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 synapsis 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, mosasaur es, 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 skeleton ized 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 eco spae 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.
Following the Ordovician mass extinction re diversification of surviving groups occurred throughout the Silurian and Devonian. In addition, the Devonian saw the first appearance of sharks, bony fish, and ammonoids. During the Devonian the world's oceans were dominated by reef builders such as the stromatoporoids, and corals, and some of the world's largest reef complexes were built. Terrestrial newcomers in the Devonian including amphibians, insects, and the first true land plants, giving rise to the first forests. The Devonian mass extinction occurred during the latter part of the Devonian at the Frasnian-Famennian boundary. The crisis primarily affected the marine community, having little impact on the terrestrial flora.
This same extinction pattern has been recognized in most mass extinction throughout Earth history. The most important group to be affected by this extinction event were the major reef-builders including the stromatoporoids, and the rugose, and tabulate corals. This late Devonian crisis affected these organisms so severely that reef-building was relatively uncommon until the evolution of the scleractinian (modern) corals in the Mesozoic era. Among other marine invertebrates, seventy percent of the taxa did not survive into the Carboniferous.
Amongst the severely affected groups were the brachiopods, trilobites, conodonts, and acritarchs, as well as all jawless fish, and placoderms. Evidence supporting the Devonian mass extinction suggests that warm water marine species were the most severely affected in this extinction event. This evidence has lead many paleontologists to attribute the Devonian extinction to an episode of global cooling, similar to the event which is thought to have caused the late Ordovician mass extinction. According the this theory, the extinction of the Devonian was triggered by another glaciation event on Gondwana, as evidenced by glacial deposits of this age in northern Brazil.
Similarly to the late Ordovician crisis, agents such as global cooling and widespread lowering of sea-level may have triggered the late Denvonian crisis. Meteorite impacts at the Frasnian-Famennian boundary have also been suggested as possible agents for the Devonian mass extinction. Currently, the data surrounding a possible extraterrestrial impact remains inconclusive, and the mechanisms which produced the Devonian mass extinction are still under debate. The Permian Mass Extinction The Permian Period ranged from 286-248 million years ago.
During the Permian period terrestrial faunal diversification occurred. However, when the mass extinction occurred 90-95% of marine species became extinct making the Permian extinction the largest and most tragic extinction. With the formation of the super-continent Pangea in the Permian, continental area exceeded that of oceanic area for the first time in geological history. The result of this new global configuration was the extensive development and diversification of Permian terrestrial vertebrate fauna and accompanying reduction of Permian marine communities. Among terrestrial fauna affected included insects, amphibians, reptiles (which evolved during the Carboniferous), as well as the dominant terrestrial group, the therapsids (mammal-like reptiles).
The terrestrial flora was predominantly composed of gymnosperms, including the conifers. Life in the seas was similar to that found in middle Devonian communities following the late Devonian crisis. Common groups included the brachiopods, ammonoids, gastropods, crinoids, bony fish, sharks, and fusulinid foraminifera. Corals and trilobites were also present, but were exceedingly rare. The Permian mass extinction occurred about 248 million years ago and was the greatest mass extinction ever recorded in Earth history; even larger than the previously discussed Ordovician and Devonian crises and the better known End Cretaceous extinction that felled the dinosaurs. Ninety to ninety-five percent of marine species were eliminated as a result of this Permian event.
The primary marine and terrestrial victims include the fusulinin foraminifera, trilobites, rugose and tabulate corals, blast oids, acanthodians, placoderms, and pelycosaurs, which did not survive beyond the Permian boundary. Other groups that were substantially reduced included the bryozoans, brachiopods, ammonoids, sharks, bony fish, crinoids, eurypterids, ostracods, and echinoderms. Although the cause of the Permian mass extinction remains a debate, numerous theories have bee formulated to explain the events of the extinction. One of the most current theories for the mass extinction of the Permian is an agent that has been also held responsible for the Ordovician and Devonian crises, glaciation on Gondwana.
A similar glaciation event in the Permian would likely produce mass extinction in the same manner as previously, that is, a global widespread cooling and / or worldwide lowering of sea level. Another theory which explains the mass extinction of the Permian is the reduction of shallow continental shelves due to the formation of the super-continent Pangea. Such a reduction in oceanic continental shelves would result in ecological competition for space, perhaps acting as an agent for extinction. However, although this is a liable theory, the formation of Pangea and the ensuing destruction of the continental shelves occurred in the early and middle Permian, and the mass extinction did not occur until the late Permian. A third possible mechanism for the Permian extinction is rapid warming and severe climatic fluctuations produced by concurrent glaciation events on the north and south poles. In temperate zones, there is evidence of significant cooling and drying in the sedimentological record, shown by thick sequences of dune sands and evaporates, while in the polar zones, glaciation was prominent, This caused severe climatic fluctuations around the globe, and is found by sediment record to be representative of when the Permian mass extinction occurred.
