Mass Extinctions
“Every mass extinction marked a watershed in the evolution of life.” ~ American geologist Jon Erickson
Earth has seen a staggering array of biota, beyond imagination in variety. This proliferation derives from evolutionary impulse to diversely adapt: either toward generalized hardiness, or, more often, to fit into environmental niches, to more adroitly exploit available energy resources.
Specialization proves risky in time, as environments change, often dramatically. The causes are various, but all ultimately involve changes in temperature and the availability of water.
There are 2 extents of extinction event: background and mass. Background extinction is the demise of a relatively few species. This occurs where adaptation fails, often in a rapidly changing biome. Mass extinction indiscriminately wipes out many species. The difference between the two is a matter of degree. Species are always going extinct.
Outside mass extinction, species diversity tends to hold as new opportunities arise, but, as the number of extinctions rises, speciation invariably declines for a time.
2 temporal vectors commonly lead to speciation via population separation: dispersal and vicariance. Speciation by dispersal happens when a subpopulation migrates outside the range of the main population, adapting to the new habitat to eventuate into a new species. Variation by vicariance occurs when a new geographic barrier arises, separating a population. Isolation precedes speciation in either case, but by geology in vicariance rather than overt dispersal behavior.
“Many of the major biotic turnovers, extinctions, and radiations that had once been attributed to direct competitive replacements or adaptive breakthroughs are now seen as physically mediated.” ~ American geophysicist David Jablonski
Background extinction and mass extinction are typically provoked by environmental changes such as climate. The rapidity and severity of change dictate the degree and duration of an extinction event.
“There is no commonly accepted definition of the term mass extinction other than as a vague generic reference.” ~ American paleontologist Norman MacLeod
Method
“The nature of the geological record is complicated, so it is not trivial to decipher it correctly.” ~ paleontologist Michal Kowalewski
As a guide to extinction, the fossil record readily fools. Life forms have specific ecological requirements. They may disappear from a biome because it did not suit them, but still survive in other habitats. If a location within that region becomes a fossil dig site, it may indicate an extinction that did not occur.
(A habitat comprises the relevant aspects of an environment in which a species population lives. By contrast, a biome is an area where organisms live with similar conditions, both geographically and climatically. Habitat is the environment from the perspective of a species, whereas biome characterizes a similar environment for all species within it.)
Paleontologists have seldom been methodical enough to cross-survey disparate sites of the same geological age: a difficult and expensive proposition. Instead, finds have been typically taken at face value wherever found. The methodological rigor necessary to accurately assess extinction has largely been lacking.
“Methods assuming uniform recovery potential of fossils falsely supported stepwise extinction patterns among studied species and systematically underestimated their stratigraphic ranges.” ~ paleontologist Rafal Nawrot et al
Extinction Events
There have been many mass extinction events on Earth. (By convention, extinction events terminate a labeled geological period, age, or epoch.) 10 have been especially catastrophic: where most of the existing diversity succumbed. (Conventional accounting tallies only 5 major extinction events. In 2015, American biologists Peter Ward and Joe Kirschvink cited 10 such massive extinctions (omitting only the Botomian event).) Another such horrific extinction event is presently gaining momentum.
1. Great Oxidation Event (2.45 BYA) (W–K): marine cyanobacteria infuse the atmosphere and sea surface with oxygen, slaughtering anaerobes who couldn’t tolerate O2.
2. Snowball Earth (~800–630 MYA) (W–K): thick ice covers most of the planet in 3 episodes, dropping both diversity and biomass.
3. Ediacaran (period) (542 MYA) (W–K): early worms ravage marine microbial populations.
4. Botomian (age) (517 MYA): a severe extinction pulse in the Early Cambrian epoch.
5. Cambrian (period) (488 MYA) (W–K): many weird wonders lose their lease on life, including the first flush of trilobites, in an extinction event called the Dead Interval.
6. Ordovician (period) (455 MYA): glaciations again take their toll with wholesale extinction in the tropics.
7. Devonian (period) (374–364 MYA): a series of extinc-tion pulses involving invasive species and volcanoes.
8. Permian (period) (252 MYA): Earth’s most severe mass extinction event – the Great Dying.
9. Triassic (period) (201 MYA): life on land and in the oceans takes a major hit from volcanic climate change.
10. Cretaceous (period) (66 MYA): the finale for large dinosaurs.
The chronicle of mass extinction events is incomplete. New discoveries keep being made of extinction pulses which profoundly affected life.
