life of a radioactive isotope and the ratio of radioactive to stable isotopes in a fossil enables us to tell how old the fossil is. Radioisotope dating cannot be used directly on fossils since they don’t contain the unstable radioactive isotopes used in the dating process. To determine a fossil’s age, igneous layers (volcanic rock) beneath the fossil and above it are dated, resulting in a time range. Thus organisms are dated with respect to volcanic eruptions.
There are a variety of other ways to date rock layers as well. One is to determine the magnetic fields in rocks from different geological eras. Another is the noting of the position of rocks. Sedimentary rock layers (strata) are formed episodically as earth is deposited horizontally over time. Newer layers are formed on top of older layers, pressurizing them into rocks. Generally, deeper rocks and fossils are older than those found above them.
A chemical and mineral analysis of the dead zone reveals much about conditions at the time of the extinction. Scientists search for the existence of altered forms of quartz, the overabundance of rare Earth elements, and soot in a layer. Soot in a layer indicated huge fires. The overabundance or rare elements can be indicative of collision with a chondritic meteor.
Fossils show that extinction rates have varied through time, with four or five episodes of mass extinction interrupting periods when the diversity of organisms increased. The ability to explain periods of mass extinction hinges on interpreting the fossil record to assess the timing of individual extinctions. (See Figure 1 for a diagram representing Earth’s history).
Extinctions
Earth is a complex web of life that is dependent upon the global climate. This consists of a delicate balance of sunlight, air, water, and other factors. A significant disruption of the complex order would result in large scale environmental changes that could wipe out many species.
Paleontologists estimate that at least 99.9% of all the species that ever existed are now extinct. The actual cause of extinction is probably environmental change, either in the living or the non living parts of the environment.
Extinction is the death of all the members of a species. The three major changes that drive a species to extinction are: competition among species, novel predators or parasites, and habitat destruction. Extinctions are the result of biologic and earth processes and are a common occurrence in the geologic past. Extinctions are not necessarily caused by major catastrophes or horrendous climatic changes. Most are caused by small changes in climate or habitat, depleted resources, competition, and other changes that require adaptation and flexibility.
A species becomes extinct when its last representative dies. This has happened quite a lot of times on Earth. There is also something known as mass extinction.
Mass Extinctions
Extinction is, quite literally, the end of a particular evolutionary line, the end of a species, a family, or a larger group of organisms. While it may be bad news for the victims, it’s a ‘natural’ vent in the history of life on Earth. Extinctions, mostly at the level of species, have been occurring constantly at a low ‘background rate’, usually matched by the rate at which new species appear, with the result that biodiversity is constantly increasing.
But there have been periods in the Earth’s history when biodiversity crashes. Indeed this has been a powerful force in evolution, wiping the slate clean of up to 99% of all species, and providing the survivors with a world full of opportunities into which they can diversify. These are mass extinctions, when more than 50% of the Earth’s species vanish in the geological instant of a few million years.
Reasons for mass extinctions are: competition, reduction in food resources, atmospheric changes, climatic changes, sea-level changes, and plate tectonics. To be a mass extinction, the following must occur: extinction occur all over the world, a large number of species go extinct, many types of species go extinct, and the extinctions must be clustered in a short amount of geological time.
Paleontologists have been able to recognize patterns within and between extinction events. The extinction process is as follows: 1) extinction strikes both inland and the sea 2) on the land while animals suffer repeatedly, plants tend to be highly resistant to mass extinctions 3) preferential disappearance of tropical forms of life during mass extinctions 4) tendency of certain groups of animals to experience them repeatedly 5) alleged equal spacing, or periodicity in geological time (occurring about every 26 million years).
Mass extinctions are of widespread interest because current habitat destruction worldwide may be generating a modern mass extinction. Rates of habitat loss are highest in tropical countries, where the diversity of species is also high. Lists of endangered species are growing rapidly in much of the world, and rates of extinction are likely to rise sharply as the human population increases.