The fourth and final suggestion that paleontologists have formulated credits the Permian mass extinction as a result of basaltic lave eruptions in Siberia. These volcanic eruptions were large and sent a quantity of sulfates into the atmosphere. Evidence in China supports that these volcanic eruptions may have been silica-rich, and thus explosive, a factor that would have produced large ash clouds around the world. The combination of sulfates in the atmosphere and the ejection of ash clouds may have lowered global climatic conditions.
The age of the lava flows has also been dated to the interval in which the Permian mass extinction occurred. The Cretaceous Mass Extinction Following the Permian mass extinction, life was abundant but there was a large diversity of species. However, through the Triassic, Jurassic, and Cretaceous, major faunal radiation resulted in a large number of new species and forms. New terrestrial fauna that made their first appearance in the Triassic included the dinosaurs, mammals, pterosaurs (flying reptiles), amphibians (including frogs and turtles).
In addition, the first birds appeared in the Jurassic. Among the terrestrial flora, the gymnosperms of the Permian remained dominant until the evolution of the angiosperms (flowering plants) in the Cretaceous. In the Cretaceous there was also major radiation occurring in several established groups including the marine reptiles, rudest bivalves, ammonoids, beliemnioids, and scleractinian corals, bivalves, and brachiopods. Marine groups that were present but did not undergo major evolutionary expansion in the period included the gastropods, bryozoans, crinoids, sea urchins, and sponges.
About 65 million years ago some event took place that wiped out more than half of all life on Earth. In addition to this mass extinction there was a major face-lift for the Earth as a planet. The continents were shoved to new locations, and inland seas moved on and off the continents, as major ridge expansion took place, particularly in the Atlantic and Indian Oceans. In the process there was major mountain building, volcanic eruptions, and some places sunk beneath the sea, while others rose from the sea. There is evidence of massive destabilization of the oceans and atmosphere. Powerful storms wrecked havoc as indicated by vigorous erosion and greatly increased continental weathering.
Some elements, known as isotopes, show drastic shifts in abundance. Masses of sand, clay, and mud deposited layers everywhere, while other areas show missing sedimentary layers. The oceans show rapid desalination events, temperature transitions, and chemical changes. The latter part of the Cretaceous period was a time of high tectonic activity (continental drift) and accompanying volcanic activity. The super continent Pangea was splitting up and the continents were taking on their modern-day forms. Many mountain ranges were formed.
The sea levels rose during the mid-Cretaceous, covering about one-third of the land area. Toward the end of the Cretaceous, there was a drop in sea level, causing land exposure on all continents, more seasonality, and greater extremes between equatorial and polar temperatures. The earth was getting colder. There have been many extinctions throughout the history of the Earth. Probably the most famous is the extinction that finally saw the end of the dinosaurs reign on the Earth, 65 million years ago. It wasn't just the dinosaurs (See Figure 2 for a dinosaur timeline) that died out in this extinction.
Whatever caused the death of the dinosaurs also caused the death of around 85% of all of the species on the Earth. Although the dinosaurs had been in a period of decline, it is thought that their recovery was prevented by some sort of catastrophic event. There are many theories about why the dinosaurs finally became extinct, some of which are more than a little outlandish. The two most serious theories are the asteroid and volcano theories.
Both of these theories make use of the analysis of the rocks in and around the K-T boundary (the Cretaceous - Tertiary boundary). The use of K comes form the Greek word for chalk (K reta) which is found in great quantities in the rocks of the Cretaceous. (See Figure 3 for a diagram of the thin clay layer that marks the boundary between the Cretaceous and Tertiary rocks). We will examine these theories made by a variety of scientists hypothesizing the reason for the mass extinction of the Cretaceous epoch. Asteroid Impact Theory This widely accepted theory, proposed in 1980 by physicist Luis Alvarez and his son Walter Alvarez, a geologist, is that an asteroid four to nine miles in diameter hit the Earth about 65 million years ago (See Figure 4 for diagram of a representation of the asteroid). This impact would have penetrated the Earth's crust, scattering dust and debris into the atmosphere, and caused huge fires, volcanic activity, tsunamis, severe storms with high winds, and highly acidic rain.
The impact could have caused chemical changes in the Earth's atmosphere, increasing concentrations of sulfuric acid, nitric acid, and fluoride compounds. The heat from the impact's blast wave would have incinerated all the life forms in its path. The dust and debris thrust into the atmosphere would have blocked most of the sunlight for months, and lowered the temperatures globally. There are many impact craters on Earth. A 120-mile-wide, 1-mile-deep impact crater, Chicxulub, is found at the tip of the Yucatan Peninsula, in the Gulf of Mexico (See Figure 5 for a map of the location). This crater dates back to 65 million years ago, and is probably the site of the K-T meteorite impact.
