Illustration 40: The skeleton of an American Mastodon, source Kameno Doba by Jovan Zujovic, pub. 1893.
Cuvier now firmly believed that animal species went extinct. In his 1796 paper on elephants, he wrote, “All of these facts, consistent among themselves, and not opposed by any report, seem to me to prove the existence of a world previous to ours, destroyed by some kind of catastrophe.” A belief that history was filled with natural catastrophes, which caused many species to become extinct, formed the basis of the geological school of thought called catastrophism.
Appointed Inspector-General of public education and State Councilor by Napoléon Bonaparte, Curvier outlasted the Napoleonic Empire. He continued as a state councilor under three successive Kings of France after the Bourbon Restoration. Showing an amazing talent for survival, he managed to serve under three different, opposing French governments; the revolutionary French First Republic, the Napoleonic French Empire, and the restored monarchy. In 1826, he was made grand officer of the Legion of Honour, and in 1831, he was elevated by King Louis Philippe to the rank of Peer of France.
Illustration 41: Megatherium, subject of Cuvier's 2nd paper.
Through all this, he helped establish vertebrate paleontology as a scientific discipline and created the method of comparative anatomy. For a brief period of time, starting in November of 1831, Louis Agassiz, whose work we discussed in Chapter , was Cuvier's student and colleague. Cuvier died in April 1832, and though their relationship had lasted only six months, Agassiz credited Cuvier with opening his eyes to the workings of nature. For the rest of his life, Agassiz promoted and defended Cuvier's geological catastrophism and classification of animals.
After Cuvier's death, the catastrophic school of geological thought lost ground to uniformitarianism. This school of thought claimed that the geological features of Earth were best explained by observable forces, such as erosion and volcanism, acting gradually over an extended period of time. Uniformitarianists, or gradualists, were among the first to argue that Earth must be several billion years old.
Championed by Scottish geologist Charles Lyell (1797-1875), who had been influenced by fellow Scotsman, James Hutton, this theory of slow, gradual change became the dominant mode of thinking in geology and paleontology. In his widely read, three volume Principles of Geology, Lyell showed that Earth must be very old based on the supposition that it was subjected to the same sort of natural processes in the past that shape the land in the present.
So strong was the influence of uniformitarianism, that it was impossible to deviate from its tenets up until a few decades ago. Uniformitarianism is credited with much of the resistance to the Alvarez Theory that an asteroid strike caused the KT extinction. The strength of that theory, and its eventual widespread acceptance, has led to a re-evaluation of catastrophism and the work of Cuvier. Nowadays, it is recognized that portions of both schools of thought are correct. Earth and living creatures do undergo gradual long-term changes, but sometimes catastrophes occur causing more abrupt shifts.
The Tree of Life
When scientists discuss extinctions, they rate the severity of an event based on how many species were extinguished. But they also consider the impact on groups of species that are closely related. There are several complicated classification schemes for living things. Among the most recent is the three domain system introduced by Carl Woese in 1990.
The phylogenetic tree, shown in Illustration 42, reflects the very top of this classification scheme. This type of organized classification is called a taxonomy, and the entries are called taxa (singular taxon). A taxon name can designate an organism or group of organisms. A taxon is assigned a rank and is placed at a particular level in the systematic hierarchy reflecting evolutionary relationships.
Illustration 42: A phylogenetic tree of living things, based on RNA data and proposed by Carl Woese, showing the separation of bacteria, archaea, and eukaryotes. Wikipedia.org.
Aristotle divided the living world between animals and plants. This system was followed over 2,000 years later by the first hierarchical classification of Carolus Linnaeus, who we mentioned earlier in this chapter. Originally, scientists assigned taxa based on the shape or appearance of organisms, their morphology. More recently, a school of thought has arisen where taxa are classified only by their evolutionary relationships. Called Cladistics, from the Greek word klados, meaning branch or twig, this approach has caused much reorganization of the Tree of Life. To add to the confusion, Cladists sometimes refer to taxa as clades.
The taxon hierarchy, from the largest units to the smallest, looks like this: Domain, Kingdom, Phylum (or Division), Class, Order, Family, Genus and Species. There is a mnemonic to help remember this scheme, “Distinguished Kings Play Chess On Fine Green Silk.” These classes can be further subdivided into Sub-Classes, Sub-Orders, Sub-Families, etc.
All members of a species are, at least in theory, able to interbreed and produce fertile offspring. Several species may belong to a genus but, if they interbreed, members of different species within a genus are unable to produce fertile offspring. An example of this is when horses and donkeys are bred to produce sterile mules. Our genus, Homo, has one surviving species, sapiens. The other members of our genus, twenty-two species at last count, all died out in prehistoric times.184 But, for a time, modern humans cohabitated with other human species, most notably Homo neanderthalensis. In recent times, there has been much speculation that modern humans interacted, and may have interbred, with Neanderthals.185 Though there is no proof186 of this, there have been a number of science fiction novels based on this premise.