After more than two decades of intensive research and debate, perhaps only one thing remains clear about the causes of mass extinction, and that is how little we know for sure. The situation is complicated by the complex relationship between many of the suggested causes, which are often inextricable linked to one another. Climate change, for example, could result in changes in sea level which could, in turn, result in changes in the oxygen content or salinity of the oceans, and all of these factors could, by themselves, result in major mass extinctions.
It is important to remember that, as is so often the case in biological science there may be many causes for an observed effect, and that it is highly unlikely that there is only one reason for each of the mass extinctions There are undoubtedly many exciting discoveries which remain to be made about the causes of mass extinction.
There have been a total of five major mass extinction events through geologic time: the late Ordovician period (about 438 million years ago) in which 100 families became extinct and more than half of the bryozoan and brachiopods species became extinct, the late Devonian (about 360 million years ago) when 30% of animal families became extinct, the end of the Permian period (about 245 million year ago) where all trilobites became extinct, 50% of all animal families, 95% of all marine species, and many trees died out. The late Triassic (208 million years ago) was a mass extinction as well. In this extinction 35% of all animal families died out. Most of the early dinosaur families went extinct, and most synapsids dies out (except for mammals). And finally the Cretaceous Tertiary boundary (about 65 million year ago) where about half of all life forms died out, including dinosaurs, pterosaurs, plesiosaurs, mosasaures, ammonites, and many families of fishes, clams, snails, sponges, sea urchins, and many others.
Each mass extinction has had its own cause and result. There have been many theories on the reasons for these mass extinctions. Many of the theories are similar to each other, but each extinction is different in its own way. There have also been a large number of other mass extinctions.
Precambrian and Vendian Extinctions
The Precambrian period lasted form the creation of our planet (approximately 4.5 billion years ago) to 523 million years ago. The Vendian period lasted from the end of the Precambrian period (523 million years ago) to 543 million years ago. Both Precambrian and Vendian periods host to at least one mass extinction each.
The Precambrian era was a period in Earth history before the evolution of hard-boiled and complex organisms. Throughout the extent of both periods, dominant Precambrian and Vendian organisms were soft-bodied, simple, and entirely marine. Diversification of the hard-boiled organisms did not occur until the beginning of the Cambrian, when the first fauna appeared.
Extinctions are proposed to have affected even life’s earliest organisms. About 650 million years ago, seventy percent of the dominant Precambrian flora and fauna perished in the first great extinction. This extinction strongly affected stromatolites and acritarchs, and was also the predetermining factor that encouraged the diversification of the following Vendian fauna. However, this distinct fauna, resembling modern-day soft-bodied organisms such as sea pens, jellyfish, and segmented worms also perished in a second extinction at the close of the Vendian. This event, responsible for the demise of the Vendian organisms, may have been responsible for the ensuing diversification of the Cambrian shelly fauna.
The first extinction of the Precambrian has been correlated with a large glaciation event that occurred about 600 million years ago. This event was of such severity that most microorganisms were completely wiped out.
The Vendian extinction, occurring near the close of the Vendian period, is currently under debate as to whether an extinction event occurred of not. Many paleontologists believe that the Vendian fauna were the progenitors of the Cambrian fauns. However, others believe that the Vendian fauna have no living representatives. Under this latter hypotheses, the Vendian fauna is believed to have undergone an extinction, after which the Cambrian fauna evolved. Until more information can be collected, details on the Vendian extinction event will remain open to debate.
The Cambrian Extinctions
The Cambrian period ranges form 543-510 million years ago. Four major extinctions occurred during the course of the Cambrian.
During the Cambrian period, the world was largely covered by seas, and existing organisms were entirely marine. At the beginning of the period, only small skeletonized sponges and mollusks were present, but by about the middle of the Cambrian, diversification of the shelly fauna occurred. The most important phyla present in Cambrian communities included trilobites, archaeocyathids, brachiopods, mollusks, and echinoderms.
