When Life Nearly Died: Reconstructing Mass Extinctions
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.
During the late 18th and early 19th Centuries paleontologists such as Georges Cuvier helped establish the reality of natural extinctions. Significantly, they discovered that there were some intervals of time with tremendous turnover of diversity, which would in the 20th Century be named mass extinctions:
So a mass extinction must:
The 19th British uniformitarian gradualist Charles-es (Lyell and Darwin) did not accept these revolutions as real, and instead thought that these apparent catastrophes were the byproduct of missing data:
In the mid-20th Century (1950s-1960s), renewed interest in the study of "revolutions". M. Newell coined the term "mass extinction", recognizing these as "crises in the history of life." He was able to show that the new mathematical and analytical methods of estimating ages and durations in the geologic record removed the "gap" argument did not work: there wasn't enough time missing to transform gradual rates of background extinctions, so there had to be intervals of time when the extinction rates rapidly increased. He argued primarily for rapid sea level changes and atmospheric changes as likely causes for these events. At around the same time, Otto Schindewolf proposed cosmic causes for mass extinctions (such as asteroids).
During the 1970s and the 1980s the beginning of the modern era of mass extinction of studies begins. In 1971 paleontologist Dale Russell and astrophysicist Wallace Tucker proposed that a supernova (exposing star) sleeted Earth with radiation to cause the Cretaceous/Paleogene Extinction. In 1980s geologist Walter Alvarez and his team (more about them below) provided evidence for an asteroid impact as a superior explanation for the K/Pg. More broadly, paleontologists Dave Raup and Jack Sepkoski used computer-based statistical analyses of taxon ranges of marine fossils, and were able to quantify a distinction between background extinction rates and the Big Five mass extinctions.
Eve since, the number of research papers on the topic of mass extinction in general and of each particular one has exploded. Researchers from many diverse fields have contributed to this, and it is one of the richest field of paleontologically-related subjects in terms of the literature.
Identifying proximate and ultimate causes in particular extinctions and mass extinctions is possible, with varying levels of success.
While identifying reasonable causal agents, we must keep in mind that the agent:
Some have indicated a distinction between pulse and press extinctions. Pulse extinctions are rapid catastrophic events that do not allow for adaptive changes during the episode. Much like the "Game of Thrones", you either Win or you Die (the taxa present either survives or doesn't). Traits that allow survival (or favor extinction) sorted by this pulsed event, rather than for changes in ecology as such. In contrast, press extinctions are a series of events spread over 100s of kyr. In principle, this allows for selection process to sort for adaptations to conditions during this extended interval. (That is, it might be possible for a taxon to be doing poorly in the early phases of the event, but doing better over time with the accumulation of adaptive traits.)
One problem addressing mass extinctions is the problem of stratigraphic resolution. Because the chance of any one individual, or locality, or species actually entering the fossil record and being discovered is not very high, it is exceedingly unlikely that anyone will find the youngest individual of a species right at the extinction boundary. Thus it might be that a gradual extinction event and an instantaneous one might look the same. Paleontologists Signor and Lipps observed this, and indeed this is now referenda to as the Signor-Lipps Effect: instantaneous extinctions get smeared out to look like gradual ones. By collecting more individuals from more horizons this effect might be reduced, but never eliminated.
Various studies may lose some of these mass extinctions as truly larger than general (especially the D/S), and some mass extinctions may represent multiple events (dividing up P/Tr into two; turning the D/S 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.)
Let's take a look at the characteristics of the Big Five:
Ordovician/Silurian (444 Ma): The Ordovician Period saw a dramatic increase in biodiversity, but crashes at the end. Perhaps 85% of species are lost. 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 (359 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 speices with it. Additionally, there are losses of primitive grades 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 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 95% species level extinction in the marine realm. (Or in other words, only 5% survivorship at a species level, and given that a species could survive with very few individuals, it was much greater than 95% of individual 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. Victims include Paleozoic corals (tabulates and rugosans), trilobites (their final extinction), eurypterids (ditto), primitive groups of plants, primitive insect groups (the ONLY mass extinction to cause clade-level extinctions in the insects!), and many types of echinoderms. 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.
