An Overview of Mass Extinctions
"Amidst the vicissitudes of the earth's surface, species cannot be immortal, but must perish, one after another,
like the individuals which compose them. There is no possibility of escaping from this conclusion." -- Sir Charles
Lyell, 1837, Principles of Geology, Vol. 2
BIG QUESTION:What is a mass extinction, and how do we recognize one?
"Dead as a Dodo": Defining Extinction
What is extinction? It depends on your context, with different definitions (or at least different emphases) according to different types of scientists:
- Classical biologist/Paleontologist: When last member of species or clade dies out.
- Geneticist: When that particular genome is no longer passed on.
- Ecologist: When species or clade range goes to zero.
Only two types of taxa can go extinct: species and clades. Old-fashioned gradistic paraphyletic groups could "go extinct" even though their descendants (and thus their genome) persisted on.
The extinction of the dodo (Raphus cucullatus) of Mauritius after the 1660s indicated that species could be driven to extinction by human activity. But there was a great debate in the latest 18th/earliest 19th century as to whether natural extinctions might be possible. Some argued that the Creator would not create something merely to destroy it; others argued that the interactions within the natural world were too closely knit to allow any one species to go extinct without the entire ecosystem collapsing. But the weight of the evidence-- backed by the comparative anatomical studies of Cuvier and his colleagues, as well as more complete exploration of the world-- indicated that there were indeed fossil species no longer present in the world. So people accepted the existence of natural extinctions.
A History of Mass Extinction Studies
Even more, Cuvier demonstrated the existence of mass extinctions (or "revolutions" as he called them): the loss of great parts of the diversity of life, to be replaced by other forms. Furthermore, Cuvier assumed that these revolutions were the products of catastrophes of some sort.
Regardless of their cause, geologist John Phillips (in 1841) used the pattern of fossil diversity, and especially the crash of diversity to define the Eras of the Phanerozoic: thus the boundaries between the Paleozoic and Mesozoic Eras and between the Mesozoic and Cenozoic Eras were mass extinctions.
But not everyone was convinced that mass extinctions were real phenomena. 19th Century geologist Charles Lyell and Charles Darwin thought that there were no mass extinctions: rather, what we thought were Cuvier's "revolutions" were no more than extended gaps in the rock record, making the gradual loss and addition of species at the ordinary rate appear to be the sudden loss and appearance. In Darwin's words (from The Origin):
"So profound is our ignorance, and so high our presumption, that we marvel when we hear of the extinction of an organic being;
and as we do not see the cause, we invoke cataclysms to desolate the world, or invent laws on the duration of the forms of life!"
In the early 20th Century at least some paleontologists resurrected Cuvier's ideas of 'revolutions' under the name of global diastrophism ("diastrophism" being an old world for "geological uplift, mountain building"). Leading scientists such as the American Museum's H.F. Osborn thought that periods of mountain building occurred worldwide, resulting in major climate shifts and causing intense droughts, climate changes, etc.
As it turns out, both the Lyell/Darwin and Osborn ideas were testable by the same observation. Both required that the extinction boundaries would occur at unconformities (erosional surfaces): in the former case, because all the missing record was lost; in the latter, because global diastrophism would cause uplift and erosion. But when many extinction sites were sampled, no such unconformities were found. Something else must be going on.
In the mid-20th Century paleontologist Norman Newell finally coined the term "mass extinction" and recognized these as "crises in the history of life". Because of advances in understanding geologic time, it was recognized that Lyell and Darwin's "gap" argument did not work: there simply wasn't enough missing time to turn these boundaries into "simply business as usual, with missing rock". As for causes, Newell (who worked primarily on marine invertebrates) argued they were mostly due to rapid sea level changes and/or changes in atmospheric composition. Around the same time, German paleontologist Otto Schindewolf argued that some mass extinctions might have been caused by cosmic causes (such as comets colliding with Earth.)