The first extinction occurred at the Early Cambrian epoch boundary. During this event, the oldest group of trilobites, the olnellids, perished as well as the primary reef-building organisms, the archaeocyathids. The remaining three extinctions were irregularly distributed around the Late Cambrian epoch boundary, and as a whole, severely affected trilobites, brachiopods, and conodonts.
The two most accepted current hypothesis for the Cambrian extinction are: glaciation in the early Ordovician, and cooling and depletion of oxygen in marine waters. The advancement of the theory of glaciation as the predetermining agent of the Cambrian extinctions has been developed by James F. Miller of Southwest Missouri State University. Through research undertaken by Miller, evidence of early Ordovician sediment of glacial origin has been uncovered in South America. Miller suggests in his hypothesis that this evidence of continental glaciation at the Cambrian-Ordovician boundary is responsible for a decrease in global climatic conditions. Such a decline in temperature is implied by Miller to destroy Cambrian fauna which are intolerant of cooler conditions, producing a mass extinction of mostly warm water species. He also suggests that a significant continental glaciation would bring large amount of ocean water onto the land in the form of frozen glacial ice. This trapping of ocean water inevitably results in the decrease of sea-level and the withdrawal of shallow seas. Miller implicates that this reduction in sea-level would produce many consequences, perhaps acting as a driving agent for extinction.
The development of a hypothesis invoking the cooling and depletion of water in marine waters as a causative agent for the Cambrian extinctions has been advanced by several geologists, primarily Allison Palmer and Michael Taylor of the U.S. Geological Survey and James Stilt of the University of Missouri. The cooling and oxygen depletion would occur when cool waters from deep zones of the ocean spread up onto the continent, eliminating all organisms not able to tolerate cool conditions. The cooling would also result in stratification on the water column. Thus, species would ultimately perish due to their inability to tolerate dramatic shifts in such limiting factors as temperature and oxygen availability. Further research is required to more fully test the validity of the above outlined Cambrian extinction hypotheses.
The Ordovician Mass Extinction
The Ordovician period reigned form 510-438 million years ago. The Ordovician mass extinction occurred 440-450 million years ago. The Late Ordovician extinction was the second most devastating in Earth history.
The Ordovician period was an era of extensive diversification and expansion of numerous marine classes. Although organisms also present in the Cambrian were numerous in the Ordovician, a variety of new types including cephalopods, corals (including rugose and tabulate forms), bryozoans, crinoids, graptolites, gastropods, and bivalves flourished. Ordovician communities typically displayed a higher ecological complexity than Cambrian communities due to the greater diversity of organisms. However, as in the Cambrian, life in the Ordovicain continued to be restricted to the seas.
The Ordovician extinction occurred at the end of the Ordovician period, about 440-450 million years ago. This extinction, cited as the second most devastating extinction to marine communities in Earth history, caused the disappearance of one third of all brachiopod and bryozoan families, as well as numerous groups of conodonts, trilobites, and graptolites. Much of the reef-building fauna was also decimated. In total, more than one hundred families of marine invertebrates perished in this extinction.
The Ordovician mass extinction has been theorized by paleontologists to be the result of a single event; the glaciation of the continent Gondwana at the end of the period. Evidence for this glaciation event is provided by glacial deposits discovered by geologists in the Saharan Desert. By integrating rock magnetism evidence and the glacial deposit data, paleontologists have proposed a cause for this glaciation. When Gondwana passed over the north pole in the Ordovician, global climatic cooling occurred to such a degree that there was global large-scale continental decrease in temperature resulting in widespread glaciation. This glaciation event also caused a lowering of sea level worldwide as large amounts of water became tied up in ice sheets. A combination of this lowering of sea-level, reducing ecospae on continental shelves, in conjunction with the cooling caused by the glaciation itself are likely driving agents for the Ordovician mass extinction.
The Denvonian Mass Extinction
The Denvonian period ranged form 408-360 million years ago. A major intra-Denvonian extinction occurred at the Frasnian-Famennian boundary.