"The way in which many Paleozoic life forms disappeared towards the end of the Permian Period brings to mind Joseph Hayden's Farewell Symphony where,
during the last movement, one musician after the other takes his instrument and leaves the stage until, at the end, none is left." -- Curt Teichert, 1990
"The Mother of All Mass Extinctions" - Douglas Erwin, 1996
"Nature was quick to pass the sponge of her deluges over these awkward sketches [i.e., dinosaurs and other Mesozoic megafauna], these first nightmares of Life. And yet, what curious prints might have been made of all these creatures! Alas, the vision is lost forever." -- Auguste comte Villiers de L'Isle-Adam, L'Ève future (1886)
and
"Night comes to the Cretaceous." -- J.L. Powell, 1998
BIG QUESTION: How do the Cretaceous-Paleogene and Permo-Triassic mass extinctions compare?
When Life Nearly Died
Permian/Triassic (251.902 Ma): The "Mother of All Mass Extinctions" (so named by Doug Erwin of the Smithsonian), this is the greatest diversity crisis known. If this was the single terminal Permian event, then it was an event with 55.7-82% of the marine genera went extinct (which corresponds to an 80-96% species level extinction). Or, to put it another way, there was only 4-12% survivorship at a species level, and given that a species could survive with very few individuals, it was much greater than 96% of individuals lost!). In comparison, the K/Pg had a 40-47% genus loss.
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.
Jack Sepkoski's studies for the evolution of marine life (which led to the recognition of the "Big 5" mass extinctions) 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 therapsid synapsids (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: a tremendous volume in a geologically-short interval. The Siberian Traps are the greatest example of a flood basalt: a form of eruption where vast volumes of lava (along with dissolved gases) spread out over huge regions in burst after burst. They are also called "large igneous provinces". The don't represent eruptions from a single conical volcano: rather, a huge region would have fissure eruptions.
The Earth Goes to Hell: Killing Agents of the Siberian Traps
However, although the Siberian Traps are the causal agent, the killing agent isn't the lava (except for the unlikely animals and plants of latest Permian Siberia!). Instead, the killing agents are produced mostly from the gases and aerosols released by the eruptions, and the side effects of these.
First and foremost is the carbon dioxide release, producing some of the most extreme global warming in Earth's history. Today (2018) there is about 873 Gt (gigatons) of C (carbon) in our atmosphere, reflecting about 410 parts per million CO2 (1 ppm = 2.13 Gt C). Before the Industrial Revolution, that value was around 600 Gt C. The Siberian Traps dwarfed these values by several orders of magnitude. Initial estimates were that over the course of the eruptions about 12,000-18,000 Gt C were released; newer modeling points to values closer to a mind-staggering 170,000 Gt C!! This didn't happen as a single burst, so there was no moment when Earth's atmosphere had 80,000 ppm CO2 during this interval, but it was still tremendously higher than today (or, more important for this issue, compared to the time before the extinction). Values aren't certain, but carbon dioxide levels had an increase of at least 8 times, and possibly more! This lead to extreme global warming (surface temperatures rising by more than 7°C, possibly much more), 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 because of the carbon dioxide mixing with water, causing damage to shell-forming organisms.
Recent work points to the hyper-global warming as being first a jump from 710 to 2800 ppm, with a 38°C global average temperature. (For reference, in 2018 the global average temperature is 15.6°C [60.08°C].) After the full rise of CO2 and CH4 the estimates are that the atmosphere reached 5600 ppm, with global average 45°C (113°F)!!! This is hotter than a hot tub!
Additionally, the new studies point to as much as 18,000 Gt HCl (that's hydrochloric acid!) being released by the melting of underground rock salt deposits. Between the HCl, sulfuric acid (a byproduct of sulfur dioxide, a common component of eruptions), and carbon dioxide there would likely have been catastrophic levels of acid rain and oceanic acidification, worldwide.
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. 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.
A consequence of the ocean anoxia is the promotion of sulfur bacteria. These prokaryotes prefer anoxic and dysoxic conditions. Their photosynthesis does not release oxygen as a waste product; instead, they release hydrogen sulfide (H2S), a gas highly poisonous to both aquatic and land animals. With shallow sunlit water devoid of their oxygen-generating competitors, the purple bacteria spewed out hydrogen sulfide, further making the shallow seas and land a hellish condition for other life.
