When Life Nearly Died: The Permo-Triassic Mass Extinction
"The Mother of All Mass Extinctions" - Douglas Erwin, 1996
However, some models suggest that this is a two-phase extinction, with an earlier one between the Middle and Late Permian (the Guadalupian extinction), then EACH of these is among the greatest extinction events. As with the K/Pg there are many names for this event, including the Changhsingian/Induan extinction, or (using older terminology) the Tartarian/Scythian extinction.
This was a "game changing" event. Most estimates show that diversity of marine organisms was essentially stable from the Late Ordovician until the end of the Permian (minus the crash-and-recovery intervals at each mass extinction). In contrast, life ever since the Permian has been relatively steadily increasing in diversity, surpassing Paleozoic levels sometime in the Cretaceous.
Sepkoski's studies did not merely look at general levels of diversity. He found that there were generally three different sets of organisms (not necessarily close relatives) who tended the share the same fates: when one member of each "evolutionary fauna" did well, the others were doing well, and when one suffered, they all suffered. He named these three evolutionary faunas the Cambrian, Paleozoic, and Modern faunas. (Don't let the names fool you! The "Cambrian fauna" is still present, but very rare; the Paleozoic fauna survives at moderate levels, and the Modern fauna goes back all the way to the Cambrian.)
The groups in each of the evolutionary faunas tend to share similar types of general traits:
The marine realm of the Permian was thus, like most of the Paleozoic, dominated by sessile epifaunal suspension feeders. There were nektonic predators around, and clams burrowing and snails crawling, but they were rarer than you would see in the seas today. On land the forests contained both conifers and other primitive seed plants and a great diversity of spore plants (horsetails, other ferns, club mosses, etc.): this is long before the rise of flowering, fruiting plants. The heyday of giant insects and millipedes and so forth was over, but there were still some of these around. Freshwater systems were patrolled by large amphibians, and the land dominated by the therapsids (protomammals), with reptiles (including the early precursors of the crocodilian-dinosaur group Archosauria showing up by the very end.)
In the marine realm, this was a major overhaul of diversity, with the world of the Mesozoic and Cenozoic radically different from that of the Paleozoic. Victims include:
There are major diversity losses in bryozoans (again), brachiopods (ditto), ammonoids (ditto), conodonts (ditto), those echinoderm groups that do survive, and terrestrial and marine vertebrates.
After the extinction, diversity was greatly reduced. Very, very few species were present, but some of these survivors were very common. We'll look more at patterns of survivorship in a bit.
Causes for the Greatest Extinction
Up until recently this was thought to be a very gradual event (see paleontologist Curt Teichert's quote at the top of these notes.). However, work in the 1990s established the catastrophic nature of this event. Some ideas were suggested as to what could cause such tremendous death at this time:
The ultimate cause appears to be the Siberian Traps, a huge lava field in Siberia with an area of 5 million km2, and a volume of about 3 million km3! (This would cover North America to a depth of 121 m (nearly 400'!). It erupted over the space of less than 1 Myr, releasing 12,000-18,000 Gt C (in comparison, modern atmospheres are only 800 Gt C, and preindustrial levels were about 600 Gt C.) This produced a tremendous increase (8x background level) in atmospheric CO2. This lead to extreme global warming, which lead to warming of the sea floor, which lead to melting of the methane clathrates (methane frozen in ice on the sea floor), which bubbled into the atmosphere, which led to even more global warming. The oceans would also become more acidic, causing damage to shell-forming organisms.
New evidence shows that there would likely have been catastrophic levels of acid rain and oceanic acidification, worldwide. One effect of the increased sulfates injected into the atmosphere is that the ozone layer would be damaged, increasing the level of dangerous UV radiation reaching the Earth's surface.
Additionally, there were tremendous drops in atmospheric and oceanic O2 due to the mass death of so many land plants and phytoplankton and to oxidation of the methane. Furthermore, there is strong geochemical evidence of a burst of hydrogen sulfide from sulfur bacteria, making the oceans become sulfidic and becoming poisonous to animals (and also destroying the ozone layer.) Also to make matters worse, increases in global temperature would decrease the temperature differential around the world, decreasing oceanic and atmospheric circulation, and thus reducing the churning up and oxygenation of the ocean water. Continued eruptions for the space of 100s of kyrs and the slow recovery of the atmosphere, ocean, and pedosphere meant that the ecosystems continued to suffer for millions of years.