End-Ordovician Extinction
Beginning 455 MYA, the mass extinction event ending the Ordovician (485–443 MYA) was incited by 2 glaciations with global impact separated by a million years. The 1st glaciation came on the heels of an extraordinarily warm world during the early and middle Ordovician; this despite the Sun basking Earth with 5% less radiation than today.
There must have been a 10% greater concentration of greenhouse gases for such warmth. How that came about, whether by volcanism or a different carbon cycle, is not known. An elevated level of greenhouse gas during hothouse meant low atmospheric oxygen levels: something which organisms had adapted to by the Late Ordovician.
The toll of the Ordovician–Silurian extinction event was: 12% of marine families, encompassing 96% of marine species, at a time when most macroscopic life lived in the seas. More than 60% of marine invertebrates died. In terms of diversity loss, this was the 3rd-worst extinction in the history of life.
During the Ordovician Gondwana shifted south, into the polar region. This, along with the freshening impact of plants’ arrival on land, led to global cooling. The cooling initially delivered anoxic (low oxygen) bottom waters, especially beneath regions of high sea productivity, such as those zones fed by nutrient-upwelling currents. This anoxia accelerated extinction.
The onset of glaciation altered the carbon cycle, with the atmospheric oxygen level rising. This was after much extinction had already transpired.
Glaciation lowered sea levels by 70–100 meters. The drop devastated habitats on the continental shelves.
After the 1st glaciation, the planet reeled from frigid icehouse to hothouse in a half-million years. A strong thermocline developed, returning the deep ocean to its anoxic state.
Sea levels rose rapidly, blanketing shallow marine habitats with anoxic waters. The benthic faunas on the continental margins that had managed to survive the freeze succumbed from the warming.
The species that survived tended to be small and simple. Reducing body size shortens the time to reproductive maturity. This evolutionary strategy provides more opportunities for adaptation from one generation to the next.
The end-Ordovician extinction pulses favored life forms with greater tolerances to changing conditions. This adaptive stratagem would replay in succeeding extinction events.
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Extinction events are gauged by diversity loss, not population decline. The toll is measured in disappearance from the fossil record, not deaths, which are uncountable.
Ecological generalists – species with considerable tolerances to environmental changes – are the most successful migrants into new habitats. When invaded, generalists hang tougher than specialists, which are well-adapted to their niche but lack robustness in challenging times.
Specialists make the most of their habitat. Generalists fare better during rapid environmental changes, but there are fewer species of them. Owing to their inherent adaptability, generalists are less prone to speciation.
End-Devonian Extinction
Oceanic reefs teemed with life during the Devonian (416–359 MYA). Various carbonate-secreting organisms coalesced into communities that built cities on the ocean floor which served all sorts of sea creatures.
Extinction at the end of the Devonian came as a rampant reign of invasive species in the oceans catapulted into catastrophe. The world’s watery realm was hard hit: loss of 95% of shallow-water species, 60% extinction of deep-water species, and 22% of all marine animal families lost.
Ocean life at the end of the Devonian declined because opportunities for new diversity did not exist. In a death spiral that lasted 10 million years (374–364 MYA), what invasive species started was finished off by environmental changes, as oceanic oxygen levels plunged.
Jawed fish fared fairly well, especially considering the devastation of the reefs; but then, these predators may well have been the invaders that were the extinction initiators. While their populations eventually plummeted, these generalists survived and thrived through coming eras.
In the early history of life, speciation by vicariance was 3 times more common than dispersal. That pattern was flipped upside down during the Devonian extinction event: speciation by vicariance happened only 28% of the time, with 72% by dispersal.
Life on land suffered a more modest decline at the Devonian–Carboniferous boundary, owing to tectonic activity and climate changes. The tumult was not only of species dying out but also a failure of new ones to form. Diversity plunged as opportunities for new life declined.
“Recoveries from mass extinctions can be unpredictable.” ~ English paleontologist Michael Benton
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The worst extinction event of all time was 252–248 MYA, at the end of the Permian. The Paleozoic era closed with an extinction event that drove life’s diversity on land down to 5 to 10% of what it had been. At least 90% of terrestrial species and over 50% of marine families were wiped out during the Great Dying.
The multiple-pulse Triassic–Jurassic extinction event 217–199 MYA profoundly affected life on land and in the oceans. Most large amphibian species vanished. 20% of all marine families went extinct. Plant biodiversity was not severely affected, as plants adapted, though there was considerable species turnover in the mix of vegetation.
