Humberside Geologist no 11
published 1994
Palaeontologists have long recognized that large numbers of species disappeared simultaneously from the fossil record. Those disappearances often mark the end of a geological period. The causes behind these events have been obscure, but in the last decade Earth Scientists have uncovered much new information about these disappearances and have put forward numerous theories to explain their findings.
There are several aspects of dispute currently under discussion. What is a major extinction? When does a crisis or a catastrophe become an extinction? How can the severity of a crisis or a catastrophe be determined?
Do localized surges in extinction merge with global ones? How do these differ from "background extinctions" -- the continual dying out of species all over the world? Is a mass extinction just the intensification of the background extinction rate? (Extinction has claimed 90% of all species that have ever lived -- most of them victims of background extinction.)
The number of bodies cannot be counted because the fossil record is too incomplete. Estimates that use families are more accurate because they often include many species whose ranges in geological time are relatively well known. However, family samples only include those whose members possessed hard bodies that have been fossilized. Creatures made of soft tissues are rarely preserved in the rocks. It is highly probable that many biological groups which have existed and became extinct are unknown, simply because their body parts have not been preserved.
At some time in the late Precambrian the earliest form of life was developed -- the primordial slime. At this time the Earth's atmosphere and oceans were oxygen-free (anaerobic). Carbon dioxide was the dominant gas.
About 3000 Ma ago certain bacterial members of the primordial slime developed a type of photosynthesis that released oxygen as a waste product. The waste built up, firstly oxygenating the oceans, then the atmosphere. This change has been described by microbiologists as "the worst case of pollution in the Earth's history".
Oxygen is a reactive gas and deadly to organisms adapted to anaerobic conditions. Many of the anaerobic micro-organisms were wiped out -- the first great extinction was caused by poisonous gas attack.
These oxygen-producing microbes existed in warm, shallow seas building diverse bacterial communities -- mounds of micro-organisms. The fossils of these mounds are called stromatolites.
Beds of layered stromatolites up to several kilometres thick are found in shallow water deposits laid down between 2 500 Ma and 600 Ma ago. Today, stromatolites are only found in conditions hostile to predators such as in hypersaline or swift-flowing waters of Western Australia or the Bahamas.
The stromatolite-producing cyanobacteria (formerly called blue-green algae) dominated the world for some 2 000 to 3 000 millions of years until they fell prey to a new form of life -- animals.
Cyanobacteria made their own food from water and carbon dioxide using photosynthesis.
Other life forms made a huge step forward. They had developed high energy, oxygen-burning metabolism which enabled them to become grazers or hunters.
Animal life exploded across the Earth at the start of the Cambrian and within 25 million years life had evolved nearly all the phyla or basic body plans it uses today.
The dominant fossil group of the time was the trilobites. These innocuous creatures were up to 15 cm long. They scavenged on the sea floor all over the world.
On at least three occasions, towards the end of the Cambrian, trilobites suffered from mass extinctions, with many species lasting less than a million years.
The reasons for this are unknown, but it has been suggested the trilobites were destroyed when cold, oxygen-depleted water from the ocean depths was washed into the shallows where they lived.
During the Ordovician Period there occurred a severe mass extinction -- more than 20% of marine families that left fossil remains were eliminated. Early fish survived, but marine invertebrates and primitive reef-builders were hard hit.
This suggested cause has been plate tectonic processes.
The super-continent Gondwana drifted over the South Pole started a period of prolonged glaciation. The glaciers caused a wide-reaching climatic chill. Species began to disappear and floating animals and bottom dwellers moved to warmer waters nearer the equator. The sea-level also dropped as great volumes of water were locked up in the ice.
Shallow seas dried up in some places and the lowered sea-level exposed vast areas of continental shelf reducing the area for species to live.
These events decimated many forms of life.
There is also evidence to show that the surface waters of the oceans became depleted of oxygen. This was caused by the continual movement of Gondwana which started to head back over the South Pole. This and perhaps the effect of an asteroid, caused anaerobic waters from the deep to flood over the continental shelves. This suggests the extinction extended over a period of several million years.
During the Devonian sea life was abundant. Massive fossil reefs were being built, and a huge explosion in predatory fish took place. Land plants and the first amphibians evolved.
In the late Devonian most of the world's fish and up to 70% of its invertebrate species perished. Two main possible theories exist to explain this "great extermination".
Some late Devonian rocks reveal a thin band of sulphur thought to have been precipitated as oxygen vanished from the upper layers of the oceans. This suggests cold deep anaerobic waters from the deep were forced into the upper layers and over the continental shelf. The anoxic waters decimated life forms.
Other rocks show traces of Iridium, a mineral rare on Earth but abundant in meteorites.
