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

Spring Semester 2018
Death from Above! The Cretaceous/Paleogene Extinction

Detail from "Dead T. rex" by Douglas Henderson (1999)

"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)


"Night comes to the Cretaceous." -- J.L. Powell, 1998

BIG QUESTION: What caused the Cretaceous-Paleogene extinction and the end of the Age of Dinosaurs?

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 (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.


    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:

    To Lecture Schedule

    Last modified: 12 March 2018

    Detail from "The Cretaceous-Tertiary Impact" (2007) by Joe Tucciarone