Global warming had an impact but does not explain the sudden demise of marine life during the Triassic–Jurassic mass extinction. No massive meteorite craters that can be considered causal have been found. Some evidence indicates voluminous volcanic eruptions at the time, though conclusions to causes are still speculative.
The last great extinction event, at the Cretaceous–Paleogene (K–Pg) boundary, 66 MYA, set the stage for the large life forms found today. There was an 80% elimination in marine invertebrates, a drastic drop in mammal species, and the utter demise of dinosaurs, save the birds that soar to this day. All told, 60–80% of all animal species were snuffed. The only marsupial to make it were opossums. Plants were profoundly affected by the abrupt changes caused by a bodacious bolide bashing into the Yucatán peninsula.
There have been numerous minor extinction events: minor only by comparison to those that were devastating. The disruptions to life in the lesser extinction pulses were dramatic, though not nearly as drastic as the vast numbers lost in major events. As an example: 183 MYA, a relatively minor mass extinction – the Early Toarcian oceanic anoxic event – wiped out more than 80% of the marine bivalve species, such as clams. Many shallow-water species went extinct. Oceanic methane release from tectonic plate movement was the probable cause.
Causes
“Major extinctions happen when a set of causal factors that might not be of serious consequence by themselves become aligned in time.” ~ Norman MacLeod
The causes of extinction events are various. A major meteorite impact creates a planetary shock, as it did 66 MYA (the K–Pg event), finishing off the large dinosaurs.
Comet storms and bolides occasion extinction events, as has happened in at least 3 events, with shock waves causing raging wildfires, massive floods, acid rains, and withering winters.
Radiation from supernovae sterilizes and kills surface life, as do solar flares. Geomagnetic reversals forge a flux of cosmic rays to similar effect. Disruptive radiation factored into the extinction events that ended the Ordovician, Devonian, Permian, and Cretaceous periods.
Continental drift has been another facet in mass extinction via global glaciation or warming, increased aridity, and volcanism. The configuration of continents has a pronounced effect on the viability of macroscopic life everywhere.
Volcanoes spew toxicity into the atmosphere, on land, and in the oceans, affecting ecology worldwide, setting up stepwise extinctions. Tectonic plate movements and volcanoes are the 2 sides of the same coin. Volcanism played a role in many mass extinction events, notably the most severe.
Changes in sea level, salinity, and oceanic oxygen levels contributed to several extinction events, as have pattern disruptions in ocean-atmosphere circulation. Rising sea levels from deglaciation can prompt volcanic activity as mantle plumes are put under more pressure.
Finally, life itself creates extinction events: by disease, predation, and other changes into the food web. Most notable is the evolution of new plants, with better protections against herbivory: depriving animals reliant upon the plants of the past.
All mass extinctions stem from a selfsame biotic dynamic: relatively rapid changes in the environment with which life forms are unable to cope quickly enough via adaptation. An extinction event ensues by a cascade of ecological dependencies, or by rapid environmental changes that simultaneously decimate numerous species. Extinction events invariably involve both dynamics.
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Mass extinctions are generally non-selective. The species that go extinct or are diminished are not those ill-adapted; just unlucky. The most evolutionarily advanced species may have their candles snuffed by the stiff wind of an extinction event, leaving behind lesser-but-luckier life.
The only selection factor in extinction events is tolerance to adversity. The hardiest organisms tend to be the most archaic: little ones that have persevered through tough times before. That withstanding, microbial species do go extinct.
“Most bacterial lineages ever to have inhabited this planet are estimated to be extinct.” ~ microbiologist Stilianos Louca et al
The safest place to be during an extinction event is the deep ocean. Benthic life fares better than anywhere else.
Mass extinction is typically followed by high species origination rates within a few million years. The availability of nutrients is a key factor in the revival of life.
Glaciation is a particularly subtle seeding for the next generation of species. Organic remains from the previous period are preserved by the cold, only to be released when the ice melts. At the other extreme, heading to hothouse by global warming provides no such head start.
“Environmental change correlates closely with extinction but not with speciation.” ~ American geologist Steven Holland
Extinction Theories
“What escapes the eye is a much more insidious kind of extinction: the extinction of ecological interactions.” ~ American evolutionary ecologist Daniel Janzen
The history of scientific endeavor is littered with virtual mountains of discarded theories, commonly developed by scientists who grip a belief system based solely on facts in favor while studiously ignoring inconvenient contradictions. Personal esteem often determines whether theories are rapidly rubbished or polished through time by discarding bits of discredited dross. The obscenely esteemed Darwin, who got much more wrong than right, is exemplary.