But wait, there's more! The sulfates ejected by the eruptions and the H2S from the bacteria can destroy ozone. As these gases made their way into the higher atmosphere, the Earth lost its protection from dangerous UV radiation reaching the surface. Malformed pollen had already been known from latest Permian rocks; a recent experimental study showed that these malformations can be produced by subjecting plants of the same general groups present in the Permian with high levels of UV. Such malformations would greatly decrease plant fertility. (On top of that, the UV can kill the plants themselves, as well as animals.)
The sulfur aerosols did more than this, however. Their first effect was to block insolation, plunging the Earth into a temperature drop of 10-15°C for at least 10 years (some estimates suggesting a more profound extended 10s of kyrs cold snap.) But once the sulfate rains out of the atmosphere, it allowed the hyper-global warming to occur.
Stratigraphic evidence demonstrates that the main pulse of extinction took a mere 60 kyr, plus or mins 48 kyr, to occur!
And keep in mind, these killing agents are produced independent of living things. The eruptions would continue for 100s of kyrs, so that there would be long slow recovery for the atmosphere, ocean, and pedosphere (soil). This meant that the ecosystems continued to suffer for millions of years.
Patterns of P/Tr Extinction:
There are strong patterns to survivorship vs. extinction at the event:
In the terrestrial realm, temperature stress, anoxia, hypercapnia (too much CO2), UV radiation, and probably acid rain were major killing agents. In the marine realm these plus sulfidication (okay, probably also affected the land!) and ocean acidification were at play.
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.
After the End
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.)
In the seas aftermath of the P/Tr, there are many algal mats (stromatolites) in the shallow seas. Stromatolites are more characteristic of the world before animals; the rise of grazers like snails meant that stromatolites only form in conditions where animals can't live (hypersaline water, for instance). Also, as mentioned, most of the world's fauna and flora is made up of a few common species worldwide (rather than smaller numbers of individuals but greater diversity of species, with different species in different parts of the world.) There are no reefs and no calcareous algae in the seas in the Early Triassic; they only show up later in the Triassic. And there is some evidence that water temperatures were lethally hot at the equator: 40°C (104°F) or HIGHER!, rather than todays 25-30°C (77-86°F). Also, warm water holds less nutrients and less oxygen than cold water, making water not merely hot but starved. (Land temperatures would have also been phenomenally hot, but we cannot measure these as directly.)
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 hundreds thousands of years, keeping the planet's ecosystems destabilized throughout the Early Triassic. Only after they had settled down could Life get back 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.
A "One-Two" Punch?
This is not the only major mass extinction associated with a flood basalt/large igneous province. The Triassic/Jurassic mass extinction is associated with the rifting of Pangaea and the eruption of the Central Atlantic Magmatic Province (CAMP). As we will see, the Cretaceous/Paleogene mass extinction has its Deccan Traps (although we will see there is something else also at play!). Some smaller extinctions also have LIPs associated with them: the Paleocene/Eocene and the North Atlantic event; the Early Jurassic/Middle Jurassic and the Karoo-Ferrar; etc.. V. Courtillot has shown there does seem to be a strong correlation, at least from the Early Permian onward. (Curiously, the mass extinctions seem to be linked to eruptions that followed long periods of magnetic stability, with a lag time of about 20 Myr after the "superchron" ends).
Recent evidence suggests that LIPs actually deliver a "one-two" punch. In both the P/Tr and Tr/J there is evidence for a short term (90 kyr or less) cold snap right at the beginning of each extinction. This might be due to the sulfates erupted into the atmosphere: stratospheric sulfates are long known to reflect a lot of sunlight, cooling the world beneath. Indeed, so much seems to be erupted at these two extinctions that there is evidence of a mini-duration but intense-level ice age, enough to cause a major drop of sea level and erosion (unconformities). (So maybe both Lyell and Cuvier were right: there is both a gap and a catastrophe at some extinctions!) The extended extreme volcanic winters themselves bring about extinctions, as might the crowding on the continental shelves as sea level drops. (A well-known aspect of ecology is that the area of a region is directly related to its species abundance: a rapid decrease of area produces a loss of species.)
Indeed, a "one-two" punch (or maybe in the case of the P/Tr a "one-two-three-four-five-six-seven-etc." punch!) might be necessary to produce one of the Big 5 mass extinction events?