Several genera (plural of genus) can belong to the same family, several families to an order, and so on up the hierarchy. Eventually, kingdoms are placed into one of the three domains; Bacteria, Archaea, or Eucaryota. Humans belong to the Eucaryota domain, in the kingdom Animalia (i.e. The animal kingdom), phylum Chordata, class Mammalia, order Primates, family Hominidea, genus Homo, species Sapiens. Even for scientists, this is too long a naming scheme for daily use. Instead, the binomial naming scheme is commonly used. Binomial designations use both the genus and species names, with the genus capitalized. In this scheme, we are Homo sapiens. There is also a trinomial naming scheme that adds a subspecies designation. Under this scheme, we are Homo sapiens sapiens, Latin for “wise wise man.”
Illustration 43: Skull of a Diapsid.
Membership in classes, orders and the other higher taxa, are decided on structural or anatomical similarities among life-forms. For example, both modern birds and dinosaurs belong to the sub-class Diapsida. Diapsids (Latin for “two arches”) are a group of animals that developed two holes in each side of their skulls (temporal fenestra).
These creatures first appeared about 300 million years ago during the late Carboniferous period. Living diapsids are extremely diverse, including all birds, and all reptiles except turtles. Over time, lizards lost one skull hole and snakes both holes, but they are still classified as diapsids based on their ancestry. Modern bird skulls also diverge significantly from the skulls of primitive diapsids, but they are also classified as Diapsids.
The reason we mention all of this is because paleontologists will often express the impact of an extinction in terms of lost genera or families, instead of species. A genus going extinct is worse than a single species going extinct, and a family going extinct is even more noteworthy. When the dinosaurs were wiped out, their entire order, Dionsauria, disappeared. But birds, who belong to the same sub-class, survive to the present.
Major Phanerozoic Extinction Events
Since the advent of complex life on Earth there have been five major mass extinctions. Recently, evidence has been found for another extinction during the early-Cambrian, 512 mya. This event is so far in the past that not much is known about its causes, but it is an indication that major extinctions have been happening for half a billion years.
The most famous extinction is also the most recent, the KT or end-Cretaceous Extinction, 65 millio
n years ago. The subject of many TV shows, most people know the story of the asteroid that killed the dinosaurs. What most people don't know is that, along with the dinosaurs, 85% of all species on Earth vanished during that time. Because it was the most recent extinction event, scientists know more about the KT event than the other great extinctions. Here is a summary of the six major extinctions:
Early-Cambrian (512 mya): earliest recognized mass extinction eliminated 50% of all marine species.
End-Ordovician (439 mya): 85% of marine species disappeared, including many trilobites. Second largest marine extinction with 60% of marine genera and 26% of marine families.
Late-Devonian (365 mya): 70-80% of animal species went extinct, including many corals, brachiopods, and some single-celled organisms. Accounting for 57% of marine genera and 22% of marine families.
Permian-Triassic (251 mya): extinction of 96% of marine species, including all trilobites and many terrestrial animals—Earth’s biggest extinction.
End-Triassic (199 mya): extinction of 76% of species including many sponges, gastropods, bivalves, cephalopods, brachiopods, insects, and vertebrates. Mostly affecting ocean life killing 57% of marine genera and 23% of marine families.
KT or end-Cretaceous Extinction (65 mya): 80% of all species on Earth vanished, most notably the dinosaurs. Eliminated 47% of marine genera and 16% of families.
The causes of these natural catastrophes are varied. Ice ages may have played a major role in several. There is evidence that fluctuations in sea level was the primary cause of the end-Ordovician extinction, which mostly affected marine life This is unsurprising since, during the Ordovician, the majority of life was found in the seas. Joseph Sepkoski called this extinction one of the two or three worst extinctions of the Phanerozoic, noting that generic diversity dropped to about the level of the pre-early-Ordovician proliferation.187
The late-Devonian mass extinction was a prolonged marine crisis spread over 20-25 million years and punctuated by 8-10 extinction events.188 Because it most severely affected warm water marine species, leading many paleontologists to attribute the Devonian extinction to an episode of global cooling, similar to the event which caused the late-Ordovician mass extinction. Glacial deposit evidence, found in what is now Brazil, points to a glaciation event on the supercontinent Gondwana during this period. Much like the late-Ordovician crisis, global cooling and widespread drop in sea level may have triggered the late-Devonian crisis. There are suggestions that a meteor impact could be the cause, but the evidence is inconclusive.189
The Permian-Triassic Extinction is widely considered the worst of all the major extinction events, killing off an estimated 95% of terrestrial life. From rock layers in Texas and Utah comes evidence that this extinction came in two parts, called extinction pulses, separated by about 10 million years. Either of the two events alone was worse than the KT Extinction that killed off the dinosaurs. Between the two events, 82% of marine genera and 50% of all marine families were extinguished. In earlier chapters, we mentioned the extent of damage this extinction inflicted on Earth's creatures, but the most telling feature was the length of time needed for life to recover. Well into the Triassic, as many as 20 million years later, the effects were still felt. Geologists and paleontologists consider this extinction a major turning point in the history of life on Earth.190
Causes put forward for the biggest of all extinctions pretty much cover the entire range; climate change, sudden release of CO2 or methane, volcanoes and an asteroid strike have all been suggested. Douglas Erwin, in his excellent book “Extinction,” covers all the theories in detail, and finds no single explanation fully satisfying. He has suggested what he calls the “murder on the orient express” theory, a combination of several or even all of the causes listed above.
