GEOL 204 Dinosaurs, Early Humans, Ancestors & Evolution:
The Fossil Record of Vanished Worlds of the Prehistoric Past

Spring Semester 2015

Death from Above, Death from Below: Contrasting the K/Pg and P/Tr Mass Extinctions

The Cretaceous-Paleogene Extinction (66.0 Ma)
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.

Testing Ideas
Many hypotheses proposed for the K/Pg Extinction. In evaluating the hypotheses, must consider:
Most significantly: (Without those two aspects, the hypothesis is not scientific, but simply speculation)

Here are but some older proposed causes for the K/Pg event:

  • Global Diastrophism
  • Racial Senescence
  • Poison Gas from Comets
  • Caterpillars ate all the food
  • Mammals ate the dinosaurs to death
  • Allergies to Angiosperms?
  • Diseases

    Modern Approaches to the Cretaceous-Paleogene 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 (dino 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.


    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.

    The Chicxulub Impact
    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.

    Shocked Quartz:

    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).

    Question, though: was the extinction just from impact?

    Media (and some professional scientists) act as if Chicxulub impact was only global change occurring at K/Pg boundary.

    However, equally good geological evidence for some other big changes:

    Deccan Traps Volcanism:
    Long known that a period of intense volcanism begins in later part of Cretaceous. 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

    Some try to dismiss Deccan Traps as a side effect of Chicxulub crater, but begins a little too early (see paleomagnetic data above).

    So, Deccan Traps themselves were a MAJOR event, and might have contributed greatly to the extinction event.

    But there were even earlier, longer term geologic changes:

    Maastrichtian Regression:

    Maastrichtian Regression clearly happens (latest Maastrichtian terrestrial rocks on top of earlier Maastrichtian shoreline rocks on top of earlier marine rocks).


    All three events (Chicxulub impact, Deccan Traps volcanism, Maastrichtian Regression) are known to occur. Can we separate their effects in the geological record?

    Suggestions that all these systems were in effect:

    But, there are complications:

    When Life Nearly Died: The Permo-Triassic Mass Extinction
    Permian/Triassic (252.2 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.

    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 Permo/Triassic fauna wiped out most of the remaining Cambrian fauna, and was noticeably the time the
    Paleozoic fauna stopped being the dominant assemblage and the Modern fauna took over.

    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:

    So mollusks, arthropods, and vertebrates survive better than brachiopods, echinoderms, bryozoans, and cnidarians.

    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!

    After the End
    In the 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 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 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.

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    Last modified: 4 March 2015