Evidence of K-T period tsunamis all around the Gulf of Mexico has been found. In the clay layer from the K-T boundary, scientists have found chemical evidence that supports the Alvarez impact theory. The K-T layer consist of the sedimentary deposits that occurred from the end of the Cretaceous period to the beginning of the Tertiary period. It is divided into two layers, the Magic Layer, and the Ejecta Layer. The rare Earth Elements, Os, Au, Pt, Ni, Co, Pd, and Iridium, are Siderophile elements. Their abundance in the lower K-T layer is indicative of an asteroid impact.
Iridium (Ir) has been found in the K-T layer around the world. Iridium is rare on Earth, except near the Earth's center. It is relatively abundant in chondritic meteors. A meteoritic origin of this iridium layer seems likely.
These layer became know as the iridium anomaly. In the late 1970's Luis and Walter Alvarez along with a team of scientists form the University of California were making a study of the rocks around the K-T boundary in Gubbio, Italy. In particular they were looking at an unusual layer of clay at the boundary point which contained an unusual spike in the amounts or the rare element iridium. These spike revealed that the levels of iridium contained in the clay were roughly 30 times the normal levels. Tektites are quartz grains which are vaporized under intense heat and pressure, and cool into glass beads with no crystalline structure.
Tektites were probably formed during a meteorite or comet collision. Tektites are abundant in the K-T layer. Kenneth Miller has discovered a two-inch layer of glass beads in the K-T layer near the Bass River in New Jersey, USA, supporting Alvarez' theory. When quartz is put under extremely high pressure, it can cleave in parallel planes. Shocked quartz is found at nuclear bomb sites and known meteorite impact areas. Shocked quartz is abundant in the K-T layer.
Silicon Dioxide, a form of quartz created under conditions of high heat and pressure is used as an indicator of meteor impact. It has been found in abnormally high abundance in the K-T layer. Most likely formed during a massive collision. Although there is much circumstantial, biological, chemical, and geological evidence to support the theory of asteroid-impact, until recently there was no crater of appropriate size and age. Although it was possible that the crater had been destroyed by subduction, the search continued. A graduate student approached the oil company, Pemex, for help in locating the crater.
Pemex identified a circular structure of presumed volcanic origin on the Yucatan peninsula (See Figure 6 for a map of the drilling site). Abundant shocked quartz, tektites, and sedimentary breccias were known in the Caribbean / Gulf of Mexico regions. The structure was a 65 million year-old crater which was 120 miles across and at a depth of 1 mile. The energies of impact of an asteroid that could leave such a crater have been estimated at 6 x 10 MT.
On the land the effects of the impact on the flora and fauna would have been devastating, especially on the large animals which would need large food supplies and on the dinosaurs which would need sun light to keep warm. The global fires would have destroyed considerable amounts of vegetation (by the analysis of the soot in the K-T boundary it is estimated that 25% of the vegetation cover was destroyed), the immediate effect of this would have resulted in the death of large herbivores. A resulting effect of this would have killed off the large carnivores. Only the small active scavengers, like birds and mammals with the ability to find food from a wide range of sources would have survived. Analysis of the K-T boundary fossils shows that there was a short term takeover of the land by the hardy ferns, which moved into the areas where there had been fires. In the sea the effects would have been just as dramatic.
There would have been a decrease in the oxygen levels in the seawater as low oxygen deep seawater would have been brought up by massive under water currents. This would have resulted in a massive disturbance of the marine food chain through the death of much of the plankton. This would have resulted in the eventual death of the marine reptiles which would have relied on the food chain. There would also have been a massive death rate amongst the shelled sea animals like the ammonites.
There could have been a serious increase in the acidity of the seas caused by the acid rains. This may have also killed off some the sea species. As there is much evidence to support this theory, there are also many problems with the impact theory. One is that the site of the proposed impact at Chicxulub, Yucatan, as part of the Caribbean Plate, was undergoing uplift, and plate rotation from the Pacific to the Atlantic during the Cretaceous, which is extremely difficult to reconcile with an impact. Second is that recent coring at the Chicxulub indicates that the structure may be volcanic or a crypto explosive geobleme (a structure caused by an explosion ejection from the Earth). A third problem is that the iridium found is almost global and is found in strata that is not the same date everywhere, when it should be found mostly in the Yucatan region and bare the same date.