The realities of Nature are seldom neat, but the human mind demands that theories be kept tidy, else comprehension succumbs to conceptual chaos. What follows are extinction events for extinction theories, including denial of extinction altogether in favor of smooth evolution.
Catastrophism
“Theories of evolution rest on two arbitrary suppositions; the one, that it is the seminal vapor which organizes the embryo; the other, that efforts and desires may engender organs. A system established on such foundations may amuse the imagination of a poet; a metaphysician may derive from it an entirely new series of systems; but it cannot for a moment bear the examination of anyone who has dissected a hand, a viscus, or even a feather.” ~ Georges Cuvier
Beginning with 2 papers in 1796 which compared fossils to living animals, French naturalist Georges Cuvier became a major figure in the natural sciences in the early 19th century. Cuvier’s studies established extinction as a fact, and mass extinction as a theory.
Cuvier criticized evolutionary theories proposed by contemporaries Jean-Baptiste Lamarck and Geoffroy Saint-Hilaire: the notion that one life form gradually transforms into another. He repeatedly emphasized that there he could see no evidence of one fossil form changing into another.
Cuvier pointed out that mummified animals thousands of years old seem no different than those living today. Lamarck dismissed this by arguing that evolution happened slowly.
Cuvier retorted how Lamarck and other evolutionists had conveniently contrived hundreds of thousands of years “with the stroke of a pen” to justify their wild theories. Cuvier argued that one can judge what happens over a long time by multiplying what a lesser time produces. Since a lesser time showed no organic changes, there was no reason to think that a longer time would be any different.
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. ~ Georges Cuvier
Cuvier came to believe that the fossils he had examined were remains of species now extinct. This led Cuvier to catastrophism: catastrophic events caused mass extinctions, as evidenced by geological features, notably rock layering (stratigraphy). Each catastrophe set the stage for a new wave of creation. Cuvier was a devout Lutheran.
Cuvier’s conversion to catastrophism was abetted by collaboration with French chemist, mineralogist, and zoologist Alexandre Brongniart. Together they correlated fossils to their place in the geological column (sedimentary rock layers). To others stratigraphy told a different story.
Uniformitarianism
“The present is the key to the past.” ~ Uniformitarianism creed
A group of English geologists, William Buckland and Robert Jameson among them, interpreted Cuvier’s work much differently: as support for the biblical flood. Cuvier was Christian, but never floated his catastrophic boat that far downstream into the thought pool known at natural theology: an influential branch of theology in the early 19th century, founded on reason and ordinary experience, but taken to godly ends.
On the other bank of evolutionary theory, sedimentary rock inspired a steady-as-she-goes school of thought. Uniformitarianism was the brainchild of Scottish geologists in the late 18th century.
James Hutton, the father of modern geology, coined this concept of constancy, with a side dish of gradualism: that the same processes and natural laws that operate in the universe now have been constant everywhere since time immortal.
“We find no vestige of a beginning, no prospect of an end.” ~ James Hutton
Scottish geologist Charles Lyell propounded uniformitarianism in his 1830 book Principles of Geology. Lyell was a close friend of Darwin and a considerable influence on him. Lyell, a devout Christian, had trouble accepting the idea of evolution without it being part of God’s handicraft.
Lyell’s legacy includes naming the geological epochs of the Cenozoic era, which English lexicographer H.W. Fowler characterized as “regrettable barbarism,” lamenting Lyell’s laxity in not consulting a philologist in coining terms.
English polymath and Anglican priest William Whewell minted the term uniformitarianism in the mid-19th century, as well as coining catastrophism for the creed that Earth was shaped by a series of sudden, violent events.
From 1850 to 1980, geologists generally endorsed uniformitarianism and geological gradualism, rejecting that cataclysmic events played any significant role in Earth’s formation. Uniformitarianism was embraced partly as a rejection of what was on the other side of the same theoretical coin: that the catastrophists of the early 19th century granted God as the shaper of Earth’s history.
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Charles Darwin published his book on evolution, On the Origin of Species, in 1859. Embracing uniformitarianism, Darwin denied mass extinction, as it didn’t fit his natural selection hypothesis. For Darwin, extinction was a slow process, affecting each species via ecological competition.
“The extinction of old forms is the almost inevitable consequence of the production of new forms. The utter extinction of a whole group of species may often be a very slow process.” ~ Charles Darwin
Mass extinction was a well-established fact by Darwin’s time. Seeking simplicity, Darwin blindsided himself with his sophistic “survival of the fittest” story.
