The End of All Things: An Overview of 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.
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.
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):
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:
When identifying reasonable causal agents for any given mass extinction, we have to evaluate if that agent:
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.
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, 252.17 Ma), the Triassic/Jurassic (Tr/J, 201.3 Ma), and the Cretaceous/Paleogene (K/Pg, 66.0 Ma). We will look at the Era-ending events (P/Tr and K/Pg) in detail in the next lectures, so 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.)
Let's take a look at the characteristics of the three non-Era ending events:
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. A fair amount of evidence suggests there were two pulses of extinction, separated by about 1 Myr.
Despite some news items in the 2000s suggesting a "hypernova" or "gamma ray burster" (extremely huge supernova) explosion being the cause, there is zero (zip, nada, no) evidence for this. Instead, 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.)
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 pseudosuchians 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.
Some minor mass extinctions include:
Mass extinction is thus a "Game of Thrones": you only survive or you die. It happens when environmental change happens with such severity that organisms cannot adapt to the shifting conditions. The resultant pattern of life history is long periods of relative stability in terms of groups represented, punctuated by mass extinctions and resorting of the players of the game.
There are some recurring patterns after many mass extinctions. These include:
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.
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