The greatest abundance of iridium was found on the Hess Rise in the mid-Pacific, some 10,000 kilometers away from Chicxulub. In Rant on Basin, New Mexico, the iridium deposited during normal polarity of the geomagnetic field, not the reversed polarity of other sites. Many irregularities in iridium occur worldwide. A fourth reason is that some evidence indicated that the shocked quartz did not originate by impact, but may be volcanic or tectonic in origin.
A fifth reason is that some areas, such as at Gubbio, Italy, display a long interval of shocked minerals which is bisected by the boundary. Also at Gubbio, there are five iridium peaks, indicating the need for five impacts, and therefore, five craters with no other impact structure of the right age. Similar extended zones can be found in the Pacific, Atlantic, Denmark, Spain, France, Germany, and New Zealand. A sixth fault with the asteroid impact theory is that in most situations the iridium and other noble metals are associated with organic compounds from dead biomass, which is likely to be the source of the metals. The seventh reason is that the abundances of noble metals is more consistent with earthly compositions than extraterrestrial sources at many sites. Also other metals typical of meteoritic materials are missing in some sites with iridium or the ratios are not typical of impact debris.
Moreover, the shocked quartz at some sites is more consistent with water transport (ocean erosion) rather than atmospheric (as would occur with impact). An eighth point is that other times of impact did not cause such extensive mass extinctions. This is apparent in the Barringer Meteor Crater (See Figure 7 for picture) instance in northern Arizona. A ninth outstanding problem is that an impact is theoretically less likely to initiate widespread tectonic activity, and sea level rise, which occurred at the end of the Cretaceous. A tenth factor is that the climatic shift should have gone from a drastic drop in temperature (with sunlight blocked) to progressively hot temperatures (the Greenhouse Effect).
Not withstanding, the temperature went from warm to progressively colder temperature with only a short period with a temperature rise. The atmosphere never rebounded, but was colder than it had been and would remain colder than any time throughout the Cretaceous. In the Cretaceous there was no glacial cover anywhere, and the Arctic regions were much warmer than today with places like Alaska having temperature similar to San Diego, and the treeline and temperate species were much farther north than at present. After the boundary the climate is much more like that of today, with glaciers developing in the following periods. The eleventh reason is that the mass extinctions of the time do not fit the impact theory for many reasons.
One, the extinction were not instantaneous and were selective. Two, many species were in decline before the time of the proposed impact. Three, if the Yucatan region were the impact site then the greatest mass extinctions should be in North America, Central America, and northeastern South America, but were not (it seems that the greatest dinosaur fossil graveyard is in the Gobi Desert, on the other side of the Earth, and most extinctions were along mid-latitudes, not tropics). Four, the huge dust and water vapor cloud should have caused plant extinctions the most, but it did not, and equatorial species should have been hit the worst, but it was mid-latitude species that were affected the most, and most mass extinctions were animals. Five, photosynthetic nannoplankton survived into the Tertiary, and Cretaceous and Tertiary species even coexist in land-based marine sections of the Tertiary. Six, tropical insects should have become extinct, but persist into the Tertiary.
And Seven, the dinosaurs appear to have undergone gradual extinction in at least some locations. The twelfth reason for disagreement with the asteroid impact theory is the high-energy terrestrial explosions, called geoblemes or crypto explosions, have not been studied, nor have laboratory simulations been tested. Therefore, much of the evidence for impact is somewhat biased by not considering the evidence in light of all the possibilities. For example, a number or journal articles have shown that the craters on Mars and other planets are dynamically related to the core, and therefore, are internal in origin.
Also, the structural similarities of multi-ringed craters with a central peak are too uniform, regardless of size and proposed angle of impact, for them to be impact craters; laboratory experiments show different structures for different angles and impactor size. This suggests that multi-ringed craters with a central peak, like that of the Chicxulub, are internally produced. The thirteenth reason is that amino acids typical of organic-rich meteor and asteroid remnants were found in Late Cretaceous formations. However, they occur over a long period of time and show no signs of being burnt by entry into the atmosphere, which is not typical of an extraterrestrial source. The fourteenth, and final argument against the impact theory is that other times of proposed impacts did not cause mass extinctions.
Volcanic Theory Another main theory, proposed by Officer and Drake which suggests that the K-T mass extinction was the result of hot spot volcanism. (See Figure 8 for representation of a volcano). The impact theory has been debated with a scenario of huge volcanic eruptions. Like the impact theory, the volcanic event could have brought about drastic weather changes that included a darkened sky, climatic cooling and acid rain. Both iridium and shocked minerals can be caused by volcanic eruptions. Furthermore, the hot lava could cause widespread wildfires.
