Extinction, a fact of life
Extinction of species, genera and sometimes families has been a part of the history of life on earth since life first appeared approximately 3.8 billion years ago. Whenever a species reaches a point on its timeline when the average death rate of its component members exceeds the replacement rate whether by birth or germination or whatever may be the appropriate method, the complete disappearance – extinction – of that species becomes inevitable. This is the process called background extinction. It goes on all the time, and is thought to be responsible for the demise of 95% of all species that have ever gone extinct. It is not associated with any one environment changing event or process.
At times however there have occurred sudden and massive declines in life at all levels known as mass extinctions that may indeed be linked to some cataclysmic event; one that so impacts the environment as to threaten all living things. Although the past has undergone many such events science currently recognizes five particularly lethal episodes, collectively known as major mass extinctions.
The Ordovician – Silurian extinction event
The earth of 445 million years ago, at the end of the Ordovician was a very different place than it is today. Modern man would not recognize its geography, nor could he survive for very long in its atmosphere. Almost all of land above water was concentrated in the two super continents of Gondwana and Laurentia and this land was devoid of all life saving extremely rudimentary plant forms. The atmosphere contained just 68% of the oxygen than we currently enjoy, while Co2 levels were at 15 times today’s concentration. Sea levels were higher by as much as 1500 feet; temperatures averaged 2 degrees warmer than at present.
The great, warm, shallow seas teemed with life; trilobites, graptolites, bryozoans, mollusks, cephalopods and early fishes abounded.
And then, 445 million years ago, everything changed. Gondwana, having drifted over the South Pole, began to glaciate. Ice accreted, sea levels and temperatures plunged. CO2 concentrations dropped to nearly half of their former levels. Around 50% of all marine genera vanished forever, the trilobites, graptolites and cephalopods succumbed nearly en mass. Struggling plant life on land also declined.
For many years it was thought that advancing and retreating glaciations with their attendant climate change was sufficient explanation for the die off, but recently Dr. Adrian Melott, astrophysicist at the University of Kansas has suggested a triggering mechanism. Melott believes that a supernova, exploding within 10,000 light years of the planet may have sent forth such a burst of gamma radiation as to cook the poorly shielded sea dwellers and rudimentary land plants, while at the same time changing the very nature of the stratosphere, enveloping earth in a brown fug of nitrous oxide, dropping temperatures and adding to the severity of the overall disaster. It is an intriguing and plausible idea that is currently gaining widespread scientific support.
The late Devonian extinction
Life rebounded and diversified after the great Ordovician calamity during the Silurian and throughout most of the Devonian age. Bony fishes became commonplace, true plants evolved on land and vertebrates like amphibians began to flourish as well. Insects and spiders appeared and began to diversify. Then, at a point about 375 million years ago on the boundary of the Frasnian and Femennian stages of the late Devonian disaster struck once again.
Marine life was particularly hard hit; approximately 70% of all marine taxa are assumed to have succumbed. Invertebrates and jawless fishes were particularly hard hit, and as in the Ordovician die off shallow dwelling organisms suffered most. Reef building essentially came to a halt as the reef building organisms were unable to adapt to the changing environment.
Global cooling with its attendant rising and falling sea levels is again thought to be the driving force behind the second major extinction event. Meteor impacts have also been suspected but the evidence is circumstantial at best. With or without the arrival of extra terrestrial bodies the effect on marine life was shattering.
The Great Dying
This is the evocative name given to the massive extinction episode that occurred 250 million years ago at the boundary of the Permian and Triassic ages, and which very nearly ended all life on earth. Also known as the P/T extinction the die off affected every form of living entity; 9 out of 10 marine species vanished, as did 7 of 10 land species. It is, as far as anyone knows, as close as this planet has come to becoming a blasted, sterile rock.
There are many suspects when it comes to finding the causal event for this cataclysm. Volcanism in the form of massive eruptions in what is now Siberia ejected over 1.5 million cubic kilometers of lava, and incalculable volumes of greenhouse and toxic gasses as well, more than enough to kill outright. At the same time decreased sunlight caused by this debris in the atmosphere would chill the planet and promote a drop in sea levels, as was the case in the Devonian extinction. The earth plate tectonics had conspired to create essentially one enormous land mass, Pangaea, and one super ocean, Panthallasa with disastrous results for life forms everywhere.
And yet, these are all very slow processes, and modern stratographic techniques put the period of the dying at a maximum period of 100,000 years, and possibly at as little as 8000 years. This is a wink of an eye on the geologic timeline.
Dr. Luann Becker, a geologist with the University of California working with a team funded by NASA, may have found the smoking gun. Working in rock formations from the P/K boundary, Becker found molecules associated with meteor impact, called fullerenes or Bucky balls. These molecules, shaped like geodesic domes are capable of capturing gases, and the gases which these captured are isotopes called helium3 and argo36 – gases rare on earth and common in space. Something carried those gases to earth and it appears that the bearer was an asteroid between 6 and 12 kilometers in diameter.
The impact of that gigantic rock, the titanic blast, the firestorm, the winter caused by the debris coupled with the volcanic and tectonic events already underway just about finished life as we now know it on this planet.
The end Triassic event
Hard on the heels geologically speaking of the P/K extinction, at the other end of the Triassic age there occurred a fourth major extinction event, one that may in fact have ushered in the long rule of dinosaurs as the earths dominant life form. Occurring just about 205 million years ago, this cataclysm claimed about one half of all marine species and a whopping 80% of land based quadrupeds. The dominant quadrupeds at the time were not the early dinosaurs nor their precursors, the archosaurs. Instead, mammalian type animals known as therapsids were gaining ascendance.
The end Triassic extinction hit the therapsids unusually hard, changing the balance of power by sweeping the dominant contender from the playing field. This allowed the dinosaurs to flourish and diversify. The therapsids nearly became one of paleontology’s footnotes; fortunately a very few mammaliformian lines survived, eventually giving rise to mammals and therefore, to man.
The causes of the end Triassic extinction are currently poorly understood; a key suspect is the sustained undersea eruption of a gigantic – 4.3 million square miles – region known as the Central Atlantic Magma Province. A boost from yet another cosmic visitor has not been ruled out but the evidence for this is tenuous at best.
The K/T extinction
If one extinction event cleared the field for the dinosaurs, another surely cleared the field of them. Sixty five million years ago, on the boundary of the Cretaceous and Tertiary ages another massive dying took place. This time the global disaster killed all large land vertebrates, all dinosaurs, plesiosaurs, mosasaurs and all the flying reptiles. It destroyed plant life in abundance, plankton needed to sustain sea life, and most tropical invertebrates.
There clear evidence that once again, multiple events were responsible for the devastation. A huge volcanic event, the eruption of the Deccan traps in what is now India spewed many millions of tons of ejecta into the atmosphere, with the familiar toxic and greenhouse gases. Today, the remains of this massive eruption cover nearly 200,000 square miles to a depth of a mile or more in places. It was a massive and potentially killing event in and of itself but we have evidence, and very clear evidence this time of a meteor impact that was almost certainly the ultimate K/T kill shot.
The Yucatan peninsula enfolds a 180 kilometer wide crater, dubbed the Chicxulub crater, which is the impact site for a small asteroid or large meteor that impacted earth exactly at the K/T boundary. Adding its impact to the already stressed environment caused by the eruption of the Deccan traps was enough to end the reign of the dinosaurs, and much else as well.
Is there a similar event in the future? Given the evidence of past it seems clear that this can be a matter of time.