There are strong patterns to survivorship vs. extinction at the event:
In order to better test what was going on at the event, we need to be able to look at detailed stratigraphic ranges of fossils and long records of geochemical changes. In both cases, we need a record where we have rocks from before, at, and after the boundary. Thankfully, the American southwest, the Ural region of Russia, South Africa, and parts of China have all of these.
New evidence (from 2014) demonstrates that the main pulse of extinction took a mere 60 kyr, plus or mins 48 kyr, to occur!
On land we see meandering streams temporarily disappearing (due to loss of ground-cover plants), and a great increase in the amount of fungal spores and hyphae in the fossil record (the decay of the rotting corpse of the Paleozoic Era.) The recovery fauna and flora was exceedingly depauperate (low in diversity): a small handful of taxa characterize both terrestrial and marine communities. Very recent work suggests a second round of extinctions a mere 180,000 years or so after the P/Tr boundary (although once again, we have to be concerned about stratigraphic resolution.)
On land, the best survivors among the amniotic vertebrates are: those that nest in burrows; those that may have been mountain-dwellers; and those which were semi-aquatic. All of these are groups which survive very well in low oxygen, high carbon dioxide conditions. Furthermore, groups of animals (like advanced protomammals and archosaurs) which very sophisticated methods of apparatus survive quite well.
Even though the main pulse of extinction occurred quickly, the Siberian Traps continued to erupt for several million years, keeping the planet's ecosystems destabilized throughout the Early Triassic. Only after they had settled down could Life get bak to some form of normality. But the make up of the world had changed. The oceans became dominated by swimming and crawling and burrowing forms. And on land the Age of the Protomammals was over. The Age of Reptiles, and soon the Age of Dinosaurs, was at hand.
But what if it is death from above rather than below? With the discovery in 1980 of the K/Pg asteroid impact, D. Raup suggested that ALL mass extinctions (and maybe even all extinction) were due to impacts!! The success of the Chicxulub explanation for the K/Pg, many people sought possible impacts for the other Big 5. A possible Tr/J impactor--the Magnicoagan astrobleme in Canada, was proposed, but it turns out to be millions of years too early. (It MIGHT be the cause for the mid-Late Triassic faunal turnover in the American Southwest, but that is just about it.) Also, there have been reports of an iridium spike in the American Northeast at the boundary, but these have not been duplicated elsewhere, and this region was definitely the site of deep volcanic activity at that time.
Impactor madness reached its peak in 2006, when scans of the surface of Antarctica under continental ice saw a circular object even bigger than the Chicxulub crater. Even though there is no data whatsoever to show this is an asteroid impact rather than a collapsed volcanic caldera or other object, and even though there was no evidence that this object was emplaced 252 Ma, and even though there are not impact-related proxies (tektites, iridium spikes, etc.) at the P/Tr boundary anywhere, this was touted as being the "impact that caused the greatest mass extinction." No further data supporting it has been forthcoming, but social media being what it is, the story raises its head every few years.
An addition to the "it's all impacts" is "it is a cycle of impacts". Raup and Sepkoski (in 1984) found an apparent 26 Myr periodicity in the extinction data. Two teams of astronomers independently argued that a dark distant object periodically came close enough to the Oort comet cloud (the farthest edges of the Solar System) to disrupt cometary orbits, sending them plummeting towards the Sun (and any planet unlucky to be in their path!). One team even gave this object a name: Nemesis. However, the periodicity is a statistical artifact of the analysis, Additionally, the dates of many of the extinctions have been changed by millions of years or more due to revised geochronology, so it is a "garbage in, garbage out" situation.
An alternative to Nemesis is independently proposed every few years (by people who don't search the published literature, apparently…). The Solar System passes through the Galactic Plane about 2.7 times per orbit around Galactic Center. (It takes about 225 to 250 Myr for the Solar System to complete one orbit). Also, it passes from the (relatively empty) inter-arm spaces of the Galaxy into the more cluttered arms. In both models, the hypothesis is that as it passes through the plane or into the arms, the chances of close passes with objects that knock comets out of the Oort Cloud increases, sending showers into the planets of the Solar System.)
In February 2015 a suggestion was made that "dark matter" was the culprit. This is practically a non-answer, though, because we know essentially no properties about dark matter. And the evidence used was the supposed periodicity--which does not appear to be real! So invoking a substance whose properties we don't know to both cause supervolcanism and knock asteroids out of their orbit on the basis of a periodicity that doesn't exist is not good science!
There IS a common pattern to mass extinctions, though: "The Game of Thrones." All of these seem to be associated with environmental changes of size and intensity far beyond normal, such that adaptation to the changes are not possible. During the mass extinction, you either win or you die.
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