Up until recently this was thought to be a very gradual event. Paleontologist Curt Teichert wrote (in 1990):
However, work in the 1990s established the catastrophic nature of this event. It's 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. 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. Nektonic, planktonic, and infaunal benthic organisms suffered least strongly; motile epifauna more intensely, and sessile epifauna very strongly. Additionally (and probably related to this), animals which are physiologically "buffered" (that is, have gills or other structures that moderates the contact of the external condition with the internal ones) tended to survive better than those that were "unbuffered" (whose "insides" were in more direct contact with the sea water.) So mollusks, arthropods, and chordates survive better than brachiopods, echinoderms, bryzoans, and cnidarians.
In the aftermath of the P/Tr 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.
Triassic/Jurassic (201 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 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 (65.5 Ma): Not the biggest, but the most famous by far! Since the beginning of the Cenozoic Era was once called the "Tertiary Period", this was for a long time called the "Cretaceous/Tertiary" or "K/T" extinction. 50% or higher genus extinction rate is observed, including basically all land animals of greater than 5 kg mass. Consequently, this took out the non-avian dinosaurs, the pterosaurs, many land and aquatic crocodilian groups, and (although they were smaller) all toothed birds and many mammal groups. In the marine realm the ammonoids FINALLY died out, as did several groups of large-bodied bivalves, the large marine predators (big sharks, big ray-fins, most marine reptiles). There were also major extinctions within the phytoplankton and zooplankton, but these recovered to one degree or other.
There is a major volcanic event associated with this: the Deccan Traps of western India. This was an eruption of at least 2 million km3, with all the effects brought with these events. This began more than 350 kyr before the K/Pg boundary. Nevertheless, the primary killing agent seems to have been the impact of an asteroid in Mexico.
1980: Walter Alvarez was investigating a layer of clay in Gubbio, Italy at the K/Pg boundary. Wanted to determine length of time represented by the clay layer. Consulted dad (Nobel winning physicist Luis Alvarez) for possible solution. Suggestion:
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 (iridium spike) at base of clay: clearly not an "average" of infall.
Hypothesized: an asteroid impacted Earth at the K/Pg boundary
Modern analogue: fear of nuclear war during 1980s concerned with nuclear winter, the likely consequence to a large-scale nuclear war first proposed shortly after (and suggested by) the Alvarez scenario
Biotic prediction fits most of the predictions; search for geological signature was on.
Melt Glass (Tektites):
Tsunami ("tidal wave") and ejecta deposits:
So, great evidence for an impact at K/Pg independent of extinction. Also, pattern consistent with proposed effects (although some versions of the superacid rain, global fires, and global super tsunamis do not have good evidence and are probably "overkill" scenarios).
Suggestions that all these systems were in effect:
Extinction of non-avian dinosaurs paves the way for the rise of mammals as the dominant group of terrestrial animals. Marine realm recovery represents survival of many groups, but less change in structure.
Some minor mass extinctions of interest include:
Lazarus Taxa: Species which are present in the fossil record, disappear, and then return seemingly from the grave. In a sense, ALL fossil taxa are Lazarus taxa, since there is always some gap (however small) between successive depositional horizons. But the term is used for cases where the taxon disappears for millions, even tens of millions, of years.
Elvis Taxa: At first glance they seem to be Lazarus taxa. But examination of the later form demonstrates it is not actual a survivor of the old form, but an example of convergence ("impersonation") of the extinct taxon.
Zombie Taxa: Specimens of extinct (dead) organisms that get reworked out of older sediment and redeposited in younger ones.
"Dead Clade Walking": Named by Dave Jablonski in 2001. Some taxa may survive the immediate extinction event, but fail to re-establish and go extinct during the next geologic interval.
To Lecture Schedule
Back to previous lecture
Forward to next lecture