Was my lecture on the Permo-Triassic Mass Extinction not enough for you? Here is another one, by Beniot Beauchamp (University of Calgary):
We'll start our look at particulars of mass extinctions by looking at the most famous of all: the end of the Age of Dinosaurs. This is the boundary between the Mesozoic Era and the Cenozoic Era (and thus between the Cretaceous and Paleogene Periods, the Late Cretaceous and Paleocene Epochs, and the Maastrichtian and Danian Ages). Currently dated (as of 2013) as being 66.0 Ma, this is the extinction event that eliminated the dominance of dinosaurs and allowed the adaptive radiation of mammals.
(By the way, this event is by no means the largest of mass extinctions: we'll see that one next lecture! It was also not the final extinction of the dinosaurs, for as we'll see later in this course Dinosauria is still alive and flying!)
This event goes by many names:
Some of the major victims and survivors of the event:
In the marine realm, among the main victims were:
In contrast, there does not seem to be too much in terms of extinction among smaller bottom-dwelling organisms.
In the terrestrial/continental realm, major victims include:
In the terrestrial realm there is a transition from a gymnosperm-dominated flora to an angiosperm (flowering plant)-dominated one. The toothless crown-group birds survived, as did the living groups of amphibians, turtles, lepidosaurs, and crocodylians. The long-snouted champsosaurs (distant kin to the archosaurs) survived and thrived in the early Cenozoic, but have subsequently died out.
Some Older Ideas
Many hypotheses proposed for the K/Pg Extinction. In evaluating the hypotheses, must consider:
Here are but some older proposed causes for the K/Pg event:
Modern Approaches to the Cretaceous-Paleocene Extinction
The global nature of the K/Pg extinction would seem to favor some causal agent which could affect the whole planet. Cosmic (extraterrestrial) phenomena might be a good possibility.
1971: Suggestion by Dale Russell (dinosaur paleontologist) and Wallace Tucker (astrophysicist): a supernova killed the dinosaurs.
Supernovae are exploding stars: put out TREMENDOUS amount of energy. If a star in a nearby solar system exploded, it would bombard surface of planet with radiation, bringing radiation sickness, cancer, etc.
Modern analogue: during 1950s through 1970s, greatest fear about nuclear war was radioactive fallout.
Predictions:
Fits prediction. However, problem because it is an untestable (and thus non-falsifiable) hypothesis:
So, remains as a potential but no reason should be supported. Was the leading candidate during the 1970s.
Confirmed Potential K/Pg Causal Agents
While the above phenomena are largely insufficient to explain the event, there are three confirmed large-scale environmental changes during the end of the Maastrichtian that potentially are involved with the extinction. In order of appearance (and of increasing severity) they are:
Maastrichtian Regression:
The Maastrichtian is last Age of Late Cretaceous Epoch; regression refers in this context to any period of sea-level drop. In this particular case, it was triggered by reduction in mid-ocean ridge activity. As the mid ocean ridges shrank, the water that they had displaced onto the continents throughout the Late Cretaceous drained away.
For the marine realm, this caused a very large drop in the area for the shallow marine communities. Additionally, it removed a major source for marine productivity and modified global circulation patterns. For the terrestrial realm it made climates more continental (hotter summers, colder winters) because now not every place was essentially close to the sea shore. This would also mean that habitats would shift.
The Maastrichtian regression unquestionably occurred (although as with all such changes, there were local and global smaller scale sea level rises mixed in with the pattern of the drop.) One would predict that the environmental effect of the Maastrichtian regression would play out over the period of a few million years.
Deccan Traps Volcanism:
The Maastrichtian has long been known as a period of intense volcanism in parts of the world (see "Global Diastrophism" above). For instance, in North America, associated with change in mountain building in Rockies (the beginnings of the Laramide Orogeny). But the biggest aspect of this volcanism is the Deccan Traps.
The Deccan Traps were a GIGANTIC series of lava flows in western India. They were the most major flood basalt event since the Siberian Traps at the Permo-Triassic boundary and the Central Atlantic Magmatic Province volcanism that formed by the break up of Pangaea at the Triassic-Jurassic boundary. In some places the Deccan Traps are 2.4 km (1.44 MILES) thick. An area of 2 million km3 was covered. As with all flood basalts, it was not a single continuous event; instead, there would be eruptions; periods of cooling from sulfates; global warming from excess greenhouse gases; stabilization of the environment after the eruption had ended; then a new eruption.
