Following the Ordovician mass extinction rediversification 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, blastoids, 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, ostracodes, 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, rudist 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 (Kreta) 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 cryptoexplosive 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 Ranton 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.