Near the end of the Cretaceous, India had vast lava flows, known as the Deccan Traps. These lava flows covered an area the size of France. The Deccan Traps were the outcome of the largest volcanic catastrophe since the beginning of the Mesozoic. Areas in the Pacific Basin, northeastern Asia, Peru, and the Western Interior of North America also had a fairly high level of volcanic eruptions. However, volcanic activity had already been spent by the end of the Cretaceous. Eruptions in Britain, Baffin Island, Canada, and northern and western Greenland began shortly after the end of the Cretaceous.
In some ways the volcanic theory can explain many of the facts that the impact theory cannot. There are a number of iridium peaks with irregular distribution, which can be explained by the fact that the eruptions covered a longer time-span than an impact. It may also account for some of the selective extinctions. But just as with the impact theory, the volcanic theory has many outstanding problems too. Number one, it is very difficult to reconcile lava flows in India, which was surrounded by seas, with being responsible for global wildfires. This seems especially so when considering that the continents were being pushed and inland seas were either forming or retreating due to major ridge expansion.
Problem number two, basalt lava flows can produce iridium, but none has been found in the Deccan Traps themselves. Furthermore, it is difficult to imagine volcanic eruptions sending iridium globally. The third problem is that shocked quartz can form from violent eruptions, but usually the grains are so large that they do not travel very far. Again, quartz was found at a number of sites. The fourth outstanding problem is that a volcanic eruption cannot explain the sharp boundaries at some sites. A fifth reason is that some extinctions, such as the global extinction of the dinosaurs, seem impossible to reconcile with just volcanic eruptions.
Number six, the massive climate changes including temperature and ocean perturbations, cannot be accounted for by the volcanic eruptions of the time. And finally the seventh problem is that at least three of the times of massive flood basalts are not connected with mass extinctions. An extension of this theory known as the Volcano-Greenhouse theory was proposed by Dewey McLean. He proposed that a large volcanic eruption could have filled the atmosphere with carbon dioxide and allowed for a large rise in temperature. According to the theory, the carbon dioxide would have let the suns radiation enter the atmosphere, but not to escape This rise in temperature would have caused some species to become sterile. Infertility could have caused the extinction of the dinosaurs as well as mammals such as the mastodon and the sager-toothed tiger.
A rise in temperature might have killed plankton in the sea, disrupting food chains and making the carbon dioxide factor worse because of its job converting it to oxygen. Without clean air to breath, many animals would die. The Climate Change Theory One of the simplest theories of why the dinosaurs became extinct is that the earth's climate gradually changed. As the continents of the Earth continued to shift 60 million years ago, new mountains rose and sea levels gradually dropped.
As a result, the average air temperature around the earth could have fallen 17 degrees or more, changing weather patterns and given rise to new kinds of plants. It's possible, think some scientists, that dinosaurs were not well adapted to such cool weather. If they were, indeed, cold-blooded animals, then the colder air temperature would have made them more sluggish and less able to hunt or forage for food. Another possibility is that dinosaurs were not well suited to the new vegetation, such as flowering plants and leafy tree, which thrived in a cooler climate.
Convincing as this theory sounds, it does not explain why dinosaurs did not simply move to the tropical regions of the world where the temperature is far warmer, or why dinosaur fossils have been found above what was then the Arctic Circle. Glaciation, global cooling, and the creation of huge ice-sheets, is perhaps the single most important type of climate change to be implicated in mass extinctions Evidence of these 'ice-ages' comes both from characteristic glacial sediments, such as those under the floor of the North Sea, and form the extinction of groups adapted to warm climates. Glaciation was probably a major factor in a least three mass extinctions, in the late Ordovician, the late Devonian, and the end Permian. It many also have been a factor in the extinction event at the end of the Cambrian period. Another type of climate change, an increase in rainfall, has been suggested as the cause for another extinction, that of the late Triassic. Climate change causes extinction by altering the type and availability of ecological niches (potential homes for animals and plants), resulting in conditions that are unsuitable for previously thriving species, which must migrate or adapt, they are driven to extinction.
Radiation from a Nearby Supernovae Theory The suggestion that the dinosaurs were killed off by cosmic radiation from a nearby supernova is pure speculation. (See Figure 9 for an example of a supernova explosion). As no supernova remnant has been, or is ever likely to be, identified, the notion can be neither proved nor disproved. However, it is difficult to explain why certain organisms succumbed to the radiation while others survived. First of all, a supernova would have had to be very close to the Earth for the radiation to have penetrated the atmosphere to a sufficient degree to have completely wiped out all species of dinosaurs. If it had been close enough, and the radiation intense enough, to have also penetrated the ocean (water is a remarkable good radiation screen) to wipe out so many lineages of marine organisms, it should have virtually sterilized the land.