Illustration 44: The Manicouagan impact structure seen from space, Quebec, Canada. Source NASA/JPL.
The end-Triassic Extinction doesn't get much press, coming on the heels of the worst ever extinction, and before the dramatic meteorite impact that extinguished the dinosaurs. At least two impact craters have been found from around the time of this extinction. One is in Western Australia, where scientists have discovered the faint remains of a 75 mile (120 km) wide crater. The other is a 212 million year old crater in Quebec, Canada, forming part of the Manicouagan Reservoir. The Manicouagan impact structure is one of the largest impact craters still visible on the Earth's surface, with an original rim diameter of approximately 62 miles (100 km).
Others have suggested that a sudden, gigantic overturning of ocean water created anoxic conditions causing the massive die-off of marine species. About 23% of terrestrial families also died out,191 so there is doubt that such an aquatic event could account for all of the vanished species. There is recent evidence that the end-Triassic experienced an extended period of massive volcanism called the Central Atlantic Magmatic Province (CAMP). This event is associated with the breakup of the super-continent Pangaea and the appearance of a giant rift that eventually formed the basin of the Atlantic Ocean. Carbon isotope anomalies at the Triassic–Jurassic boundary reflect the effects of volcanically derived CO2, possibly combined with methane release from gas hydrates due to global warming.192 This makes volcanoes the current favorite trigger for this extinction.
Illustration 45: Impact point of the Chicxulub asteroid. Source Tufts.
The final major extinction in our list is the KT or end-Cretaceous Extinction. We have already mentioned, in Chapter Error: Reference source not found, how Luis and Walter Alvarez, having found an unexpected spike in iridium content in sediment from the Cretaceous-Tertiary boundary, hypothesized that this extinction was due to an impact with an extraterrestrial body. They later found evidence of that impact in the Yucatan peninsula of Mexico. That evidence came in the form of a crater between 105 and 185 miles (170 to 300 km) in diameter.
Impact models estimate the object was between 6 and 10 miles (10-15 km) in diameter, and would have ejected 25,000 cubic miles (100,000 km3) of rock and debris. Such an impact would have directly killed every-thing for thousands of miles, and also triggered earthquakes and tsunamis. As bad as these effects were, the real killing was caused by ash and vaporized rock that filled the atmosphere, which formed dust clouds that blocked the Sun. These clouds are thought to have lasted many months, stopping plant growth and chilling the planet.
Illustration 46: Marine diversity during the Phanerozoic showing major extinction events. Modified from a figure by Jack Sepkoski (1984).
A summary of the major extinctions is shown in Illustration 46, depicting diversity in terms of marine families. Starting with the newly discovered early-Cambrian event, the extinctions are numbered from zero. This is to avoid changing the normal numbering of the “big five” extinctions, as they are widely called in the literature.
The three differently shaded areas of the graph represent the numbers of families belonging to the Cambrian, Paleozoic, and Mesozoic periods. As you can see, the early Cambrian life forms started being superseded by the intermediate forms of the Paleozoic prior to the end-Ordovician Extinction and had almost vanished by the late-Devonian. The life-forms characteristic of the Paleozoic undergo a major decline until the Permian-Triassic Extinction, and some related species linger to this day. Even so, the double blow of the Permian-Triassic followed by the end-Triassic cleared the way for the rise of the dinosaurs and, eventually, mammals.
The Causes of Extinction
As we have seen, there is no single answer to the question of what causes mass extinctions. Even the cause of the KT extinction is in question. Though there is no doubt that there was a major asteroid impact at the Cretaceous Tertiary boundary, there is good evidence that the dinosaurs had been declining for millions of years. Some speculate that only a few species were left in the late Cretaceous when the Chicxulub object collided with Earth.193
Much like the controversy surrounding the Permian Triassic extinction, there were other events taking place at the time of the Chicxulub eve
nt. Prior to the KT boundary, a massive series of volcanic eruptions had taken place in what is now western India and Pakistan. Called the Deccan Traps or lavas, these eruptions produced over 10,000 cubic kilometers of lava during continuous eruptions lasting a million years. The eruptions peeked a half million years before the K/T boundary and continued for some time beyond it. These eruptions could have changed the atmosphere, the ocean and the global climate, causing the extinction of many species.
At this same time there was a noted decline in sea levels, exposing wide swaths of previously covered continental shelf. This change may have been responsible for the extinction of families of ichthyosaurs and plesiosaurs, aquatic dinosaurs which were in decline long before the impact. Some tiny forms of marine live, such as coccoliths, vanished while diatoms and benthic foraminifera survived unscathed. The KT, Permian Triassic and other extinctions may have been caused by the coincidence of several factors, not a single earth shaking event. Looking back 65 million years or more, extinctions taking place over a million years can appear to have occurred suddenly. The Chicxulub impact may have just been the coup de grâce that finished what nature had already started. As with most things in science, the arguments continue.
The Resilient Earth: Science, Global Warming and the Fate of Humanity Page 11