The Deccan Traps began around half a million years before the K/Pg boundary. The lowermost beds erupted during a global magnetic normal time (subchron C30n), while the impact of the asteroid and the boundary itself would be later in a magnetic reversed interval (subchron C29r). Since the switch from C30n to C29r happened 350 kyr prior to the boundary, the asteroid impact could NOT have been the cause of the Deccan Traps. That said, there is some evidence that a few thousand years after the impact the intensity of the eruptions increased: this timing would be consistent with magma chambers being disrupted by impact-generated mega-earthquakes, and then percolating slowly up to the surface.
As with the Siberian Traps and CAMP, the Deccan Traps would provide both a cooling and a warming component. There is a 5-6°C increase in annual temperature in western North America during the pre-impact eruptive phase, that might reflect the volcanic greenhouse contributions. Furthermore, we would expect ocean acidification as a side effect.
So, Deccan Traps themselves were a MAJOR event, and might have contributed to the extinction event. Had just this eruption plus the Maastrichtian regression occurred, there probably would have been a mass extinction. But perhaps it would have been much less severe, and not an "era-ending" one. But Nature had one more, far more spectacular event to unleash.
The Chicxulub Impact:
The discovery of an asteroid impact at the end of the Cretaceous began in the latest 1970s and was published in 1980. Geologist Walter Alvarez was investigating a layer of clay in Gubbio, Italy at the K/Pg boundary. He wanted to determine length of time represented by the clay layer. He knew that the limestone below was latest Maatrichtian and those above were earliest Danian (early Paleogene). But how much time was there for the clay layer? A few years? Decades? Millennia? More? (The biostratigraphy above and below showed it couldn't be more than a million years.) Being clay he couldn't radiometrically date it, and there is no magnetic flip-flop right near the boundary. So he consulted his dad (Nobel winning physicist Luis Alvarez) for possible solution. After teaming up with a few chemists, the decided to look for meteoritic material as a possible clock. This was based on the following observations:
The element used: iridium (a platinum-like metal, common in metallic asteroids but very rare in Earth's crust).
When examined Gubbio clay, found a huge increase in iridium (the iridium spike) at base of clay. Using their method it would mean that there was a few million years missing, which was clearly not the case. So which assumption was not correct? They eventually realized that this wasn't a case of an average rate of infall. Instead, they hypothesized that an asteroid impacted Earth at the K/Pg boundary. Luis Alvarez had been part of the US nuclear weapons program, and later had worked with NASA on calculating the energy of impacts from asteroids and comets. Both of these sets of skills proved useful in determining the effect of the end-Cretaceous impactor
Although there had been a very good chance that the impact had hit oceanic crust that had since been subducted back into the mantle, the crater was actually recognized in 1990s. It is in the Yucatán Peninsula of Mexico. It is not visible from the surface except by radar scanning, because it is covered by 300-1000 m of Cenozoic sediments. But cores drilled during petroleum exploration had found a disrupted and melted layer right around the K/Pg boundary; with the publishing of Alvarez paper and subsequent search for evidence to test their hypothesis, these cores were re-examined and re-interpreted as being from a crater. Seismic and gravity scanning revealed the presence of a 180-km diameter crater, exactly the right size for hypothesized impactor. There are even sinkholes (cenotes) in the rock above that trace out the disrupted layer, because groundwater in the limestone preferentially drains down into the crustal fractures. The crater is named Chicxulub after a town near the first core where the disruption was recognized.
Between the initial Alvarez paper and the discovery of the crater, several proxies of the impact beyond the iridium spike were recognized. These include:
The map of the K/Pg sites and the thickness of the disrupted layers form a bullseye around Chicxulub, showing that this rather than some other spot was the source of the blast.
Effects of Chicxulub
An asteroid impact is a very different kind of causal agent, and its effects are more instantaneous than the other types of mass extinction causes we've explored. (Some of the effects, however, are extended over long periods.) Here is what we have reconstructed so far.