In the later 20th Century paleontologist Jack Sepkoski finally gave us a good working definition for mass extinctions:
"Any substantial in the amount of extinction (i.e., lineage termination) suffered by more than one geographically wide-spread higher
taxon during a relatively short interval of geologic time, resulting in an at least temporary decline in their standing diversity."
- Happened to many different distantly related large groups of organisms, not limited to some particular group (e.g., not just big land mammals or to amphibians, etc.)
- Happened quickly, at least in geologic terms
- Resulted in reduced diversity of life
- Consequently, resulted in fewer niches present after the event
The Big Five
The 1970s and 1980s saw renewed interest in mass extinction studies, in response to research of the K/Pg event 66 Ma (the one that ended the giant dinosaurs): first, the Russell and Tucker supernova hypothesis, and then the Alvarez et al. discovery of the iridium spike (more about these specifics in a few lectures). While this was going on, invertebrate paleontologist Jack Sepkoski was assembling a huge data base of the diversity of marine life through time. This data (like Phillip's in the 1840s, and Newell's in the mid-20th Century) helped reveal the presence of mass extinctions. In this case, however, Sepkoski worked with fellow paleontologist David Raup in 1982 to reveal statistically a difference between "background extinctions" and "mass extinctions".
In this study, Raup and Sepkoski idenfitied what came to be called "The Big Five": mass extinctions which were statistically quite different from the ordinary level of diversity collapses. Raup and Sepkoski identified the Big Five mass extinctions: the Ordovician/Silurian (O/S, 443.8 Ma), the Devonian/Carboniferous (D/C, 358.9 Ma), the Permian/Triassic (P/Tr, 252.2 Ma), the Triassic/Jurassic (Tr/J, 201.3 Ma), and the Cretaceous/Paleogene (K/Pg, 66.0 Ma). Let's briefly look at the O/S, D/C, and Tr/J.
Various studies may lose some of these mass extinctions as barely larger than background (especially the D/C), and some mass extinctions may represent multiple events (dividing up P/Tr into two; turning the D/C into two; etc.) the count maybe be more than 5. But we'll stick with the Big Five for this course. (This doesn't meant that there aren't minor mass extinctions: some moderate mass extinctions do seem to exist.)
The Big Five:
Ordovician/Silurian (443.8 Ma): The Ordovician Period saw a dramatic increase in biodiversity, but crashes at the end. Perhaps 85% of species are lost (although the total number is less than later ones: there were fewer total species known for the Ordovician than for later in the Phanerozoic.) There is devastation within brachiopods, bryozoans, graptolites, trilobites, and conodonts, but more major clades were lost.
This event coincides with a major period of glaciation. It has been suggested that this might have been responsible for the extinction by perhaps reducing the available habitat area in the shallow marine realm (because of a major sea-level drop), and/or a major period of anoxia from global eutrophication, due to the influx of considerable amount of nutrients from material scraped off the continents from glaciers.
Devonian/Carboniferous (358.9 Ma): For a long time (and to some researchers, some still consider it) to be two extinctions: a first strong one between the Frasnian and Famennian Stages of the Late Devonian Epoch, and the final Devonian/Carboniferous event (also called the "Hangenberg Event"). But newer studies tend to support a single major terminal Devonian event, with the Frasnian/Famennian event as a result of the Signor-Lipps event. This is characterized by a collapse of major reef community (the tabulate-stromatoporoid reef community); as today, reefs in the mid-Paleozoic were MAJOR centers of biodiversity, so collapse of the reef community dragged numerous other species with it. Additionally, there are losses of primitive grades and clades of fish ("ostracoderms", "placoderms", and acanthoidians). There are also major extinctions within trilobites (again), eurypterids, echinoderms (with some major clade losses), conodonts (again), bryozoans (again), and ammonoids (for the first but not the last time). Many of these survived with only a few handful of species to repopulate the Carboniferous.