Yet a surprising majority of land organisms, from plants and insects to small land and freshwater vertebrates, survived virtually unscathed. This theory continually reappears because there is evidence of its effects in the fossil record. For example, Upper Cretaceous fossil form Nem egt Valley, Mongolia, had greater radioactivity levels than fossils of upper and lower strata in the same location. Many fossils suggest the effects of radiation, such as mummified dinosaurs, fossilized delicate dinosaur embryos, and pine cones without any signs of predation, decay, or the effects of insects, scavengers or de composers.
Even preserved amino acids may have been discovered, though there is a debate that amino acids could not survive such an extended time period, but if they were buried and the environment irradiated they could survive. A generally overlooked fact is that mass extinctions are eventually followed by mass speciations, not just during this event, but throughout evolutionary history. Again, this suggests radiation. According to molecular evolution, radiation could split chromosomes, and through the effects of recombination, crossing-over, and aberrations, as well as RNA and DNA mutation, they would be shifted into a different order, hence new species (or in the case of extinction, radiation poisoning or unworkable mutation). Furthermore, research over the past 80 years has shown that nothing is more capable of causing genetic mutation than radiation. This could explain many other facts.
The big problem for the radiation theory is that there is no confirmed source for the radiation. Theories have suggested a nearby supernova (exploding star), a super solar flare, and a super solar flare during a magnetic reversal. However, a supernova would leave behind a molecular cloud (nebula), but none has been detected in a region near Earth. The super solar flare would not cause a major problem as the Earth's atmosphere and magnetosphere (a magnetic sphere around the Earth resulting from the effects of the geomagnetic field) would deflect the flare.
When a super solar flare occurs along with a magnetic reversal, the magnetosphere no longer being protective, the possibility looks promising. However, no known such event has occurred to compare it to. In August of 1971, an expedition, while in the Gobi Desert, uncovered the skeletons of two dinosaurs still locked in mortal combat. Something very sudden had to overcome them or obviously both would have defended themselves against a common threat.
Furthermore, both had their back arched as if something had severely cramped their back muscles. Radiation affects high atomic-number elements more. The most abundant high atomic-number elements in biological organisms are calcium and potassium. These two elements are utilized in muscle function, and ionizing radiation may have caused extreme cramping of their muscles, arching their backs. Some have claimed that the tendons tighten after death producing this arching.
Regardless, something had to overcome these dinosaurs very quickly in order for them to still be locked in combat. Sun Variability Theory As indicated earlier climate changes is almost always the reason for mass extinctions. It is possible that also the changes in Sun's (See Figure 10 for a complete diagram of the Sun) activity could be causing some changes in terrestrial climate. The eleven year solar cycle is the best known variability in the Sun. However, all solar cycles are not equally intense, and it is possible that longer period modulations are operating. For example, long periods of almost no sunspot activity at all have been recorded.
Especially the Manuder's minimum is well known; it is perhaps noteworthy that it coincided with a "small ice age". One possible mechanism operating is that during high activity levels that decreased amount to galactic cosmic rays could lead to reduced clouds formation in the atmosphere, and hence to increased temperatures. Some research suggests that the Sun's variability could account for virtually all of the global warming measured to date. Experts are now scrutinizing three solar variables as likely agents of terrestrial change; the Sun's overall brightness (See Figure 11 for a picture of the brightness of the Sun), which is seen as affecting temperatures; the Sun's ultraviolet rays, which are seen as affecting winds and ozone production high in the atmosphere, and the Sun's storms of magnetic fields and subatomic particles, which are seen as affecting rainfall and the amount of cloud cover.
Just a 1 degree change in the Earth's temperature can result in mass extinctions. Magnetic Reversals Theory The Earth's magnetic field, like the ozone, acts as a barrier against outside forces. Periodically our Earth's magnetic field, changes polarity. (See Figure 12 for a timeline of the magnetic field).
In terms of magnetic arrangement the poles reverse. Approximately 700,000 years ago, the Earth undertook its last reversal. After a relatively brief stretch of time, the Earth poles shifted to where they are located now. Minerals found in magnetic rocks (See Figure 13 for reversal in rock), when formed, align themselves with the immediate arrangements of the poles.
(See Figure 14 for a drawing of the Earth's magnetic field). Ozone Depletion Theory Dr. M.L. Keith, professor of geochemistry at Pennsylvania state University has developed a possible theory for the extinction of dinosaurs. He claims that volcanic gases could have depleted the upper atmospheres protective ozone layer. Base-skinned creatures such as dinosaurs would have been particularly vulnerable to the ultraviolet radiation.