Phase I (the same day): Shockwave and Tsunami: A magnitude 10.1 earthquake would be expected from this blast. In the modern world this would likely topple buildings all over the planet; in a Cretaceous world it might be very disruptive to forests and cause landslides (and make dinosaurs fall over...), but wouldn't be a primary killing agent. A shockwave of hurricane-force winds would spread over southern North America and northern South America would also be locally dangerous, and the energy flash from the impact would incinerate everything in line-of-sight, but again these are not mass extinction-causers. A tsunami of 100-250 m would surge out, but again while regionally disruptive would not bring an era to an end.
Phase II (later the same day): "Easy Bake Oven" and the Canopy Collapse: However, other events of that first day would be globally catastrophic. Infalling material would mostly burn up in the atmosphere. Your average meteor doesn't put out much heat, but so much infalling material generates substantial infrared radiation. This heat raises air temperature by only about 10C° (18F°), but would be fully absorbed by rock, leaf, flesh, and any other opaque material. It is predicted that the increase in infrared radiation would be 8-10x that of high noon at the hottest spot of the Earth, and persist for many minutes to hours. Living tissue would bake, unless underground 10 or more cm (heat wouldn't have time to make it that deeper) or underwater (upper few microns of water might boil off, but that would be it). I have nicknamed this the "Easy Bake Oven" effect, and may be the reason that no land animal larger than 5 kg seems to have survived.
Related to this, some of the material WOULD make it down to surface, and seems to have sparked off global forest fires. This is called the canopy collapse, and is shown by an increase of soot and charcoal and by an increase of fern spores (the fern spike in sediments post-impact. (Ferns are excellent at recovering from periods of forest fires.)
Phase III (the first decade or so): Impact Winter: Material vaporized by the impact is kicked up into the stratosphere, blocking insolation. This was the primary killing agent suggested by the Alvarez team. Probes on Mars show big temperature drops when fine particles are spread to the high atmosphere. And in human history, the eruption of Tambora in Indonesia in 1815 produced chilling effects worldwide for more than a year later; later eruptions, such as the 1991 eruption of Mount Pinatubo, while not as severe, were better studied and analyzed.
Estimates of duration of the Impact Winter have varied from a year or so to a few months to just a few weeks, but a model published in January 2017 puts a duration for a 26°C (46.8°F) temperature drop in global surface temperature for 3-16 years and a greater than 30 year duration until recovery! Recovery on land goes much more quickly (basically once the skies are clear), but some deep-water sites show the cooling required 10 kyr to recover.
Phase IV (100 kyr or so): Greenhouse Warming: The impact site was covered by carbonate rocks; when you oxidize carbonate rocks, you release CO2 (this is the same as when limestone is converted into cement). The Deccan Traps already released enough CO2 to raise the atmosphere from ~500 to 1400 ppm; the new addition brought it up to 2300 ppm or so. Once the dust was cleared the full effect of the greenhouse gases could be in play. Recent studies show a warming of 5C° in the shallow ocean for 100 kyr; air temperature warming might be between 4.3 and 13.5C°C (average of 7.5°C). See the discussions of the P/Tr and the PETM a few lectures ago about the effects of this on the living world.
General Patterns of the K/Pg Impact
In the marine realm, plankton (including the larval ammonoids) and nekton suffer worse than benthos. Photosynthesizers, and the creatures that feed on them directly (and those that fed on THEM directly) suffer worse than bottom feeders (which eat food "stored" in sediment). Groups with symbiotic algae also suffer strongly. Among marine vertebrates, larger ones in the open seas suffer worse than those on the coast, and those with higher metabolic rates worse than those with lower ones.
(Note: the basic pattern is the exact opposite of the P/Tr, where the benthos got clobbered relative to nektonic and planktonic forms.)
In the terrestrial realm, freshwater animals and those that feed on the water ecosystem tended to do better than those which fed on land. Larger animals and medium-sized animals with high metabolic rates suffer worse than small animals and medium-sized animals with low metabolic rates.
For the terrestrial realm, it seems that "Easy Bake Oven" is the primary selective filter, with the Impact Winter taking out some survivors and the Greenhouse Summer even more. For the marine realm the "Easy Bake Oven" is likely not a factor, but the Impact Winter is the dominant selective force.
The pattern of extinction looks like it was global and essentially instantaneous: hours to days to months to a few years. This suggests that the Chicxulub impact is by far the major causal agent. But all three events (Chicxulub impact, Deccan Traps volcanism, Maastrichtian Regression) are known to occur, so the earlier scenarios may have destabilized the ecosystems to some degree.