Although Schindewolf proposed an asteroid impact as the cause for this (without any direct evidence), this is currently best explained as a result of the spread of vascular land plants! With the first rain forest and the first seed plants in the continental interior, there was considerable burial of excess carbon (leading to cooler temperatures and reduced continental shelf space). More importantly, increased mechanical and chemical weathering on the continents leads to increased nutrients into the sea leading to marine phytoplankton booms leading to eutrophication and anoxia. (Indeed, modern coral reefs are suffering from these effects locally because of increased runoff due to human activities in these regions.)
Permian/Triassic (252.5 Ma): The closest that Life has come to being extinguished outright. If this was the single terminal Permian event, then it was an event with 55.7-82% of the marine genera went extinct.
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.
The marine realm of the Permian was 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.)
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. Moreover, deposition of organic-rich marine sediments and changes in sedimentation styles on land suggest that this extinction involved not just a reduction in diversity, but an absolute reduction in biomass.
Causes for the Greatest Extinction
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: Mollusks, arthropods, and vertebrates survive better than brachiopods, echinoderms, bryozoans, and cnidarians.
New evidence (from 2014) demonstrates that the main pulse of extinction took a mere 60 kyr, plus or mins 48 kyr, to occur! 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) with advanced respiratory systems survive quite well.
Triassic/Jurassic (201.3 Ma): In some ways a repeat of the P/Tr on a smaller scale. The final extinction of the conodonts, a major diversity crash in ammonoids (AGAIN!), loss of once-major groups of brachiopods and bryozoans, collapse of some corals and sponges, and on land loss of primitive seed plant groups, various diapsid reptiles (especially the majority of the crurotarsans outside of Crocodylomorpha), and of various therapsids (basically everything but mammals and their immediate closest kin).
The causal agent here is the Central Atlantic Magmatic Province (CAMP for short), a major field of volcanics and intrusive igneous rocks associated with the break-up of Pangaea and the formation of the Central Atlantic Ocean basin. It is comparable in scale to the Siberian Traps of the P/Tr in terms of the area covered and volume erupted, but apparently lower levels of carbon released: "only" 2200-2500 Gt C as CO2 and 4300 Gt C as CH4.
Cretaceous/Paleogene (66 Ma): This extinction struck primarily at three major parts of the food-web:
- Oceanic primary producers and plankton (coccolithophorids, foraminiferans)
- The creatures that depended upon them directly (ammonoids, reef-forming clams, giant suspension-feeding actinopterygians)
- Organisms at the top of the food web on land and sea. (plesiosaurs, mosasaurs, hesperornithiform birds, most terrestrial vertebrates larger than 5 kg., pterosaurs.)
The primary causal agent here is the Chicxulub impact event - a major impactor first recognized through the K-Pg iridium anomaly - a global chemostratigraphic enrichment in extraterrestrial iridium, but subsequently identified in the subsurface of Yucatan.
There is evidence that the Chicxulub impact was not the only causal agent. Other contributors include:
- The Deccan Traps flood basalt - one of the largest continuous volcanic eruptions in history - was occurring at the same time.
- The Maastrichtian Regression - resulted in a reduction in shallow-marine habitat and changes in global circulation pattern.
Some minor mass extinctions include:
- The Eocene/Oligocene event (33.9 Ma), trigged by a shift from greenhouse to icehouse planetary conditions with the rise of the circum-Antarctic current, sees major extinctions in plankton and in land vertebrates.
- The Paleocene/Eocene event (56.0 Ma), triggered by the PETM. Again, plankton and land vertebrate extinction.
- The Early Jurassic/Middle Jurassic event (174.1 Ma), triggered by a flood basalt event. Primitive dinosaur groups are replaced by more advanced forms, amount the most noticeable changes.
- And many extinctions during the later Cambrian.
Agents of Destruction
When talking about "causes" of mass extinction, we have to recognize we are talking about several different things:
- The causal agent (or "trigger"): the initial phenomenon that starts the extinction event. For example, an asteroid impact, a volcanic eruption, etc.
- The killing agent: the particular aspect of the phenomenon that does the killing. For example, atmospheric dust, carbon dioxide, thermal pulse, etc. (NOTE: a causal agent can produce multiple killing agents.)