Furry mammals, feathered birds, and select sea creatures must have had enough protection to account for their survival. Galactic Dust It has been suggested that as our Solar System orbits around the center of the Milky way galaxy, that it might pass occasionally through clouds of unusually thick galactic dust, or clouds of hydrogen gas. Such clouds are known to exist within the galaxy, so the possibility undoubtedly exists. It is further proposed that higher than normal concentrations of dust falling into the Sun might caused it to burn brighter and hotter, thereby increasing the amount of Solar radiation received by the Earth, leading to elevated temperatures and global warming. While all of this is entirely possible, the problem with it as an extinction theory is that there is no evidence to support the ideas that global warming caused, or was even associated with, any of the major extinction episodes.
It is true that temperatures appear to have been universally warmer during the Age of Dinosaurs, so perhaps it was the Sun exiting form a dust could, resulting in global cooling, that caused the K-T extinction. It would have had to be an awfully big dust cloud, however, as the Jurassic and the Cretaceous, nearly 150 million years, both appear to have been warm. Stress on Thin Eggshells Theory This theory can be largely attributed to Heinrich K. Erbe n of Bonn University in Germany. His research led him to the conclusion that a species of dinosaur procreated a shell that, through time, grew progressively thinner. He continued to reason that luxurious swamps and warm climate activity led to "Biological Prosperity". These factors contributed to problems such as under population, overcrowding and other such related stresses.
Assuming the relation of dinosaurs and birds, stress could have increased estrogen in female dinosaurs. This in turn, would result in thin eggshells, thus the extinction of the species. A flaw that this theory suffers is which species were affected and which weren't. Egg Predation by Mammals Theory Mammals eating dinosaurs' eggs has been suggested as a cause of the K-T extinction. This theory has many problems with it. First of all, evidence was found suggesting parental care and feeding of the hatchlings.
It is difficult to imagine that dinosaurs would have survived if they couldn't defend their nests from predators. Secondly, dinosaurs appear to have nested in large rookeries, with many nests together, each separated by about one adult dinosaur-length. This undoubtedly made predation more difficult, as a predator would have to run the gamut of many dinosaur feet to get to any but the outermost nests. Epidemic Diseases Theory Dr. Robert T. Baker explains in his book "The Dinosaur Heresies" that the mass extinction that wiped out the dinosaurs was caused by the lowering of sea level at the end of the Cretaceous.
Prior to its close, the oceans of the Cretaceous covered about 90% of the land surface, forming vast shallow waters. As these drained off, land bridges such as the Bering Straits were created between formerly isolated continents, allowing large animals to migrate form one continent to another. Each population of large animals had evolved with its own unique parasites and diseases, and had developed a degree of immunity to them. When populations mixed at the close of the Cretaceous, each acquired and succumbed to each other's diseases. Smaller animals were spared simply because they were incapable of such long migrations. Marine species died out because their favorite habitat, the shallow seas, dried up and went away.
The problems with this theory is that sea levels today are not tighter than they were at the K-T boundary, and there are still abundant enough shallow sea environments to support a rich variety of organisms. Also, epidemic diseases may wipe out large segments of a population, but rarely if ever has disease caused the complete extinction of a species, let alone an entire lineage of dozens of genera. Drop in Sea Level Theory Changes in sea-level are commonly suggested as a cause of mass extinctions. Oceans contain many different habitats, which in turn contain a wealth of ecological niches in which animals and plant can flourish. When sea-levels change, these habitats are disrupted, or lost completely, along with any species that lived in them, which were unable to move away or adapt to the new conditions. Sea-level is closely liked to the formation of ice-sheets in periods of glaciation, as seawater is taken up into the ice-sheets the oceans shrink until the ice melts once again, and sea-level rises.
Shallow, warm water seas (such as those that contain coral reefs) are some of the most biologically diverse habitats, but are also among those most susceptible to even small changes in sea-level. Changes in sea-level may have other, secondary, effects. Changes in the distribution of oxygen of the oceans, or in their salinity, have both been associated with fluctuations in sea-level, and could easily have altered marine environments enough to cause extinctions A change in sea-level has been implicated in most of the mass extinctions, and almost certainly played a major part in those of the late Cambrian, the late Ordovician, and the end of the Permian. Global Cooling Theory Changes in the Earth's orbit that could have caused climactic cooling might have caused the extinction. In this scenario, the dinosaurs couldn't adapt to the cold, but the furry mammals could. This is consistent with the climate in the late Cretaceous; toward the end of the Cretaceous, there was a drop in sea level, causing land exposure on all continents, more seasonality, and greater extremes between equatorial and polar temperatures.
Seventy million years ago, the Earth's surface was changing very quickly. These changes in the Earth's surface caused weather patterns and climates to change. These changes caused global cooling and bigger changes in seasons. All of these changes in the Earth's surface caused volcanoes to erupt, thus spreading lots of dust into the air.