A possible cause of this could have been the impact of an extraterrestrial body. As well as forcing deep water to the surface huge tides were caused. The tremendous rise and fall of up to 90 metres placed terrific stress on marine life.
Such an event would have been short-lived -- perhaps only 20 000 years.
Some scientists reject this idea and declare that the extinction was spread over millions of years.
Gondwana had started to head back over the South Pole. The iridium in the rocks was concentrated by mats of organisms and the continental movement caused deep ocean waters to flood the continental shelves. The influx of these anoxic waters would cause sulphur to be precipitated.
During the previous periods life was confined to the seas. By the end of the Permian coal swamps were extensive, insects swarmed and pig-sized amphibians roamed the warm surface of the Earth. Dog-sized mammal-like reptiles dominated the land surfaces.
The most catastrophic extinction in the fossil record is the one at the end of the Permian Period about 250 million years ago. This crisis ended the Palaeozoic Era and swept away up to 95% of all animal species on Earth -- a gigantic catastrophe. Most groups of mammal-like reptiles and many kinds of marine animals were wiped out. Over half of all marine families with good fossil records were eliminated. The Permian Extinction was the first to affect terrestrial life significantly.
The most acceptable theory for this extinction is that plate tectonic processes were responsible. These were forging another supercontinent -- Pangaea -- as all the planet's continents were joined into a single colossal landmass. With only one continent the amount of shallow offshore water shrank drastically -- these were probably the richest habitat on the planet. Also Pangaea extended to both Poles starting Ice Ages in both hemispheres.
The result was the sea-level rose and later fell some 200 metres and coastlines advanced and retreated 1900 kilometres. Tropical seas became chilled and on land the climate grew progressively drier and the winters Siberian.
Reptiles with the beginning of mammal-like features, some of which had migrated to the land, suffered at least six mass extinctions during the last eight million years of the Permian, reflecting strong climatic swings.
Survivors at the end of the Permian appeared to be warm-blooded and thus better able to cope with the cold. They were also small -- small creatures need less food and can find shelter easily.
Ichthyosaurs roamed the seas, crocodiles inhabited the swamps, pterosaurs took to the air and the first dinosaurs appeared. These were small, fast and described as "big pheasants on the run". However, they were unable to displace the mammal-like creatures.
At the end of the Triassic Period, at about 200 Ma, up to 75% of marine invertebrate species and several land dwellers vanished.
The suggested cause of this is extraterrestrial. A huge crater is to be found in Quebec. The fireball alone would have scorched an area of several thousand square kilometres and created the kind of global havoc suspected at the Cretaceous/Tertiary (K/T) boundary. This will be commented on in the next section.
During the Jurassic's 55 million years the great reptiles developed. The gigantic Apatosaurus (Brontosaurus) browsed on tall coniferous trees, the armoured Stegosaurus grazed the vegetation and the Allosaurus hunted its prey.
At the end of the Jurassic these monsters vanished along with many smaller dinosaurs and marine creatures.
The Jurassic killer has left many questions but very few clues. Was there a global environment change? Why was life in the seas hit hard? Why did a new generation of small-beaked dinosaurs emerge? Few theories exist to account for this extinction.
Dinosaurs were the dominant land life form during the Cretaceous. New dinosaurs developed after the Jurassic, up to 30 genera (perhaps a hundred species) inhabited the planet during the last 10 million years of the Cretaceous. These included the duckbilled dinosaurs of the swamps, the huge herds of the Triceratops and their relatives feeding on the plains and the fearsome Tyranosaurus rex.
In this period blooming and fruit-bearing plants, the angiosperms, exploded across the land. Their pollen and seeds were spread rapidly by animals and insects. They reproduced rapidly and colonized quickly. The end of the Cretaceous is marked by a pencil-thin layer of iridium-rich clay. Before this was deposited there were dinosaurs and after it, in the Tertiary, there were none. Scientists name this change the K/T boundary.
This boundary marks the time when the most widely-known extinction occurred -- on land the dinosaurs were wiped out after 140 million years of existence.
Life in the seas was seriously affected. Most plankton (the primary food source) died out. All the large marine reptiles disappeared. The rudists, giant coral-like clams whose shells helped build the Cretaceous reefs, were obliterated. Ammonoids, survivors of previous extinctions, were wiped out.
Not all these creatures disappear from the fossil record at the K/T boundary. There is some evidence that ecosystems were decaying for at least two million years before this. During this period the rudists, which were the giants of the molluscs, were in decline. A million years before their final disappearance the reefs were biologically impoverished and poorly developed.
Another group of giant bivalves, the Inoceramids, died out just before the end of the Cretaceous. Terrestrial vegetation underwent a sudden change and many species of hardwood trees and flowering plants died out.
The cause of the extinctions at the K/T boundary have been the subject of much debate in recent years.