- The killing mechanisms: the physiological change produced by the killing agent. For example, starvation, asphyxiation, starvation, heat load, etc.
When identifying reasonable causal agents for any given mass extinction, we have to evaluate if that agent:
- Be capable of actually generating the extinction (and if there are both terrestrial and marine components to the event, be capable of effecting both)
- NOT be overkill! (After all, every organism that lived after the extinction event is a descendant of survivors of the event)
- Be identifiable as being present at the time of the event from evidence independent of the extinction event itself
Some paleontologists have described a distinction between pulse and press extinctions. Pulse extinctions are rapid catastrophic events, which do not allow for adaptive changes during the episode itself. These are the "Game of Thrones" events: you either win or you die. In contrast, press extinctions would be series of events spread out for 100s of thousands of years. In principle, it allows for evolution to select variations throughout this extended interval, so a taxon which is doing poorly early in the event might become progressively better at surviving it over time. (That said, no one has made a good case for any pulse aspect to mass extinctions.)
A related set of terms are proximate vs. ultimate causes. Proximate causes are the events that directly brings about the change. Ultimate causes are the phenomena that sparks the proximate causes. For example, the proximate cause for the sinking of the Titanic was the flooding of the decks through the hole ripped in its side by the iceberg; the ultimate causes were (among other things) failure to spot the iceberg sufficiently in advance to turn it. For any given extinction (or mass extinction), we have varying levels of success in identifying the proximate or ultimate cause.
A problem with all mass extinction studies is the issue of stratigraphic resolution: that is, how closely does our observed record match the actual event itself. The same set of observed data of species disappearing one-by-one running up to a stratigraphic boundary might potentially be explained as a gradual extinction OR it might be explained by the fact that we are statistically unlikely to recover the last population of any given species. This is the Signor-Lipps Effect: the observation that the stochastic nature of fossil preservation and recovery will tend to "smear" out an instantaneous extinction to make it look gradual.
In order to counteract the Signor-Lipps effect we need to have very high sample sizes of fossils and of stratigraphic horizons running up to the possible event. But nature doesn't always provide us with this, so we always have to remember that our ability to definitively say when a particular taxon went extinct is limited.
There are some recurring patterns after many mass extinctions. These include:
- The Liliput Effect: survivors of mass extinctions (or other diversity crises, like PETM), are often markedly smaller than the pre-extinction populations. At present we cannot resolve if this is environmentally-induced dwarfism (that is, stunted growth) or if it is positive selection for the smallest members of the normal distribution of the population.
- Lazarus Taxa: Not necessarily associated with mass extinctions, but any taxon which temporarily disappears from the fossil record. Presumably the lineage persisted either at some non-preserved location, or at highly reduced abundance, only to recover later. This makes it looks like it "rose from the dead".
- Elvis Taxa: Species which converge upon the form of a taxon which went extinct. Thus they are "impersonators" of the original.
- Zombie Taxa: Cases where fossils of already extinct fossils become reworked (from a channel deposit, cliff, etc.) and deposited in geologically-younger sediments, thus making it look as if the fossil species were alive long after it was actually dead.
- "Dead Clade Walking": When a taxon survives at low diversity and abundance after a mass extinction, but fails to become reestablished and goes extinct at the next geologic interval.
It has also been noted that after a mass extinction event, taxa that live close to the sediment-water interface typically recover well, but extend upwards and downwards from it as time goes by.
Because this is a phenomenon which has happened multiple times in Earth's history, some have speculated that there is a common cause behind all of these. One idea that has been proposed is that mantle plumes (jets of heat from the core) periodically cause Large Igneous Provinces (LIPs) and thus massive flood basalt events. We already have associations between LIPs and several mass extinctions (P/Tr and the Siberian Traps; Tr/J and CAMP; the K/Pg and the Deccan Traps; 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).
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!! 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.
Next lecture we'll look at the two best studied: the Permian/Triassic one then the Cretaceous/Paleogene one.