The dust blocked the sunlight, causing even cooler temperatures. The cooler weather killed many of the dinosaurs. The cold weather also caused thinner egg shells. Thinner egg shells have been seen in birds that have been exposed to cold, chemical poisoning, or overcrowding. Mammals that were smaller were better able to survive the temperature changes, and so they competed successfully for food. Arctic Ocean Spillover Theory This theory suggests that for a brief time the ocean surface waters became cooler, not warmer.
During the Mesozoic continental drift the arctic ocean could have become isolated. Brackish or even freshwater would result form such an event. Colder. lighter arctic water would have mixed with the warmer Atlantic, forming a frigid layer on top of heavier salty sea-water. Drop in world temperatures as much as ten degrees Celsius would have resulted. If this happened, according to Stefan Gartner and James P. Mcguire of Texas A&M University, a devastating chill would have spread over much of the world. However, no evidence supports the idea of the Arctic ocean ever being fresh or Brackish.
Other Unusual Theories Some scientists have suggested that subatomic particles called neutrinos, released as stars collapse, could have bombarded the earth and resulted in widespread cancer among animals. However, such events are rare, and do not necessarily explain all the extinctions, but further research may yet prove that they have played some part in the mass extinctions. An extension of the asteroid impact theory of mass extinction suggests that nickel, which is common in asteroids, could be spread across the globe in the cloud of debris released by such an impact. Nickel prevents plants from photosynthesizing, stopping them from growing. There would have been a huge shortage of food for both plant-eating animals, and the predators with ate them. A quantity of other absurd theories have been proposed over the years.
For a list of these theories see the appendix. Species Affected During the End-Cretaceous (K-T) extinction (65 million years ago) 85% of all species disappeared, making it the second largest mass extinction event in geological history. This mass mass extinction event has generated considerable public interest primarily because of its role in the demise of the dinosaurs. One of the foremost problems that any successful theory of dinosaur extinction must explain is the fact that not just dinosaurs were affected. In fact, dinosaurs represent but a small portion of the species that became extinct at or near the end of the Cretaceous Period. The extinction even that brought the Cretaceous Period to a close was truly a "mass extinction", in that a wide variety of taxonomic groups from many different habitats were wiped out essentially at the same time.
Many of the groups of organisms that were hit hardest by the K-T extinction lived in the ocean. Ammonites and belemnites, shelled cephalopod mollusks related to the octopus and squid, were abundant in the seas of the Cretaceous Period, but had disappeared entirely by its end. Another squid relative, the nautiloids, were also severely affected; only two species, the Chambered Nautilus and the King Nautilus, have survived to the present. Plesiosaurs, long necked, fish-eating marine reptiles, and mosasaur's, ferocious giant seagoing lizards, also vanished form the seas at this time, although their smaller land-dwelling reptilian cousins survived mostly unscathed. Bivalve mollusks and other shelled invertebrates of the sea floor also suffered greatly at the end of the Cretaceous. Ru dists, bivalve mollusks and the dominant reef builders of the Cretaceous seas, declined sharply towards the end of the Period, and disappeared entirely at its close.
Brachiopods, which are bivalves but not mollusks, also suffered greatly, but managed to survive in severely restricted numbers to the present. Perhaps the most dramatic extinctions in the sea were among the nannoplankton, minute calcium-secreting algae, and the foraminifera ns, calcium-secreting protozoans. Their abandoned shells piled up in immense thickness to form the great chalk cliffs that give the Cretaceous Period its name. Marine sediments during the Cretaceous Period were comprised almost entirely of this chalk, with only a small percentage of clay particles. Sediments deposited immediately after the K-T boundary is dominated by clay particles, with only 20 to 40% being chalk. This clay later, known as the "Fish Clay" in Europe, is widely accepted worldwide as the boundary between Cretaceous and Tertiary sediments.
It ranges in thickness from less than one-half inch to over three feet in thickness. Thus the K-T boundary exhibits a drastic reduction in the abundance of calcium secreting organisms. Assuming that the clay particles, derived from the erosion of nearby continents, continued at the same rate across the K-T boundary, this represents approximately a 97% reduction in the abundance of marine calcareous algae. Dinosaurs were the undisputed rulers of life on land, right up to the catastrophic K-T event, but they were not the only creatures to suffer. Although fossil birds are rare during the Cretaceous (due more to the scarcity of preservation than to a lack of abundance), there were apparently several distinct lineages of Cretaceous birds, only one of which survived the extinction event, to give rise to the birds of today. However, many species within that one lineage survived, as many of the modern bird orders were represented prior to the close of the Cretaceous.
Many species of mammals also survived the extinction, as many mammalian orders also have Cretaceous representatives. Cretaceous mammals, however, tended to be quite small, and probably were predominantly nocturnal. Freshwater animals and the smaller terres t.