Perhaps flowers were an agent of extinction. Dinosaurian overgrazing threatened many low-growing plants with extinction. These were replaced by the angiosperms which could have been unsuitable diet for the dinosaurs.
Cooling of the climate could have taken place changing the type of plants. There seems no obvious reason for this as the next Ice Age was not to occur for millions of years.
Fossils of mammals have been found in late Cretaceous rocks. The mammals developed rapidly and one school of thought suggests "they ate the dinosaurs out of house and home".
Although the large dinosaurs became extinct at the end of the Cretaceous smaller animals such as turtles, crocodiles, many lizards and most mammals made it into the Tertiary -- perhaps because they needed less food and were small enough to find shelter.
In 1980 a team of physicists and geologists led by Walter and Luis Alvarez (father and son) made the discovery that an abnormally high concentration of the rare element iridium was laid down in a thin layer of sediments in many parts of the world. This layer corresponded to the K/T boundary. The Alvarez team recognized that iridium is far more abundant in meteorites than in the Earth's crust. They suggested that the extinction of the dinosaurs and their contemporaries was the result of a large meteorite or comet (a bolide) hitting the Earth. From the amount of iridium they calculated a bolide with a diameter of 10 km was involved. This would have created a crater 100 km in diameter under the sea or 125 km diameter on land.
The effect would have been catastrophic. Alvarez comments: "In the first days after the Earth was hit, dust blanketed the entire world. It grew pitch dark for one to three months. If the impact was on land, it probably got bitterly cold. If it hit at sea, the water vapours could have created a greenhouse effect, making things hot. Hot nitric acid would have rained out of the atmosphere -- a life-threatening rain that would have dissolved the shells of organisms".
Widely controversial at first, the theory has since been backed up by abundant and convincing evidence from around the globe.
With the iridium layer has been found an enormous amount of soot. When the meteorite hit the atmosphere it would have pushed the atmosphere aside. A fireball would have occurred with a radius of several thousand kilometres. Winds of hundreds of kilometres per hour would have swept the planet, drying out the vegetation. Rock vapour from the meteorite and the impact would have spread rapidly, condensing to white hot grains that would have started additional fires. The quantity of soot found indicates as much as 90% of the world's forests must have been burned.
But to date no crater of the required dimensions has been found. Perhaps the meteorite broke up in the atmosphere and the pieces were scattered causing small craters.
In Iowa there is a circular geological structure which seems to record an impact. It is dated to the time of the K/T boundary but is only 35 km in diameter. This crater lies beneath sediments.
Researchers from the University of Arizona have studied seismic evidence from the Caribbean. They claim to have identified an impact crater about 300 km across buried beneath deep sediments. This is currently under investigation -- and findings of supporting evidence continually being reported. A further suggestion in that a meteorite impacted in the Deccan area of India and the crater is buried beneath the lava it caused to erupt. One of the greatest outpourings of lava the Earth has known occurred here. Massive volcanic eruptions may have temporarily dropped global temperatures by throwing particles of dust into the atmosphere which blocked out sunlight.
However the Deccan lava was expelled relatively calmly and it is doubtful much debris would have reached the upper atmosphere.
The history of the Earth has been punctuated by massive extinctions. How and why they occurred is a matter of debate and argued by scientists from many fields including geophysics, astronomy and palaeontology.
At one end of the spectrum there are those who believe mass extinctions are triggered by brief, cataclysmic events such as the impact of a celestial body or periods of intense volcanic activity. Two University of Chicago researchers have suggested that extinction crises have occurred every 26 million years during the past 250 million years.
However, it is difficult to be precise about datings over such a long period and they are subject to error of + or - 10 million years.
Others argue that the extinction process is gradual, caused by environmental changes brought on by rapid tectonic, oceanic and climatic fluctuations.
Many believe the truth is a combination of both theories, a combination of Earthly and extraterrestrial causes.
Are we experiencing an extinction crisis at present?
Over a period of 500 to 1 000 years (a very short time geologically) starting about 11 000 years ago most large mammals (sabre-toothed cats, mastodons, mammoths, huge ground sloths, bears and dire wolves) perished abruptly.
One argument is that the climate grew drier. This reduced the food supplies for the herbivores. As these died out so did the carnivores that preyed on them.
Many scientists suspect an alternative reason -- Homo sapiens. This new hunter emerged at the end of the last Ice Age with the ability to hunt using traps, snares and weapons.
Today man continues to have an increasing effect on the biosphere. Extensive destruction of rain forests in the developing countries occurs on a massive scale. A recent survey in the Philippines reported that 42% of species found in 1930 have not been collected since. Numerous other such examples have been written about.
Perhaps for the first time a living organism can do something to halt a mass extinction
copyright Hull Geological Society 2021